PREVIOUS | TABLE
OF CONTENTS
ANNEX TO APPENDIX III
SUGGESTED FORM OF GUIDANCE TO THE MASTER ON AN
APPROXIMATE DETERMINATION OF SHIP’S STABILITY
BY MEANS OF THE ROLLING PERIOD TEST
Introduction ^
1. If the following instructions are properly carried out, this method
allows a reasonably quick and accurate estimation of the metacentric height,
which is a 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.
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 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 vessel 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.
- 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 against
such movement. The free surface effects of slack tanks should be kept as
small as is practicable during the test and the voyage.
Determination of the Initial Stability ^
4. Having calculated the period for one complete oscillation, say T
seconds, the metacentric height GMo can be calculated from the
following formula:
![T seconds, the metacentric height GMo](/web/20071224080122im_/http://www.tc.gc.ca/MarineSafety/tp/Tp2534/images/Image332.gif)
where F is ...(to be determined for each particular vessel by the
Administration).
5. The calculated value of GM should be equal to or greater than the
critical value which is ... (to be determined for each particular vessel
by the Administration).
Limitations to the Use of this Method ^
6. 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.
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 GMo 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 be regarded as a very appropriate estimation. If
such a 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.
![Metric Unit](/web/20071224080122im_/http://www.tc.gc.ca/MarineSafety/tp/Tp2534/images/metric.jpg)
![Feet Unit](/web/20071224080122im_/http://www.tc.gc.ca/MarineSafety/tp/Tp2534/images/feet.jpg)
PREVIOUS | TABLE
OF CONTENTS
|