MULTI-HULL CRAFT

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					                                                                                   CHAPTER 45
Antony Armstrong


CATAMARANS

45.1     INTRODUCTION                                    which relies on the buoyancy of the oars or the
                                                         dynamic forces generated by the oars to remain
          The vast majority of water-borne craft have    upright.
a single-hull, because this provides a simple solution             An obvious solution to this dilemma of poor
to the problem of transporting a given payload across    stability is to add one or more stabilizing hulls to the
the water, and at a minimum cost. The conventional       main hull, and this is the basic reason for the success
mono-hull solution to the transportation problem is      of the multi-hull as a high-speed craft.
usually a compromise, because, as with most                        The advantages of a high L/B ratio in
engineering problems, there are many different           minimizing resistance, are such that a catamaran (L/b
requirements such as payload and stability as            = 15) will have considerably less resistance than a
illustrated in Figure 45.1. Prior to about 1850, the     mono-hull (L/b = say 6) of equal  and L, despite
principal requirement of a craft was to provide as       having two hulls.
much stowage space for payload as could be fitted                  Adding another hull of the same dimensions
into a given length and sail layout. The speed of the    as the main hulls results in a craft generally termed a
vessel was never an issue, and it is only since the      “catamaran”. There are other types of craft with
introduction of mechanical propulsion and the socio-     more than one hull. If the other stabilizing hull is
economically-driven development of international         smaller, a commonly used term is a ‘proa’, as
trade, that the concept of maximizing speed has          illustrated in Figure 45.2. Adding two small hulls to
become important. Today, greater speed can provide       provide additional stability results in a craft termed a
a commercial edge in getting the product to the          “stabilized mono-hull” (Monostab) or “trimaran”.
market as well as suiting the modern rapid lifestyle.    When the stabilizing hulls, or pontoons are
Greater speed might be achieved by greater installed     somewhat larger, the craft is generally termed a
propulsion power, but usually it is more efficiently     “trimaran”.
obtained by the minimization of resistance.
          Unfortunately, such a minimization of
resistance affects the compromise engineering
solution that so favors the simple mono-hull at lower
speeds.                                                                   Figure 45.2 - Title

                                                                   All of these above examples are multi-hull
                                                         craft. Many of the characteristics that they exhibit
                                                         can be applied to craft that are powered by
                                                         mechanical means, or by person-power, for example,
                 Figure 45.1 - Title                     a “pedallo,” or by the wind.
                                                                   High-speed sailing catamarans however are
          Analysis of the somewhat simplified            somewhat different because they are usually
relationship illustrated in Fig 45.1. shows that the     designed to operate with some heel angle, and
hull form determines the resistance of the craft. As     trimarans may, in hydrodynamic terms, be
will be further examined in Chapter 45.2, the            considered as monohulls as one hull may be above
principal hull characteristics that determine the        the water surface. Sailing proas such as Crossbow
resistance at a given high speed are the ratios of       (Figure 45.3) may only be operated on one tack, and
slenderness L/ 1/3 and thinness L/B, where L is the     may be designed to operate as a mono-hull with the
waterline length, B is the individual hull beam, and    side hull above the water surface.
is the displacement.
          The solution for a mono-hull to achieve low
resistance is a minimum displacement, a minimum
beam and the greatest practical length.                                      Figure 45.3 -
Unfortunately these three hull characteristics not
only affect the resistance, but the stability                    The remainder of this Chapter will therefore
characteristics and the payload capacity. A long thin    concentrate on mechanically propelled craft and
lightweight mono-hull may have the minimum of            catamarans.   (See Chapter 46 -SWATH and
resistance, but would exhibit poor and probably          Trimaran Vessels for discussion of other multihull
inadequate stability. An example is a rowing shell,      craft). A discussion on catamarans requires the


                                                     46-1
introduction of additional nomenclature, as               developed based on the sea. Catamarans were ideal
illustrated in Figure 45.4. The overall beam B of the     for this trade, as they offered a large deck area and a
craft is made up of the beam of the individual hulls      high-speed resulting from their low resistance.
b, and the hull separation Sc. The space between the      Westamoen (later Westamarin) was at the forefront
hulls is generally called the tunnel, and when            of the construction of catamarans built from
discussing resistance effects the tunnel width or the     aluminum, and a typical vessel is illustrated in Figure
clear distance between the inside hull surfaces ST        45.5. These passenger craft were all less than 35mm
may be of interest. When considering slamming, the        in length.
height of the underside of the structure between the
hulls HT may be important. Such structure is
generally called the “bridging structure” and the
depth of this structure HB is an important structural
characteristic.                                                               Figure 45.5 -
          For a catamaran, the individual hulls are
often called demi-hulls.                                            In 1975 the first high-speed catamaran was
                                                          built in Australia, following the collapse of a bridge,
                                                          which split the city of Hobart into two. A ferryboat
                                                          operator Robert Clifford, recognized the ability of a
                                                          catamaran layout to carry large numbers of
                    Figure 45.4 -                         passengers, and rapidly built a number of such craft
                                                          in steel to provide a service between the two severed
45.2 BRIEF            HISTORY           OF       THE      parts of the city. These craft were immediately
CATAMARAN                                                 successful, and Clifford recognized from this that a
                                                          lightweight version of the same craft could provide a
          The earliest catamarans were probably           means of transporting tourists from the Australian
developed amongst the islands of the Pacific and          mainland to the Great Barrier Reef, Australia’s main
Indian Oceans. Catamaran is a Tamil word meaning          tourist attraction, a trip which could take up to two
“tied wood” (1). This type of craft almost certainly      hours by conventional ferry.
developed because the only available material was a
hollowed-out tree trunk. Such a craft of necessity
would have a large L/b ratio, and very poor stability
characteristics, which could have been dramatically                           Figure 45.6 -
improved by lashing cross-bracing to the main hull
and adding another hull or outrigger to the cross-                  The first aluminum passenger ferry in
bracing. Such craft had exceptionally good stability      Australia was built and demonstrated with immediate
characteristics and allowed trans-oceanic journeys to     success by Clifford’s new company, International
be made between islands over a wide distance,             Catamaran (Incat). After building thirty or forty of
particularly throughout the Northern Pacific, where       these boats, one of which is illustrated in Figure
the Polynesian canoe is still in use today.               45.6, attempts were made to improve the sea keeping
          The earliest mechanically propelled             performance in head seas by making the hulls
catamarans were designed at the end of the 19th           extremely fine at the bow and removing the flare of
century to operate across the English Channel. The        the hulls above the waterline. In this way it was
first mono-hull ferries were driven by paddle wheels,     thought that the hulls would pierce through the
and were operated directly across the prevailing sea      waves rather than to ride up and over them. This
direction, as a result of which the vessel would roll     concept was demonstrated with a 5m prototype
and expose the paddle wheel on one side, and deeply       “Tassie Devil” and then a 30m prototype “Spirit of
immerse the other, resulting in an inefficient            Victoria”, illustrated in Figure 45.2.7.
propulsion system. A catamaran layout with the
paddle wheel at the center of the craft solved this
problem, as well as providing for a more attractive
layout for passengers.        However, it was also                            Figure 45.7 -
recognized at this early stage that the catamaran
layout offered a large deck area and a lower                       Recognizing that additional buoyancy was
resistance.    Unfortunately the catamaran layout         needed at the bow, the fore body of the catamaran at
resulted in a rolling characteristic that increased the   the centerline was molded into a ship bow shape, but
levels of motion sickness.                                located above the waterline such that it only became
          The modern history of the catamaran starts      immersed when pitching down into waves, and so
in Norway in the 1960’s. With towns situated on the       provided hydrodynamic and hydrostatic lift when
edges of the fjords and with high mountains between       necessary. These craft were termed “wave piercing
them, which were usually impassible in wintertime, it     catamarans”.
is easy to understand how a transportation system


