Speeding up the Motor Boat

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					       SPEEDING UP THE MOTOR
                BOAT
                            BY LAWRENCE LARUE

                HE question of speed in a     horsepower were much larger and more
                 motor boat is an intricate   clumsy than the present-day engines de-
                 one, as there are many        veloping over twice that power, and
                 factors to be considered;    while, under the same conditions, a
                 and these do not always      greater bore and stroke will give an in-
                 give the results that are    creased power, it is the actual test of
expected. A fleet of boats may be built,      the motor on the blocks that really
all of exactly the same size, constructed     counts.
from the same patterns and equipped               But a more powerful motor is not an
with the same make and size of motor,         Aladdin's lamp that may merely be
and yet there may be a consistent differ-     rubbed with the starting crank to shoot
ence of from ten to twenty per cent be-       the boat off at a greatly increased speed.
tween the speed of the fastest and slow-      There are, in fact, certain limits beyond
est. It is such results as these, consist-    which a boat may not be driven, no
ent by virtue of their inconsistency, that    matter how great the power installed,
puzzle not only the owner, but the de-        and this applies to the motor canoe and
signer and builder as well; and it is safe    converted cat boat as well as to the es-
to say that not a single one of the most      pecially designed racer. The tendency
expert of the latter can accurately deter-    to "overpower" is one of the great faults
mine what speed a motor boat will make        of the average amateur and of many
until she is actually launched and tried      builders; every design has a certain
out under the best of running conditions.     speed beyond which any increase in num-
   If the expected results are not then       ber of cylinders or size of motor is an
obtained, it may require no small amount      actual waste of power.
of experimenting before the real diffi-           An incident illustrating the futility of
culty is found; when it is believed that       crowding excess power into a hull that
an increase in power will produce the de-      is already being driven at the top speed
sired speed, it may be discovered that this    for which it was designed is found in the
will not be nearly so effective as a simple    case of one of the well-known racers of
change in the pitch of the wheel. It is        a few years ago. This particular craft
small wonder, then, that both expert and      was thirty feet long, and when equipped
amateur will be found making frequent         with a forty-eight horsepower motor,
changes in the details of his boat in         could maintain an average of twenty-
search of that elusive extra mile-an-hour.    four miles an hour—a speed considered
   The power plant will probably receive      remarkable in those days. But her
more than its share of praise or censure,     owner was not satisfied and decided that
according to the performance of the boat,     if he could get twenty-four miles with
 and it is to this that the ordinary owner    forty-eight horsepower, he would have
will first direct his attention if any in-    no trouble in making a world-renowned
crease in speed is desired. The greater       racer if a one hundred and fifty horse-
the speed desired, the more powerful          power motor was installed in her.
must be the motor installed to drive the          He made the change, but instead of
boat. But it must be remembered that          being able to travel over the water at the
by power is meant the actual ability to       rate of thirty-five miles an hour, as he
do work, and not necessarily the size of      expected, he found that his craft could
the motor or bore and stroke of the           make scarcely a mile an hour more than
cylinders. The old-style motors of ten        she could with her old motor, and that
                                          751
752                       THE OUTING MAGAZINE

