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  #371  
Old 04-12-2012
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Re: Full or fin keel?

Pvajko and Skygazer, Thank you for your kind words of support.

Quote:
Originally Posted by GBurton View Post
In the case of a wooden boat, I would think that a fin or a full keel would sustain more damage than a glass boat, but the fin keeled boat would always be more susceptible to damage than a full keeled boat. That is just basic physics.
As far as the Westsail 32 you helped build with concrete and scrap steel, 95% of these boats left the factory with the ballast installed (this is from the general manager of the factory at the time)
Unfortunately you have contributed to building a not so desirable Westsail 32 ..what a shame as the boat is potentially a great cruiser.
Starting from the bottom up, I agree with you entirely about the Westsail 32 with the concrete and scrap metal ballast and specifically said that to the fellow building the boat at the time. I tried to make the case that the Westsail started out with less ballast than the Eric which it was patterned after, so going to a low density ballast made less than zero sense. He claimed that he had sailed on one and talked to "A bunch of guys who were building their Westsail 32's that way, and they sailed fine." I'll take your word that these other not so desirable Westsail 32s were a rarity. But that is another story.

Getting back to the original topic, as you acurately note, due to the physics of the situation either a wooden boat or a glass boat with a fin keel will potentially experience much higher stresses in a grounding than a full keeled boat.

One of the clear advantages of a full keeled boat with external bolt-on ballast is that the ballast takes the initial impact and spreads the load over a much larger area of the boat's structure. Because the ballast is comparatively low aspect, and placed outside of the structure, the forces imparted into the hull and fastenings are predominantly in compression and sheer, which is what conventional boat building techniues can absorb best.

The stresses on a fin keel are higher in part because of the greater lever arm of the keel, but also because of the smaller contact are of the fin limiting area within which the connections need to be made, and so the fastenings work at an inverse mechanical advantage. Not an inherently good thing.

The higher stresses of the fin keel, require much more careful engineering and construction techniques if the same margin of safety is to be achieved. (This was in part why I said that properly engineered fin keeled boats are typically more expensive than a simialr strength full keeled boat.)

But that does make any specific fin keel automatically weaker than any specific full keel. Properly engineered, the fin keel needs to be design to equal or greater safety factors than the full keel design intended for the same purpose. That means designing an internal structure and fastening scheme which can safely withstand the fin keel's greater loads, and distribute them over a large enough area of the boat that damage is not incurred during its service life.

In my mind the problem with discussing this in the abstract, vs analyzing this in the specific, is that for the most part, the majority of fin keel boat whoch have been built have been aimed at the racing, coastal cruising and value oriented communities. These boat have purposely smaller safety factors than boats intended for dedicated offshore passage making and cruising. By the same token, a much larger percentage of full keel boats built in recent years were designed with the intent of offshore use.

So, when this comes down to debating the strength advantage of a specific design feature citing specific boats, it is easy to suggest a range of examples of full keeled offshore cruiser that are way more robust than some collection of fin-keeled coastal cruisers, or more glaringly yet, compare dedicated offshore full-keeled cruisers to the strength of some dedicated race boat. But it gets harder to make the point when the same comparison uses dedicated offshore cruisers with fin keels that in theory have been designed to the same safety factor as any other purpose built offshore cruiser.

The same argument applies in the hull form debate as well. It is easy to say that a purpose built, offshore cruiser- no matter what its keel type, should have a more comfortable motion, more carrying capacity, and a more seaworthy hull form no matter what its size or displacement than would be expected on a dedicated race boat, racer cruiser, coastal cruiser, or even a boat designed to make occasional offshore passages.

Where these debates go off the rails is that comparasons are often made between purpose built offshore cruisers versus purpose built race boats, racer-cruisers, value oriented family cruisers and coastal cruisers and so on, when each may be well suited and optimized for their secific intended use and so do not represent a fair example for comparason on the issue being debated.

Jeff
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Last edited by Jeff_H; 04-12-2012 at 05:41 PM.
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  #372  
Old 04-12-2012
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Re: Full or fin keel?

Having sailed both, I recall loving the full keel when I had to heave to and hating it when maneuvering in marinas. Island hopping in the Caribbean, I'd opt for fin keel. Good sailing
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  #373  
Old 04-12-2012
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Re: Full or fin keel?