                                                      46-2
         In 1988, Hoverspeed instigated the design                From the year 2000 it is anticipated that the
and subsequent construction of a 74m length wave-        established and proven designers and builders of
piercing catamaran arranged to carry passengers and      high-speed catamarans will continue to improve the
cars. This craft was the first of five intended to       product and to expand the range of capabilities,
supplement the two SRN4 hovercraft that were             possibly with technology spin-offs into the military
operated by the company. The initial wave piercer        area. Reliability of structure and machinery will be
“Hoverspeed Great Britain”, illustrated in Figure        improved, the sea keeping characteristics will be
45.2.8 was the first vehicle-carrying high-speed         exploited to result in reduced motions, and the
catamaran, and proved the speed capability on the        effects on the environment will be specifically
delivery voyage in June 1990 by successfully taking      considered.
the Hales Trophy for the fastest crossing of the
Atlantic by a passenger craft, a trophy that had been
held since 1953 by the SS United States.                 45.3       CATAMARAN CHARACTERISTICS

                                                                  Catamaran high-speed craft have been
                                                         developed to exploit the inherent advantages of such
                                                         craft, namely:
 Figure 45.8 - Hoverspeed Great Britain Leaving
  New York for the Successful Attempt on the                       A large deck area.
            Hales Trophy, June 1990
                                                                   Reduced hull resistance leading to higher
                                                                    speeds or lower fuel consumption.
          In the ten years since 1990, the size of the
catamarans has grown as the available engine power                 Increased safety levels
has grown and the complexities of the structural                   Attractive layout possibilities resulting
design have become understood. The larger vessels                   from the wide beam.
can now carry trucks and coaches. Some catamaran
car ferries are operating at very high speed, above 50             The challenge is to maximize the
knots. The principal manufacturers of high-speed         advantages whilst addressing the structural
car ferrying catamarans have been Incat and Austal       difficulties in the design of these craft.
Ships in Australia, although the largest craft have                A catamaran essentially contains three
been built in Finland and Canada. The principal          elements:
Norwegian builder of aluminum catamarans,
Fjellstrand has been the major manufacturer of                  1) The hulls, which are principally intended
passenger high-speed catamarans, including a                       to provide buoyancy and to house the
subsidiary shipyard in Singapore, with over 50 craft               propulsion machinery.
built at about 40m size. Figure 45.2.9 illustrates the          2) The connecting structure, sometimes
growth of the industry between 1989 and 1999, and                  called the ‘bridging structure’, between
the size and number of vessels in service at the                   the hulls. This structure provides the
beginning and end of the ten-year period.                          transverse strength of the craft.
          Since the introduction of “Hoverspeed                 3) The superstructure is fitted above the
Great Britain”, the high-speed catamaran industry                  hulls and the bridging structure
has been a volatile and slowly maturing one. The                   containing the passenger accommodation
desire for ever-increasing speed, sea keeping ability              space deck. Some manufacturers have
and deadweight capability has ensured that new                     fitted the passenger accommodation as a
designs have been one step ahead of regulations and                module onto rubber mounts, mounted on
legislation. A constant flow of ideas has led to                   the hulls and bridging structure, and in
confusion in the industry as designers have promoted               this way ensured that no structural loads
their own ideas, which were mostly unproven.                       are transmitted into the superstructure
          Several established builders of conventional             and isolated the passengers from noise
craft, often with many years of experience in steel                and vibration from the propulsion
construction, have failed with their attempts to build             system. This system is also used by Incat
lightweight catamarans as a result of not appreciating             under the passenger cabin on the
that different materials require different design and              vehicular ferries, as illustrated in Figure
construction techniques.                                           45.10.




                                              Figure 45.9(a)-(c)


                                                     46-3
                                                                           The straight-line stability, or course-
                                                                 keeping ability of multi-hulls is generally poor, akin
                                                                 to an arrow without flight feathers. This can be
                                                                 improved by the addition of skegs at the stern. Hard
                                                                 chine shapes exhibit improved course-keeping
                    Figure 45.10 -                               ability, and consequently some commercial builders
                                                                 have developed a hull shape being round bilge in the
45.4     HULL SHAPES                                             forward part and with a hard chine in the after part.
                                                                           A special type of round bilge form is the
          Early catamarans were characterized by                 semi-swath, so called because it demonstrates the
asymmetrical hull shapes, as illustrated in Figure               main characteristics of a SWATH described in
45.5. These can be described as being a mono-hull                Section 45.6 as having a small waterplane. The
shape split along the hull centerline and the two parts          semi-swath generally has a fine angle of entrance at
separated, with a (usually) vertical inside surface,             the waterline in the forward part, as illustrated in
and provided a minimum of additional resistance                  Figure 45.11. The after part is generally more full in
from the interference effects resulting from having              order to accommodate the propulsion machinery and
two hulls in close proximity. The small hull                     the waterline may be wedge shaped, with the widest
separation reduced the structural loads by                       part almost at the transom. A chine may be
minimizing the span of the bridging structure. These             introduced at the stern to improve course keeping
loads were not fully understood, but as knowledge of             and sea keeping. Figure 45.11 illustrates the largest
the behavior of the multi-hull structure in a seaway             high-speed catamaran built to date “Stena Explorer”,
developed, it became possible to design with larger              which as a patented semi-swath hull shape,
spans.                                                           developed for a minimum of ship motion in a seaway
          This lead to a reduction in interference               and with a low resistance.
effects on resistance, as discussed in Section 45.2.3,
and allowed the hulls to each become symmetrical in
shape. This simplified construction and minimized
the wetted surface area, allowing for further
reduction in resistance, such that almost all                            Figure 45.11 - HSS Stena Explorer
catamaran craft by 2000 were of the symmetrical hull
type.
          Almost all high-speed catamaran hull                   45.5     HULL CHARACTERISTICS
shapes feature a transom stern, which offers three
distinct advantages. Firstly, it is simple to construct.                   The hull forms associated with high-speed
Secondly, it provides an abrupt change to the hull               multi-hulls     are     generally   simple    shapes.
shape along any flow streamline, causing the flow to             Considerable leeway is possible in the hull
separate from the full at a fixed point. This prevents           characteristics without unduly affecting the hull
any problems associated with flow separation as                  performance, unlike conventional slower speed hulls
could be anticipated with conventional shapes such               where many parameters have an optimal value, and
as a canoe stern, although it also causes additional             may be determined by simple empirical formula. For
drag as discussed in Section 45.4. Thirdly, the                  example, there is no obvious relationship between
transom stern provides a simple and effective means              hull speed and form coefficients, such as that
for the mounting of water jets, a favored propulsion             demonstrated by the Alexander formula relating the
system for high-speed craft.                                     ship speed V and the block coefficient CB. This
          The sectional hull shapes are generally                difference from conventional slower speed hull
characterized into two types – round bilge and hard              shapes may be because as the speed increases, the
chine. The hard chine shapes are somewhat simpler                component of resistance associated with friction also
to construct and are favored by less experienced                 increases, as illustrated by Figure 45.12. The effect
builders. A round bilge hull form at high speeds will            of the transom stern may also be a factor.
generate lift, and the center of this force can move
longitudinally by a substantial amount, such that a
poorly designed round bilge hull may exhibit large
amounts of dynamic trim at high speeds, with an
associated increase in resistance.                                                  Figure 45.12 -




                  (a)                                      (b)                                   (c)



