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  #21  
Old 08-16-2006
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Quote:
Originally Posted by captnnero
Ok, so if you reduce the draft by concentrating in a bulb you need to increase mass of the keel to make up for the shorter lever arm. On the other hand by shortening the fin to make the shallower bulb you've also reduced your lift area affecting pointing. What about that fatter profile for the bulb ? Isn't that more turbulence or at least more wetted surface without lift ? Maybe that is why I've seen some fin keels that taper outward as they go down, to concentrate weight lower like a bulb yet maintain some Bernoulli lift effects.
You don't necessarily increase the mass of the keel, as the keel's effects are based on the center of mass for the keel, and using a bulb may in fact move the center of mass lower, without a corresponding need to increase the weight.
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  #22  
Old 08-16-2006
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Two comments -- many "modern" bulbs also have wings of some sort. And most shoal draft keels have several hundred pounds more ballast than a deep draft fin keel for the same boat. Of course, there are exceptions to every rule, including this one.
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  #23  
Old 08-16-2006
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no free lunch either

Quote:
Originally Posted by sailingdog
You don't necessarily increase the mass of the keel, as the keel's effects are based on the center of mass for the keel, and using a bulb may in fact move the center of mass lower, without a corresponding need to increase the weight.
If you're not going to increase the keel mass then the profile and therefor lift effects are reduced as mass has to be shifted from the upper keel into the bulb in the shortened keel to move center of mass lower to maintain the righting moment. To compensate some lighter fairing material could be used to maintain the profile but the remaining support structure metal has to bear more load in the thinner upper keel. That is not a trivial engineering exercise. I'm just saying there does not appear to be a free lunch by shifting mass into the bulb.

The popular aftermarket cruising conversion involves chopping the fin keel and adding additional mass beyond the original chopped portion when the bulb halves are mated to the new bottom of the keel. The additional mass has to be used to maintain the righting moment. Instead of doing that I'd find a well engineered shoal draft boat to replace the deep draft one. Once you chop that keel your boat becomes a special case when it's time to sell her. You may attract some buyers but you certainly will turn many others off.

Last edited by captnnero; 08-16-2006 at 10:25 AM.
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  #24  
Old 08-16-2006
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In a general sense, there is nothing currently available that is more efficient in terms of lift to drag than a deep fin keel. For a given draft and an equal weight, a bulb keel will get the weight of the keel lower than a fin keel, but that comes at the price of increased drag. Designers using bulb keels are making a trade off between greater stability vs. decreased lift and increased drag. Properly designed bulbs are shaped to provide an end plate for the keel and reduce keel vortex, which to a small degree offsets the decreased lift and increased drag.

Few, if any, so called wing keels provide any lift at all and at best they are a glorified bulb keel, which like a well designed bulb, provide some end plate effect and reduce keel vortex (although their own wider spans may produce tip vortexes equal to or even larger than keel's tip vortex.) Wing keels, especially the low aspect wings used on production boats, are extremely hard to design so that they have a proper incident angle and since the angle of attack relative to the water flow greatly affects drag, and also and since the incident changes dramatically in waves, despite the marketing claims to the contrary, in practice, the kinds of low aspect ratio wing keels used on production boats tend to have enormous drag compared to deep fin versions of the same design (or even bulb keel versions) As a result performance oriented production boats are just about doing away with the option of the type of wing keels that were popular just a few years ago.

There was a question about the Naval Academy study. This was a study that was undertaken a couple years ago. The Naval Academy was getting ready to move onto its next generation of 44-foot cutters that are used for training purposes. As it turns out the NA44's really get abused. They apparently are in collisions once a week on average, and are rung aground with fair regularity (sometimes as a training drill). Since the NA44's are also raced, there was some concern about performance as well.

The study involved a rather large hull model of the proposed new NA44. There were a variety of model keels constructed that could be switched out as part of the testing program. The bottom of the testing tank was lined with a wedge of sand chosen for its consistency and linear behavior. The model boat was towed at a constant speed and force into the wedge of sand and the forces of impact and depth of penetration precisely measured. Then the boat was extracted and the force of extraction measured. They also measured rotation forces as well, since rotation is often a component of extraction.

The experiment was repeated with a number of different wing, bulb, fin, and long keel configurations as well as at various heel angles. If I remember correctly the results were then calibrated by towing full sized 44's with mock-up keel bottoms into the section of Bay bottom with speed and tow load made as constant as possible. Impact loads and withdrawal loads were then measured.

