Full or fin keel? - Page 10 - SailNet Community
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post #91 of 847 Old 03-12-2012
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Re: Full or fin keel?

Paulo,

I'm reading as he can sail higher upwind and faster at any point of sail than a lighter fin keel! Not sure I would believe it myself............

marty

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post #92 of 847 Old 03-12-2012
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Re: Full or fin keel?

Fuzzy,

I have some pics of wrapping a crab pot line tween the top of the rudder and the hull. Not fun. Fortunetly it was AFTER we won the race that night to claim 1st place in that race series. Still trying to figure out why a crab pot was out, being as the season did not open for a week.......... I did ge the 2011 bent prop from YC out of that one.......that is when the spouse found out about this misadventure.......double OUCH!.......

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post #93 of 847 Old 03-16-2012
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Re: Full or fin keel?

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Originally Posted by PCP View Post
So why a fin keel if properly designed and produced is lesser safer or make a boat less seaworthy?
Just wonder if anybody here read Marchaj's book 'Seaworthiness: The Forgotten Factor' where he shows why: the most important reason is that a full keel with its bigger surface area damps the rolling motion better.

It is a very interesting book, it discusses the dynamics of sailing in great detail and points out that while a fin keel performs much better in ideal conditions (flat water), stormy weather with big seas is a whole different story.

Last edited by pvajko; 03-16-2012 at 08:59 PM.
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post #94 of 847 Old 03-16-2012
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Re: Full or fin keel?

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Originally Posted by pvajko View Post
Just wonder if anybody here read Marchaj's book 'Seaworthiness: The Forgotten Factor' where he shows why: the most important reason is that a full keel with its bigger surface area damps the rolling motion better.

It is a very interesting book, it discusses the dynamics of sailing in great detail and points out that while a fin keel performs much better in ideal conditions (flat water), stormy weather with big seas is a whole different story.
Good book, but old book. Are you familiar with the term "tripping on the keel"?





Regards

Paulo
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post #95 of 847 Old 03-17-2012
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Re: Full or fin keel?

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Good book, but old book.
Can you recommend a newer one, at least as good as his, backed with solid physics and math?
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post #96 of 847 Old 03-17-2012
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Re: Full or fin keel?

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Good book, but old book. Are you familiar with the term "tripping on the keel"?



Regards

Paulo
Paulo, I'm interested in this subject, only having had experience with full keels. Here is what I see, please go into a bit more detail if you see something else.

The drawing of the "tripping" on the keel looks like they are located where waves hit shallow water and break. An area we all try mightily to avoid. Seems that with a small keel or no keel the boat would be "flipping" instead of tripping. Not sure being tossed from the boat is better than filling the cockpit. Plus, trying to get some rest if the boat is rolling with the steepness of the wave fronts, that seems like it would be difficult.
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post #97 of 847 Old 03-17-2012
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Re: Full or fin keel?

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Originally Posted by pvajko View Post
Just wonder if anybody here read Marchaj's book 'Seaworthiness: The Forgotten Factor' where he shows why: the most important reason is that a full keel with its bigger surface area damps the rolling motion better.

It is a very interesting book, it discusses the dynamics of sailing in great detail and points out that while a fin keel performs much better in ideal conditions (flat water), stormy weather with big seas is a whole different story.
Thank you for mentioning this book. I searched, got a look at some inside pages, and ordered one a minute ago.

One page I saw talked about the motion within a wave, and how a short keel is affected only by the motion where it is located, but a longer keel averages the motion out more, throwing the boat around less. This particularly applied to a course other than broadside to the waves. Also, I think it said that the turbulence coming off the short fin keel reduced the effectiveness of the rudder, the idea seemed to be that in flat water the fin keel worked better, but in rough conditions the long keel worked better.

Overall, the book looked a bit "dry" for pleasure reading, but I could digest at least a bit of what I read, so I think I'll learn some things from it.
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post #98 of 847 Old 03-17-2012
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Re: Full or fin keel?

Marchaj's book was a seminal work when it was written. Like Tony Marchaj's other books it did a great job of gathering existing testing, studies and performing targeted testing and explaining the principles in ways that a moderately technically savy amatuer could understand. For us normal sailors Tony's book really progressed the understanding of the factors involved in producing a seaworthy boat.

