New (possibly) idea for monohull stability in rough seas

I think what Skippy meant to say was that stability is achieved by increasing the horizontal distance from the Center of Gravity (CG) to the Center of Buoyancy (CB). On the whole I have to agree with Mr. Sponberg in that seaworthiness can be achieved in most any size boat if the designer pays attention to the factors that will deliver good ultimate stability. The ideas being talked about are a lot of fun, but the goal of a creating a stable boat for the worst weather is easily achieved without going to the excesses in complicated design and expense being discussed here. Remember - when things are at there worst is when equipment fails. Call me old fashioned, but keeping it simple often keeps it safe.

Michael

 

With everybody putting a bulb bow on everything afloat. What happens in a rogue wave, 60-80' ? Does the bow lift as quickly as a non-bulb? Or does it do something different? My gut guess it would do nothing.

 

Yes it is too difficult to implement in the capacity you are talking about, i.e. preventing capsize. Just imagine trying to lower the mast as conditions are starting to degrade- 20+ foot waves, high winds, long metal pole moving around, crew trying to hang on and not get killed. Heck, in the rough stuff, it can take 15-20 minutes just to get below and take your gear off when you're going off watch, let alone lower the friggin mast.

Then you've got the engineering of it. If this thing is supposed to stick down in the water, you're talking a whole 'nother realm of design loads. Hydrodynamic forces would be LARGE, so the thing would have to be heavy, and the supporting structure would be akin to the structure holding the keel in place. If you want to lower the CG, why not have it low all the time, make the keel heavier, or deeper, or make higher weights light. If the lowerable mast portion is to have enough impact to appreciably lower the CG, then it would have to be a significant percentage of the boat's displacement, otherwise it's pointless. And if it's heavy, you'd have to have weight down low to counteract it when the pole is in the up position.
If you want roll damping, sailboats in general have plenty of that already with the keel. Sticking a skinny pole in the water won't help that much.
Sorry to pooh-pooh the idea, but it's not feasible in my opine.

 

if you lower the keel and/or mast in heavy weather, its not going to make your boat very comfortable. The boat will start snap rolling, so basically anyone on deck or anything not lashed down to the deck tight enough is headed overboard.

 

I'm interested in this question of roll damping through the rotational moment of inertia. Eric said it's reduced when you lower the mast, which is true, since you're bringing most of its parts closer to the cg. But I shudder at the thought that a boat with a short mast designed specifically for heavy weather only would actually have stability problems because it's not high enough! Can that be true???

 

I think you have roll damping, roll period, and roll inertia mixed up. Roll damping isn't reduced when you lower the CG, roll inertia is. Roll damping is largely assumed to be independent of roll period, in fact in simplified nav.arch. calculations, the roll period is assumed independent of damping to make the problem workable.

 

water addict: Roll damping isn't reduced when you lower the CG, roll inertia is.

That's true, but inertia and damping can both reduce oscillation. A higher moment of inertia reduces rolling accelerations, for instance in gusts, making the motion smoother. Technically speaking that's not dissipative damping, but it does reduce the amplitude of high-frequency oscillations, and it sure is more comfortable. And of course the cg isn't really the issue, it's the distribution of R^2. The comment of Eric's that I'm wondering about is that the lower mast makes the boat "unbearable". He even refers to accelerations, which are reduced by the tall mast. Is it really that bad? Does the mast really have to be full height just so the crew doesn't get seasick?

 

A fact that has bearing. In the navy at sea we ran into a condition called ground swells. They were about 1/2 mile crest to crest apart . Our destroyer simply and comfortably rode up and down them. The super carrier on the other hand would ride up, pitch over the crest, point down and rev the exposed prop and shafts in the air for a couple of seconds. on reaching the bottom of the wave she procedded to put 1/4 to 1/3 of her main flight deck under for 3 to 4 seconds. I know no one on her had a wonderful sunny day like we all did. Size and wave resonance or harmonics made all the difference on ground swell days. I am not to sure designers would like to see her flexed like that.

 

isnt that pitching, rather than rolling?

 

Yes. Till they changed course. At that point they became smoother and we had our hard time . The point is, it almost impossible to design out rough sea conditions effects with one blanket solution due to so many hull designs in use today. Weather info. has given us the option of avoiding storms to a very high degree. We really can control how badly our boats are tossed around.

 

Sryth,

The idea of a keel-mast moving up or down in a slot, leaving aside all the attendant engineering, is not so far fetched an idea. In fact it's a good idea, if you are into far fetched boats.

Eric's comment about an all round well proportioned boat makes sense of course, but the notion that if the mast comes down you lose rotational inertia is only right when the boat carries normal beam, say between 3-4 beam to length ratio.

The narrower the boat, the less roll is induced by waves.

So if the boat is narrow, and the keel is deeper (because the keel-mast slides down through it's slot), there will not be any loss in rotational inertia. In fact it could be that the rotational inertia of the boat will be enhanced.

Here are some numbers:

Say centre of gravity of a 400 lb mast, is 20ft above centre of rotation, 2 ton bulb keel 7 ft below, then combined rotational inertia = 400*20^2 + 4,480*7^2 = 160,000 + 220,000 = 380,000 lb ft^2, in the normal position.

Lowering the mast-keel unit by 10ft say, we have 400*10^2 + 4,480*17^2 = 40,000 + 1,294,720 = 1,334,720 lb ft^2

This is a very big increase in inertia, in fact four times as much. On a wide boat this could lead to fast accelerations, but it would have to be very wide, and light as well. But on a narrow boat would be a solid as a rock. I'm not sure you would want a boat so solid though. It would have a tendency to take surface waves on the chin, like the big tanker do. They are always getting their superstructures dammaged from big waves.

So the idea is fair enough. Couple this with a self standing mast, and the engineering does not look all that insurmountable.

At any rate, I agree, that a well proportioned normal boat is well equipped to handle what the ocean can dish out, and this alternative appraoch will lead to other difficulties and complications that cruisers can very well do without. On the other hand, for some futuristic racing boat say, it does have some merit.

 

I say nay! The only way to assure stability is a good set of ratios! Yay yacht design ratios...

 

Dionysis, I don't think you can fit all the ballast in the bottom of the mast. If you filled the mast up high enough to hold all the ballast, the cg of the ballast in the mast-up position would be much higher than if it were in a bulb. If you have some ballast in a bulb, and some in the bottom of the mast, then maybe the mast-down position is an emergency "buoy-mode" setup. Between strength, drag, pitching moment, and vulnerability, it sounds questionable underway.

 

I was thinking that he meant that the keel was attached to the mast, not to the bottom of the hull; a hammer made of mast and bulb, stuck through the hull like a daggerboard, but from underneath. Shoal waters, pull the bulb up, put in a reef or three to bring the center of effort down a bit; deep waters, bad weather, put it way down with only a trysail.

 

why not instead of lowering the mast or keel for increasing stability, how about putting an oversized centreboard or daggerboard on the boat rather than lowering the mast through the hull? The area the board would present to the sea would slow down rolling without causing the boat to "snap" back and you wouldnt need to have a complex system for the mast to be lowered.