                                                       46-4
                                                   Figure 32.13 -
          The choice of hull parameters is therefore        are beneficial effects reducing resistance, as
usually driven by more pragmatic requirements than          suggested by Everest (1974) and later by Soding
optimizing the hydrodynamic performance.                    (1997). Similar effects could be evident if one of the
          The length L is usually chosen to suit the        demi-hulls was smaller, as would be the case with a
required deadweight and speed. Generally a longer           proa, or when two smaller “demy-hulls” were fitted
hull results in an improved hydrodynamic                    as with a trimaran.        However, it is probably
performance, however the construction cost of a             impractical to take advantage of these troughs in the
vessel is also directly related to the length, as may be    humps and hollows of the resistance curve because
the operating costs. The choice of hull length is           the effects are only valid for a given displacement,
therefore a balance between cost and performance.           craft speed and depth of water. If these are altered
Figure 45.13 (a)-(c) illustrates the broad relationship     then what was the advantage of the trough may
between length and payload for many multi-hull craft        become the disadvantage of a crest, requiring a
built between 1990-2000. The hull beam b is usually         change to the hull geometry.
the minimum width chosen to suit the chosen                           The vessel draft T may be determined by
propulsion machinery layout. This dimension is              the propeller diameter, required propeller shaft
typically dictated by the necessary width on the            angle, or by the immersion of the water jets and the
transom to mount the water jets, or by the width of         associated machinery and transmission.           There
the main propulsion engines and associated                  appears to be an optimal value of b/T associated with
gearboxes with the necessary allowances being made          a minimal resistance, but this only a weak
for access to the machinery and for any structural fire     relationship, as illustrated in Figure 45.16, and it
protection against the hull structure. By minimizing        may be varied considerably without a significant
b it can be anticipated that the resistance will also be    effect on resistance.
reduced, as may be the characteristics of the bow
wave. Most multi-hulls do not utilize the space in
the hulls. If it is desired to place accommodation or
other spaces below deck, then the hull width may
need to be enlarged.                                                             Figure 45.16-
          The overall beam B is largely determined
by the tunnel width ST, as illustrated in Figure 45.4.                The block coefficient CB is typically chosen
It is known that the presence of two hulls operating        between 0.45 and 0.55. The half angle of entrance ie
alongside each other leads to interference affects and      should be chosen as small as practical in order to
changes to the residuary resistance. This has been of       reduce the bow wave height and period. Valves of
interest to researchers for many years, for example         3° - 5° for the larger vessels at speeds of 40 knots,
Everest (1974) and Doctors (1994), and it is                varying up to 12° for the smaller craft at speeds of 30
apparent that at some values of ST, there will be           knots are typical.
greater or lesser total resistance. Generally the                     The deadrise angle α is obviously a
resistance will decrease as the hull separation is          parameter that affects the midship section area. It
increased, but above a certain value the effects might      may be anticipated that different values have an
be considered to be small, as illustrated in Figure         effect on resistance, but exactly what this
45.2.14. (a) and (b) taken from Muller-Graf (1994).         relationship may be is not yet understood. Flatter
                                                            deadrise angles generate greater hydrodynamic lift,
                                                            and a small planing craft, for example a 25m long
                                                            craft operating at 35 knots, would have a dead rise
                                                            angle of about 14°. A larger craft operating more in
                                                            a displacement mode, would have a higher dead rise
                                                            angle, for example a 74m long craft at 35 knots may
                                                            have a dead rise angle of up to about 26°.
                                                                      The displacement  is the principal
                                                            parameter of importance in the design of high-speed
                     Figure 45.14 -                         craft. Hydrodynamic resistance is directly related to
                                                            displacement, which must as a consequence be
          The relationship between tunnel length L          minimized. The design of successful high-speed
and tunnel width ST for designs built between 1990          multi-hulls is therefore directly associated with
and 2000 is illustrated in Figure 45.15, suggesting a       efficient structural design and the minimization of
mean minimum value of 7. All commercial high-               lightship weight l.
speed catamarans built to date have been designed                     The rake of keel and the amount of
with demi-hulls arranged alongside each other in the        longitudinal curvature in the hull, called rocker, are
longitudinal direction. It should be theoretically          other parameters that may be considered at the
possible to design the hull placement with a stagger,       design stage. The rake of keel may influence the
with one hull fitted ahead of the other, such that there    amount of lift generated at increasing speeds, or


                                                       46-5
perhaps more importantly the hydrodynamic trim.           where CT is the non-dimensional total resistance, CR
Generally a rise of keel may only be of value at          represents the residuary resistance, CF the frictional
higher speeds, but may be used to influence the           resistance and (l+k) is a form factor to account for
position of the longitudinal center of buoyancy           the three-dimensional effect of the shape on the
(LCB). The amount of rocker varies from design to         chosen frictional formula.
design, but it generally should be small for higher                 The two basic characteristics of high-speed
speed craft, because the rocker will introduce            multi-hulls are high speed and lightweight. The
dynamic trim, thereby increasing resistance as            result of high speed is the generation of high
discussed in Section 45.7 and frequently causing          dynamic pressures at the hull surface, and when
course instability.                                       combined with the lightweight nature of the craft the
         The parameters , L, b, T, Cb, ie, α, rake of    result is a substantial movement of the hulls in
keel and rocker are obviously all inter-related, and a    translation (sinkage) and in rotation (trim). These
choice of one will affect the value of the others. The    dynamic effects are substantially greater for high-
hull shape should generally be developed with a           speed craft than conventional craft, and unfortunately
minimum of wetted surfaces area SA, however it            the result in a changed underwater geometry
should be noted that many features affect the             compared to the hull form under the static waterline.
dynamic trim which in turn changes the wetted             This dynamic squat changes the wave making and
surface area, and therefore some shapes with an           frictional components of drag.
apparently low value of SA, have a high frictional
resistance, as further discussed in Section 45.8.
         Unlike conventional slower speed craft, the      45.7      WAVE MAKING DRAG
position of the longitudinal center of buoyancy LCB
is not important from a hydrodynamic viewpoint, and                 CR is substantially made up of the wave
it is common to position the LCB at the longitudinal      making resistance CW, however the effect of the
center of gravity LCG as dictated by the vessel           transom stern on wave making drag must also be
arrangement. The coefficients normally of value for       considered. The hull shapes associated with multi-
conventional craft, prismatic Cp, block CB, and           hulls are generally long and slender, and
midships section CM, appear to have little                consequently fit the thin ship and slender ship
significance for high-speed multi-hulls. The two          theories very well. The formula for CW first
coefficients of most use in the design of high-speed      published by Michell (1898) and developed by
multi-hulls are L/b and L/n1/3, where n is the volume     Lunde (191) has been shown to provide an accurate
of displacement. Maximizing these values usually          predictive method for CW for catamarans as
leads to reduced resistance, suggesting that L should     illustrated in several papers by Doctors (1990). The
be maximized and b and n minimized. However, as           use of Computational Fluid Dynamics CFD based on
previously noted, increasing L can lead to increased      solutions to the Navier-Stokes equations can also
weight and n as well as cost.                             provide accurate estimates of CW, but in both these
                                                          cases special care must be taken to allow for flow
                                                          separation at the transom. Doctors (1990) has found
45.6     RESISTANCE                                       good correlation using the Mitchell Integral by
                                                          extending the transom back with a ‘virtual
         As discussed in the previous sections, the       appendage’.
various hull shapes and characteristics are developed               If the wave making drag is calculated by
substantially to minimize resistance.         This is     taking wave cuts in the towing tank, as first
achieved by minimizing the vessel weight, wetted          suggested by Eggart (1971) and first used for high-
surface area and characteristics driving the wave         speed catamarans by Insel (1990), then an additional
making resistance Cw such as L/b and L/n1/3. Figure       component termed transom drag CTR has to be
45.17 illustrates the Cw values for a wide range of       introduced. This drag represents the additional force
practical catamaran hull shapes according to              caused by the imbalance of hydrostatic pressure on
Werenskjold (1990). See                                   the hull as a result of flow separation at the transom
                                                          causing the entire transom to be at atmospheric
                                                          pressure. It is part of the residuary resistance and
                                                          scales with Froude number Fn and can be easily
                                                          calculated from the hydrostatic pressure on the
 Figure 45.17 -Residuary Resistance Coefficient           transom.      If CWP represents the wave pattern
  from Tank Test Results on Many (unrelated)              resistance as measured from wave cuts, then:
    Models, according to Werenskiold (1990)
                                                                    CR = CWP + CTR and the total resistance
          The total resistance of a catamaran may be      becomes
split into component parts in the normal way.
                                                                    CT = CWP + CTR + CF (l+k)
                  CT = CR + CF (l+k)


