Full or fin keel?

[mitiempo]

Steve Dashew's "Offshore Cruising Encyclopedia" has a good explanation of keel, rudder, and hull design and its effect in real offshore conditions as well as showing the advantages of computer programs for comparing design changes.

[Waltthesalt]

While we often don't do it here, a longer keel is more accommodating in supporting the boat upright when drying out your boat in a tide to work on it.

[pvajko]

Originally Posted by Jeff_H

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.

Jeff, don't misunderstand me, I'd love to be mistaken on this. I have a fin keel boat which is perfectly safe I guess for where I sail it.

I always had the impression though that for off shore sailing a full keel is safer, based partly on anecdotal evidence and Marchaj's book (which, I'm happy to admit, I may be misinterpreting, or at least oversimplifying it's reasoning).

If this is not true, that's great news for me because it means there is a lot more (and cheaper) boats to choose from in case I wanted to go off shore.

I wish there was a book or study like Marchaj's available for the general public, summarizing the past 30 years progress on this subject, because, you see, the reasoning like PCP's "[Tabarly] was not a theorist but you can be sure he knows what he was talking about", with all due respect, is not very appealing to me.

Originally Posted by Jeff_H

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.

I'd really be interested to learn what these tools are. Could you give more information on that, or at least point out where to look?

And please, don't get me wrong, I'm not arguing against the fin keel or any other keel or feature, I'm perfectly aware of the fact that my knowledge on this subject is way to limited to be able to argue for or against anything.

[skygazer]

Originally Posted by PCP

......family boat (that he loved) the Pen Duick, an old and beautiful old full keeler.



Regards

Paulo

Paulo, thank you for the further details on the wave motion etc. I especially like the photo of the Pen Duick, wow! Those old style low lean boats thrill my blood. I have to say new boats look like they sit on top of the water in the wind, while the old style seem to knife through the water.

Nevertheless, I like the way I can stand up in a modern 27 ft. boat, instead of having to increase size to a 40ft plus boat. So I'm casually looking at more modern boats with headroom in the smallest size that will sail well. They all have fin keels.

I hope I can find some of the information that has been gained in the last 30 years, at a layman's level that I might understand. I've read some good points on this thread, like the fact that with wood, they could not build a strong fin keel like we can with modern materials. Also, today's better understanding of airplane type wings (keels) and their ability to generate lift.

[PCP]

Originally Posted by pvajko

,,,the reasoning like PCP's "[Tabarly] was not a theorist but you can be sure he knows what he was talking about", with all due respect, is not very appealing to me.

...

Well, as Leonard da Vinci once said: "Experience is the mother of all Knowledge" and Marchaj had some but not much in what regards sailing.

On other hand, experience, with full keelers and fin keelers was a thing that Eric Tabarly had in huge amounts: “A former officer in the French navy who is often considered the father of French yachting”.

Éric Tabarly - Wikipedia, the free encyclopedia

Éric Tabarly - Wikipedia, the free encyclopedia
Éric Tabarly - Wikipedia, the free encyclopedia

Marchaj made a notable theorist work trying to explain reality (based on 35 year old sailing boats) trough equations and mathematical calculations but let me tell you that even if static stability is a simple and forward thing, dynamic stability is a very complicated subject (the interaction of a variable sea motion with a boat and its effects on boat stability) and even today with all tank testing and mathematical computer aided models it is far from being a settled matter.

But if the science is still messing around with dynamic stability Naval Architects have been working on the model of the best boat to fast and safely cross oceans. Probably nothing as contributed as much to the knowledge of dynamic stability than the 35 years of designing small boats to solo crossing oceans, what is called the Mini class racing:

Classe Mini

What was learned with these boats influenced bigger sailing boats, racing and cruising, in an extent that can be considered probably as the biggest influence on modern yacht design, and what was learned had all to do with dynamic stability.

Regards

Paulo

[PCP]

Originally Posted by skygazer

.....

Nevertheless, I like the way I can stand up in a modern 27 ft. boat, instead of having to increase size to a 40ft plus boat. So I'm casually looking at more modern boats with headroom in the smallest size that will sail well. They all have fin keels.

I hope I can find some of the information that has been gained in the last 30 years, at a layman's level that I might understand. I've read some good points on this thread, like the fact that with wood, they could not build a strong fin keel like we can with modern materials. Also, today's better understanding of airplane type wings (keels) and their ability to generate lift.

Regarding standing up on a 27ft boat, probably not the best way to increase seaworthiness, but related with what I said in the last post I will post in the interesting sailboat thread a boat that taking into account what was learned in 35 years of Transats on minis (including a non stop circumnavigation), is proposed as the prototype of the (very) small offshore fast boat.

Regards

Paulo

[BryceGTX]

Originally Posted by mitiempo

Fin keel or full keel, a boat doesn't have to be fat

There are two reasons to be fat. First is to get more interior room. Second is to create more righting moment. It is the easiest way to get righting moment. And of course more stiffness.

However fat hulls make for poor handling in rough water without significant keel mass. And therein lies their downfall as a substitute for a full keel boat.
Bryce

[BryceGTX]

Originally Posted by Jeff_H

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.

There really is nothing wrong with what he says, but it is incomplete. It has been well documented in 100 year old yacht design books that full keels have the advantage in big water because the rotation of the water in the wave causes the full keel boat to heel toward the wave rather than heel with the wave as with other keel designs.
Bryce

[BryceGTX]

Originally Posted by PCP

Marchaj made a notable theorist work trying to explain reality (based on 35 year old sailing boats) trough equations and mathematical calculations but let me tell you that even if static stability is a simple and forward thing, dynamic stability is a very complicated subject (the interaction of a variable sea motion with a boat and its effects on boat stability) and even today with all tank testing and mathematical computer aided models it is far from being a settled matter.

Dynamic stability to illustrate the difference between hull and keel designs is fairly trivial and can easily be simulated on any PC. Keep in mind that yacht designers from 100 years ago understood the concepts quite well. And all their calculations were with pencil and paper.

But if the science is still messing around with dynamic stability Naval Architects have been working on the model of the best boat to fast and safely cross oceans. Probably nothing as contributed as much to the knowledge of dynamic stability than the 35 years of designing small boats to solo crossing oceans, what is called the Mini class racing:

Lol... The million dollar question is "best boat". This is kind of a silly statement. What is best boat? If you talk to one person, it it speed at the the expense of everything. As with a racer. Such a boat is great for a boat full of excited sailors whose only goal is to get from point a to b in the shortest time.

However, the average cruising couple does not is not so motivated to put up with being thrown around like a rag doll in a washing machine. So the typical cruiser needs a more rough water friendly boat. The typical IP couple will heave to and go down and sip coffee and read books while the storm rages outside.

That is the difference between a true rough water boat and the typical lightweight cruiser.
Bryce

[PCP]

Originally Posted by BryceGTX

.....

Lol... The million dollar question is "best boat". This is kind of a silly statement. What is best boat? If you talk to one person, it it speed at the the expense of everything. As with a racer. Such a boat is great for a boat full of excited sailors whose only goal is to get from point a to b in the shortest time.

....

I did not say that there is a "best boat" or a type of boat that suits all to cross oceans following the trade winds. That is obviously a sailor's choice and some few still prefer old heavy slow full keel boats.

What I have implied is that that modern NA are not interested in slow heavy full keeled boats and said they are working on boat models to cross oceans safely and fast. You just have to look at their contemporary work, in what concerns modern passage makers, to understand what I mean.

Regards

Paulo