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MECAL TECHNICAL NOTE
MTN 11-3
Issued by the MECAL Technical Committee
CONDUCTING AN INCLINING EXPERIMENT
APPLICATION – Sailing & Motor Monohull Vessels that require Stability Information Books
These notes give guidance on conducting inclining experiments on small craft and completing
MECAL Inclining Experiment Data Form.
Drawings and other background information should be supplied to MECAL well in advance of the
Inclining Experiment date whenever possible, this will allow us to build the computer models for
stability analysis and provide additional guidance on issues such as the amount of weight to be used
for individual cases.
Make sure that you are familiar with these guidance notes and have all the necessary equipment
before you set out to conduct an Inclining Experiment.
Check the general conditions:
1. The weather conditions must be suitable for taking accurate heel angle measurements. If in
doubt; erect a pendulum before you do anything else. If the boat does not settle sufficiently to
read the pendulum then the inclining experiment cannot be carried out.
2. Obviously circumstances will dictate the practicality of re-arranging the experiment for
another time, if this is truly impractical then you must get the best results you can. It may be
possible to move the boat to a more sheltered mooring, lying to a single bow line can often
lessen the motion sufficiently.
3. Inevitably there are times when the pendulum will not settle completely so that readings must
be ‘estimated’ from the swinging pendulum, a few millimetres is not unusual but the more
unreliable the readings are the more you should take. A second pendulum with a different
person reading it is useful where ‘estimation’ becomes significant. Using a liquid trough to
dampen pendulum swings can be useful
Check the mooring:
1. The boat needs to be floating freely; apart from the more obvious restraints above the waterline
you should ensure that there is plenty of water underneath the keel and that this will remain the
case throughout the experiment.
2. The boat should not be restrained by her mooring. You should cast off any which are not
necessary, before you start the measurements.
3. Make sure the boat is not heeling significantly, if necessary re-arrange items to get her more or
less upright.
4. The boat is going to be heeled about 2 degrees from the upright, to port and starboard, check
that rubbing strakes and the like on the boat or wall will not snag when this happens.
5. Check that the vessel is not being held against a wall or pontoon by side wind pressure, if so
move, or re-arrange moorings accordingly.
MTN 11-03 Iss. Jan 08
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Check the boat:
1. All bilges should be dry.
2. Any tank cross connection valves should be shut.
3. Anything that may move as a result of heeling should be secured. (booms etc lashed
amidships)
On the Form:
Vessel’s Name Record the name of the boat you are on
Builder Record the builder and model if appropriate
HIN / Reg NO. Either or both, if available
Test Location i.e. “Sutton Harbour, Plymouth”
Test date and time Date and time the experiment was started and finished
Mooring i.e. “to slack lines alongside a pontoon”
Wind Record wind strength (approx) and direction relative to the boat
Tide Time if nearest high or low water, any significant current
Depth of Water From the echo sounder, or estimated depth under keel
Personnel Who is conducting the experiment and the total number of people on board
Condition of the Boat Record that the bilges are dry, cross connection valves shut etc.
Arrange inclining ballast:
1. As close as practical to LCF so as not to alter trim significantly.
2. Establish reference lines parallel to boats centre equally and as far outboard as practical to P
& S. Ballast spread is the distance between these lines deal initial arrangement as below (40
weights) i.e. ensure that there is room to place weights on either side, evenly about each
reference line and without altering the longitudinal or vertical centre of the total ballast.
Erect pendulums:
1. The pendulums (2) do not have to be on the centre line, they should be as long as possible;
1800mm (a door height) is about the minimum height. Put them well out of the way of
people who will be moving weights across the deck.
2. Ensure that the top fixings are secure and cannot be dislodged by passers by i.e. if one is
hanging over a hatch coaming or similar; secure the string at the suspension point with
masking tape. If it’s a sliding hatch; use masking tape to secure the hatch in position.
3. A 300mm (1ft) ruler or longer, should be secured at the bottom, athwart ships, so that the
pendulum weight is below the ruler and the string rests light against it at about its mid
length. A companionway step or the sole by a lifted board are ideal, the ruler can be taped
to a batten spanning a void etc. The important issue is that the ruler will not move between
measurements, again lots of masking tape is usually the answer. “G” clamps and battens are
often useful.
