Full or fin keel? - Page 65 - SailNet Community
Old 03-07-2013
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Re: Full or fin keel?

according to the ratio calculators (which I don't particularly trust) the angle of vanishing stability on my boat is 183degrees (which means 177 degrees on the other side, in other words there is no point where the angle of stability vanishes).... if someone has an accurate calculator and/or can figure the AVS I would be glad to give all the specs of my boat. My boat likes to heel around 30degrees and performs very well at that heel. Because it is better ti find the limits of your boat rather than be shown them, I decided to have some fun...in 15kts of wind with main and 180 genny (800-900sqft combined) I sailed for 3 hours at a 40 degree heel, doing hull speed with cap rail under 3" of water. If a gust of wind were to push the boat over to 45 degrees it would immediately swing up into the wind. The base of my rudder is 5' down....the ideal size of the rudder is said to ne 15% of of the waterline length....as the original rig on my boat had a nasty weather helm the designer chose to make that 25%.
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Old 03-08-2013
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Re: Full or fin keel?

Quote:
Originally Posted by wolfenzee View Post
according to the ratio calculators (which I don't particularly trust) the angle of vanishing stability on my boat is 183degrees (which means 177 degrees on the other side, in other words there is no point where the angle of stability vanishes).... if someone has an accurate calculator and/or can figure the AVS I would be glad to give all the specs of my boat. My boat likes to heel around 30degrees and performs very well at that heel. Because it is better ti find the limits of your boat rather than be shown them, I decided to have some fun...in 15kts of wind with main and 180 genny (800-900sqft combined) I sailed for 3 hours at a 40 degree heel, doing hull speed with cap rail under 3" of water. If a gust of wind were to push the boat over to 45 degrees it would immediately swing up into the wind. The base of my rudder is 5' down....the ideal size of the rudder is said to ne 15% of of the waterline length....as the original rig on my boat had a nasty weather helm the designer chose to make that 25%.

I would be curious what numbers you used to come to this conclusion.

Brian
Living aboard in Victoria Harbour
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Old 03-08-2013
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Re: Full or fin keel?

The angle of vanishing stability is not a ratio. I don't see how that can be found by a calculator without full measures and all the weights of boat parts. What calculator are you talking about?

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Old 03-08-2013
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Re: Full or fin keel?

According to the US Sailing calculator http://www.sailingcourse.com/keelboat/design_winds.htm my AVS is 193.64

http://www.sailtrain.co.uk/stability..._stability.htm and http://www.radford-yacht.com/stablty1.html both explain the AVS
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)

Historically the older style boats had a much higher AVS even though thier narrower beam. Typical sailboats produced from the early 70's on have LPS's (limit of positive stability = AVS) in the 100-120 degree range. "Designs typical of the 30's and 40's (e.g. many Alden designs) have LPS in the 160 degree range. from "http://dan.pfeiffer.net/boat/ratios.htm

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

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.

Last edited by wolfenzee; 03-08-2013 at 01:34 PM.
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Old 03-08-2013
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Re: Full or fin keel?

The calculator called for draft (not including keel) but the formula called for draft at beam/b away from center line. Which when figured in on my boat would make my AVS 167.7degrees....alot more realistic than 193.

calculator used: http://www.sailingcourse.com/keelboat/cal__avs.htm

numbers used were beam: 8.75'
total weight: 15,000lb
draft at hull B/8 from centerline: 2.5'
ballast & keel weight: 4400lb
(4000lb ballast and approx keel wieght 400 figured on 31lb/cf for fir)

Last edited by wolfenzee; 03-08-2013 at 02:12 PM.
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Old 03-08-2013
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Re: Full or fin keel?

Quote:
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 15º. 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.

Quote:
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.

Quote:
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 180º 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 90º, 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 180º 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.

Quote:
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":

http://www.dft.gov.uk/mca/blue.pdf

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 122º. 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 110º 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 110º? 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 120º 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.

Regards

Paulo

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Last edited by PCP; 03-08-2013 at 07:38 PM.
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Old 03-09-2013
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Re: Full or fin keel?

AVS refers to a boats righting ability.....there is another, sometimes called a "capsize screening ratio" which addresses how easily a boat capsizes, a good AVS doesn't mean a bad capsize ratio SloopIT - Boat performance calculator. The capsize ratio of my boat is 1.42....which is considered to be very good (2 or higher is not considered safe at sea).

William Atkin was famous for designing safe ocean cruising boats, the design of mine was commissioned with the intent of using it for ocean cruising. Of the 40-50,000nm (alot of which were ocean miles) the only comment regarding it's performance at sea was "It's a good sea boat" (from the owner that had here for 15 years and sailed everywhere from Mexico to Alaska on my boat and all over the world on others). I would take my boat to sea ahead of 95% of the boats in the harbor, I truly feel safe in my boat.
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Re: Full or fin keel?

Modern day sailboats try and emulate racing boats while being as spacious and/or comfortable as possible in their accommodations...while at the same time keeping production costs down. Speed is the main performance consideration....safety in "blue water conditions" is not factored into design as much as being byproduct of design. Boats of the sort I have were designed specifically with ocean cruising in mind, not trying to behave and/or look like a racer, not trying to be comfortable for the weekend sailor, not designed around what the marketing industry is trying to make popular.
My boat was designed around work boats of the time (lines are similar to a Gloucester fishing schooner). Just because a boat is of a modern day design doesn't automatically make it better, just because elements of rig are no longer used, doesn't make them inferior. I am not talking down modern boats just because they are new, but rather defending mine (there is nothing wrong with it just because it is old)....My boat was designed 75 years ago by one of the leading designers of ocean cruising yachts of the time (his designs are used in new "blue water boats" to this day), it was commissioned specifically as a single handed ocean cruiser with more of an emphasis of performance than accommodations (boats built for the American market seem to try and squeeze as many bunks in them as possible)
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Old 03-09-2013
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Re: Full or fin keel?

Wolf your boat is a magnificant design. No one in their right mind would say otherwise. Both a gas operated semi auto 30.06 and a muzzleloader bring down deer. To an collector/hunter both have appeal. The deer is just as dead so he feels the same about both as well. Still, the physics of the ocean and weather remain the same and folks will always strive to find a better solution to deal with it. You are right as regards many production boats but realize they are aimed at a different segment of the boating public then your boat is and was when it was constructed. Compare apples to apples and for the same segment you refer to there are boats still being made which incorporate the genius of subsequent generations. Some are the equal to your vessel and the audience who buys them probably know what a gem you own.

s/v Hippocampus
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Old 03-09-2013
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Re: Full or fin keel?

Pease note my boat was designed to cross oceans, interior set up for 2 to comfortably live aboard ( will sleep 7 with no hot bunking) and one to sail. Same for the same size Morris', HR s etc. Yes a lot of creature comforts and some improvement in the expected days work have been added. Isn't that to the good. I hope I or who ever sails my boat in 75y from now to be able to feel about it the way you feel about yours. Now that's a successful design.

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