But the book was written 30 years ago. The research data was based on the testing available at the time. Most of the basic principles remain 100% valid, but many of the conclusions relate to the specific hull forms, keels, rigs and so on that existed at the time, and did not have the advantage of the ongoing testing which has occured in the 30years since.

In the countervening 30 years, designers, using much more sophisticated testing and design tools, have considered the basic principles that were identified in 'Seaworthiness' and designed boats which purposefully address these issues.

I would respectfully suggest that that Pvajko is mistaken when he says:
"The most important reason is that a full keel with its bigger surface area damps the rolling motion better."
"While a fin keel performs much better in ideal conditions (flat water), stormy weather with big seas is a whole different story. "

I would suggest that, in the first quote, Pvajko is in part misinterpreting what was being said, but in both cases, these conclusions were based on the type-forms and limited testing available at the time that the book was written.

Today, seaworthiness is no longer forgotten. Designers have long since learned how to design around the concerns raised in 'Seaworthiness' and have the tools to make both of the quoted statements inaccurate.


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post #99 of 847 Old 03-17-2012
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Re: Full or fin keel?

Thank you Jeff. I wish I could say that I understand this stuff, but I have only a vague grasp and can be readily influenced by logical sounding information.

Perhaps I don't need to worry too much, as I can't really afford a boat less than 30 years old!

I've spent most of my time on wooden sailboats with full keels, so I have no comparison. I am however quite interested in the fin keel concept.

In the non-consecutive pages of the book that I was allowed to look at, I had the impression that it was not the surface area of the keel that was so helpful. It was the linear length (bow to stern), which when running downwind put the different parts of the keel in different (and differing) parts of the wave motion, thus no one part has such a great effect. Plus the old style V hull went deeper, divers know that just a slight increase in depth greatly lowers the wave motion effect.

Of course, I can't help thinking that very deep fin keels would also reach down into less violent motion.

Have you any suggestions on a good place to look for newer information? And are computer models a big part of the new design? I'm a bit suspicious of computer modeling at the moment. We just had a long winter of totally failed computer modeling of atmospheric motion, which gave very poor weather predictions, the worst I've ever seen actually.

Last edited by skygazer; 03-17-2012 at 01:39 PM. Reason: clarity
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post #100 of 847 Old 03-17-2012
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Re: Full or fin keel?

Quote:
Originally Posted by pvajko View Post
Can you recommend a newer one, at least as good as his, backed with solid physics and math?

Quote:
Originally Posted by skygazer View Post
Paulo, I'm interested in this subject, only having had experience with full keels. Here is what I see, please go into a bit more detail if you see something else.

The drawing of the "tripping" on the keel looks like they are located where waves hit shallow water and break. An area we all try mightily to avoid. Seems that with a small keel or no keel the boat would be "flipping" instead of tripping. Not sure being tossed from the boat is better than filling the cockpit. Plus, trying to get some rest if the boat is rolling with the steepness of the wave fronts, that seems like it would be difficult.
Hi!,

No I cannot recommend another one. Taking into consideration what Jeff has said, there are 30 years of sailboat design between that book and today and 30 years where mathematical computing models of boats and hydrodynamics and tank testing have assumed a main role to understand dynamic stability and the way a boat reacts with the sea.

But more than what those studies have shown I would say that more was learned with the pragmatical work of many Naval Architects and many thousands of designs and the assessment that was made of those designs mostly by racers, that in some cases were also the designers.

Skygazer, regarding waves, breaking waves are the only real danger for a monohull and you don't find them only near shore. With a formed sea with big waves and over 35K winds the top of the waves break and if the wave is big, the top of the wave are many tons of water. On that drawing is that what is happening: it is not all the wave that is breaking (like in a beach) but just the top.

By any mean I want to say that full keels are dangerous, just showing that they have also some disadvantages in what regards seaworthiness and dynamic stability.

Those are pages of a book written by one of the biggest sailors of all time, Eric Tabarly. He was not a theorist but you can be sure he knows what he was talking about. He raced what was then modern boats (transats, circumnavigations) and him and the guys that were behind the designs he sailed had an important role in the development of today's modern hull shapes, rudders and keels. And I am saying that he knows about what he is talking about because he did not only sailed extensively racing boats but also its family boat (that he loved) the Pen Duick, an old and beautiful old full keeler.



Regards

Paulo

Last edited by PCP; 03-17-2012 at 03:06 PM.
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