4. The pendulum weight may be suspended in a bath of oil or similar ‘thick’ liquid to dampen
its motion but this is not always necessary.
MTN 11-03 Iss. Jan 08
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5. The second pendulum is used as a check in case of scale reading anomalies found during
later analysis of the results.
6. Record the length of the pendulums which is the height from the ruler to the suspension
point.
Alternatively a transparent water tube can be used, the end parts of which are taped to vertical
rulers port & stbd. Two people are required for readings, one at each station
Measure the water density:
Use a floating hydrometer, or fish tank type hydrometer, to measure the density of the water, (you
will lose fewer hydrometers if you measure a bucket full rather that the sea itself!)
Fresh water would be 1000 kg/m3 and very salty sea water will rarely be more than 1025 kg/m3 . lf its
outside these limits then you probably need a better hydrometer. If in doubt take a sample, at least 0.5
litres, and measure it later with a more accurate instrument.
On the Form:
Inclining ballast Description i.e. “40 steel ingots of 25kg each totalling 1000kg”
Ballast spread Record the transverse distance for each ballast move.
Pendulum length Length from suspension point to ruler for each.
Water density Record the measured density
Take freeboard measurements to establish Draft and Trim:
1. Measurements should be taken on the centre line fwd and aft, if it is necessary to measure
off the centre line then both P & S measurements should be taken.
2. Measure from points which can be clearly related to the drawings i.e. deck at prow,
transom on CL as below:
3. Ensure that personnel taking measurements do not alter heel or trim, use personnel or
ballast opposite to each measurement point to balance out.
4. It can be difficult to obtain accurate freeboards due to choppy conditions & the wind
bending metal tape rules. Using a folding rigid rule with a small plastic “tray” blue tacked
on to the base, can help accuracy
Enter measurements on the form:
Location Freeboard
If the vessel has draught marks, record the readings as well as freeboards.
Ensure that total displacement and trim is constant throughout the experiment:
1. Number of people and things on board at free measurement must remain throughout inclining
readings.
2. All items must be in the same positions at each inclining reading i.e. the only difference at
each reading should be the inclining ballast movement. Get people to pick a spot at the outset
and return to it whilst the pendulum is being read.
Take inclining readings:
1. Arrange ballast in 4 groups of 25% weight each group.
MTN 11-03 Iss. Jan 08
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2. Commence with 50% ballast each side & measure drafts/freeboards & pendulum reading(s)
3. Move 25% of ballast from port to stbd & record pendulum reading(s)
4. Move remaining ballast from port to stbd & record pendulum reading(s)
5. Reverse the moves until you are back to the even distribution, record mass to P & S and the
pendulum reading after each move. Re-measure drafts / freeboards
6. Continue in 25% increments until all ballast is to port
7. Reverse the moves until you are back to the even distribution, record mass to P & S and the
pendulum reading after each move. Re-measure drafts / freeboards.
This will give a total of 9 readings for 5 different arrangements of the ballast:
Mass to port Mass to stbd Reading
Move kg kg mm
1 EVEN 0.5 total 0.5 total A
2 >P 0.75 total 0.25 total B
3 >P 1.0 total 0 C
4 >S 0.75 total 0.25 total D
5 >S 0.5 total 0.5 total E
6 >S 0.25 total 0.75 total F
7 >S 0 1.0 total G
8 >P 0.25 total 0.75 total H
9 >P 0.5 total 0.5 total I
On the form:
Shift of weights and readings:
1st Pendulum 2nd Pendulum
Length:
Mass to port Mass to stbd Reading Reading
kg kg mm mm
EVEN
Record the length of each pendulum and in the columns below it the actual deflection reading from
the ruler, after each move. Do not record the differences between moves or the deflections from the
initial reading; these are simple sums that are best done later.
Check the readings for consistency; you should expect a millimetre or two discrepancy & in the ideal
world:
The readings for moves: 1,5 & 9 should all be equal.
The readings for moves: 2 & 4 should be equal.
The readings for moves: 6 & 8 should be equal.
