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Old 01-07-2017, 20:15   #1
er9
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wave frequency, rolling frequency?

Are sailboats vulnerable to rolling when broadside to a wave from syncing of wave frequency and rolling frequency as are big ships? i know broadside to breaking waves is a concern of itself but was curious about frequency sync as it can cause an inward (facing oncoming waves) direction of roll.
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Old 01-07-2017, 23:11   #2
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Re: wave frequency, rolling frequency?

Not an issue from a capsize point of view with your typical ballsted sailing boats, too much roll damping from the keel, and too much reserve stability. But it can certainly be uncomfortable when the waves get in sync with your roll period.

Some motor boats have a safety issue with Parametric rolling. One capsized in the Mediterranean a few years ago, and parametric rolling was thought to be a cause.
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Old 02-07-2017, 15:17   #3
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Re: wave frequency, rolling frequency?

Not a problem for catamarans!

Generally, beam seas will make a mono roll. Just how much seems to have to do with the shape of the boat under the water. Both keel and mast offer some resistance to rolling. With fin keels, the deeper the keel, the more resistance to rolling.

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Old 27-07-2017, 22:29   #4
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Re: wave frequency, rolling frequency?

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Originally Posted by JPA Cate View Post
Not a problem for catamarans!

Generally, beam seas will make a mono roll. Just how much seems to have to do with the shape of the boat under the water. Both keel and mast offer some resistance to rolling. With fin keels, the deeper the keel, the more resistance to rolling.

Ann
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But then again on cats you get those waves that roll under your windward hull and slap up against the inside of the lee hull and then that wave accelerates and slams up under the bridge deck right where you bunk is.
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Old 13-09-2017, 14:14   #5
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Re: wave frequency, rolling frequency?

Good video of wave knock-down. You can see how the boat surges at the top of the wave and is dragged back in the trough. Quite moderate waves until they reach shallower water. A more powerful motor would have helped. Turning back was a good idea but who knows what the forcast was, may have been trying to escape something worse??

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Old 13-09-2017, 14:28   #6
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Re: wave frequency, rolling frequency?

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Originally Posted by JPA Cate View Post
Not a problem for catamarans!

Generally, beam seas will make a mono roll. Just how much seems to have to do with the shape of the boat under the water. Both keel and mast offer some resistance to rolling. With fin keels, the deeper the keel, the more resistance to rolling.

Ann
Interesting discussion that I missed earlier. Iíve noticed that our full-keel, 6í draft boat resists initial rolling, but once it gets going will seemingly keep rolling in certain wavelengths.

I guess this makes sense Ö but itís damn annoying sometimes
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Old 14-09-2017, 13:11   #7
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Re: wave frequency, rolling frequency?

"Not a problem for catamarans!"
but the rotational energy around the longitudinal axis imparted by a steep & fast moving wave certainly is! nearly did our Wharram cat in & was maybe only 8 or 10' high. could have been our subjectiv perception, but the cat seemed to teeter & things that hadn't budged in nearly 40.000nm of rtw violently flew around.
convinced me that it is not necessary for a wave to actually lift the windward hull high enough for the boat to capsize, imparting a sufficiently strong rotational impulse suffices
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