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  #381  
Old 04-20-2012
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Re: Full or fin keel?

Quote:
Originally Posted by souljour2000 View Post
...I am glad to have a train-tracking, full-keeler that can hold a course on a 1-2 or 3 day leg without always having auto-helm turned on...but instead just lashing off the wheel...and I can possibly take 15 minute nap,if single-handed (with multiple set alarms) and hold a course too without autohelm.....
Have you keep up with the thread?

Regarding tracking ability Bob Perry said (in one of his books, particularly about boat design):

"It’s often argued that a full- or modified full-keel boat has better directional stability, which is often referred to as “tracking ability.” My experience is just the opposite. I have found that the further I can separate the keel form the rudder, the better a boat tracks… I’m a believer in the ‘feathers on the end of an arrow’ theory. In other words, keep the rudder as far aft as possible.”

of course there are bad designed fin keelers and bad designed full keelers but implying a broad generalization assuming that full keelers track better than fin keelers just does not make sense. In fact Bob Perry says the opposite, not as a rule but as a tendency.

Regards

Paulo

Last edited by PCP; 04-20-2012 at 05:10 PM.
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  #382  
Old 04-20-2012
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Re: Full or fin keel?

50 years ago virtually everybody sailed a full keel boat - whether daysailing, coastal cruising, or crossing an ocean. They raced them as well.

The last decades have seen many changes to design, some from racing rule influence, some from design knowledge.

Today more boats with fin keel and aft rudders, either on a skeg or a spade, are crossing oceans than full keel boats.

As the keels were shortened to reduce wetted surface the forefoot was cut back and the rudder moved forwards, ending up close to mudships in some boats. This doesn't make for a very efficient rudder and added to its location it was usually raked forward at a steep angle.

The aft rudder was the solution. Leaving the keel to be designed as an efficient foil.

I agree with Paulo and the Maestro - the feathers should be on the end of the arrow.

And as with everything there are both good and not so good designs of either type.
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  #383  
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Re: Full or fin keel?

This discussion could be tightened up a bit, I think, if we could get away from vague terms like "Full" vs. "Fin" and focus on the relevant parameter: aspect ratio. This is basically depth divided by chord (length in the direction of water flow) A more formal definition involves area but is just a refinement.

A "Full keel" boat has a low aspect ratio underwater profile, maybe 0.5 depth to chord, and a "Fin keel" boat has an underwater aspect ratio of at least 1:1, and as high as perhaps 7:1 in the various Open class racers.

The sail plans usually follow these trends, low aspect ratio (think CCA or "America") on full keel hulls and tall rigs on the more racy vessels.

If you look at the L/D curves in C A Marchaj's Theory and Practice of Sailing (best book ever, on the subject, IMHO) you see that there is no question that the high aspect ratio system is best in terms of absolute performance, i.e. getting to windward. (I won' t get into going downwind as damn near anything can do that.)

The L/D ratios at optimal angles of attack are very high for high aspect ratio foils, whether air or hydro, and correspondingly lower for low aspect ratios. This is just simple physics -- the end, which is a leak, is smaller and farther away.

The thing which is important as to this discussion, i.e.which is preferrable, is to be found in examining how the L/D ratio varies with angle of attack. Keep in mind that angle of attack is the angle between the apparant wind vector and the span of the foil. It is determined, hopefully, by how you steer, but also by wind shifts, pitching, rolling, yawing of the vessel, elsticity in the rig and on and on.

The L/D plots for high aspect ratio systems have very high peaks BUT this high performance peak occurs over a very narrow range of angles of attack.

The same plot for a low aspect ratio system shows a broader range where the L/D is not as high as before, but does not change so severely.

So, what's best?

If you love clawing to windward and ekeing out a 1/32 point advantage over a competitor and are willing to stand white-knuckled at the helm to do it then the high aspect ratio, i.e."fin-keel" is the way to go. But bear in mind that if you get out of the groove with either the sail plan or the detached spade rudder, either or both will fall off of that peak in the L/D curve, i.e "stall" and you may well end up ass-over-teakettle with your boat wondering what the hell you were trying to do.

If you'd rather let the boat sail herself for the most part, the broader range of the low aspect ratio is more appropriate. The extreme of this is the old Tahiti ketch with the helm lashed and the skipper sipping rum all the way across the Pacific.