the greater part of the increased power          The power plant of a boat can be
only served to drive her into the water,     increased or reduced to suit the desires
instead of over it. When the motor was       of the owner, but once the shape and
opened to its full power, the hull almost    size of the hull have been decided upon,
submerged itself in its own wave, and it     changes of this nature are not easily
was found that the fastest running was       made. Consequently, the "lines" of the
obtained when only about one-third of        boat become really of foremost impor-
the available power of the engine was        tance in a consideration of speed. The
used.                                        best designed hull for any practical pur-
                                             pose is that which is sufficiently large
          Vagaries of Power
                                             and heavy to carry the load intended for
   This, of course, was an extreme case      it, is seaworthy, and yet one that will
of ignorance on the part of owner and        travel over the water and will make as
builder (or re-builder, rather), but re-     little wave and disturbance as possible.
sults of this nature are liable to be ob-    A boat which "pulls the whole river be-
tained, to a less extent, when applied to    hind her" is either overpowered and is
smaller boats of low power and slow          being forced through the water at an
speed. "Let well enough alone" is a          uneconomical speed, or she is poorly de-
good maxim to follow, although this          signed and her lines offer a greater re-
does not mean that better results can-       sistance than should be the case.
not be obtained often by increasing the          Although many boats are designed to
power of a motor boat. Many hulls are        "draw down" at the stern when under
not equipped with an engine sufficiently     way, this characteristic should not be
large to obtain the speed for which the      carried too far, as an undue drag is then
craft was designed.                          formed by the after half of the hull.
   Even in this case it will require a       The once popular canoe model of speed
greater amount of additional power than      boat gave way to the torpedo stern, and
the owner will probably deem necessary,      this, in turn, has been replaced by the
for the resistance to the passage of a       moderately wide hull with the "V-tran-
boat through the water may be said, in       som" stern, a design well adapted for
general, to increase as the square of the     almost any purpose, from a small, one-
speed, and the power necessary to obtain      man "runabout" to a racer or a large
a greater speed has been found to vary       cruiser. This model raises slightly at
as the cube of that speed. This, per-        the bow at the higher speeds, but the
haps, is a sufficient reason why the          under side of the stern is flattened so
power in a certain hull may be doubled        that the submerged portion is pushed
with an attendant increase in speed of        over the surface of the water, rather than
but one or two miles.                         through it.
   On the other hand, even though the            This is the principle applied to the
increased power necessary seems out of        hydroplane, except that the under side of
all proportion to the results obtained,       this model is shaped in a series of steps
this is not necessarily all lost. Take,       slanting from stern to bow so that, as
for instance, the case of a heavy, thirty-    the speed increases, these successive steps,
foot pleasure boat, constructed more for      or planes, will rise out of the water and
service and comfort than for speed. A         the hull will finally be riding on a single
twelve horsepower motor may drive this        surface that will glide, or slide, with
craft at eight miles an hour, and yet         much less attendant resistance than
double this power would scarcely serve       would be the case were the entire hull
to shove the heavy boat through the           forced through the water. Other vari-
water a mile an hour faster. But while        ations may be applied to the design of the
this added power would not exhibit its        hydroplane, but all are developed on the
presence by a noticeable increase in the      theory that a hull that skims over the
speed of the boat, other craft could          water meets with less resistance than
be taken in tow with no apparent re-          does one that must move aside a great
duction in the original eight or nine-mile    amount of water in order to push its way
speed of the tug boat.                        through.
                     SPEEDING UP THE MOTOR BOAT                                      753