                                                       46-6
         When calculating CW by numerical means,              value from the coefficient CF that the wetted surface
it is important that the effect of dynamic squat is           area S, only includes the hull surface and does not
included in the calculation.                                  include any of the transom. This also raises the
                                                              difficulty of what value of S should be used, the
                                                              value at the static waterline or the value associated
45.8        FRICTIONAL DRAG                                   with the dynamic value, which may vary
                                                              substantially as illustrated in Figure 45.18. It is
         The frictional drag can be estimated using           normal practice to use the static waterline value,
the standard friction lines, based upon the Reynolds          excluding the transom, as this is easily calculated and
number Rn, such as the 1957 ITIC formula:                     the dynamic value is extremely difficult. Molland et
                                                              al (1994) has shown that in scaling from model tests
            CF = 0.075/log (Rn-2)2                            to full size using the static value or the dynamic
                                                              value makes a negligible difference as long as it is
         Many of this formula were derived from               consistent.
experimentation on essentially two-dimensional flat
plates, and it is standard practice to apply a form
factor to the value of CF to account for the three-
dimensional effects. High-speed multi-hull forms are
usually slender and have little longitudinal curvature,
and it may be anticipated that the form factor
correction for these hull forms would be close to                                Figure 45.18 -
unity. The ITTC formula is in reality a correlation
line and contains some allowance for the three-                         In addition to the form factor allowance on
dimensional effects, so it is not surprising if form          CF, it has become standard practice with
factor values for high-speed multi-hulls are less than        conventional craft to add to correlation allowance
unity when applied to the ITTC formula. Armstrong             CA. This may consist substantially of an allowance
(2000) calculates values of form factor (l+k) relating        for hull roughness CF and to account for unknown
to the ITTC formula for the NPL standard series               differences between model tests and full-scale trials.
described by Bailey (1974) at full scale and given            High-speed craft depend on achieving a high speed,
by:                                                           and as frictional resistance is the major component of
                                                              the total, most if not all operators keep the hull free
            (l+k)ship ITTC = 1.72 - f(L/n1/3)g (b/T)-0.1      from marine growth in order to minimize the
                                                              frictional resistance. Most multi-hulls are built from
for Fn values of between 0.6 and 1.0 only, and where          aluminum which has considerably greater resistance
the factors f and g vary with Fn according to Table           to corrosion than does steel, and manufacturers may
45.I.                                                         also go to greater lengths to make the hull
                                                              hydraulically smooth in order to minimize the hull
                       TABLE 45.I -                           resistance. The consequence of these factors is that
                                                              the hull roughness CF is usually very small for
                                                              multi-hulls, and several authorities suggest a value of
                                                              zero. Muller-Graf (1992) gives a table for various
                                                              hull surfaces, which is reproduced in Figure 45.19.

          At model scale the Reynolds Number Rn is
different to that at full scale, and the boundary layer
is considerably thicker. As a result the form factor at
model scale may be quite different to that at full                               Figure 45.19 -
scale. For Rn about 106 to 107, Armstrong (2000)
suggests:
                                                              45.9 AIR RESISTANCE
            (l+k)model ITTC     = 1.45 – 0.145(L/n1/3)0.6
(b/T)-0.1                                                              Many high-speed craft are operating at
                                                              speeds where the resistance of the part above the
valid for values of Fn between 0.45 and 1.0, and              waterline may be significant. At speeds above about
L/n1/3 values between 7.0 and 10.0.                           60 knots it would be worthwhile to streamline the
          The limit of Fn = 0.45 is important because         superstructure shapes and to remove features
this is approximately the speed at which the flow             associated with high-drag such as the bridge wings
separates clearly at the transom. At speeds below             and handrails. At speeds below 60 knots, the air
this limit the form factor may be quite different.            resistance may be reasonably calculated using the
With no flow over the transom at higher speeds, it is         front profile area of the vessel above the waterline
important that in calculating the frictional resistance       and ignoring the tunnel opening, and applying a drag


                                                           46-7
coefficient CD of 0.45. This may vary between 0.40          pressure acting on the bottom plating, and hence
and 0.55 depending on the degree of streamlining,           produced different resistance values.
angle of the wheelhouse windows and after end
shape of the superstructure.
                                                            45.12    SEAKEEPING
45.10    APPENDAGE RESISTANCE
                                                            45.12.1 ??????
         The resistance of appendages can be                           The waterplane of each hull of a catamaran
calculated from drag coefficients such as those given       usually has a high L/b ratio by comparison with a
by Holtrop (1988) and reproduced in Table 45.II.            monohull, and fine angles of entrance to minimize
                                                            resistance. Therefore it may not be surprising that
 TABLE 45.II - FACTORS TO APPLY TO CF                       the longitudinal moment of inertia of the waterplane
   VALUES WHEN CALCULATING THE                              is substantially smaller than that of a mono-hull. In
 RESISTANCE OF VARIOUS APPENDAGES,                          addition, having two or more widely separated hulls,
    ACCORDING TO HOLTROP (1988).                            the transverse moment of inertia can be higher than a
                                                            similar-sized mono-hull. The result is that a multi-
          Most high-speed multi-hull craft are fitted       hull craft will have motion characteristics that are
with ride control systems, incorporating foil control       quite different to that of a monohull.
systems. The drag associated with these foils in                       In beam seas a monohull will roll. High-
isolation may be calculated by standard means, such         speed monohulls having hard chines experience
as Hoerner (1965), however a substantial portion of         some resistance to rolling, but these types of craft
the total drag comes from tip vortex effects and            also suffer from other phenomena owing to the
interference effects, and it is recommended that the        dynamic pressures generated around the hull. The
resistance values be provided by the manufacturers.         rolling motion is characterized in that the transverse
                                                            motion of the water relative to the hull is around the
                                                            girth, i.e. it is a radial motion. The amount of roll
45.11    MODEL TESTING FOR RESISTANCE                       can be reduced by the fitting of bilge keels and fins,
                                                            although these are not desirable features for high-
          The measurement of resistance at model            speed craft owing to the additional frictional
scale, and the scaling of the results up to full scale is   resistance. On the other hand, catamarans will roll
a well-proven technique for most conventional hull          in beam seas, but the amount of roll is considerably
forms, and is used by many designers of high-speed          less, and the rolling motion is characterized as one
craft. At model scale, the Reynolds Number Rn is            hull heaving up whilst the other hull heaves down.
known to be far too low, however the effect at lower        In other words the transverse motion of the water
speeds and particularly on conventional monohull            relative to the hull is in a vertical direction as
shapes may be small enough to be ignored.                   opposed to the radial direction of the mono-hull.
However, it appears that on multi-hull shapes at high       Because of this the rolling of a catamaran cannot be
speed the low value of Rn and the resulting relatively      successfully reduced by fitting bilge keels, and the
thicker boundary layer results in a change to the           most effective way is to fit horizontal control
dynamic pressures developed around features such as         surface, as will be further discussed.             This
the transom flaps or wedges, and the amount of lift         fundamental difference means that many of the
produced by these stern-lifting devices is noticeable       formula developed to predict the motion and
reduced. A consequence is that the dynamic trim is          characteristics of roll for monohulls will not work
different between model scale and full scale, as            successfully for catamarans.
illustrated in Figure 45.20. This difference in trim                   Generally a catamaran will have
results in a difference in resistance, and for this         considerably lesser roll angles than a mono-hull, but
reason, model testing high-speed catamarans may be          the radial accelerations levels will consequently be
an inaccurate method to determine the resistance at         much higher.
full scale, unless the correct trim and sinkage can be                 The fine angles of entrance and lack of
applied to the model by applying additional external        substantial flare on most multi-hulls result in reduced
forces.                                                     resistance to pitching motions by comparison with a
                                                            mono-hull. The vessel will pitch and heave more
                                                            than a mono-hull but will have reduced acceleration
                                                            levels. In following seas a catamaran may run down
                                                            the face of a wave and then strike the back of the
  Figure 45.20 - Dynamic Trim of a High-speed               preceding wave. The bow down attitude of the craft,
   Catamaran at Full scale and at Model scale.              striking another wave at high-speed can result in the
                                                            bow of the vessel becoming buried in the wave, with
       It should also be noted that model tests with        green water shipped over the bow, and the vessel
and without model waterjets produced different              experiencing a severe reduction in speed. This is
dynamic trim effects, owing to the changes to the           termed deck-diving or nose-diving, and has been a