Having said that, there are qualifications:

There are some very nice designs out there which offer a sensible compromise -- but they are all pretty old and therefore not very fashionable. To my mind the epitome of yacht designs of this sort was in the postwar years but before the CCA and subsequent rules began to seriously deform common sense. These boats had tall, but not preposterous rigs, deep but not overly long "full" keels, strongly cut away in the forefoot and after body (aspect ratios around 1:1, thus somewhere in the middle of the current arguement) but blended structurally, and hydrodynamically -- look at the bottom of a C Morgan boat to see what I mean -- also perhaps a bit structurally sounder than the more recent bolt on fins.

I think the unfortunate trend in modern yacht design is that the buying public seems easily beguiled into thinking that the design principles of a Volvo 60, manned by a bunch of professional athletes standing watch on watch and probably terrified nearly every second (but loving it, I am sure) is somehow the right thing for the average guy and his mate, or vice versa, who want to go to sea.

I kind of don't think it is and will offer two observations to support this:

1. Most of these wide beamed, high aspect ratio fin-keeled (with spade rudders) and similar tall thin rigs spend their time as patios.

2. When you do see them sailing they usually do so with an enormous amount of twist in the sail rig -- Harken even advertises windward sheeting mains'l travellers to promote this -- so as to lessen the "twitchiness" of the high aspect rig referred to earlier. With a twisted foil, there is always at least some part of it working properly -- but never all of it. If they could figure out how to make twisty fin keels and twisty spade rudders we'd see fewer "crash jibes," "death broaches" and all the other terms recently invented to describe the behaviour of these things.

Paolo and/or Polux opines that the NAs design and the builders build these sort of vessels because that is what the buying public wants. I suspect that he is right but that the buying public is, as usual, seriously stupid, at least in the short term -- did we really need tail fins on our cars in the 70's?

And I remember a recent (year or so ago -- that's recent to me) in Cruising World where all the Senior Editors chose their own ideal cruising boat. There was not a single modern design in the lot. They were mostly of the type I described above or perhaps a bit later when the "Brewer Bite" became fashionable. None of the types heavily advertised and reviewed in the same pages, ironically.

In the end -- to each his own -- but do some research and have a bit of a clue as to what to expect from the physics of the various design parameters.
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Old 04-20-2012
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Re: Full or fin keel?

Quote:
Originally Posted by keforion View Post
....
I think the unfortunate trend in modern yacht design is that the buying public seems easily beguiled into thinking that the design principles of a Volvo 60, manned by a bunch of professional athletes standing watch on watch and probably terrified nearly every second (but loving it, I am sure) is somehow the right thing for the average guy and his mate, or vice versa, who want to go to sea.

I kind of don't think it is and will offer two observations to support this:

1. Most of these wide beamed, high aspect ratio fin-keeled (with spade rudders) and similar tall thin rigs spend their time as patios.

2. When you do see them sailing they usually do so with an enormous amount of twist in the sail rig -- Harken even advertises windward sheeting mains'l travellers to promote this -- so as to lessen the "twitchiness" of the high aspect rig referred to earlier. With a twisted foil, there is always at least some part of it working properly -- but never all of it. If they could figure out how to make twisty fin keels and twisty spade rudders we'd see fewer "crash jibes," "death broaches" and all the other terms recently invented to describe the behaviour of these things.

....
The one that know me know that I am a firm believer that each sailor should have a boat adapted to his sailing tastes and not otherwise. The big choice in the boat market, specially on the European one is a clear sign that choice exists. Boatbuilders that make boats that no one wants go bankrupt and out of the market. It is as simple as that.

This thread is not about choice of sailing style but about the means to provide that sailing style. What I have been saying is that even for the ones that like heavy boats (and that are some still made today) a fin keel or a modified fin keel on those boats makes everything a full keel would do, only better.

This is not about the choice of the type of boats (performance cruiser, voyage offshore boat, coastal cruiser, heavy, medium or light sailboat or the type of sailing each one prefers, this is about sail design: the best solutions to attain a mean that is, a given type of sailboat.

Regarding what you say concerning the difficulty of sailing fast performance boats directly derived from racing boats, it seems to me that you are the one that is partial. I do understand and see as natural that others have different sailing options regarding the ones I prefer. It is you that think that for offshore work all the boats should be like the ones you prefer for that kind of sailing.