    Although some of the highest speeds        shape of the after section does not pre-
 attained by a motor boat have been made       vent this undesirable dragging and con-
 in a hydroplane, this is not yet adapted      sequent reduction in speed.
 for an all-round pleasure, speed, or work        This trouble can be easily remedied,
 boat as is the ordinary hull, and conse-      however, by simply attaching a piece of
 quently such a design does not attract        sheet iron to the stern at the waterline
 the interest of the average boatman.          so that it will act as a plane when the
There is no reason why the generally ac-       after portion of the hull tends to drag
cepted type of hull should not remain          down. This piece of sheet iron should
the standard for any motor boat, except        be cut the same shape as the stern deck
racing freaks, for years to come.              so that no portion of it will project too
    If a hull does not seem capable of         far beyond the hull and be damaged by
 attaining the speed expected of it, the       a wharf or rock, and it should be well
impulse of many a builder seems to be          supported by struts riveted to the iron
to "cut her in two and add five or six         at one end and screwed into the plank-
feet to her length." Although this will        ing at the other.
be a time-consuming operation, it will be
effective to a certain extent, for the speed
                                                  When the Stern Draws Down
of a boat increases with its waterline
length. On the other hand, the greater              The value of such an attachment has
the beam of a boat, the slower will be          been demonstrated many times, but in
its maximum speed.                              no instance of my personal knowledge
    The observation of these two rules          more strikingly than in the case of an
will result in the conclusion that a hull       old twenty-five foot cat boat equipped
 that is "stubby and square" will never         with a twelve horsepower motor. When
succeed in landing a racing trophy if           first installed, this motor seemed to be
it is competing with a long, slim craft.        too powerful for the craft, the stern
But craft of "toothpick" proportions are        dragged down, and eight miles an hour
unstable and unseaworthy and can be of          was the highest speed that could be ob-
no possible interest to the man looking         tained. A sheet iron plane was at-
for a practical boat that will combine          tached in the manner described above,
utility and speed in the proper degree.         and by forcing the hull to travel on the
For pleasure craft of from twenty to           lines for which it was designed, the speed,
forty feet in length, a beam of from           with the same power plant, was in-
one-fifth to one-sixth this measurement        creased to eleven miles an hour.
is a good proportion which will admit              Changing the location of the motor
of both stability and medium speed.            will sometimes have the effect of obtain-
    An increased speed may sometimes be        ing a slight increase in the speed of a
 obtained from a motor boat by making          boat, particularly if the stern is found
 changes other than those in size of engine    to draw down too much. A speedy
 or design of hull, and these are the re-      boat will never be found with the motor
 finements that help make the average          located in the extreme stern, and the
 pleasure boat of to-day from two to five      majority of racing boats have their
 miles faster than its sister of a few years   power plant placed in a compartment
 ago. It has already been pointed out          well forward. Consequently, by mov-
that the bow of a boat should not rise         ing a heavy motor from the stern to the
 from the water more than a certain            bow, the position at which the boat
amount when it is under way, and that          travels will be changed on account of
 the V-shaped transom was designed to          the shifted weight, and in many in-
keep the proper length of the hull un-         stances the speed will be noticeably in-
 der water. But the tendency of all            creased.
hulls when traveling at normal speed               When an owner has made all changes
or above is for the stern to settle an         imaginable in his boat in the endeavor to
undue amount, and in the case of old           obtain more speed; when he has enlarged
style boats with the fan-tail stern and        the power plant; when he has refined the
such craft as the converted cat boat the       lines of the hull until it seems to run
754                         THE OUTING MAGAZINE