What was observed was that the bulb keel went the least distance into the bottom, and had the least extraction loads. It was found that the bulb also had the lowest rotation forces, pivoting easily on its rounded bottom. Fins came next in terms of ease of extraction. Fins and bulbs had near equal straight back extractions but fins lost out because its narrow frontal area allowed it to penetrate deeper and so made rotation much more difficult. Wing keels were a distance loser in this study, which is consistent with the in the field observations of the TowBoatUS operator that I mentioned.

Respectfully,
Jeff
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  #25  
Old 08-16-2006
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Two cents here,

But with no grounding experience in wings (thank God! lots of rock here and very little mud).

Sailed two production boats with wing keels (Catalina, Hunter), and raced (as crew) on hull #1 Shock 34PC.

All three would stall upwind consistantly. The Schock was the prototype 34PC and started with a better than normal production boat wing. It stalled bad enough the skip replaced it with an elliptical, and shortly after moved the elliptical 3 inches aft to improve the balance. The last version was great.

Wings in general....suck.

Dewey
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  #26  
Old 08-16-2006
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Jeff-
"Few, if any, so called wing keels provide any lift at all " Wasn't part of the ballyhoo about the Scheel Keel that it was supposed to generate lift and act as an end plate on the wing (keel) increasing lift by that principle?

I suppose on any given boat you'd have to contact the designer to find out how much of the keel was there for lift, versus for mass at the end of the lever arm. Once there is enough for lift, the rest if just excess drag. And if you cross over into that area...if you are not racing, and need more mass low down but don't need more lift, you can always drill a core into the keel and insert a tungsten sabot. Twice the density of lead, not radioactive or restricted--except probably illegal to race with.

Damned cheap sailors, always trying to make do with old fashioned LEAD keels and canvas sails.
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  #27  
Old 08-16-2006
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The Scheel keel was actually a very sophisticated bulb keel that was optimized in shape to provide maximum endplate effect, and minimum wetted surface and induced drag. It was a great improvement over a simple torpedo type bulb and is the basis for many, if not, most modern bulb keel designs.

Most cruising boat designers do things a lot more by guess and by golly, than by sophistcated analysis. There is precious little non-proprietary data out there on the trade offs between a little bigger drag and a little more stability. That said, the leading edge designers often have access to computational fluid dynamics programs that can provide a general sense of the tradeoff of any shape or weight distribution.

Jeff
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  #28  
Old 08-16-2006
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Jeff--

Were the NA boats actually manufactured for the academy? Having seen them many times out on the water, I always thought they were J-44s.
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  #29  
Old 08-16-2006
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Quote:
Originally Posted by Jeff_H
...There was a question about the Naval Academy study. This was a study that was undertaken a couple years ago. The Naval Academy was getting ready to move onto its next generation of 44-foot cutters that are used for training purposes. As it turns out the NA44's really get abused. They apparently are in collisions once a week on average, and are rung aground with fair regularity (sometimes as a training drill). Since the NA44's are also raced, there was some concern about performance as well.

The study involved a rather large hull model of the proposed new NA44. There were a variety of model keels constructed that could be switched out as part of the testing program. ...
...
The experiment was repeated with a number of different wing, bulb, fin, and long keel configurations as well as at various heel angles. Respectfully,
Jeff
Jeff,

Thanks for the study details. I was able to find a 2001 Navy study http://web.usna.navy.mil/~phmiller/grounding_report.pdf. It compares laboratory grounding tests of a deep fin model with a proposed bulb design for their new Naval Academy fleet. The proposed bulb design won hands down. Unfortunately, I couldn't find the larger study that you mentioned about other keel designs. The 2001 study does mention wanting to test in the future with different bottom materials other than the sand that they used.

If you know where to find the latter study that you have summarized, please let us know.

Last edited by captnnero; 08-16-2006 at 03:50 PM.
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  #30  
Old 08-16-2006
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The Naval Academy 44's are built by Pearson Composites (formerly TPI) just like the late lamented J-44's but they are very different boats all around. The old boats were a McCurdy and Rhodes design, and the new NA44's were designed by David Pedrick. http://www.bwsailing.com/01articles/issue/0305/bwb.htm.

I heard about this study at a lecture a couple years back and then spoke to some of the researchers after the presentation.

Jeff
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