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post #646 of Old 03-08-2013
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Re: Full or fin keel?

Originally Posted by wolfenzee View Post
According to the US Sailing calculator Sailboat Design and Stability my AVS is 193.64

The actual formula is: AVS = 110 + ( 400 / (SV-10) )
SV is the screening value and is calculated by:
SV = B2 / (R x T x V1/3)
Expanding SV in the first equation gives:
AVS = 110 + ( 400 / (B2 / (R x T x V1/3))-10) )

B = beam of the hull (no rub rail) in meters
R = ballast ratio
T = hull draft at B/8 from centerline in meters
V = displacement volume in cubed meters
That is what I thought. That formula, at best, works for a certain kind of boat typical of the 80's (Fin boats with non bulbed keels) and even so gives errors superior to 15. It gives not meaningful results to old boats like yours or modern boats with deep draft and a bulb.

As I have said one of the most important factors regarding the determination of AVS is the position of the CG. That formula in what regards that has only into account the B/D. it is evident that regarding boats with the same weight a boat with 40% of B/D, distributed uniformly by a a long keel with 1.4m provides a lot less righting moment than 25% of B/D ratio in a bulb at 3.0m (for instance on a cruising Pogo 12.50). Regarding that formula the Pogo is regarded as a boat with a much higher CG when it is the opposite and by far.

For finding the AVS it is needed to find the center of buoyancy, the center of gravity and the meta-center. You can only do this with careful calculations regarding the shape of the hull and the weight of all boat. I don't mean the displacement but the weigh of all parts of the boat, including ballast and their relative position.

You can do that also with inclining experiences to determine RM an adequate computer program That is made all the time in what regards serious racing by specialists, to be able to rate the boats in ORCI.

Simplistic formulas like that one or the capsizing ratio does not deserve any credibility, specially in what regards very old or modern boats.

Originally Posted by wolfenzee View Post
...the 110 degree AVS that someone stated was typical of fin keels...
It seems to me you are mistaken. I don't remember anyone stating that. I would have noticed because that it is not true.

Originally Posted by wolfenzee View Post
It's not so much as a wide boat being more stable right side up, but upside down....I wide boat is not going to want to right as quickly as a narrow boat.
Sure, it is needed more force to re-right a beamy boat than a narrow one and the opposite is also true, it is needed more force to capsize a beamy boat than a narrow one.

A perfect example of a shape with a 180 AVS is a cylinder, a tree trunk for example. Very easy to "re-right" and very easy to roll. nobody wants a boat like that. what is more important, providing the AVS is enough to right the boat easily from 90, is the proportion between the energy needed to capsize it and to re-right it. If the energy to capsize the boat is 4 times bigger than the energy to re-right it, it means that if a boat can be capsized by a 3m breaking wave, it only need the energy of a wave with 0.75m to be re-righted.

A sea condition capable of forming 3 m breaking waves will have plenty of 0.75m waves that will put it back on its feet very quickly.

Now imagine a boat with 180 AVS but that needs only a 1.5m breaking wave to capsize it. A situation with 2.0m breaking waves would capsize that boat easely while the other one would have not any problem. Sure, it will come up again...without a mast and with an interior in complete disarray.

Do you really think an easier boat to capsize is better, just because it has a bigger AVS?

Of course we are only looking at it in what regards static stability. In what regards dynamic stability a really beamy boat with a small submersed area (including the keel area) will have advantages, being able to convert more wave energy in a kinetic movement instead of a rolling movement. The best example of this situation are multihulls. Centerboarders and boats with narrow keels come next.

Originally Posted by wolfenzee View Post
According to the RADFORD "Safety of Small Commercial Sailing Vessels Code of practice"(see table below and/or link), the 110 degree AVS that someone stated was typical of fin keels would not even be safe in Catagory 2 (up to 60 miles from a safe haven)
Safety of Small Commercial Sailing Vessels Code of practice is not a Radford document but one that was made by the wolfson unit to the British government more than 20 years ago.

Here you have the actual full document that don't use that table but makes the AVS demands increase with size:

"Safety of Small Commercial Sailing Vessels Code of practice":

That regards commercial sailing vessels, not private boats and that's a huge difference.

Basically what they don't want is small sailingboats being used as training boats in offshore situations. If we consider a 50ft boat the AVS requirements are of about 122. That is not far from the average. Many modern mass production boats have more than that.

Anyway, as all the rules it has flaws and on this case one of them is using only AVS to access boat stability. In that regards the RCD, that is a more recent document and has been upgraded more times, is much more effective, I mean in the ways the stability is evaluated: They use not only AVS but much more data taken from stability curves, including max load and min load curves.

Regarding a sailboat not being safe offshore with an AVS of arouind 110 and "would not even be safe in..up to 60 miles from a safe haven", may I remind you that a Sabre 402 has only 114 and that an OVNI 43 had about 110? The OVNI 43 was the boat chosen (after having many other boats and several circumnavigations) by Jimmy Cornell for his circumnavigations and extreme navigation in high latitudes and one that he recommends to everybody.

Again, as I have said, the AVS is important but providing it is enough to right the boat easily from a knock down, there are many other important factors in what regards stability and seaworthiness to consider.

Even if I would prefer that the OVNI or the Sabre had an AVS around 120 I would fell safer going offshore on one of those boats than on your's. Not that I would have any problem in going offshore in your boat, but I would feel more secure in any of the other two: They have a superior safety margin.

I Understand your love for old boats (that I share too) but I find a bit odd that you keep thinking that XIX century designed boats are safer than modern designed sailboats.



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Last edited by PCP; 03-08-2013 at 07:38 PM.
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