I hope that the posters here take to heart Jeff's message. Its hard to compare apples to apples in this kind of discussion but without it there is little relevance. You can't compare a heavily built boat purposely designed to get caught in a storm to a Catalina 27 and say that the Catalina is a lesser boat because it has a fin keel. That doesn't make any sense. Compare a Panda 40 to a Saga 43. Purpose built ocean cruising boats with different keels. I know which one I prefer.

As a previous owner of a full keeled boat designed and built in the 60's I can say that I would rather quit sailing altogether than bob around in that scow again. We put a man on he moon in the sixties but my boat might as well have been the mayflower. Albeit she was a thing of beauty.
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  #374  
Old 04-13-2012
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Wink Re: Full or fin keel?

....As a previous owner of a full keeled boat designed and built in the 60's I can say that I would rather quit sailing altogether than bob around in that scow again. We put a man on he moon in the sixties but my boat might as well have been the mayflower. Albeit she was a thing of beauty......



.....yeah, but I'll ask...what kind of sailing do you do now...vs.. back then?
I don't like marinas....and keep me boat on a hook...manouevering a full-keel in a marina situation would be a nightmare I'd rather skip...but the old narrow ones ride well at a mooring...and even better on the high seas...and I get me exercise rowing me dinghy out to her mooring..whoever (she) is over the last few years...A full keel gives me a good feeling if ever aground too..but if we all wanted full keels...or vice versda..world of sailing would be more boring place...

Last edited by souljour2000; 04-13-2012 at 09:31 PM.
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  #375  
Old 04-15-2012
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Re: Full or fin keel? Quotes from Robert Perry

Robert Perry’s, “Yacht Design According to Perry” (2008, pp. 118-122) has a short chapter, titled “Full Keels” which complements the descriptions above. Expositions are lucid and reader-friendly. And a firm point of view, aligned to an extent with Paulo’s, and contradicting some (see points on “stability” and “tracking”) -- a position which supports an evolution away from the full/modified-full keel toward at minimum, a hybrid keel, properly and creatively designed and engineered (a provocative topic in itself).

Here are some extracts:

“A full or modified full keel has several disadvantages, one of which is that it lacks the aspect ratio to develop good flow across the chord for lift. In order to achieve an appropriate thickness ratio on a long-chord keel, you have to make it excessively thick, which gives too much added displacement and frontal area. If you reduce the thickness ratio to achieve acceptable displacement and frontal area, the keel may become too thin to keep flow attached and will stall at early angles of attack. Stalling eliminates the keel’s lift and causes drag. The good news is that when a full keel stalls and loses life, you’re still left with all that planform area to prevent the boat from being shoved sideways. I use a 7 to 9 percent foil thickness ratio for my modified full-keel designs.“

“… wetted surface has a severe negative effect on boat speed in light air, therefore an effort should be made to reduce wetted surface. From my perspective though, the biggest disadvantage of a full or modified full keel is the additional displacement it adds to the canoe body compared with a far less bulky keel. On the other hand there are pragmatic benefits … “enough internal volume to house the ballast” “room left over for a tank or two” “contributes to a lower VCG” (most of these boats “use cast iron at 450 lbs/cubic foot (or even less dense materials) rather than cast lead at 700 lbs/cubic foot and in doing so fail to take full advantage of their keel volume to lower the VCG. Lead is always better. It’s nice to have a big, long keel on which to rest your boat when you haul out. People also say that a full keel [incl. modified full] protects the rudder, but I wonder. Most full-keel boats still have the heel or gudgeon of the rudder at the lowest point, so there is still a chance of damaging the rudder when you hit bottom. I design the rudder bearing or “Gudgeon” to be at least 4” above the lowest part of the keel to help prevent damage.

I do not regard having the propeller in an aperture [in such keels] to be an advantage … The worst boats to operate in reverse are [such] boats… Of course you may sail in waters littered with lobster and crab pots, or maybe your boat sits on the mud at low tide. A full keel is also a strong shape to have in a catastrophic grounding or if you are pounding on a beach. Haulouts with crude gear may be easier…”

“A sailor must balance these virtues against the full keel’s performance vices. To me performance is paramount. This is not to say that all full/modified full keels are slow. Starting with the Tayana 37, I made an effort to separate the full-keel shape as much as possible from the canoe body by reducing the garboard radius and increasing the span. My Taishiba series boats all sail beautifully, and I’m sure that Chuck Paine’s similar full-keel boats do also. Look, for example at the new Cabo Rico 45 … [possibly] an example of a modified full keel or the very elusive modified fin keel.”

