New (possibly) idea for monohull stability in rough seas

[sryth]

I'm not sure if this idea is new, or if it would even work, so I figured I'd ask.

I was reading a usenet discussion regarding the stability of monohull boats in rough seas, and the writers seemed a bit paranoid about rough weather in smaller craft (< 40'). What if you made part of the keel detachable, so that it could be lowered (by a line) in rough seas. Would this provide additional stability? Basically, it'd be an anchor that originates from the lowest part of the boat.

My original idea was for a keel-stepped mast that could be lowered (along with a small portion of the keel) beneath the boat. In rough seas, you lower the mast, which (I would think) would provide awesome stability. The only problem in designing such a system would be dealing with the standing rigging and boom.

Any thoughts?

[yokebutt]

There are a few practical issues with your proposal, like how to deal with the standing rigging while the mast is lowered, for one. Sailboats tend to have an easier motion in waves partly because of their keel, but also partly because the mast increases their moment of inertia, and thus slows the motion down. No quick and easy answers, every configuration has it's down-side, for example, in breaking seas, the keel can get stuck in relatively still water while the hull is in water that is sliding down the wave, resulting in lots of turning moment. Not an enviable situation.

Yokebutt.

[Eric W. Sponberg]

Sryth,

I find this interesting--do we not go offshore then in boats less than 40'? My wife and I sailed our Bianca 27 for a year and a half from England to California (it was the largest boat we could afford) and we felt perfectly safe in heavy weather, which we had plenty of. A properly designed boat of any size should be able to withstand the sea. Yes, larger boats do have better resistance to being rolled in waves than smaller boats, but that does not mean small boats are not adequate.

To lower the mast in heavy weather would make life on board the boat unbearable. Rolling accelerations would make you sick. The mast indeed does add tremendous mass moment of inertia to the rolling motion and contributes to the safety of the boat by resisting capsize. To drop the keel by a line is not a solution--it too adds to mass moment of intertia in rolling, and detached from the boat would make the boat capsize all the more easily. If you suspend the keel by a line, you take it out of the rolling motion where it is doing the most good.

The best defense against bad weather is to avoid those situation with a generous helping of prudence. The second defense is to have a boat with the right stability, hull shape, weight distribution and strength so that if you are caught out, you have a good chance of survival.

Eric

[Skippy]

Eric W. Sponberg: To lower the mast in heavy weather would make life on board the boat unbearable. Rolling accelerations would make you sick. ... the keel ... adds to mass moment of intertia in rolling, ...

That's funny, the keel/anchor idea sounds like the TP52 thread. The answer is no, lowering the ballast has no effect on righting moment. As Eric pointed out, releasing the ballast onto a line reduces the boat's moment of inertia because the boat/ballast combination no longer acts as a rigid body. The keel does not exert a righting moment when the boat is upright. It only starts to right the boat when the boat is heeled, and the center of gravity (cg) is offset horizontally from the center of ballast (cb). Vertically lowering the ballast on a flexible line has no effect on righting moment at all. The only solution is a lower but fixed cg as in a bulb or a deeper keel, which swings out farther horizontally when the boat heels. Or more initial stability, as you get in a multihull. I would also be worried about the effect of an anchor tied to the bottom of the keel.

As for the mast, remember he's talking about somehow stepping the mast in a hole that passes all the way through the hull, and lowering the mast so that its lower end protrudes below the hull bottom. In addition to providing extra roll stabilization through drag, how low would the moment of inertia really drop with the mast still at its full length? And lowering the cg should be a big help. The biggest problem would probably be that it's very difficult to implement.

[Edit: minor corrections after rereading Eric's post]

[Wynand N]

Skippy

Thats the way I saw it as well........

[usa2]

skippy is right

[Skippy]

Wow, thanks for the support. Just to bring the subject back to reality a little, isn't the mast usually right above the keel on a sloop? How the heck would that work? And with a cat rig or maybe a schooner or something like that, a mast sticking down forward of the keel would be vulnerable to bashing into things.

[sryth]

That's kinda what I had in mind. I understand the standing rigging would be the main hurdle in implementation, but I know how it could be done.

A hollow mast (aluminum) acting as a sleeve for an inner-mast. The inner-mast would lower, leaving the main mast stayed.

[usa2]

what about a telescoping mast?

[water addict]

Oh c'mon guys. A keel suspended by a line? a mast that drops through the bottom of the boat? What have you been smokin?

[SailDesign]

Oh c'mon guys. A keel suspended by a line? a mast that drops through the bottom of the boat? What have you been smokin?

Been watching "Waterworld" again, I suspect...

[John Perry]

The deep water navigational bouys maintained by Trinity House (in the UK) are a bit like your yacht with the mast projecting down through the keel. These bouys have a cylindrical float below which is a pole extending a fair way below the water with a ballast weight at the bottom. I understand that you dont have to go far below the surface to encounter still water even in a storm, so if you can place a large paddle like object (sail?) at the end of your immersed mast you should get a steady motion in a rough sea, but I agree with other people here that the practical difficulties probably outweigh any benefit.

As for suspending ballast from the bottom of the boat on a line I can see that this might have some advantage in that if the boat heels beyond 90 degrees the line will wrap around the hull and maintain an approximately constant righting moment as the boat rolls further, whereas with ballast on a keel the righting moment decreases as full inversion is approached. Again, the practical difficulties of having a line wrapped round the hull in a rough sea are discouraging.

John

[sryth]

What's impractical about having a lowerable inner-mast? Is this really that difficult to implement?

Maybe stability wasn't the right word to use. I figured if you implemented either of these, you would turn capsizes into mere knockdowns.

[gonzo]

One of the problems of a telescoping mast is that in rough weather, as it flexes, it will jam preventing the sections from sliding. A mast of proper height is a good thing. If you want a rig that can be lowered, there are several working examples. Gunter, lugs and sliding topmasts are some of them. Another problem with a lowering inner mast is the mechanical device that would be housed in the mast. It would be a lot of extra weight and complication.

[cyclops]

ALL the ships are built wrong! Beach them. Replace them with cylinders 20' X 100'. Only 5' of one end is above water, clear, to see the beauty of the storm waves coming at you!