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ANNEX
SUGGESTED FORM OF GUIDANCE TO THE MASTER ON AN APPROXIMATE
DETERMINATION OF SHIP'S STABILITY BY MEANS OF THE ROLLING PERIOD TEST
1. Introduction
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(1) If the following instructions are properly carried out this
method allows reasonably quick and accurate estimation of the
metacentric height, which is measure of the ship’s stability.
(2) The method depends upon the relationship between the metacentric
height and the rolling period in terms of the extreme breadth of the
vessel.
2. Test Procedure
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(3) The rolling period required is the time for one complete
oscillation of the vessel and to ensure the most accurate results in
obtaining this value the following precautions should be observed:
- The test should be conducted with the vessel in harbour, in
smooth water with the minimum interference from wind and tide.
- Starting with the vessel at the extreme end of a roll to one
side (say port) and the vessel about to move towards the upright, one
complete oscillation will have been made when the vessel has
moved right across to the other extreme side (i.e. starboard) and
returned to the original starting point and is about to commence the
next roll.
- By means of a stop-watch, the time should be taken for not
less than about five of these complete oscillations; the counting of
these oscillations should begin when the vessel is at the extreme
end of a roll. After allowing the roll to completely fade way, this
operation should be repeated at least twice more. If possible, in
every case the same number of complete oscillations should be timed
to establish that the readings are consistent, i.e. repeating
themselves within reasonable limits. Knowing the total time for the
total number of oscillations made, the mean time for one complete
oscillation can be calculated.
- The vessel can be made to roll by rhythmically lifting up and
putting down a weight as far off middle-line as possible; by pulling
on the mast with a rope; by people running athwartships in unison;
or by any other means. However, and this is most important, as soon
as this forced rolling has commenced the means by which it has been
induced must be stopped and the vessel allowed to roll freely and
naturally. If rolling has been induced by lowering or raising a
weight it is preferable that the weight is moved by a dockside
crane. If the ship’s own derrick is used, the weight should be
placed on the deck, at the middle-line, as soon as the rolling is
established.
- The timing and counting of the oscillations should only begin
when it is judged that the vessel is rolling freely and naturally
and only as much as is necessary to accurately count these
oscillations.
- The mooring should be slack and the vessel "breasted
off" to avoid making any contact during its rolling. To check
this, and also to get some idea of the number of complete
oscillations that can be reasonably counted and timed, a preliminary
rolling test should be made before starting to record actual times.
- Care should be taken to ensure that there is a reasonable
clearance of water under the keel and at the sides of the vessel.
- Weights of reasonable size which are liable to swing (e.g. a
lifeboat), or liable to move (e.g. a drum), should be secured agains
such movement The free surface effects of slack tanks should be kept
as small as is practicable during the test and the voyage.
3. Determination of the Initial Stability
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(4) Having calculated the period for one complete oscillation, say T
seconds, the metacentric height GM0 can be calculated from
the following formula:
![Image](/web/20071224084750im_/http://www.tc.gc.ca/MarineSafety/TP/TP2072/images/Image302.gif)
![](/web/20071224084750im_/http://www.tc.gc.ca/MarineSafety/TP/TP2072/images/Image303.gif)
where F is ... [to be determined for each particular vessel by
the Administration]
(5) The calculated value of GM0 should be equal to or
greater than the critical value which is [to be determined for
each particular vessel by the Administration].
4. Limitations to the Use of this Method
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(6) A long period of roll corresponding to a GM0 of 0.20 m.
or below, indicates a condition of low stability. However, under such
circumstances, accuracy in determination of the actual value of GM0
is reduced.
(7) If, for some reason, these rolling tests are carried out in open,
deep but smooth waters, inducing the roll, for example, by putting over
the helm, then the GM0 calculated by using the method and
coefficient of paragraph (3) above should be reduced by figure to be
estimated by the Administration to obtain the final answer.
(8) The determination of stability by means of the rolling test in
disturbed waters should only be regarded as a very approximate estimation.
If such test is performed, care should be taken to discard readings which
depart appreciably from the majority of other observations. Forced
oscillations corresponding to the sea period and differing from the
natural period at which the vessel seems to move should be disregarded. In
order to obtain satisfactory results, it may be necessary to select
intervals when the sea action is least violent, and it may be necessay to
discard a considerable number of observations.
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