Considerations on the
Metacentric Stability of Narrowboats APPENDIX 2
AN APPROXIMATE DETERMINATION OF A SMALL VESSEL’S STABILITY BY MEANS OF THE ROLLING PERIOD TESTS
(From the Official Journal of the European Union) 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 leaning on the nearest gunwale when it is at its highest point and taking one’s weight off as soon as the boat starts to move downward.
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 T seconds, can be calculated.
7. The boat’s metacentric height can then be estimated using formula 5 above.
The Report • March 2019 • Issue 87 | 41
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