“…given the amount of volume in a full keel, all else being equal, a full-keel boat will be less stable through the normal sailing range, 0 to 30 degrees of heel, than a fin-keel boat. Picture the midsections of a full-keel boat and a fin-keel boat with both boats heeled 20 degrees. The immersed portion of the fin-keel midsection is almost entirely to leeward of the centerline, where it contributes to righting moment via buoyancy. The windward portion of the hull is mostly out of the water, where it, too, contributes to righting moment via gravity. The immersed portion of the full-keel midsection, on the other hand, is still perhaps 40 percent to windward of the centerline, where it contributes to further heeling… technically expressed, the volume of a full keel reduces the righting arm (the distance from the VCG to the transverse center of buoyancy). Thus, not surprisingly, I get a lot of calls from owners of older full-keel boats complaining about lack of stability."

"The stiffest boats are those with deep, high-aspect fin keels with some type of bulb at the tip. It’s all about getting the VCG low… On the other hand, it’s important to remember that bulbs in themselves are not hydrodynamically desirable. A clean fin has far less drag… A relatively thin, high-aspect fin will have less frontal area than most other keels, which can reduce drag and add up to a downward wind speed advantage, but the thinness comes at a price. [It] poses a structural challenge due to the short chord where the fin attaches to the hull. A short fin makes it difficult or impossible to spread the fin loads over a big section of the hull. You wouldn’t want to bounce a fin like that off a reef for a day or two, nor would you want to sit the boat’s entire weight upon it when you haul out. Short-chord, high-aspect fins are unsuitable for most cruising boats, which hold durability as a primary desirable feature.”

“Remember, more stability means a boat that will stand up to its sail better and present a more efficient keel shape to the water. People like stiff boats… 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.”

“… [L]imiting yourself to more ‘normal’ cruising keels, you arrive at hybrid keels like those in the accompanying illustration. This is an external keel for the 57-foot cruising sled ‘Mobisle.’… I have used this hybrid keel shape on three of my ‘cruising sleds.’… Obviously you want to limit draft on any cruising boat, and this severely ties the hands of the designer."

Last edited by Daily Alice; 04-15-2012 at 12:45 AM.
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  #376  
Old 04-15-2012
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Re: Full or fin keel? Quotes from Robert Perry

Thanks for posting. Interesting and informative read.


Quote:
Originally Posted by Daily Alice View Post
Robert Perry’s, “Yacht Design According to Perry” (2008, pp. 118-122) has a short chapter, titled “Full Keels” which complements the descriptions above. .. a position which supports an evolution away from the full/modified-full keel toward at minimum, a hybrid keel, properly and creatively designed and engineered (a provocative topic in itself).

Here are some extracts:


“…given the amount of volume in a full keel, all else being equal, a full-keel boat will be less stable through the normal sailing range, 0 to 30 degrees of heel, than a fin-keel boat. Picture the midsections of a full-keel boat and a fin-keel boat with both boats heeled 20 degrees. The immersed portion of the fin-keel midsection is almost entirely to leeward of the centerline, where it contributes to righting moment via buoyancy. The windward portion of the hull is mostly out of the water, where it, too, contributes to righting moment via gravity. The immersed portion of the full-keel midsection, on the other hand, is still perhaps 40 percent to windward of the centerline, where it contributes to further heeling… technically expressed, the volume of a full keel reduces the righting arm (the distance from the VCG to the transverse center of buoyancy). Thus, not surprisingly, I get a lot of calls from owners of older full-keel boats complaining about lack of stability."

"The stiffest boats are those with deep, high-aspect fin keels with some type of bulb at the tip. It’s all about getting the VCG low… On the other hand, it’s important to remember that bulbs in themselves are not hydrodynamically desirable. A clean fin has far less drag… A relatively thin, high-aspect fin will have less frontal area than most other keels, which can reduce drag and add up to a downward wind speed advantage, but the thinness comes at a price. [It] poses a structural challenge due to the short chord where the fin attaches to the hull. A short fin makes it difficult or impossible to spread the fin loads over a big section of the hull. You wouldn’t want to bounce a fin like that off a reef for a day or two, nor would you want to sit the boat’s entire weight upon it when you haul out. Short-chord, high-aspect fins are unsuitable for most cruising boats, which hold durability as a primary desirable feature.”

“Remember, more stability means a boat that will stand up to its sail better and present a more efficient keel shape to the water. People like stiff boats… 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.”

