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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 ^

(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 ^

(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:

1. The test should be conducted with the vessel in harbour, in smooth water with the minimum interference from wind and tide.
   
   
2. 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.
   
   
3. 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.
   
   
4. 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.
   
   
5. 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.
   
   
6. 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.
   
   
7. Care should be taken to ensure that there is a reasonable clearance of water under the keel and at the sides of the vessel.
   
   
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 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 ^

(4) Having calculated the period for one complete oscillation, say T seconds, the metacentric height GM₀ can be calculated from the following formula:

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 ^

(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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