ESTIMATING A SMALL CRAFT’S STABILITY BY MEANS OF
BY JEFFREY CASCIANI-WOOD HONFIIMS ROLLING PERIOD TESTS
“The metacentric height is the lever by which the sea rocks the ship.” Kenneth Barnaby
When carrying out the survey of small craft for whatever reason, the marine surveyor must always bear in mind that, among the many factors that determine whether or not she is seaworthy is an adequate metacentric stability. To determine that accurately requires a full inclining experiment and a considerable amount of detail about the vessel’s hull form that is generally not available at a pre-purchase or similar stability. This method described here gives a simple rule of thumb by which a marine surveyor may make a sufficiently adequate guesstimate of the vessel’s stability from which he may make a decision as to whether or not he should recommend an inclining experiment. The evaluation of a number of inclining and rolling tests according to various formulae shows that the following gives the best results and that it has the advantage of being the simplest:
GMo where BWL
GMo TR
kRP The factor kRP
= = = =
waterline breadth
initial metacentric height rolling period
m m s
constant (the rolling coefficient) - is of the greatest importance (it’s about 1.25 for a narrowboat)
and it should be noted that the greater the distance of masses from the rolling axis, the greater the rolling coefficient will be. Therefore, it can be expected that:
• the rolling coefficient for an unloaded vessel (i.e. for a hollow body) will be higher than that for a loaded vessel.
• the rolling coefficient for a vessel carrying a great amount of bunkers and ballast both groups are usually located in the bottom (i.e. far away from the rolling axis) will be higher than that of the same vessel having empty fuel and ballast tanks.
44 | The Report • March 2018 • Issue 83 ≈ (kRP BWL /TR )2 m (1)
• the rolling periods observed may not be free oscillations but forced oscillations due to seaway.
• frequently, oscillations are either irregular or only regular for too short an interval of time to allow accurate measurements to be observed.
• specialized recording equipment is necessary.
However, sometimes it may be desirable to use the vessel’s period of roll as a means of approximately judging the stability at sea. If that is done, care should be taken to discard readings which depart appreciably from the majority
Experiments have shown that the results of the rolling test method get increasingly less reliable the nearer they approach metacentric height values of 0.2 m and below.
For the following reasons, it is not generally recommended that results be obtained from rolling oscillations taken in a seaway:
• exact coefficients for tests in open waters are not available.
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