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  #21  
Old 10-11-2013
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Re: Stability question

[quote=MarkSF;1103026]
Quote:
Originally Posted by PCP View Post
....
Ahh, so now you have separated boats into "cruisers" and "voyage boats". I thought we were talking about typical cruising designs? The ones I see all around me don't have bulb keels 10ft down. Yes, I firmly believe that the main priority of the typical Beneteau / Hunter / Catalina et al is cruising accomodation, not storm survivability.
It is not me that makes that separation. Market has many niches and one of them, not a big one, regards voyage boats, boats that are made and designed with voyage in mind since it is what the owners are going to do with them.

One of the members of this forum has come to Europe to buy one of them, a Boreal 44 ( a very beamy boat by your standards) and is sailing now back to the states.

You don't need to have bulb keels with 10ft down. One of those (very) beamy boats that has fast voyage as one of their main objectives, the Pogo 12.50 has a AVS bigger than the Vaillant (that has already a good one). The Pogo has a swing keel and with the keel up the draft is 1.20m.

Even with a 2/2.25m high profile keel with a torpedo on the bottom is possible to bring the CG much more down than with a full keel, or fin (not bulbed) keel even if that one is made of lead. Older cruising boats have also generally a smaller draft than the new ones.

I do not consider that Hunters, Catalinas, Benetaus or Bavarias are designed taken as the principal goal storm survivability. I don't know of any sailboat designed with that goal as main objective but even so, if rightly sized and conveniently fitted they can circumnavigate without problems and many have done so. However, boats designed specifically for voyaging are more suited to do that .

Regards

Paulo
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Last edited by PCP; 10-11-2013 at 03:23 PM.
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  #22  
Old 10-11-2013
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Re: Stability question

Formula 1 cars, NASCAR Grand National Stock Cars, Indy 500 cars, and my car all have 4 wheels and one engine
The CatBenHuns with in-mast furling mains and 4 foot draft wing keels only vaguely resemble an Open 40 or Open 60. A race boat is designed to win a race, not be easy for 2 likely-elderly and likely not-expert sailors to handle. Also note this fat-beam design HAS been known to end up inverted and stay that way. Even if that doesn't happen, a very deep bulb is a terrible cruising design. They are prone to damage, tend to snag things, keep you out of many desirable cruising areas, and don't appear to be able to take drying out. When I see them out of the water they are on cradles that do not support the hull weight on the keel.
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  #23  
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Re: Stability question

I am totally with Paulo on this. He has a very good idea.

I don't want to sound cavalier but I'll risk it.

Very, very, very few of us are ever going to capsize our boats. I never have and I don't know anyone who has. I have been knocked down before to about 90 degrees more times than I care to remember, maybe even slightly past 90 degs. It's exciting but not fun.

But lets rule out boats with a AVS less than 120 degrees. I've done boats as low as
111 degs and that boat is still a fabulous boat and used hard racing. But I know numbers like that scare some of you so lets keep the AVS above 120 degs.

So, comparing Rm's at 90 degs would show which boat is going to pop up quickly compared to the others from a typical knockdown. Most of you, if you are cruisers are never going to drive you boat hard enough to ever knock it down to 90 degs. It takes hard work to do this. You need a lot of breeze and a spinnaker up in most cases. Most boats will round up and right themselves well before 90 degs. But if Rm concerns you, follow Paulo's advice and get the Rm, at 90 degs.

The problem is this. Where do you get that number? The designer has it. But if it's an older boat you are not going to find it I'm sure. There's a slim chance but very slim. Keep in mind that prior to computer hull design programs most designs NEVER HAD STABILITY STUDIES. Some did but most did not. Trust me. I've been there. I didn't do a stability study for the Valiant 40 till well after it was in production. I relied on my instinct, my experience and the whole world of yachts that had gone before the Valiant to make my design decisions.

I think most sailors today don't have the background to understand these numbers and they treat them like absolutes. Stability studies on a computer do not include waves. The computer just tips the boat over in calm, flat water and measures the Rm. How the hell does that correspond to a gale blowing 60 knots and 35 high reaking waves?

