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Old 19-10-2010, 21:42   #1
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Swell Dampening

I found on my 4 ton sloop I can dampen ocean swell completely keeping the boat virtually roll-free by moving my body in anti-phase with the rolling motion of the boat. This act enables me to sail in light winds, and I no longer need a preventer on the boom.

The problem is I can only keep this up for minutes, and I need a better solution to allow me to cover many miles at 1 and 2 knts winds! The only thing I can think of (other than training a llama or something) would be a gimbaled gyroscope.

Has anyone else any better success for this? Maybe multihull boats dont have this problem?
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Old 19-10-2010, 23:03   #2
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Why is rolling bothering you.

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Old 20-10-2010, 09:17   #3
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rolling is fine with me, but the sails can't stay full because of very light winds in large swells.
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Old 20-10-2010, 10:20   #4
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Host a weight up the mast. Adjust the mass and height until the response of the boat is sufficiently far from the forcing function (the wave height and period). The boat shouldn't roll then.

BTW, I have never tryed this however there are sound engineering reasons why it may help!! You can really slow the boat's roll period using this approach (I have done this part.)
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Old 20-10-2010, 10:24   #5
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Quote:
Originally Posted by geckosenator View Post
I found on my 4 ton sloop I can dampen ocean swell completely keeping the boat virtually roll-free by moving my body in anti-phase with the rolling motion of the boat.
Keep yourself and any crew on the leeward side of the boat during extreme light-winded conditions. That will help to keep the sails full a little bit, maybe. To reduce the ocean swell, command the sea to do so, using any god-like powers you may possess.
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Old 20-10-2010, 10:25   #6
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if you find a real solution to the light-winds/large-swells problem, you'll make millions.
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Old 20-10-2010, 11:35   #7
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Host a weight up the mast. Adjust the mass and height until the response of the boat is sufficiently far from the forcing function (the wave height and period). The boat shouldn't roll then...
The sails and the standing rig provide inertial damping, even at anchor, due to the weight of the rig and its distance from the roll centre (increased roll moment of inertia).
If you think of the boat’s roll as a pendulum, just moving from side to side, I think LakeSuperior’s theory is to add a second pendulum (sometimes known as a roll tank) which is moving 180 degrees out of phase with the first pendulum. This would result in no boat rolling, as one pendulum cancels out the other. The higher above the centre of gravity, the less mass required in the roll tank.

I have no idea how one could practically accomplish this on a typical cruiser.
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Old 20-10-2010, 11:48   #8
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The sails and the standing rig provide inertial damping, even at anchor, due to the weight of the rig and its distance from the roll centre (increased roll moment of inertia).
If you think of the boatís roll as a pendulum, just moving from side to side, I think LakeSuperiorís theory is to add a second pendulum (sometimes known as a roll tank) which is moving 180 degrees out of phase with the first pendulum. This would result in no boat rolling, as one pendulum cancels out the other. The higher above the centre of gravity, the less mass required in the roll tank.

I have no idea how one could practically accomplish this on a typical cruiser.
Actually, what Gord has proposed is one step in sophistication above what I was proposing. I was trying to just drive the natural roll frequency of the boat far away from the periodic wave forcing function frequency. This will prevent coupling the wave energy into the boat roll. The weight would be fixed or tied to the mast.
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Old 20-10-2010, 11:52   #9
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Use a light air sail and beam-ish reach. With a standard Dacron jib the sail is just way too heavy in light air. With someone nice and light the sail will stay full and keep more tension on the rig. Makes a world of difference. Being able to switch out to a hanked on light air drifter is a big reason I'm not into roller furlers which severly restrict how creative you can get.
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Old 20-10-2010, 12:18   #10
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I remember reading somewhere that the old square riggers used a barrel hoisted up the mast to stop rolling. One line tied to the top and one to the bottom to keep it from swinging.

I'm wondering if you couldn't accomplish the same thing with some sort of planing board attached to a rail and lowered just below the water. It would need to be on a pole that could stabilize it on both up and down movements. One could accomplish the same thing by attaching it to the stern ladder and extending it to one side. I can't see any reason why it would need to extend on both sides.

but that's just my feeble hyperactive brain at work. Please commence to shoot down the idea......
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Old 20-10-2010, 14:33   #11
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I wonder if you can't achieve the same thing with a traditional set of roll-dampening interconnect tanks, as used on ships. See picture for principle. Say one 30 - 50 liter tank on either side, interconnected with a suitable 2" - 3 " (50 or 75 mm hose or pipe) with an adjustable ball or gate valve. The idea is to get the transfer from one side to the other out of sync with the rolling. It will take some experimenting to find the right sizes etc.

Thomas
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Old 20-10-2010, 14:40   #12
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There are many effective means of reducing rolling.
"Passive" means:
- changing the boat natural rolling frequency away from the peak in the wave spectrum; it can be done by modifying the arrangement of mass onboard, e.g. hoisting something heavy (the skipper?) in the rigging or by altering the stability curve, e.g. heeling the boat;
- adding a pendulum with a carefully tuned oscillating frequency (either a solid one, as proposed by LakeSuperior, or a fluid one, as proposed by Gord);
- adding more fluid damping, e.g. by increasing the keel area or adding other appendages.

"Active" means:
- increasing the damping effectiveness of existing appendages by increasing the speed;
- selecting the course in such a way that the swell doesn't excite rolling: steering with head or following swell;
- generating an anti-roll hydrodynamic torque, either with active fins or just with the rudder (as done on USN DDG51-class destroyers, and some other warships).

Alain
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Old 20-10-2010, 15:08   #13
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The solution is to sail on inland waters.


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Old 20-10-2010, 15:19   #14
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Quote:
Originally Posted by geckosenator View Post
Maybe multihull boats dont have this problem?
yes, so add two floats, a couple of crossbeams and you are good. :-)
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Old 20-10-2010, 18:59   #15
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I'm going to go with the gimbaled gyroscope. Gyroscopes would allow the same type of roll compensation of a pendulum at a small fraction of the weight. I know it's kind of complex, but since I already have digital gyroscopes accelerometers magnetometers etc..., my computer knows the boat attitude roll rate and all sorts of inertial information, and can therefor command an electric winch I have (which has 100ft/lbs torque) to rotate the gyro. I have a crappy high speed low-torque 12v motor I can use for the spinning part (used to be a blower motor fan)

The interesting thing is, eventually I should be able to optimize for 3 things using a gimbaled gyroscope:
swell dampening (make boat level for light winds, or comfort)
power generation (maximize power generated from swell)
speed (maximize boat speed by forcing the best motion through the waves)

Yes, power generation is possible, but then it cannot perfectly dampen things. I doubt initially I will even try considering what I have to work with, but I like to think about it for the day I get high efficiency stuff. I'll keep the forum posted on my progress, but this is a long-term goal of mine now :-P
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