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  #1  
Old 10-11-2012
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Fin vs bulb vs wing keels

Here I am again, I'm asking about sailing characteristics of the different keel designs. The Bristol 20 that the admiral and me have been sailing on Narragansett bay since spring is a full keel boat, it would appear that most of what I have looked at is a fin or wing keel, less so the bulb keel.

So there it is, you folk have forgotten more about sailing than I have learned, so I'm asking your opinions on these keel designs. What do you guys and gals think about this as applied to sailing characteristics. I understand that fin keels point better and wing keels with a more shallow draft are good for getting in to shallows better. How do they compare one against the other on 32 to 36 foot Catalina's or Hunters while under sail.

Thank you for your knowledge and insight on this question.

Last edited by Pegu club; 10-11-2012 at 11:08 PM.
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Old 10-11-2012
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Re: Fin vs bulb vs wing keels

OK.. presuming you've read all 54 pages of this?

Full or fin keel?

and now to the various 'fin/short' keel discussion. You may have started another unwinnable argument.


IMO from a pure sailing (esp upwind) perspective, draft "rules". If you can get around with it the maximum draft your boat can be engineered for would be 'best'.. but what's best and what's practical won't likely align so nicely.

Plenty of sailing areas are so shallow that a good deep draft boat would spend more time stuck than sailing. Hence the current popularity of the various shoal draft designs. To a degree I think the current crop of wing keels is as much marketing as design refinement, but moving the ballast into wings does lower the CG and improve righting moment. The various keel/centerboard configurations can offer the 'best of both worlds' for some.

Concentrating the ballast in a bulb hanging off the tip of a short chord low drag fin is most efficient of all, but it can be a difficult thing to properly engineer, and they require a lot of skill to maximize the performance benefit. If such a boat doesn't keep the flow 'attached' there will be ridiculous amounts of leeway because such a keel has little lateral plane to resist sliding sidewise in the absence of hydrodynamic lift.

Most cruiser/racer type boats today will either have a standard deep fin (many new designs do have bulbous keels that get the ballast down low) or some version of a wing/shoal style keel. I think the difference between the two will only be seriously noticed if two equally well sailed similar boats line up, one with fin and one with wing. For the average cruiser/casual racer I think it may be a non issue, and the shoal draft may work to advantage overall.

For ourselves our cruising grounds don't really have any draft restrictions so a deep fin would be my preference. Ten feet of draft would be extreme, of course, but a 6 feet we rarely need to worry about where to go.

Another issue is that not all models even offer both versions.. some manufacturers seem to have concentrated on the shoal draft 'version' exclusively.
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Old 10-12-2012
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Re: Fin vs bulb vs wing keels

I have the shoal keel version of a hunter 25. It's got a very high ballast/displacement ratio of 50% (2200 lbs lead to 4400 lbs overall displacement. Most boats have around 40%). This is, I believe, to make up for having the weight up higher than a standard fin keeled boat (draft is 3', I think the fin keel version would have 5' draft).

I haven't sailed on the standard fin keeled version of my boat, but I have sailed on several similarly sized boats with fin keels and I must say they point significantly higher than my shoal draft boat. I can rarely tack through 90 degrees, 100-115 is usually what I can do while maintaining good speed (always speed first, then pointing). I do a little better with the working jib but not much. The shallow draft has not been much of an advantage for me. I have anchored in shallow water a few times, and I rarely worry about draft, but from sailing my fathers boat with a 6' draft I can say that there is absolutely nowhere I can go with my boat that we can't go with his, at leat in this area. And that extra pointing, and more importantly that extra vmg that the deep keel gives you with its proper foil profile, really make a difference ESP when fighting a current upwind.

Last edited by peterchech; 10-12-2012 at 01:43 AM.
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Re: Fin vs bulb vs wing keels

Faster sums it up nicely. Essentially if you sail in an area where there are a lot of shallow waters, then you will benefit from a shoal draft, but you will definitely get better performance out of a deep draft fin. So if depth is not an issue, go deep!
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Re: Fin vs bulb vs wing keels

Thank you for the info and advice everyone.
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Re: Fin vs bulb vs wing keels

The one thing I would point out is that a bulb keel is really just a modified fin.

But instead of being restricted to the chord required to get that much mass, they just sling it at the bottom ( I am simplifying, it's tricky to do right). But what this allows is the ideal fin shape for the given hull, while allowing less mass to give the same righting moment.

Basically bulbs are much faster than either a fin or a full at any given draft. The more draft you can allow for though the more efficient the mass can work, and therefore the less you need.

The formula is:
Distance from pivot around which the hull heels that the mass is located -D
Weight of the mass - M

Righting moment =DM.

Note that this is just a specialized application of the general torque formula. So the two ways to get more righting moment are 1) increase weight, 2) increase distance (draft). Because in all instances righting moment is directly related to the amount of maximum available horsepower
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Re: Fin vs bulb vs wing keels

Probably the most concise answers I've seen this group give out. Could not agree more with any of them. Fin keel is good, light fin with a bulb even better (cause it puts the weight where it can be most effective), shoal keel/wing keels are more about making due (therefore increasing weight, to give the same righting amount) for easier access to shallow waters (and/or easier trailerablity).
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Re: Fin vs bulb vs wing keels

Actually wing keels are an attempt to reduce tip vortex creation by effectively having a longer keel. If they worked as intended you could have the lift gains of a deeper keel on a shoal draft. Sadly what works on planes doesn't work very well on boats. In practice wing keels can't keep the water attached due to subtle water currents, yaw and roll, and attitude changes as a boat goes through the waves.

The only advantage a wing keel has over a fin, is the lower mass, but in practice a streamlined bulb will always be better.
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Re: Fin vs bulb vs wing keels

Not sure I agree with your police work there Lou/Stumble.
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Re: Fin vs bulb vs wing keels

Quote:
Originally Posted by Stumble View Post
Actually wing keels are an attempt to reduce tip vortex creation by effectively having a longer keel. If they worked as intended you could have the lift gains of a deeper keel on a shoal draft. Sadly what works on planes doesn't work very well on boats. In practice wing keels can't keep the water attached due to subtle water currents, yaw and roll, and attitude changes as a boat goes through the waves.

The only advantage a wing keel has over a fin, is the lower mass, but in practice a streamlined bulb will always be better.
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Last edited by jimrafford; 10-12-2012 at 06:21 PM.
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