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.
In view
of the foregoing circumstances, it needs to be recognized that the determination of the stability by means of the rolling test in disturbed waters should only be regarded as a very approximate estimation.
Although a vessel may successfully pass a roll test the results of this type of test do not provide a full assessment of a vessel’s stability and can, therefore, be misleading. Current MCA guidance on the methods that can be used to assess the stability of small fishing vessels, for example, is not sufficient to provide their owners with the information needed to understand the limitations of the various options available. Like fishing vessels of less than twelve metres registered length, pleasure craft are not required to meet any statutory stability criteria. However, in response to a number of accidents that have resulted in recommendations from the Marine Accident Investigation Branch, the Maritime and Coastguard Agency intends to introduce legislation which will require small fishing vessels of under twelve metres registered length to comply with similar stability criteria to that which already exists for small commercial vessels and the small craft marine surveyor should be aware of that fact. It is also important to appreciate that the virtue of meeting the criterion in only one loading condition does not ensure immunity against capsizing or absolve the skipper from his responsibilities.
It has to be
assumed that the vessel will maintain adequate stability throughout her whole voyage cycle and that will only be valid if best practices with regard to operation of the vessel, the use of consumables and the stowage of the catch and gear are followed.
The Rolling Period Method
As a supplement to the approved stability information, the initial stability can be approximately determined by means of a rolling period test. Vessels with a high initial stability are said to be stiff and have a short rolling period and conversely, vessels with a low initial stability are said to be tender and have a long rolling period. The following guidance notes describe the rolling period test procedure which can be performed at any time by the crew of a small vessel.
1. The test should be conducted in smooth water with the mooring lines slack and the vessel breasted off to avoid making any contact during the operation. Care should be taken to ensure that there is a reasonable clearance of water under the keel and the sides of the vessel.
2. The vessel is made to roll. That can, for example, be done by men pushing bdown on the gunwale when the boat is at the top of its roll away from them. As soon as this forced rolling has commenced the men should stop and place the vessel allowed to roll freely and naturally.
3. 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 it is necessary to accurately time and count these oscillations (approximately 2° - 6° to each side).
4. With the vessel at the extreme end of the roll to one side (say port) and the vessel about to move toward 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.
5. By means of a stop watch, the time should be taken for not
less than six of the complete oscillations. The counting of these oscillations should begin when the vessel is at extreme end of a roll.
6. After allowing the roll to completely fade away, the operation should be repeated at least twice more. Knowing the total time for the total number of oscillations made, the time for one complete oscillation, say TR seconds, can be calculated.
If the measured value of TR, in seconds, is less than the breadth of the vessel in metres, it is likely that the initial stability would be sufficient, provided that the vessel carries full fuel, stores, gear, etc. Equally, if it is greater, then the metacentric height is suspect and, for safety reasons, the marine surveyor should recommend a proper inclining test.
The rolling period TR usually increases and the vessel becomes more tender as the weight of fuel, stores, gear, etc., decreases. As a consequence, the initial stability will also decrease.
If the rolling
period test is conducted under such circumstances it is recommended, that for the estimate of the initial stability to be considered satisfactory, the calculated value of TR, in seconds, should not be more than 0.8 times the waterline breath of the vessel, in metres.
The method is suitable for vessels up to about 70 metres length but may not be applicable to vessels with a hull shape that dampens the rolling, for example vessels with large bilge keels or vessels of an unconventional design, such as high speed motor boats.
Stability – Weight Creep
Flooding, foundering and capsizing are all associated with a loss of transverse stability and whether or not a boat has adequate reserves of stability when newly launched, they can be lost through overloading or
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