Just to show how wrong you are about sailing difficulty I will post here what Eric, the happy owner of a brand new Pogo 12.50 (the cruising version of a racing 40class boat), said about its first sail on its boat (I hope he doesn’t mind).

Quote:
Originally Posted by EricKLYC View Post
…..

Last week we sailed the boat over to Nieuwpoort, which was a cold but nice and very valuable experience. I will be happy to discuss this in more detail later, but the bottom line is: the boat dislikes pointing or sailing dead downwind, keeping up the speed is the issue and then the VMG is always very correct. It is quite a different way of sailing compared to more traditional designs.

I mentioned before the statement of an experienced class 40 sailor: it’s just like a big 470 dinghy. I’ve been sailing a 470 for almost 30 years and could not agree more. “Sail the boat under the mast” and first try to build up the apparent wind. Then you get exhilarating sailing everywhere between a close and a broad reach.….

The initial (form) stability is as spectacular as the 4m50 wide (and honestly quite disgraceful) beam. Even with myself and my two basketball centre players of sons on the same side, the boat hardly moves.

Under sail, more than 20° of heel only slows the boat down. But before you get there, you have already enjoyed the enormous power of both the hull (form stability) and the 3m deep, leaded keel (weight stability).

Between l’Aber Wrach (North Brittany) and Cowes we kept all the sail (full main + solent) up in 25 knts on a broad reach. With nice, long, 3m high waves and gusts up to 35 knts the average speed was around 14 knts with some wonderful long and thrilling surfs up to 21 knts, without ever feeling out of control.

So our first experience after 450 NM with the 12.50 is: WYSIWYG.

A big 470 with visually basic, but functional and in fact quite comfortable accommodation for our crew of 6.

Eric
To make things clear that boat option is not the one I favor to myself, not because it is too radical in what regards sailing but because it has an interior “too clean” for me and because on the type of sailing I do I make a lot of upwind sailing that is not the strong point of that boat. But if I was circumnavigating, following the trade winds you can bet that it would be that the type of boat I would chose…and I am not implying that would be the right choice for you or any other that prefer slower heavy boats with a different sea motion.

Regards

Paulo

Last edited by PCP; 04-20-2012 at 06:39 PM.
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  #385  
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Re: Full or fin keel?

Paulo:

Thanks for the response. But your statement, "It is you that think that for offshore work all the boats should be like the ones you prefer for that kind of sailing," is off the mark. I never try to tell anyone what they should do. Gave that up long ago. I was pointing out the physics implications of the various design choices.

The post by EricKLYC is interesting. I would note the following:

"... but the bottom line is: the boat dislikes pointing or sailing dead downind."

"... it’s just like a big 470 dinghy."

"... on a broad reach... without ever feeling out of control." Well, I should hope so, given that on this point of sail most would be taking a nap.

This all just confirms my points that these things are "big dinghys."

I would also observe that almost anyone who goes for a first sail in a brand new boat is ENTHRALLED.

What I think this discussion needs, and what I tried to provide, is some guidance to the prospective buyer as to what sort of behaviour he or she might expect from a boat by considering some quantifiable and specific parameters of it's design, namely the aspect ratios of its underwater form and sailplan. There are many other design parameters as well, of course, but they are off topic.

What you or I or anyone else prefers is irrelevant. The point is to understand how a boat is likely to behave given these measureable parameters, and whether that is what the prospective thinks he or she might want.

You seem to want to deal in vagueries, opinions and personal attacks. I have little patience for any of that.
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
But if I was circumnavigating, following the trade winds you can bet that it would be that the type of boat I would chose…
Well, when I was young, I had a 470 and it was really great fun sailing that boat. But you know, if it's really like a big 470, then that is certainly not something for cruising.
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Re: Full or fin keel?

Quote:
Originally Posted by keforion View Post
..

"... but the bottom line is: the boat dislikes pointing or sailing dead downind."

"... it’s just like a big 470 dinghy."

"... on a broad reach... without ever feeling out of control." Well, I should hope so, given that on this point of sail most would be taking a nap.

This all just confirms my points that these things are "big dinghys."
You seem to forget some relevant facts and it is not hard to understand why

Regarding this:

"... on a broad reach... without ever feeling out of control." Well, I should hope so, given that on this point of sail most would be taking a nap.

You seem to forget that the boat was sailing with full sail, 25K wind, gusting 35K, with 10ft waves, averaging 14K with points at 21K. I find amazing that a cruising sailboat can do that without the skipper felling the slightest loss of control, even more so on the first sail on an unknown boat.