It doesn't. So go ahead and get all wrapped up with your spread sheets of numbers but you are fooling yourselves. You'd be far better off sailing more and doing some racing so you can learn how to really push a boat safely to it's limits. Seamanship and sailing skill is what you need, along with a wholesome boat if you want to stay right side up.
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Last edited by bobperry; 10-11-2013 at 05:04 PM.
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  #24  
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Re: Stability question

Say "Amen"!
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Re: Stability question

Amen Brothers!
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  #26  
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Re: Stability question

Amen
Quote:
Seamanship and sailing skill is what you need, along with a wholesome boat if you want to stay right side up.
It ain't the boat, it's the sailor.
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  #27  
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Re: Stability question

I think Joshua Slocum showed us that.
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Re: Stability question

How many Spray replicas have been built under the assumption that was the deciding factor, ignoring Slocum's 35 years experience at sea.
The man had previously built a boat from WRECKAGE (Liberdade) that he sailed from Brazil to the US with his family aboard.
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Re: Stability question

Bob - what about size and polar moment?
AFAIK most large ships are not stable much past 60 degrees at best, but they rarely if ever capsize without the cargo shifting or flooding. A 60 foot boat stable to 110 degrees is a different thing entirely than a 30 foot boat stable to 110 degrees IMHO. The larger boat will be slower to roll (higher polar moment I think is the right wording??) and a single large wave will be past the boat before she gets a chance to go as far as the smaller boat would.

Quote:
Originally Posted by bobperry View Post
I am totally with Paulo on this. He has a very good idea.

I don't want to sound cavalier but I'll risk it.

Very, very, very few of us are ever going to capsize our boats. I never have and I don't know anyone who has. I have been knocked down before to about 90 degrees more times than I care to remember, maybe even slightly past 90 degs. It's exciting but not fun.

But lets rule out boats with a AVS less than 120 degrees. I've done boats as low as
111 degs and that boat is still a fabulous boat and used hard racing. But I know numbers like that scare some of you so lets keep the AVS above 120 degs.

So, comparing Rm's at 90 degs would show which boat is going to pop up quickly compared to the others from a typical knockdown. Most of you, if you are cruisers are never going to drive you boat hard enough to ever knock it down to 90 degs. It takes hard work to do this. You need a lot of breeze and a spinnaker up in most cases. Most boats will round up and right themselves well before 90 degs. But if Rm concerns you, follow Paulo's advice and get the Rm, at 90 degs.

The problem is this. Where do you get that number? The designer has it. But if it's an older boat you are not going to find it I'm sure. There's a slim chance but very slim. Keep in mind that prior to computer hull design programs most designs NEVER HAD STABILITY STUDIES. Some did but most did not. Trust me. I've been there. I didn't do a stability study for the Valiant 40 till well after it was in production. I relied on my instinct, my experience and the whole world of yachts that had gone before the Valiant to make my design decisions.

I think most sailors today don't have the background to understand these numbers and they treat them like absolutes. Stability studies on a computer do not include waves. The computer just tips the boat over in calm, flat water and measures the Rm. How the hell does that correspond to a gale blowing 60 knots and 35 high reaking waves?

It doesn't. So go ahead and get all wrapped up with your spread sheets of numbers but you are fooling yourselves. You'd be far better off sailing more and doing some racing so you can learn how to really push a boat safely to it's limits. Seamanship and sailing skill is what you need, along with a wholesome boat if you want to stay right side up.
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  #30  
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Re: Stability question

Joie:
I don't know anything about big ship stability. I did ride out a Taiphoon for three days once in the Pacific in a Liberty ship. That was,,,dynamic.

After the Queen's Birthday Storm is the South Pacific an exhaustive stability study was done in Australia I don't recall the name of the school. What I remember from that study is that the one thing could say with certainty was that "size matters". The bigger the vessel, the less likely it is to roll over.

My theory all along has been this preoccupation with AVS, LPS call it what you will, for yachts is a waste of time. It's a simplistic, static glimpse into a highly dynamic and unpredictable situation. Now we have sailors who still don't know where to put the traveller or how to tie a bowline trying to anayze stability.

I am still going to do stability studies for my designs. But I tend to focus on the curve below 30 degrees. That's where we "live" while sailing. And of course if I run accross aboat with a LPS less than 115 I'm going to take a hard look at it. But I am pretty experienced in this and I have large data bank of stability studies I can reference.

I was driving my chest X Rays to my doctor's office. I had the big brown envelope on my passenger seat. As I sat at a red light I thought. "Let's have a peek." I pulled out a big X ray and I was startled. It looked like I was riddled with cancer. Shitski! I'm gonna die.

My doctor looked at the x rays, "You're fine" he said.
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