3.16.1 The rolling period required
is the time for one complete oscillation of the ship and to ensure
the most accurate results in obtaining this value the following precautions
should be observed:
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.1 The test should be conducted with the ship
in harbour, in smooth water with the minimum interference from the
wind and tide.
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.2 Starting with the ship at the extreme end of
a roll to one side (say port) and the ship about to move towards the
upright, one complete oscillation will have been made when the ship
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.
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.3 By means of a stop-watch, the time should be
taken for not less than about 5 of these complete oscillations; the
counting of these oscillations should begin when the ship is at the
extreme end of a roll. After allowing the roll to completely fade
away, 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.
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.4 The ship 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 should be stopped and the ship 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.
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.5 The timing and counting of the oscillations
should only begin when it is judged that the ship is rolling freely
and naturally, and only as much as is necessary to accurately count
these oscillations.
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.6 The mooring should be slack and the ship “breasted-off”
to avoid making any contact during its rolling. To check this, and
also to get some idea of the number of oscillations that can be reasonably
counted and timed, a preliminary rolling test should be made before
starting to record actual times.
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.7 Care should be taken to ensure that there is
a reasonable clearance of water under the keel and at the sides of
the ship.
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.8 Weights of reasonable size which are liable
to swing (e.g. a lifeboat), or liable to move (e.g. a drum), should
be secured against such movement. The free surface effects of slack
tanks should be kept as small as is practicable during the test and
the voyage.
3.16.2 Limitation to the use of this method
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.1 A long period of roll corresponding to a GMo of
0.20 m or below, indicates a condition of low stability. However,
under such circumstances, accuracy in determination of the actual
value of GMo is reduced.
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.2 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 GMo calculated
by using the method and coefficient of paragraph 3.16.1 above should be reduced by (figure to be estimated by the
Administration) to obtain the final answer.
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.3 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 necessary to discard a considerable number of
observations.