Regarding this:

"This all just confirms my points that these things are "big dinghys" you are wrong about that. What Eric is saying is that the boat SAILS like a big dinghy not that is a big dinghy. He knows and I know that his boat has an outstanding final stability with a great AVS. A dinghy has not that and relies on the weight of the sailor not to capsize and when capsized at 90º it will not re-right itself without the help of the sailor’s weight. It is not a big dinghy, IT SAILS LIKE A BIG DINGHY.

Regarding this:

"... but the bottom line is: the boat dislikes pointing or sailing dead downind."

You seem to forget what he wanted to say, or maybe you did not understood. He said:

"the boat dislikes pointing or sailing dead downwind, keeping up the speed is the issue and then the VMG is always very correct. It is quite a different way of sailing compared to more traditional designs".

He knows and I know that his boat is faster against the wind, VMG, than most modern performance cruisers, not to mention any heavy old keeler. What he is saying is that the way to sail the boat against the wind is different: the boat goes faster (VMG) if a bit out of the wind (compared with another performance boat, maybe at the same angle as a full keel boat) because it makes much more speed that way and in the end the VMG is better.

The same happens dead downwind and not only with this boat. With any modern performance boat is the same in what regards sailing downwind and regarding VMG and they are faster that way VMG than any older boat.

Regards

Paulo
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  #388  
Old 04-21-2012
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Re: Full or fin keel?

There's alot of folks in here who are interested in planes also I have noticed so as a one time pilot in my late teens and albeit I had only three solos in small cessnas...this discussion of different boats make me think of different sorts of aircraft....seems that this is like comparing say a Cessna 172 vs. an aerobatic plane...The Cessna 172 is generally alot slower but stable over a wider envelope of conditions...big drag- producing but stable vertical stabilizer(tail) and again a thick drag-producing wing that inhibits speed but allows high lift and high stall speeds...and good stability before it loses that lift in only highly unstable (low-speed or due to high pitch,angle of attack with correlating loss of lift, type conditions.
The aerobatic sport plane like a Pitts with thin short wings can go a hell lot faster..but the pilot has to be on top of things because of the built-in instability stemming from the thinner less-lift producing wings...greater speed of landings (stall speeds),etc...turbulence will also likeley affect the aerobatic plane more...as well as having smaller cargo capacity,fuel capacity,etc...Meanwhile the Cessna will carry four and alot of gear a fairly long way but at much slower speed..You pick...depending on what you want out of your (plane)boat...In this case obviously, the Cessna is the light cruiser and the Pitts is the "racer"...
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Last edited by souljour2000; 04-21-2012 at 01:04 AM.
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Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
Have you keep up with the thread?

Regarding tracking ability Bob Perry said (in one of his books, particularly about boat design):

"It’s often argued that a full- or modified full-keel boat has better directional stability, which is often referred to as “tracking ability.” My experience is just the opposite. I have found that the further I can separate the keel form the rudder, the better a boat tracks… I’m a believer in the ‘feathers on the end of an arrow’ theory. In other words, keep the rudder as far aft as possible.”

of course there are bad designed fin keelers and bad designed full keelers but implying a broad generalization assuming that full keelers track better than fin keelers just does not make sense. In fact Bob Perry says the opposite, not as a rule but as a tendency.

Regards

Paulo
That's fine but I suspect most would agree that in medium and up to 20 mph or so winds..the full keeler tracks just as well and with a much lighter helm....higher winds than that generate enough speed in the fin keeler's airfoil to develop the kind of lift where it will go upwind better than the full-keeler(especially shoal draught full-keeler like mine) and yes..it then "tracks better" but it's likely a bumpier ride than the guy in the full-keeler is having...and he is often only a few points more off the wind...or closer with a ketch or yawl. I will grant naturally you this might possibly be the critical difference that keeps you off a lee shore sometime....someone else's turn now...

Last edited by souljour2000; 04-21-2012 at 01:32 AM.
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Re: Full or fin keel?

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Originally Posted by souljour2000 View Post
..it then "tracks better" but it's likely a bumpier ride than the guy in the full-keeler is having...
I disagree. I think you are assuming that all fin keel boats have shallow flat bottom hulls. True some do but there are all degrees in between as well.

Do you think this boat would have a bumpy ride?
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