                                                        46-8
problem for some vessels particularly those               considered as a measure of kinetosis, using methods
undergoing trans-oceanic delivery or re-positioning       such as those developed by O’Hanlon and
voyages. To avoid this problem, one designer has          MacCauley (1974).
introduced a substantial centerfold between the hulls               The seakeeping characteristics of stabilized
of the catamaran, having a large amount of flare and      monohulls such as trimarans and proas may be quite
reserve buoyancy, and this feature is typical of the      different to that of catamarans. By careful design it
wavepiercing catamarans built by International            appears to be possible to incorporate the beneficial
Catamarans in Australia, as illustrated in Figure         elements of a mono-hull in head seas with the
45.21. Here, the centerfold is above the design           beneficial elements of a catamaran in roll. In stern
waterline in the static load condition in order to        seas a trimaran may have substantially improved
minimize resistance, but becomes immersed should          motions, but this is not yet clear as very few such
the vessel pitch down and thus provide a pitch            high-speed craft have been built, and none at any
reduction force.                                          large size.
                                                                    Several methods to minimize craft motions
                                                          have been developed. A common method for multi-
                                                          hull craft is to fit a fully submerged foil at the
                                                          forward part of each hull containing an integral
                                                          movable trailing edge control surface similar to
                                                          elevators on the tailplane of an aircraft, as illustrated
                                                          in Figure 45.24.and commonly called a “T-foil”
                                                          because of its configuration. These control surfaces
                                                          are typically activated by accelerometers in order to
Figure 45.21 - HSC Millenium, showing the Bow             produce forces opposing the pitching motion of the
                 Centerfold.                              craft. Because of the need for rapid response, the
                                                          hydraulic systems for such Ride Control Systems can
          These craft also have very fine angles of       be quite large and sophisticated.
entrance at the bows, with the intention that the
incoming waves do not cause significant pitching.
This is the reason for the term “wavepiercer”.
          A comparison of the relative motions of a
high-speed mono-hull and a high-speed catamaran is           Figure 45.24 - A Ride Control “T-foil” for a
illustrated in Figure 45.22.        Typical transfer                   High-speed Catamaran.
functions (Response Amplitude Operators) are
illustrated in Figure 45.2.23.                                      As well as the forward foil, most multi-hull
                                                          craft have a device mounted on the transom
                                                          principally to provide opposing forces to control the
                                                          roll angle. These fall into two types, the fully
                                                          activated flap and the interceptor. Both of these
                                                          interact with the water flow to promote a high-
Figure 45.22 - Relative Motions of a High-speed           pressure region to provide a lifting force on the stern.
Mono-hull and a High-speed Catamaran                      Flaps are usually hinged at the transom, or fitted in a
                                                          recess forward of the transom, and operated over a
                                                          range of about 12 to 15 degrees by hydraulic rams, in
                                                          response to accelerometers fitted to the vessel.
                                                          Because of the need for a hinge at the front of the
                                                          flap, this dictates that the transom be flat, which does
Figure 45.23 -RAOs for a High-speed Catamaran             not always suit the requirements of the hull design,
                                                          and in this case the interceptor may be more
          As previously stated, the motion                effective. The interceptor is a flat blade surface or
characteristics are different to those of monohulls.      guillotine that if fitted to the bottom extremity of
How then might passenger comfort be affected?             transom, and can be raised or lowered by hydraulics
Current levels of knowledge relate passenger              to change the immersion beyond the hull surface.
comfort to the level of vertical acceleration, but this   Typically this immersion may be up to 100 mm, and
is not the full story. The levels of kinetosis (motion    the interceptor may have any shape to suit the
sickness) on a catamaran can be higher than the           transom shape.
vertical acceleration level might suggest, particularly             Designs exist for interceptors fitted to the
in quartering seas where a corkscrewing motion of         after end of flaps and which can be fitted forward,
the craft might be evident. This is because the roll      but in this position they will be less effective than a
period and the pitch period of catamarans can be          T-foil because interceptors can only provide a
very close, and coupled motions result. However, at       vertical upward force, whereas a foil can also
the present time only the vertical acceleration can be    produce a downward force.


                                                      46-9
         The additional drag of T-foils can be quite      system.     At the present time, the maximum
high, because of induced drag and the creation of         significant wave height for which life rafts and MES
vortices from foil tips, and the design of T-foils        have been tested is 4 meters.
remains a specialist area. Characteristics such as
sweep angles, aspect ratios, materials and structural
design methods to minimize the thickness of the foil      45.13    SPECIAL TYPES OF CATAMARAN
are carefully guarded by the manufacturers. Designs
have been built to allow the T-foils to be retracted      45.13.1 SWATH and Semi-SWATH
out of the water flow, thereby allowing for a
maximum ship speed in calm water when the ride                      Because the seakeeping characteristics are
control will not be needed.                               substantially determined by the shape and properties
         Several multi-hull craft have been built with    of the waterplane, it is beneficial to reduce the size
passive control surfaces where a horizontal section       of the waterplane. A craft having a small waterplane
of the hull plating is arranged, usually at the forward   area still requires a displacement volume, and
end, to resist the vertical motion of the bow when        therefore the underwater shape is usually bulbous, as
pitching or heaving. Such a complex hull surface          illustrated in Figure 45.25. Such a craft, called a
will increase the difficulty of construction, and it is   Small Waterplane Area Twin-Hulled craft, or
debatable as to whether overall it improves the           SWATH,         can    have      excellent    seakeeping
motion characteristics. It will change the pitch and      characteristics, and where seakeeping is a major
heave periods so that it may be beneficial in some        issue then this craft is the most suitable.
seas, but perhaps it may result in worse motion in        Unfortunately a SWATH suffers from some
other seas.                                               disadvantages as well. The increased wetted surface
                                                          area results in additional frictional drag, and no truly
45.12.2 Slamming                                          high-speed SWATH craft have yet been built. The
          One unfortunate characteristic of multi-hull    propulsion system may be very difficult to arrange
craft is the need for a cross-structure to support the    with a small waterplane shape allowing no room for
two or more hulls. This means that there is a             an engine room, and when combined with a complex
structure above the waterline, which in the case of       curved hull shape the craft are extremely expensive
severe motion of the craft may come into contact          to construct. Chapter 46 - SWATH and Trimaran
with the water at high speed causing a slam. The          Vessels presents a detailed discussion of SWATH
underside of the tunnel structure is frequently a flat    vessels.
surface, and slamming may be an issue if the tunnel
height is too low. It should be noted that the
congruence of the bow waves from the multiple
hulls, superimposed on the waves passing through
the tunnel, can result in slamming occurring in the               Figure 45.25 - SWATH vessel Patria
after part of the underside of the tunnel structure.
          The vessel structure is designed to                       Many of the larger fast ferries have been
withstand such occurrences of slamming, but damage        built as a so-called semi-SWATH. These craft have
can still result in the form of indented shell plating    the features of a SWATH in the forward part, as
and cracks, and in any case there is usually a severe     illustrated in Figure 45.26, but the waist at the
shuddering of the craft causing passenger concern.        waterline decreases going aft, such that the engine
In sea conditions where slamming occurs it is             room shape is the same as a conventional catamaran.
therefore normal practice to slow down. Obviously         There is a small penalty to pay with increased
an increased tunnel height will allow operations to       frictional resistance, but usually a small benefit in the
continue at full speed in greater sea states, but a       wave making resistance, and a substantial benefit in
greater tunnel height leads to greater scantlings and     the seakeeping.
hence a heavier overall weight and a consequent
reduction in speed, so the design of the tunnel height
is a careful balance between avoiding slamming and
obtaining a maximum speed.
          Most high-speed craft are limited by the             Figure 45.26 - Semi-SWATH Hull Form
sea-state to a particular speed of operation. However
there is another factor unrelated to the hull design      45.13.2 Foil-assisted multi-hulls
that can affect the ability to sail, and that concerns
the ability to launch the life rafts. The life raft                 A limited number of foil-assisted
systems, specifically the Marine Evacuation Systems       catamarans have been built, such as the Catafoil
(MES), are type-approved up to a particular               described by Gee (1997) where the foil lift was small
significant wave height. Therefore the vessel may be      and the hull remained in the water, and the Foilcat
prevented from sailing if the forecast significant        illustrated in Figure 45.27, where the foil lift was
wave height is above the capability of the escape         designed to lift the hull completely clear of the