with scarcely a ripple; when he has                There are four things to be considered,
tuned up his engine; when he has                then, in connection with a propeller, as
smoothed and scraped the planking and           follows: Size, or diameter of the wheel;
given it its coat of friction-reducing          shape of the blades; pitch; and number
paint, there always remain the possible         of blades. The first and third of these
surprises to be found by putting on a           considerations are regulated by the nor-
new propeller. This is not a compli-            mal speed at which the motor will oper-
cated or difficult operation, and yet some-     ate at its highest efficiency, always re-
times it may be the most productive of          membering that a wheel with too much
results of any change imaginable.               pitch will work against itself and absorb
   The propeller, of course, does the act-      power from the motor without delivering
ual work of driving the boat through the        a corresponding increase in the speed of
water and any fault in its design or un-        the boat.
suitability for that particular hull or
motor will result in an absolute waste
of power. It also acts as the regulator
                                                      Getting the Right "Pitch"
of the number of revolutions at which              The problem of "too much pitch" will
the motor will run when "opened wide,"          be better understood if it is remembered
and as there is a certain normal speed at       that "infinite pitch" would be repre-
which any engine will operate at the            sented by a wheel the blades of which
highest efficiency, the size and pitch of       were set at right angles to the plane of
the propeller play an important part in         revolution, or parallel to the axis. The
obtaining the best results from the power       revolution of such a wheel would absorb
plant.                                          power with no resultant motion in either
   The propeller blades are really the          forward or reverse direction. Three-
development of a screw, and the pitch           bladed propellers are used on the major-
would correspond to the coarseness of the       ity of motor boats, so this is a problem
threads. The pitch of a propeller, how-         that need not give the owner much con-
ever, is measured in inches, and is con-        cern.
sidered as the distance a point on the             For motors under three horsepower,
blade would travel in a line parallel to        however, it is advisable to use a wheel
the shaft during one complete revolution        having but two blades, as any greater
of the wheel. In other words, the pitch         number would necessitate the construc-
is the distance that the propeller itself       tion of so small a propeller that efficient
would travel during one revolution, in          results could not be obtained without re-
a line extending along its axis, if the         ducing the pitch to a very small degree.
movement were made in a medium in               It is in the shape of the blades, then,
which there is no "give," or slip.              that the greatest latitude of selection will
   The greater the pitch of a propeller,        be found, and here is ample opportunity
then, the greater will be the resistance        for the trial of all manner of wheels.
offered to its turning; but, within certain        Whether the blade should be long
limits, if this increased resistance is over-   and tapering, thin at the hub and thick
come, the boat will be shoved a greater         at the end, or of uniform width through-
distance through the water at each revo-        out the greater part of its length, is
lution. Consequently, "easy" pitch and          largely a matter of opinion; a certain
high revolutions would produce the same         shaped propeller that will give excellent
result as "heavy" pitch and slower speed        results with one boat would be utterly
of rotation—always within certain               unsuited to a craft of different propor-
limits. The longer and wider the blade          tions and character, even though the
of a propeller, the greater will be the re-     proper size of wheel should be selected.
sistance to its revolution. The in-             But, provided a well-designed wheel can
creased blade surface, however, will            be obtained, of approximately the proper
exert a greater thrust upon the hull with       pitch and of such a diameter that the
each revolution, and consequently a             motor will be held to its normal revolu-
large wheel is "faster," for the same           tions, the owner need not worry as to
number of turns, than a small one.              what results could be obtained with a
                     SPEEDING UP THE MOTOR BOAT                                     755

different propeller, for, except in the case   the trial should always be made in a
of racing boats, it is not probable that       comparatively large body of water, for
any change would make a difference of          the full efficiency cannot be obtained in
more than five per cent in the speed of        shallow or narrow streams on account of
the craft.                                     the drag of the following waves on the
   Every propeller should be so placed         bottom or shores of the waterway.
that its entire periphery will at all times    Even the fastest racer would not be able
revolve well below the surface of the          to travel more than ten or twelve miles
water. This is necessary in order that         an hour through a narrow canal, and at
the propeller may have a solid body of         this speed it would "pull the whole river
water against which to exert its push.         behind it."
Unless this distance is at least six inches,      Owing to the greater buoyancy of
in wheels of sixteen inches in diameter,       salt water, the speed of a boat in this
and over, a vigorous whirlpool, or eddy,       medium is generally considered to be
will be formed that will reduce the effi-      greater than that on inland lakes and
ciency of the blades as they revolve past      rivers. This is true to a certain extent,
this point of disturbance.                     but the difference is usually over-esti-
   Another reason for placing the pro-         mated, for it has been found by tests
peller rather well below the surface of        that the respective speeds are as 5.66 is
the water will be seen on a rough day          to 5.5—an almost negligible proportion
when, if the wheel is not properly lo-         except in exceedingly fast racers.
cated, it will be uncovered with every            The foregoing discussion regarding
wave and so reduce the resistance on the       speed in motor boats is not intended to
motor that the latter will be allowed to       make any owner dissatisfied with the
race at each forward lunge of the boat.        present performance of his craft, for
This tendency will be overcome if the          that would be to sow the seeds of dis-
wheel is placed several feet forward of        content. But there are many boats in
the extreme stern of the hull, but as high     use that, with a little refinement or
a speed or efficiency will not be obtained     change in equipment, will travel faster
in this position as would be the case were     with the same expenditure of power, and
the propeller located as far aft as pos-       if these details are attended to, a more
sible.                                         efficient, as well as a more economical
   When a boat is tested out for speed,        craft will be the result.

				
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posted:12/30/2011
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