Your vessel rolls slowly and sluggishly. This indicates that the vessel __________.
A. has off-center weights
B. is taking on water
C. has a greater draft forward than aft
D. has poor stability
A. has off-center weights
B. is taking on water
C. has a greater draft forward than aft
D. has poor stability
D. Has poor stability
ReplyDeleteI agree the best answer is D.
ReplyDeleteThe reason why is that a stiff vessel, one with a large GM will have a fast roll, however what most people do not understand is that the roll angle will be very large. Think of it this way a Bell Buoy is the a great example of a stiff vessel. When a wave disturbs the buoy the response is a quick roll motion at large angles to ring its bell because it wants to stay perpendicular to the surface of the wave, so the wave appears to pass under it.
On the other hand a tender vessel has a slow rolling period with a relatively small angle of roll. A log floating at the surface of the water will be a good example to illustrate a tender vessel. when a wave passes by it hardly responds and most likely the wave will pass over it instead of under it.
For Metric Units where T = rolling period in Seconds from port to starb to port and B = the beam in meters . A good approximation when rolling in still water is
T= 0.8B/(the square root of GM in meters)
In if you the beam is in feet use
T= 0.44B/(the square root of GM in feet)
If you are wondering how a vessel can roll in calm waters consider rudder forces or in the case of a log ship we actually roll the ship with cargo and the ship's crane. I will publish my video of roll testing a log ship in the future.
Awaiting that interesting video
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