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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation > Conservation of angular momentum
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Thread: Conservation of angular momentum Reply to Thread
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Topic Review (Newest First)
03-02-2012 12:05 AM
BryceGTX
Quote:
Originally Posted by davidpm View Post
There is no danger of large continuous waves.
On occasion a single 1 foot roller comes in caused by the wake of of boat.

In this specific case would some extra weight in the rigging have the effect of damping the sloshing?
Adding mass at the top of the mast will never create more damping. Rather, it somewhat negates the existing damping because the inertia is higher hence the frequency is lower.

That being said, since the natural frequency is lower, you may find the frequency of the waves do not align with the natural frequency of the boat so you can actually get a cancellation.

The more fundamental problem is that the natural frequency changes as the square root of the reciprocal of the inertia. So you need to add a bunch of inertia to have a significant effect on the natural frequency.

No doubt the big benefit to adding inertia is the reduction in frequency with the associated reduction in acceleration.

Quote:
Originally Posted by davidpm View Post
However if you hoisted say 50 lbs to the middle of the mast and only in quiet water to reduce rocking due to boat wakes
A more effective way to reduce the rocking is to add damping. AS you pointed out in a following post.. "rocker stoppers". These increase damping which gets right to the heart of the problem.

I think Bene505 was trying to create a similar effect with the anchors and lines. This could also increase the damping and inertia and righting moment. Incidently, the arm to calculate the inertia and righting moment would be half the beam.

Quote:
Originally Posted by Bene505 View Post
Great posts Bryce. Really brilliant.
Thanks for the compliment Brad..

Bryce
03-01-2012 11:55 PM
Capt Len If the roll is getting to you look at paravan stabilizers or flopper stoppers.West coast boats put out their stab poles as they leave the harbour and pull them up as they return.Para vanes are good for 6 knots or so and are great at anchor or drift in offshore conditions too.
03-01-2012 11:50 PM
davidpm
Quote:
Originally Posted by Bene505 View Post
Great posts Bryce. Really brilliant.

During this thread, I've been wondering of you could tie an anchor to a pair of 60 foot lines, suspended from each midship cleats, to act as sort of an underwater mast when anchored. The weight of the anchor keeping the lines taught, combined with the angle of the lines would provide increased rotational inertia, especially for low angles of motion. There would be some additional dampening added by the anchor in the water (which I'm visualizing would keep the anchor from enscribing perfect arcs during rolling).

And the lower center of gravity of this "mast" wouldn't make for increased roll amplitude due to top-heaviness.


Regards,
Brad

There is a common trick I think they call it the flopper stopper or some such. You can make your own and they have commercial units.
It is basicaly a bucket with a bottom with a one way flap. The bucket sinks easily as the flap opens inward but when you try to lift the bucket the flap seals and it is hard to lift.
The bucked is suspected by a line from a boom which is set off the beam of the boat.

Supposed to work pretty well.
03-01-2012 11:38 PM
davidpm
Quote:
Originally Posted by BryceGTX View Post
Now given all these graphs.. would I hoist a 100 pound weight to the top of my mast during a storm? Not likely.
Me neither not during a storm and I'm not strong enough to deal with a hundred pounds and I don't like the idea of anything all the way at the top of the mast.

However if you hoisted say 50 lbs to the middle of the mast and only in quiet water to reduce rocking due to boat wakes. What does your graph say would happen then?

It is obviously no danger assuming you trust the halyard and the weather.
So the only question is would it reduce rocking and if so would it be enough so the exercise would be worth it.
03-01-2012 11:37 PM
Bene505 Great posts Bryce. Really brilliant.

During this thread, I've been wondering of you could tie an anchor to a pair of 60 foot lines, suspended from each midship cleats, to act as sort of an underwater mast when anchored. The weight of the anchor keeping the lines taught, combined with the angle of the lines would provide increased rotational inertia, especially for low angles of motion. There would be some additional dampening added by the anchor in the water (which I'm visualizing would keep the anchor from enscribing perfect arcs during rolling).

And the lower center of gravity of this "mast" wouldn't make for increased roll amplitude due to top-heaviness.

What do you think?

----- EDIT -------

I think I answered my own question. Without a rigid mast underwater, the configuration would act very similar to simply placing the anchor on top of a midships cleat. If you could re-purpose your spinnaker pole for this purpose, it could be very interesting. (Clip one end to your anchor roller, run taut lines to forward cleats on both sides, and clip the other end to your anchor.)

But if you are going to that trouble, hoist the anchor aloft with those 60 foot lines tied to the bottom of it and then tied to the midships cleats. The lines would keep the anchor from banging around up there, and you'd have your full 60 foot distance for maximum effect.

Regards,
Brad
03-01-2012 11:36 PM
Capt Len Maybe you can calculate a co-efficient of comfort. A slow roll ,even if extreme is easier to tolerate than than the quick action of a power vessel or cat in heavy action. The older west coast trollers were narrow gutted with mast and trolling poles. Rolled like a pig in the wrong period swell but they were great sea boats and comfortable.even without a steady sail .
03-01-2012 11:18 PM
BryceGTX Last, but not least. If we hoist the 100 pound weight to the top of the mast and apply the same 5 degree forcing step, we see that the acceleration is reduced even further to below 0.2 rads/s^2. And we see that the frequency reduces a bit more.

Now given all these graphs.. would I hoist a 100 pound weight to the top of my mast during a storm? Not likely. These graphs consider the dynamic effects of the added mass. WE need to also consider the static effects of the 100 pounds at the top of a 60 foot mast that produces a heeling moment of 6000 foot pounds. This is a quite significant heeling moment to add to a boat that may have a righting moment of 70000 foot pounds.

Even worse is that this heeling moment is a mass heeling moment rather than a hull moment. So as they say on tv.. "Don't try this at home.."
03-01-2012 11:11 PM
BryceGTX Even though the heeling angle is reduced without the mast, you may be more sensitive to the accelerations. In that case, without the mast, the accelerations are about three times higher than with the mast.

And since these accelerations are such low frequency, the higher accelerations may actually be more uncomfortable. And since the heeling angle is only slightly reduced, but a higher frequency, we may also find this more uncomfortable.
03-01-2012 10:59 PM
BryceGTX Now we remove the mast inertia. We notice the natural frequency increases and so does the damping. The damping causes the rolling to die out after only about 3 cycles. And because the frequency is higher, the damping cause the angular displacement to be limited to 3 degrees.
03-01-2012 10:56 PM
BryceGTX So now we run the Simulation with the mast. We see the natural frequency period of about 4 seconds and we notice that the damping causes the rolling to die out after about 5 cycles.

We notice a peak angular displacement of about +-4 degrees. THis is with a 5 degree step forcing function.
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