                                                      46-10
water. Generally these have been successful designs,     45.14.2 Materials and Material Problems
although complex to set-up and are expensive to
construct. The foils have been fitted to generate                  The great majority of high-speed
dynamic lift and hence increase the speed of the         catamarans have been built from aluminum alloy.
craft, and also to minimize the craft motion and         Designs for Surface Effect Craft do exist in steel, as
hence improve passenger comfort.                         illustrated in Figure 45.31 but no known high-speed
                                                         catamaran has been built in steel. Composite
                                                         materials are common for small private craft, but the
                                                         strict fire regulations for passenger-carrying ferries
                                                         rule out composites as a suitable material, owing to
              Figure 45.27 - Cat Foil                    the flame spread characteristics, the flammability and
                                                         the smoke and toxicity emissions. Aluminum is the
         A foil-assisted trimaran has recently been      obvious solution for high-speed catamarans, being
constructed in Australia (Figure 45.28), but the         lightweight and easy to work with.
performance of this craft is not yet known.



    Figure 45.28 - Trimaran Currently under              Figure 45.33 - SES Design in Steel, from Boote et
                  Construction                                              al (1997)

45.13.3 Air-lubricated multi-hulls                                 The plate material is typically a 5000 series
                                                         alloy, usually 5083 or 5383, having a tensile strength
          Only one catamaran with air-lubrication, mv    of MPa. The stiffeners are usually extrusions, made
Caraibe Jet, has been constructed to date.               from 6000 series alloys, as the 5000 series are
Unfortunately this craft was not successful,             difficult to extrude. 6061 is a common designation
principally owing to the decision to use waterjets for   for extruded stiffeners, having a tensile strength of
propulsion, and which were unable to operate             MPa. The welding of aluminum causes a reduction
satisfactorily with the large amounts of air being       in strength of the base material, and it is normal
ingested from the air-lubrication.                       practice to allow for this reduced strength in the
                                                         heat-affected zone where the working strength of
45.14 DETAILED DESIGN PROCEDURES                         5383 alloy is 75 MPa.
                                                                   Extrusions from other alloys such as the
45.14.1 Weight estimates                                 7000 series are possible, and have been used on
                                                         some vessels, however the corrosion of these alloys
         As with any high-speed craft, the               in proximity to 5000 series alloys is questionable.
performance is closely associated with the weight of               The corrosion of 5000 and 6000 series
the craft, and careful estimation of the weight is       alloys in a marine environment is generally excellent,
required. Typically this can be done by modification     and will far outlast the life of a steel hull if properly
of the weight estimates for craft that have been         maintained.
previously built. Where this is not possible, then a               Care has to be taken to prevent galvanic
detailed item-by-item estimate has to be undertaken.     action when other materials are present. Pipework is
Table 45.II illustrates the lightship weight of many     therefore generally made from 316 stainless steel,
catamarans that have been built, and Table 45.III        and shafting and other external fittings are also
gives the breakdown of lightship weight into the         usually of the same 316 material. Zinc anodes are
major components for catamaran fast ferries.             usually fitted, and may be recessed into the hull
                                                         plating to reduce drag.
 TABLE 45.II - LWL V. LIGHTSHIP WEIGHT                             Steel may be used in some of the larger
     FOR EXISTING CATAMARANS                             vehicle-carrying catamarans, particularly for pillars
                                                         on the vehicle decks. Steel is used in place of
                                                         aluminum in order to minimize the size of the pillar,
                                                         because an aluminum pillar would need to be
                                                         protected with structural fire protection and which
                                                         would intrude too far into the car lanes. The
TABLE 45.III - BREAKDOWN OF LIGHTSHIP                    connection of the steel to the aluminum may be made
 WEIGHT FOR EXISTING CATAMARANS                          by bolted flanges with gaskets, or, more commonly,
                                                         explosively formed bi-metallic strips are used which
                                                         allows the aluminum and the steel to be welded to
                                                         the common strip.
                                                                   Aluminum might appear to be an ideal
                                                         material from which to manufacture a high-speed


                                                     46-11
ferry, being easy to work with, relatively                 45.14.3 Structural design
inexpensive, easy to obtain, and easy to maintain.                    Multi-hulls, in common with other high-
However it does have two problem areas. Firstly the        speed craft, are designed to withstand the static and
low melting point requires that the material be            dynamic loads that can act under the operating
protected from the effect of fire. This is usually         conditions, without such loading resulting in
provided by the application of structural fire             inadmissible deformation or loss of watertight
protection to those areas of the structure where it is     integrity.
essential that failure does not occur. Various                        These loads generally fall into two broad
structural fire protection systems exist; typically they   areas, those considered to be local, such as the
may be mineral wool bats backing onto a thin steel         hydrostatic sea load, and those considered to act on
sheet, either galvanized or stainless, and supported       the whole structure and termed the global loads. For
by a framework and struts off the aluminum                 vessels having a length of less than about 50 to 60
structure, as illustrated in Figure 45.32. Such a          meters, the local loads will dominate the global
system obviously increases the weight of the               loads, and the minimum strength standard is
structure and offsets some of the advantages of            normally achieved by considering the scantlings
aluminum.                                                  required for the local loads. These smaller craft are
                                                           therefore somewhat easier to design, as the local
                                                           loads can generally be easily determined from simple
                                                           formula contained in the classification society rules
                                                           or national authority regulations.
                                                                      All of the major classification societies have
                                                           developed rules specifically for high-speed and
Figure 45.32 - Typical Structural Fire Protection          lightweight craft. They address the local loads
          Arrangement for Aluminum                         resulting from the slamming pressure and sea loads
                                                           on the hulls and allow for specific hull geometry.
          The second problem associated with the use                  For a catamaran, the transverse strength of
of aluminum is the fatigue strength and crack              the structure connecting the two hulls needs to be
propagation. There was insufficient understanding          analyzed for the moments and forces specified either
on these related problems when the first large             from simple classification society rules or from
aluminum ferries were designed and constructed, and        model tests or correlated numerical methods. The
there were several problems with some of these early       moments and forces illustrated in Figure 45.33 are
vessels.       The high number of cyclic loads             those resulting from the vertical bending moment Ms
experienced in some areas of the structure, for            on the cross-structure, together with the shear force,
example the waterjet area where cyclic loads may be        also the pitch connecting moment Mp where one hull
of the order of 5000 per minute, rapidly results in a      is rotating relative to the other, illustrated in Figure
reduced fatigue life. Design charts exist covering the     45.34, and the associated torsional moment Mt.
strength degradation of aluminum up to 108 cycles,
but very little has been published beyond that, and
yet 108 cycles might be achieved in a mere 6 weeks
of operation for a seven-bladed waterjet at 720 rpm
          The design of complex structures to account      Figure 45.33 -Transverse Bending of a Multi-hull
for high cyclic loads is extremely difficult and time
consuming, and not well understood. It has become
common practice to therefore over-design these areas
with sub-frames and additional structure and
increased thickness, based upon experience and
measurements of vibration levels from previous
designs.                                                       Figure 45.34 - Pitch Connecting Moment of a
          The majority of cracks occurring in                                   Catamaran
aluminum can be sourced to poor detail design or
poor attention to detail in the manufacturing process.     45.14.4 Global wave loads
With high cyclic loads, failure can occur at very low
stress levels, and where local stress concentrations                For all high-speed craft greater in length
occur, cracks can easily start. Once started, cracks       than about 50 to 60 meters, it is necessary to carry
can propagate easily. Prevention is the best cure,         out a detailed examination of the structural behavior
and this requires removal of all “square” corners, for     using a finite element analysis (f.e.a.) of the whole
example at window openings, soft toes on all bracket       craft. This can be a time-consuming exercise, and
connections, and sniping of the flanges at the ends of     represents the major part of the design effort. The
stiffeners where attached to structure.                    structural scantlings need to be estimated in order to
                                                           produce the f.e.a. model, which then has to be
                                                           modified to suit the results and run again. Several