Regarding this:

"The stiffest boats are those with deep, high-aspect fin keels with some type of bulb at the tip. It’s all about getting the VCG low… On the other hand, it’s important to remember that bulbs in themselves are not hydrodynamically desirable. A clean fin has far less drag… A relatively thin, high-aspect fin will have less frontal area than most other keels, which can reduce drag and add up to a downward wind speed advantage, but the thinness comes at a price. [It] poses a structural challenge due to the short chord where the fin attaches to the hull. A short fin makes it difficult or impossible to spread the fin loads over a big section of the hull. You wouldn’t want to bounce a fin like that off a reef for a day or two, nor would you want to sit the boat’s entire weight upon it when you haul out. Short-chord, high-aspect fins are unsuitable for most cruising boats, which hold durability as a primary desirable feature.”


Bob Perry, even if he considers very short fins (I call them foil bulbed keels) advantageous in what regards sailing qualities, sees them unfit for cruising boats, based on lack of durability and structural difficulties. Nevertheless they are incresingly used in fast cruising boats.

I agree with what Bob says about the structural challenge to be bigger however many boat builders resolved that with an elegant structural solution: A carbon or steel large frame where the keel is attached and that receives also the loads from the stays. This way all the keel loads are distributed by the hull and the attachment point is incredibly strong. The keel construction has also to be different from the more traditional ones to give more resistance to that thin foil. Several methods are used being the most strong a machined piece of solid steel where the lead bulb is attached.

Of course this has a disadvantage: price but an increasingly number of manufacturers are using the system among them: Salona, Comar (Comet), Grand Soleil, X-Yachts, Luffe, Arcona.

Regards

Paulo

Last edited by PCP; 04-15-2012 at 08:33 PM.
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  #377  
Old 04-16-2012
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Re: Full or fin keel?

There are certain basic atributes that apply to all full keels and others that apply to all fins (as I have mentioned earlier) each have advantages and disadvantages....but taking the best attributes of the best designed fins and comparing them aganst the worst of the full (which appears to be happening cumulatively) just isn't fair and/or defeats the purpose ofthe thread. A well designed full has these same advantages in performance over a poorly designed fin while keeping the traditional advantages of the full (in all fairness I will point out this works both ways). I pursonally prefer a well designed full...that said in a honesty there a a few fins I would be happy with.

Last edited by wolfenzee; 04-16-2012 at 01:31 AM.
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  #378  
Old 04-20-2012
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Re: Full or fin keel?

Bryce,

I got as far as finding VMG, metacenter and CB for a boat, but how do you arrive at " the ratio of mass righting moment to hull righting moment"? I mean what do you need first, for data, and what formulas are you using? I ask because your approach seems useful. Isn't hull righting moment graphed over various heel angles (like from 0-30 deg.)? Anyway, I'm curious, thanks.

Quote:
Originally Posted by BryceGTX View Post
...For me, the key number is the ratio of mass righting moment to hull righting moment. For lightweight cruisers and racers, this number is nearly zero. For heavy keel cruisers like the Island Packet, this number is closer to one. For me, my boat is closer to probably about 0.3 or 0.4. It is a relatively wide beam for speed with a huge keel mass for a significant bit of mass righting moment. It is a trade off. It is not a lightweight cruiser, but it is not a heavy weight like an IP. Bryce
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  #379  
Old 04-20-2012
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Re: Full or fin keel?

I think by asking this question,first you have to ask yourself what type of sailing you are going to do.If you want to cross oceans and want a proven multi functional design chose full keel.
If you are going to hang around the coast chose fin keel
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  #380  
Old 04-20-2012
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Wink Re: Full or fin keel?

Quote:
Originally Posted by Captain milos View Post
I think by asking this question,first you have to ask yourself what type of sailing you are going to do.If you want to cross oceans and want a proven multi functional design chose full keel.
If you are going to hang around the coast chose fin keel

I am now just really realizing that I no longer have a boat good for gunkholing . (full-keel Morgan Columbia 40)....could be worse though since she drafts only 4 and a half...

.....but for island-hopping...say... thru the Bahamas and the "thorny" on down which I hope to do someday soon...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..I have never had an auto-helm/auto-pilot on any of my former boats...so this is new ....and am not familiar with auto-pilots yet...but for now...gimme a full-keeler..or cutaway-forefoot...One day when I'm older and can afford a marina and need maneuverability...I'm sure I might spring for .5 kt faster fin-keeler

Last edited by souljour2000; 04-20-2012 at 04:48 PM.
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