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post #17 of Old 03-01-2012
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Originally Posted by Barquito View Post
So, say you are motoring in a seaway where the wave train (or a harmonic) is the same frequency as your boats roll rate. Would it reduce roll if you hoist some weight up the mast until the roll rate of the boat is different than the waves?
I think Bryce just answered that one extremely well. The answer (tongue in cheek) is to send a friend aloft. A 100 kg person at the top of Bryce's mast has this effect:

Originally Posted by BryceGTX View Post
With mast:
Sqrt(100000/38400) rad/s = 1.61 rad/s = 0.257 hz
so the natural period of my boat is about 4 seconds.

Without the mast, the natural frequency is:
Sqrt(100000/13400) = 2.73 rad/s = 0.434 hz or 2.3 second period
With 100kg buddy at the top, this adds an additional 100 * 18^2 = 32,400 kgm^2
Sqrt(100000/(38400+32400)) rad/s = 1.19 rad/s = 0.189 hz
so the new natural period is about 5.29 seconds.

But if you are motoring and your friend won't go aloft, you could change your angle to the waves, thereby changing their apparent frequency and preventing oscillation. It's David's anchoring scenerio that makes you want to hoist something aloft.

Good thread.


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Last edited by Bene505; 03-01-2012 at 10:22 PM.
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