                                                       46-12
solutions will need to be run in order to optimize the             The larger designs for carrying vehicles
design for a minimum weight. It is not uncommon to       may also have a forward and an after bridging
start with an approximate model, created by              structure, but supplemented by intermediate bridging
extruding a midships section over the length of the      frames. Other designs treat each transverse frame as
craft in order to obtain a starting point for the        equal, and although this arrangement will be heavier
scantlings to be used in a more accurate model.          in construction it is somewhat simpler to construct.
          The loads to be applied to the f.e.a. model    The superstructure of these designs are usually
are also difficult to estimate, and these are usually    arranged to take the loads produced in operation in a
provided by the classification society. Alternatively    seaway, and the whole craft is designed along the
they can be determined from numerical analysis of        lines of a monocoque shell.            Figure 45.2.37
the motion of the vessel in a seaway, modified to        illustrates these two types of design.
allow for the long-term distribution of responses that
the craft will experience during its operating life. A
safety factor of ten to account for the through-life
experiences is not uncommon.
          In order to examine the global loads, it may
be acceptable to make a fairly simple model to start         Figure 45.36 - Midships Section through a Small
with, and to determine the equivalent loads at the                             Catamaran
boundaries of the various compartments. These
equivalent loads can then be applied to compartment                The stiffening system used by almost all
models that are more complex and detailed, thereby       aluminum high-speed catamarans is longitudinal
saving on computer resources.                            stiffeners at a close spacing, typically about 200-300
          A typical global f.e.a contour plot of the     mm. Substantial transverse web frames, typically
stresses is given in Figure 45.35.                       spaced at about 1200 mm for the large vessels and
                                                         about 500 mm for the smaller craft, support these.
                                                         These dimensions have evolved to take the
                                                         maximum advantage of the scantling formulae in the
                                                         classification society rules.
      Figure 45.35 - Typical F.E.A. Plot of a                      Because it is a simple matter to produce
                   Catamaran                             extrusion dies, many catamaran shipyards have
                                                         developed their own extrusions that offer particular
45.14.5 Typical Structure of Catamarans                  benefits in manufacture or in simplifying
                                                         construction. T-bar stiffeners of various shapes and
         The catamaran originated as two hulls           sizes are popular shapes.
joined together with two crossbeams, one aft and one               Transverse struts may be fitted between the
forward. This same philosophy still continues with       web frames to break up the span. These struts are
many of the smaller catamarans. For example, one         typically I-beams or hollow sections. A longitudinal
of the most prolific designers of catamarans, Incat      watertight bulkhead is usually fitted at the boundary
Designs in Australia, continues to produce very          of the bridging structure and each hull. Typical
successful designs based upon a forward transverse       structural sections are illustrated in Figures 45.2.38
beam, or bridge, connecting the bows, and a              (a) to 45.2.38 (c).
substantial I-beam at the transom locking the after
ends of the hulls together. Frequently there may be
no deck fitted between the hulls, as illustrated in
Figure 45.36. In this design the deckhouse is a
separate module, mounted on rubber isolation
mounts in order to reduce vibration and minimize         Figure 45.37 - Typical Structural Section through
noise. The loads resulting from operation in a waves                    a Large Catamaran
are also taken entirely by the hulls, and not
transmitted to the superstructure, which can                       Because aluminum extrusions are relatively
consequently be made to lighter scantlings and           simple to manufacture, these have become a common
reduce weight.                                           structural feature. Deck plating has been replaced by
                                                         extrusions incorporating plate and stiffeners, as
                                                         illustrated in Figure 45.2.38 (a). These planks
                                                         usually have fittings at the sides so that adjacent
                                                         planks can be mechanical clipped together, thereby
                                                         allowing for excellent presentation for ease of
Figure 45.35 - Midships Section through a Small          welding. Hollow sections such as the example given
                  Catamaran                              in Figure 45.38 (b), are becoming more popular,
                                                         particularly where high deck loads are used.



                                                     46-13
Extrusions are in use with a capability of up to 10       are commercially available in a wide variety of sizes
tonnes axle loads.                                        and capabilities. There are no problems specific to
                                                          multi-hulls in the application of waterjets, except for
                                                          the lack of space with the hull beam being frequently
                                                          made the smallest possible to fit the waterjets onto
                                                          the transom.
 Figure 45.38 - Typical Aluminum Extrusions in                     The combination of waterjets and multi-
        Use in Catamaran Construction                     hulls having widely-spaced hulls results in excellent
                                                          maneuverability at low speed.
         Extrusions are being used in an innovative
way to simplify the manufacturing process, but there
is a small penalty to be paid because alloys that can     45.15    STABILITY
be easily extruded have a lower strength capability.
Fabricated hollow sections are now entering the           45.15.1 Intact Stability
market made from higher strength material and
joined by laser welding.                                            It is obvious that the inertia of the
                                                          waterplane of a catamaran will be considerably
                                                          higher than that of a mono-hull, and it is not
45.14.6 Machinery Layouts                                 surprising that the metracentric height GMT is
                                                          consequently higher. The high GMT value of
          Because high-speed craft require large          catamarans       results   in    excellent      stability
propulsion powers, the engine sizes are                   characteristics at low angles of heel, but this leads to
geometrically relatively large.        The beam of        two problems. Firstly the roll acceleration rate will
catamaran hulls is usually dictated by the machinery      become quite high, with short roll periods, so much
arrangement, and therefore the available space in the     so that passenger comfort can be affected, and
engine rooms is at a premium. Other than the lack of      secondly the GZ curve usually peaks at a relatively
space, the machinery layout is simple, particularly       low value, and can be less than the 25°
where only one engine and transmission is fitted in a     recommended by many maritime regulations, such as
hull.                                                     those of IMO contained in SOLAS regulations. An
          Where two propellers or waterjets are fitted,   example of a GZ curve for an 86m catamaran is
the engines may need to be staggered, with one fitted     illustrated in Figure 45.45.
ahead of the other, and offset gearboxes used. This
results in long transmission shafts, and these may be
made from carbon fiber in order to reduce weight.
Engine rooms are therefore usually long and thin,
and this leads to difficulties in the layout of the           Figure 45.45 - Typical GZ Curve for a Large
ancillary equipment, and care has to be taken to                              Catamaran
ensure that proper access for maintenance is
provided.                                                           Some regulations have been specifically
          Exhaust systems are large, and the silencers    regulated for multi-hull craft allowing a peak in the
can be physically larger than the engine. The             GZ curve at a lesser value than 25°, and down as low
silencers are often fitted above the engine, or           as 15°, with additional compensatory requirements
adjacent to the tunnel structure, and the exhaust is      elsewhere in the GZ curve.
frequently led into the tunnel space, thereby avoiding              It is very difficult to modify the high GMT
the need to pass through the passenger areas.             value and low angle at which the peak in the GZ
          Pipework has to be duplicated in each           curve occurs. The hull separation must be reduced,
engine room of a multi-hull, because it is impractical    and the dutiful beams also reduced; these
to share such services, given the height of the tunnel    characteristics are usually determined by the layout
and the distance to the other engine room(s).             and it may be difficult to modify them.
          The propulsion systems vary depending                     A catamaran having a small water plane
upon the duty of the craft. Propellers remain a very      area, commonly called a SWATH, will have a lower
popular method of propulsion, and these have no           GMT and therefore considerably better rolling
different characteristics for multi-hull craft as for     motion and longer roll periods.
mono-hull craft. They are less expensive than                       Trimarans offer the possibility to easily
waterjet propulsion to purchase and install, and are      manipulate the GZ curve and the GMT values.
generally more efficient up to speeds of about 30         Careful selection of the displacement of the side
knots. Some designs are in use up to 34 knots, but        hulls relative to the main hull, and the shape of the
above this speed the onset of cavitation starts to        water plane allows a desired GMT value to be
severely limit the application of propellers.             selected, and the careful shaping of the hull above
          Waterjets are the most common solution to       the waterline can provide the desired areas under the
the propulsion problem above about 30 knots, and          GZ curve. In this way a hull form can be designed


