Roll Inertia?

[Dockhead]

Originally Posted by Adelie

It doesn't increase power, it increases righting moment allowing you to hoist more sail for a given wind strength which then gives you more power.

Which increases power

[Adelie]

Originally Posted by a64pilot

I would not, I believe it would take a very specific circumstance for that to work, in all others, it wouldn’t.
I see a well balanced mono as a Weeble, they wobble but don’t fall down
And in the very unusual circumstance that they do fall down, they get back up quickly.

By putting weight up the mast you are in effect doing very similar to removing ballast in the keel.
....

Raising weight up the mast is equivalent to add a much greater mass to the keel.

Adding weight up a mast will increase your capsize resistance while at the same time decreasing your stability. Increasing stability does not increase capsize resistance they are not the same thing. Science on that is pretty clear.

There is a limiting case where the boat already has stability problems, let's say the AVS (Angle of Vanishing Stability) is already 90d, then raising weight up the mast may be a detriment. J-24 would be an example. I think it's AVS is a bit over 100d.

[Adelie]

Originally Posted by Dockhead

Which increases power

It only increases power if there is more sail to put on. And you would want stronger rigging to deal with the increased loads.

[a64pilot]

Originally Posted by Don C L

Perhaps you can think of it that way, but I think, in terms of the pendulum, you are adding mass to resist or slow the roll, which is variable depending on the height up the mast... I'm thinking of a metronome now. I too will try this next time out, but now, how to raise a weight that doesn't bang around on the mast!

From a stability perspective it’s similar to removing ballast. But how to keep it from banging around I was thinking tie a line to it and pull that tight with the winch on the mast, but also hoist it all the way up.
I think the key to making it work is to change the resonance frequency, and mass aloft ought to do that, but enough? Have to experiment I guess.
I think it’s going to take more mass than we think.

[a64pilot]

Originally Posted by Adelie

Raising weight up the mast is equivalent to add a much greater mass to the keel.

Adding weight up a mast will increase your capsize resistance while at the same time decreasing your stability. Increasing stability does not increase capsize resistance they are not the same thing. Science on that is pretty clear.

There is a limiting case where the boat already has stability problems, let's say the AVS (Angle of Vanishing Stability) is already 90d, then raising weight up the mast may be a detriment. J-24 would be an example. I think it's AVS is a bit over 100d.

No, mass aloft would be similar to removing mass from the keel, and it should decrease capsize resistance, assuming I understand capsize resistance. A boat that loses its mast would be far more resistant to capsize to one that still has it’s rig.

[LakeSuperior]

Adding mass aloft is actually not the same removing mass from the keel so roughly to first order:

1. adding mass aloft increases the moment of inertia which reduces the roll frequency but increases the amplitude for a given wave train.

2. decreasing the mass of the keel reduces the moment of inertia which will increase the roll frequency and reduce the roll amplitude for a given wave train.

It really doesn't take much weight added to the top of the mast to create a noticeable difference in roll frequency. One pound at the top of a 40 foot mast is like 30 to 35 pounds on top of the cabin to equal the same righting moment. The moment of inertia is a function of the square of the mast height so 40 squared versus 5 squared.

[Don C L]

So my question would be then how does slowing down the roll speed or dampening the roll contribute to reducing the chance of capsize? I mean I can guess that the boat is not reaching a critical point at the bottom of a wave as a result of the roll not happening fast enough to get there in time before the wave has passed, but that is just a guess.
I wonder if you could tune your roll to the waves (to be canceled) like a metronome by the height of the weight up the mast?

[Adelie]

The critical point is passing over the crest of the breaking wave.
As the wave starts to pass under the boat it is rolling the boat in the direction of wave travel. The slope of the water interacts with the shape of the hull to drive this rolling force. The inertia is resisting this rolling force. If the wave passes under the hull before the top of the mast hits the water and starts to dig in then the reverse slope on the back side of the wave reverses the force and wants to roll the boat back upright.

If on the other hand the boat rolls forward enough to bury the mast ...

Speed of wave which is related to wavelength plays a role in this, as does whether the wave is breaking or not.

The whole system is very complex, but the easy takeaway is that decreased form stability (narrower beam and slacker bilges) and increased roll moment of inertia (heavier mast) make for better capsize resistance.

[Nord Sal]

It may be helpful to note that the moment of inertia about any axis doesn't depend on the boat sitting in the water.

A righting moment is the resisting torque, it is dependent on the medium the boat is in.

A boat on stands has the same moment of inertia about any axis as it does when it is in the water. When on stands there is no righting moment.

[Adelie]

Originally Posted by Nord Sal

It may be helpful to note that the moment of inertia about any axis doesn't depend on the boat sitting in the water.

A righting moment is the resisting torque, it is dependent on the medium the boat is in.

A boat on stands has the same moment of inertia about any axis as it does when it is in the water. When on stands there is no righting moment.

Actually there is righting moment on stands, much easier to calculate too, or measure.

[Nord Sal]

Well, of course that's right. The stands push up.


I guess should've just had the boat sitting in thin air, righting moment zero, moment of inertia about any axis, unaffected.

[Adelie]

Originally Posted by Nord Sal

Well, of course that's right. The stands push up.


I guess should've just had the boat sitting in thin air, righting moment zero, moment of inertia about any axis, unaffected.

If it were sitting in thin air it would either start accelerating until something stopped it with righting moment or it was in orbit and weightless in which case tidal forces would come into play over the long term.

Roll moment of inertia is unaffected by how the vessel, or any object for that matter, is supported.

[RaymondR]

There's a fair bit of conflating of static and dynamic stability going on in this thread.

[Adelie]

Originally Posted by RaymondR

There's a fair bit of conflating of static and dynamic stability going on in this thread.

Easy to do.

[Nord Sal]

Originally Posted by Adelie

If it were sitting in thin air it would either start accelerating until something stopped it with righting moment or it was in orbit and weightless in which case tidal forces would come into play over the long term.

Roll moment of inertia is unaffected by how the vessel, or any object for that matter, is supported.

Yes. And the righting moment in that case would be zero.


Your second sentence is the point I was trying to make. There seemed to be a lot of confusion between righting torque vs. inertial properties. Without support, or with different support the righting moment is different. Inertial properties aren't.