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Old 21-08-2019, 10:26   #1
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Rollimg

I have noticed that when you are at anchor, with no wind and no current, the boat will always line up parrellel to the swell, however light, and roll uncomfortably. Do you know why?
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Old 21-08-2019, 10:30   #2
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Re: Rollimg

Never noticed this on my boats but can't recall too many times I anchored in a harbor that experienced much swell in calm wind conditions.
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Old 21-08-2019, 10:34   #3
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Re: Rollimg

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Originally Posted by Rualda View Post
I have noticed that when you are at anchor, with no wind and no current, the boat will always line up parrellel to the swell, however light, and roll uncomfortably. Do you know why?
I'm doubting it's always, but if I have only one anchor out sometime during the night my boat will be in that position until the tide changes completely which takes a while....and my boat will really roll having only an 8' beam

One night I took a breaker over the side of the cockpit......I was in a bad spot and the wind turned and came up during the night.

Now I anchor up a creek rather than just behind an island close to the ocean

Tides change every six hours here
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Old 21-08-2019, 10:39   #4
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Re: Rollimg

A cross beam swell imposes the least pull on the rode which is your pivot point, so it becomes the position of least resistance against the rode, but it sure adds to the adverse motion of the boat. But then that is why you deploy roll stoppers.
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Old 21-08-2019, 11:28   #5
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Re: Rollimg

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A cross beam swell imposes the least pull on the rode which is your pivot point, so it becomes the position of least resistance against the rode, but it sure adds to the adverse motion of the boat. But then that is why you deploy roll stoppers.
Or put out a second anchor or swing around on the one anchor and cleat
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Old 26-08-2019, 20:35   #6
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Re: Rollimg

(The following from "I haven't thought about this for a long time" memory):

Technically, it is due to the "intermediate axis theorem" of dynamics. On any rigid body (a boat counts as a rigid body), there is an axis with minimum moment of inertia (ie: resistance to being turned) and an axis with maximum moment of inertia. It would be a strange monohull where roll wasn't the lowest moment of inertia. Depending on the boat, either pitch or yaw would be the highest. My guess is that with more modern boat designs (ie: most coastal cruisers), that yaw is the highest. If your boat is at anchor, and you want the most comfort, you want the bow into the seas. However, if pitch is an intermediate axis, it will try to avoid pitching when it can yaw or roll. If there is little wind or current, then it will yaw until rolling gets initiated. The wave period can be close to the resonance frequency of rolling, so the boat will want to stay there -- beam to the sea, and rolling (sometimes extremely).
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Old 26-08-2019, 20:55   #7
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Re: Rollimg

an unscientific guess is that if your bow is turned slightly to the swell, the first thing the swell hits is the bow thus it is the part of the boat to get the first shove until the whole side is getting shoved equally. Conversely if the stern is hit first it will be pushed until the swells are beam-on.
I use a flopper stopper to mess with and dampen the rolling rhythm, especially if the swell just happens to match the resonant frequency of my hull
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Old 27-08-2019, 00:30   #8
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Re: Rollimg

Thank you for all your replies, but a friend has explained it very simply. A boat lying parallel to the swell has a lower potential energy than one lying across it. It is as simple as a ball rolling downhill until it comes to rest at the bottom of the valley.
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