                                                      46-14
having a minimum GMT and therefore the most               application to high-speed craft operating entirely
comfortable roll characteristics, but still meeting the   within their national waters.
stability criteria at larger angles of heel.                        IMO originally produced the Code of Safety
          Craft having more than three hulls might be     for Dynamically Supported Craft (DSC Code) in
similarly designed.                                       1978 in response to the number of hydrofoils and
          The majority of multi-hulled craft currently    surface effect ships that were starting to trade
in service are designed to carry passengers. This         internationally. This was an interesting approach
usually implies large superstructures, and the            from IMO, because they recognized that high-speed
stability is frequently limited by the regulatory         craft could be operated under a different safety
requirements for severe wind and weather. In the          philosophy to the more traditional craft that were
case of the High Speed Craft Code of IMO, the craft       required to be “stand-alone”. With high-speed craft,
is assumed to have rolled to windward under the           they could be on a set route within reach of rescue
action of certain waves and to then be blown by the       from the shore, and this allowed a relaxation from
wind past the upright position until equilibrium is       certain requirements of the conventional safety rules.
achieved. The area under the GZ curve to windward         Safety could also be enhanced by a strict shore-based
has to be less than the area under the GZ curve to        quality management system. This relaxation from
leeward. Unfortunately the coefficients used in the       the conventional regulations of SOLAS was
formulation of the roll to windward under the action      necessary in order to save weight and hence achieve
of waves is entirely based on the rolling                 a higher speed.
characteristics of monohulls, and are quite                         When the first high-speed multi-hulled craft
unrepresentative of those for multi-hull craft, and       appeared in 1990, IMO quickly recognized that the
therefore the multi-hull craft may be unduly              DSC Code was an insufficient method to fully cover
penalized.                                                the safety certification requirements of these sorts of
                                                          craft, and set about formulating the Code of Safety
45.15.2 Damaged Stability                                 for High-Speed Craft (HSC Code), published in
                                                          1994. This was the first safety document to
Multi-hull craft generally are arranged with all          recognize that multi-hulled craft had specific
passenger or cargo areas above the main deck, and         requirements. In particular specific stability criteria
the hull are usually void spaces with the exception of    for multi-hulls were formulated covering both intact
the engine rooms. This allows for a multiplicity of       and damaged stability. One interesting damage
watertight bulkheads, and the damage stability            stability criteria relates to the width of bottom
characteristics of multi-hulls are usually very good      damage to be assumed, particularly whether one or
and much in excess of the regulatory requirements.        more hulls may be damaged, and this was limited to
          Furthermore, with widely separated hulls,       7 meters width in order to cover the scenario of
damage to one hull is unlikely to lead to loss of the     collision with a semi-submerged 20 ft container.
craft, with the other hull remaining intact and the                 Another regulation relating to the layout of
superstructure or deckhouse also providing                multi-hull craft is the ability to substitute the
buoyancy.                                                 emergency generator set with a normal generator set,
          New regulations for high-speed craft            on the basis that with widely separated engine rooms
produced by IMO in 2000 specify extensive                 then in case of an accident in one engine room, then
longitudinal raking damage to the bottom of the           the generator set in the engine room on the other side
craft. In the case of the larger craft having a tunnel    is an effective emergency set. Specific requirements
width greater than 7 meters, then the deck angle after    for bilge pumping systems are also given.
damage of up to 100% of the length of one hull is                   The HSC Code was carefully written to
required to be less than 20°. This may be difficult to    include several features of multi-hulls as well as
achieve without arranging intermediate watertight         monohulls, such that the regulations could be simply
decks in the hulls.                                       applied. For example there is no reference to the
          The watertight bulkheads also need to be        Main deck, as this feature is not evident on several
strategically placed in order to meet the specified       designs of catamarans, rather the HSC Code refers to
lengths of different raking damage scenarios.             a deck datum.
                                                                    It should be noted that the multi-hull craft
                                                          has several inherent safety features. Two or more
45.16    SAFETY AND REGULATIONS                           widely separated engines rooms, excellent
                                                          maneuverability, vast reserves of stability, wide
         There    have     been    few regulatory         passenger compartments that allow for ease of
requirements drawn up specifically for high-speed         escape in all directions rather than being limited to
craft, with the notable exception of the IMO              fore-and-aft as is the case on a narrow mono-hull,
regulations covering craft on international voyages.      easy-draining main decks, and limited angles of heel
For this reason many Administrations have adopted         are just some of these.
the various Codes of Safety from IMO for



                                                      46-15
45.17    MANUFACTURING                                     is also easily shaped and cut using essentially hand
                                                           tools. The deck extrusions are then added, and the
          Aluminum catamarans are welded together          superstructure erected. Deckhouses may be built as
using established MIG or TIG processes. Opinion is         a separate module and added at a later stage.
divided as to the relative benefits of pulse or non-                  One shipyard has set up a simple production
pulse methods, and generally the welding methods           line where the craft is built from modules weighing
are conventional. In common with most aluminum             approximately 7 to 10 tonnes, brought together at the
welding, the porosity and inclusions have to be kept       top of the line, and as the vessel moves down the line
to a minimum, and therefore all welding is carried         it is further completed. The deckhouse is built in an
out under cover in order to prevent the shield gas         area high above the building shed, and slid across in
from being dispersed by the wind.                          a semi-completed form and added as a final module.
          Smaller craft are frequently built upside        In this case the deckhouse is fitted on rubber mounts
down for ease of construction and welding, and             and the whole deckhouse can be pre-outfitted. This
turned over when completed.                                production line approach, where up to four 90 meter
          Larger craft can be built in modules, as         long catamarans may be in production at any one
illustrated in Figure 45.2.40, but this has not been       time, is only possible where standard vessels are
commonly done except by shipyards who have                 being produced.
employed traditional steel construction techniques.
In several cases this has caused problems, because it
is difficult to predict the amount of shrinkage and        45.16     LAUNCHING
distortion caused by welding in aluminum, and
therefore the modules do not easily match up to each                  Most multi-hull craft can be supported on
other. Strict welding procedures have to be adhered        the underside of the tunnel structure, which is usually
to if the shrinkage and distortion is to be predicted,     flat and therefore simplifies transportation by road
and this requires several years of data collection and     trailer, as illustrated in Figure 45.2.41, or on a cradle
experience with aluminum. Those shipyards that             for launching.
have used a modular approach to building have been
wise to allow the seams to remain unwelded at the
ends, and to leave an appropriate amount of green
material.


                                                               Figure 45.41 - 40 m Air-lubricated Catamaran
                                                                       Caraibe Jet on a Road Trailer.
  Figure 45.40 - Bow Module for the HSS Stena
                    Explorer

          For smaller vessels the hulls are usually
built inverted, turned over and the bridging structure
at the forward and after end connected. The
superstructure is then added.
          For the large vessels the normal practice is
to construct the tunnel structure first, because this is
a rectangular structure having parallel sides and the
underside is normally parallel with the deck above,
making construction a simple matter. The web
frames and bulkheads are constructed individually,
complete with the cutouts for the longitudinal
stringers. In some cases it may be necessary to add
temporary structure so that the frame remains the
correct shape whilst being handled. The tunnel
structure is supported in the air, and the individual
frames are then positioned in the correct place and
attached to the tunnel. Once the frames and
bulkheads are in position, then the longitudinal
stiffeners are threaded through the cutouts in the web
frames, and finally the shell plating is fitted and
attached. This is a very old-fashioned method of
constructing a vessel, however the task is
considerably easier because the lightness of the
material makes handling an easy task, and aluminum


                                                       46-16

				
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