The reading for each move should differ from the previous one by the same amount. If there are any
significant variations from this pattern then the process should be repeated.
There are a number of common causes for error:
• Tank cross connection valves left open: will often result in increasing differences between
readings for successive moves in one direction, as the flow between tanks provides an
additional heeling moment. This can often become apparent during the experiment as you
can see the pendulum creeping.
• People or things altering position between moves can have any result depending on where
they have gone. Its useful to tell them in advance that inconsistency will necessitate re-doing
the whole thing. As they are generally on board to carry the weights, they soon realise the
benefit of returning to the same spot!
• Weights not moved through the same distance produce erratic results; you should check that
the weights are in the correct positions after each move, before reading the pendulum. Again
it is worth making it clear to the ‘crew’ exactly what is required at the outset so that they put
the weights down where you want them. Mark positions clearly on deck at outset of the
experiment.
MTN 11-03 Iss. Jan 08
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• Strong wind on the beam: will generally result in more heel ‘down wind’ than ‘up wind’.
• Movement of the pendulum suspension point or ruler: you should take care that they are
adequately secured in advance and ensure that nobody else goes near them.
• Human error reading the pendulum: have another go!
There is a certain amount of judgement involved in whether the readings are sufficiently consistent; a
single ‘odd reading’ can be re-taken but if they are generally erratic or obviously suffer from ‘creep’
or some other ailment then the cause should be rectified and the experiment repeated. This will, of
course never be necessary if the relevant precautions were taken at the outset and the readings taken
carefully.
The most common reason for ‘doing it again’ is simply that the boat is not settling sufficiently to
allow accurate readings, repeating the procedure will prove twice as many data points from which the
pattern should emerge.
Loading Condition:
Record weight and location (Longitude and Vertical) of all items on board which are not part of the
boats normal gear. (record locations so that they can be related to the drawings).
The calculations require that all positions be in a consistent co-ordinate system with the boat at
designed or level trim; the details are ultimately unimportant for the actual inclining experiment so
long as all locations are given longitudinally and vertically relative to fixed points which can be
readily identified on the drawings i.e. you might record the person taking the pendulum readings:
“person”, “85kg”, “0.5m fwd of the bottom companion step” and “1.00m above cabin sole”. The
Naval architect can translate this to his ‘global’ co-ordinate system provided that he has a drawing on
which he can locate the cabin sole and companion steps.
1. Inclining Ballast, record V and L central location (i.e. 2.5 aft of mast, c of g 0.2m above
deck).
2. Personnel: weights and locations at inclining readings.
3. All other items: tool kits, ‘abnormal’ locker contents, galley stores etc.
Tanks:
Record the contents of all tanks; these may be sounding or the contents recorded from a gauge. Make
sure that you check these yourself, as skippers tend to refer to a tank as full when it is anything above
three quarters capacity. (Unless a tank is “pressed” full or completely empty there will nearly always
be a free surface correction to be applied by the compiler of the stability information).
Loading
It is important to know what has been assumed to be part of the ‘normal gear’, make a note of these
items such as:
• “Furling headsail(s) on stays”, “main on boom”, sails on board in normal stowage positions”.
• “Life rafts and safety equipment in their normal stowage positions”.
• “Anchors, chain & warps in their normal stowage positions”.
This avoids later confusion as to whether these have been accounted for.
Items to be added:
It may be that some items, which will be part of the ‘normal gear’, were not on board when the
inclining experiment was carried out. The skipper should be asked to identify anything missing and
its weight and location recorded so that it can be added to the inclining results. Life rafts are a
favourite, but you sometimes find that a generator, the galley etc… have yet to be fitted.
Allowances:
Make a note of the number of people to be carried & any cargo, or unusually heavy stores, which
should be accounted for in the stability analysis.
MTN 11-03 Iss. Jan 08
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Comments and Notes:
Record anything which is significant, use extra sheets for this or any other items as necessary, but
note on the form that you have done so.
Photographs of ballast weight positions & pendulums/rulers taken during the experiment can prove
useful for future reference.
Sign and date the form and send it to MECAL and keep a copy.
MTN 11-03 Iss. Jan 08
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