Pros and cons of steel sailboats - Page 200 - SailNet Community
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post #1991 of 5317 Old 10-30-2013
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Re: Pros and cons of steel sailboats

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Originally Posted by outbound View Post
I'm confused. I thought having a heavier hull did not necessarily contribute to more favorable GZ nor to smaller area under the curve. I thought major factors were hull shape, B/P ratio and righting arm. Thought those issues plus shape of topsides and weight under water when inverted were the added issues when inverted. Spinning a steel barrow or a wood barrow floating in the water is just as easy. Thought weight of the skin of the boat has little to do with stability.
Not sure I understand everything you say but a beamier hull will give you a bigger max GZ (all other things being equal). It will give you more positive stability and also more negative stability (when the boat is inverted).

The RM is the GZ value multiplied by the weight. The GZ is actually an arm and it is as if you put the weight of the boat there to obtain a righting moment. With the same weight, the bigger the arm, the bigger the RM. For the same size of arm, the bigger the weight, the bigger the RM.

http://www.rya.org.uk/sitecollection...ty%20Intro.pdf

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post #1992 of 5317 Old 10-30-2013
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Re: Pros and cons of steel sailboats

Out:
Paulo beat me to it. He's correct.

Simplify the picture. Forget GZ. Think in terms of more tangeable components. Think Righting Moment, the force acting to right the boat.

Righting Moment is Righting Arm ( G-Z) times Displacement. So if the righting arm stays the same and the displacement increases the Righting Moment will increase. Simple as that.

Certainly having a heavy hull and deck will raise the boats overall VCG and in doing so reduce the Righting Arm.

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post #1993 of 5317 Old 10-30-2013
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Re: Pros and cons of steel sailboats

Thanks guys- my prior understanding of this was correct for a change. What I was confused about was any implication that using steel or any heavy skin ( concrete/wire) had any advantage in giving a stable hull or improving comfort motion measures. Believe that remains dependent on the skills/genius of the N.A. and the many factors not fully disclosed by a simple GZ curve. Still happy my boat has a good point of vanishing stability. Maybe BS boats hit so many reefs as they are hard to steer upside down.

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post #1994 of 5317 Old 10-30-2013
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Re: Pros and cons of steel sailboats

Out:
You are partially right.

If you have a heavier skin you will have less ballast and your VCG will go up. Generally this will reduce Rm and give you a more gentle motion. But given the scoipe of boats we are looking at here I don't think the motion change would be very much. And I do know for certain that everyone likes a stiff boat.

What is the LPS of your boat? I would guess 128 degs. Maybe 126 degs.

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Re: Pros and cons of steel sailboats

I was reading about just this subject in a book from some yacht designer just last night. I think I understood it pretty well, although the actual data was interspersed with stories from his youth.

That being said.. The first righting curve makes no sense to me at all.. The author said that the righting moments are significantly affected by how the boat is loaded, and there will be some variations between the theoretical and real-world values, but nothing like that.

And the ZCoG is .88 meters? Huh?

(Yes, I know that you have said as much in the posts above.. but I was just showing off to Bob that I am paying attention to his book.. I dreamed about this stuff all night.. seriously...)

David

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Re: Pros and cons of steel sailboats

Oh.. and I do like the green boat, too.. Hopefully they plan to put one or two lifelines between the top rail and the deck.

Not to mention people, it'd be pretty easy to lose sails over the side with those high rails...

David

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post #1997 of 5317 Old 10-30-2013
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Re: Pros and cons of steel sailboats

"The author said that the righting moments are significantly affected by how the boat is loaded, and there will be some variations between the theoretical and real-world values,"

Dave:
Hopefully the "theoretical values" will reflect the "real-world" values. That's why it is imperative to have an accurate VCG. The stability program is not that smart. You have to enter a VCG. You can put in anything. The computer takes it on faith. The only way to have an accurate VCG in the design stage is to do a weight study. If the boat is in the water you can do an inclining experiment, the way they did it to all boats back in the IOR days and derive a VCG from that.

If you have an accurate VCG and an accurate hull shape you should get reliable stability data.

Does anyone have a photo or drawing of the boat that stability curve came from?

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post #1998 of 5317 Old 10-30-2013
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Re: Pros and cons of steel sailboats

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Originally Posted by bobperry View Post
Out:
You are partially right.

If you have a heavier skin you will have less ballast and your VCG will go up. Generally this will reduce Rm and give you a more gentle motion. But given the scoipe of boats we are looking at here I don't think the motion change would be very much. And I do know for certain that everyone likes a stiff boat.

What is the LPS of your boat? I would guess 128 degs. Maybe 126 degs.
Bob can you give me your opinion on this:

It is clear that a steel boat will have a better mass moment of inertia than any other type of sailboat and there are a lot of people that sees that as primordial to dynamic stability an in what regards the capacity to resist capsizing by a breaking wave. That seems right to me if we are talking about a motor boat or a sailboat not sailing.

I think that with medium or strong winds (like the ones you find in stormy weather), a powerful sailboat while sailing has its big RM balanced by the strong wind and that factor is more important to dynamic stability than any other. The boat is so strongly tied to a side by the wind that the power of that tie is far more important than mass moment of inertia to the boat dynamic stability. That tie will provide the boat with a steady fast motion and will resist to the lateral force that side waves can provide and to some extent to breaking waves.

This makes stiffness of a sailboat of a sailboat a very important factor in dynamic stability (stiffness meaning the RM on the first 30º). The "old school" sees stiffness as prejudicial factor in dynamic stability.

These movies "explain" what I mean:



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Re: Pros and cons of steel sailboats

Paulo:
Fabulous movies!

And then he puts up the chute! Big balls and no brains!

I have my own indvidual thoughts on stability and stability calculations. I use every technical tool I have at my disposal and I consider the results. But I am a very firm believer that static calculations for stability cannot simulate the conditions found at sea in a taiphoon. I spent three days in a north Pacific taiphoon and I have yet to see a program that can replicate those waves.

The old salts actually raised weights to the masthead to reduce their stability in big waves. But that was many years ago and the boats today are not the same. I know when the university in Sydney, Australia did the stability study after the Queen's Birthday Storm, the only thing they could say with certainty was that bigger boats are less prone to rolling over/capsize. That's it. They looked at the effect of pilot houses and concluded that deck house volume did not help. This was an exhaustive study.

My thought is that you start with a healthy design with a good Rm and I look hard at the Rm up to 30 degrees. That's where we live. Beyond that, so long as the boat on paper has an LPS of over 110 degrees I think you are good to go . I would much rather sail a boat with good initial stability in any conditions than a tender boat.

I do not think having a postive Rm at 125 degrees will make you "safe" at 122 degrees.
I'll take the stifffer boat. The fact is that most cruisers are never going to sail their boats hard enough to ever even put the spreaders in the water. Yet they obsess over this LPS like it was an absolute, cast in stone. Most of them have only a rudimentary understanding of stability to begin with. They think a number will make them safe. It will not.

I'll take the stiff boat thanks. Nobosy has ever called me and said, "My boat's too stiff."
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Last edited by bobperry; 10-30-2013 at 06:56 PM.
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post #2000 of 5317 Old 10-30-2013
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Re: Pros and cons of steel sailboats

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Originally Posted by bobperry View Post
The old salts actually raised weights to the masthead to reduce their stability in big waves.
That is scary interesting. Was the purpose to slow the roll or something else?

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...Nobody has ever called me and said, "My boat's too stiff."
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