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langousta 06-12-2004 04:12 AM

Wing, fin, or bulb...what are the trade offs?
 
I am looking at various boats and see a lot of mention of the keel design. I know that wing keels allow for more stability in a shoal draft than the other models but there must be some trade offs. How do wing keels point vs. fin/bulb? Are they faster or slower? I realize that these are general questions but I was hoping to get real experience and comparisons vs. the marketing I am getting from the manufacturers.

Jeff_H 06-12-2004 05:39 AM

Wing, fin, or bulb...what are the trade offs?
 
First of all, depending on the design wing keels and bulb keels can offer equal amounts of stability in the same draft and with similar amounts of drag. The following is a part of something that I had written for a different venue but it talks about fin, wing and bulb keels. I have included the section on full keels because it explains why fins have a performance advantage.

Full keels:
These earliest keels pretty much ran from the point of entry at the bow, to the aft most point of exit at the stern. Those are full keels in the fullest sense of the word.

They have some advantages; they theoretically form a long straight plane which keeps a boat on course better (greater directional or longitudinal stability). If you run aground they spread out the load over a larger area reducing the likelihood of damage. Once really planted they keep the boat from tipping over fore and aft. They are easier to haul and work on. You can spread out the ballast over a longer distance and so they can be shallower for the same stability. You have a greater length to bolt on ballast so it is a theoretically sturdier and simpler connection.

They have some disadvantages; A larger portion of the keel operates near the surface and near the intersection of the hull and keel which are both turbulent zones. They also have comparatively small leading edges, and the leading edge is the primary generator of hydrodynamic lift preventing sideslip. Because of that they need a lot more surface area to generate the same lift. Surface area equates to drag so they need more sail area to achieve the same speed. Long keels tend to be less efficient in terms of lift to drag for other reasons as well. As a boat makes leeway water slips off of the high-pressure side of the keel to the low-pressure side of the keel and creates a turbulent swirl know as a tip vortex. This is drawn behind the boat creating drag in a variety of ways. The longer the keel bottom, the bigger the vortex, the greater the drag. So they need more sail area again to overcome this drag. To stand up to this greater sail area the boat needs more ballast and a stronger structure, which is why long keeled boats are often heavier, as well. (Of course, then the spirol starts again as more sail area is needed to overcome that additional weight as well. It is the classic weight breeding more weight design cycle) Full keels tend to be much less maneuverable.

Fin keels:
By the classic definition of a fin keel any keel whose bottom is less than 50% of the length of the boat is a fin keel. Fin keels came into being in an effort to reduce drag. Cut away the forefoot or rake the stem, as well as, move the rudderpost forward and rake it sharply, and pretty soon you have a fin keel. Today we assume that fin keels mean a separated rudder (skeg hung or spade) but in fact early fin keels had the rudder attached in a worst of all worlds situation. They offer all of the disadvantages of both full and fin keels, but with almost none of the virtues of either. Unknowing or unscrupulous brokers will often refer to boats with fin (or near fin) keels as full keel if they have an attached rudder.

Fin keels with separate rudders seem to be the most commonly produced keel form in the US these days.

Fin keels have some advantages as well. They have less drag as explained above so they typically make less leeway and go faster. You can get the ballast down lower so in theory they are more stable for their weight. They are more maneuverable. They take better advantage of the high efficiency of modern sail plans and materials.

They have some disadvantages as well, many of these have been offset or worked around by modern technology but at some level they are still accurate critiques. They have less directional stability than long keel boats so the tend to wander more under sail. Since directional stability is also a product of the dynamic balance between the sail plan and underbody, in practice they may actually hold a course as well as a full keel. In general though you can expect to make more course adjustments with a fin keel. It is sometimes argued that the lower helm loads requires less energy to make these corrections so a fin keel may also require less energy to maintain course. This I think is a product of the individual boat and could lead to a debate harder to prove than the number of angels that can dance on the head of a pin.

Fin keels are harder to engineer to withstand a hard grounding and when aground they are more likely to flop over on their bow or stern. (Although in 40 something years of sailing, I have never heard of anyone actually experiencing this.) Fins typically have deeper draft. They are easier to pivot around and get off in a simple grounding.

Shoal keel
A shoal keel is just a keel that is not as deep as a deep keel. Today the term seems to be applied mostly to shallow fin keels. Shallow full keels seem to be referred to as shoal draft boats. A shallow fin is a tough animal to classify. Like a fin keel with an attached rudder, I really think it has few of the advantages of either a deep fin or a full keel and has many of the worst traits of both full and fin. This can be partially offset by combining a shallow fin with a centerboard, which is a neat set up for shoal draft cruising.

Bulb Keel:
A lot can be done to improve a shallow fin. One way is to add a bulb. A bulb is a cast metal ballast attachment added to the bottom of the keel. They concentrate the ballast lower providing greater stability and sail carrying ability than a simple shallow keel. Traditionally bulbs were torpedo or teardrop shaped. They have been re-contoured to provide some hydrodynamic properties as well. Recalling the discussion on tip vortex from above, shallow keels need to be longer horizontally than a deeper fin in order to get enough area to prevent leeway. This means that a shallow longer fin would generate more tip vortex and more drag than a deeper keel. The bulb creates a surface to turn the water aft and prevent it from slipping over the tip of the keel thereby reducing tip vortex. This does not come free since a bulb increases frontal area and surface area which increases drag as well, but typically not as much drag as the tip vortex.

Wing keels
Wing keels are a specialized type of bulb keel. Instead of a torpedo shaped bulb there are small lead wings more or less perpendicular to the keel. These concentrate weight lower like a bulb and properly designed they also are very efficient in reducing tip vortex. There has been some discussion that wings increase the effective span of the keel when heeled over but this does not seem to be born out in tank testing of the short wings currently being used in production sail boats. Not all wings are created equal. They potentially offer a lot of advantages, but they are heavily dependent on the quality of the design and I really think that many wing designs are not really working to their potential.

Then there is the whole grounding issue. In 2002, the Naval Academy did a study of keel types and grounding. They found pretty that the popular perception that wing keel are harder to free is accurate. In their study, wing keels were extremely harder to free. Straight fins were much easier to free, especially when heeled, and the easiest keel to free was the bulb keel.

Neither a bulb keel or a wing keel with deliver the performance of a properly designed deeper fin.

Jeff

jbanta 06-12-2004 10:31 AM

Wing, fin, or bulb...what are the trade offs?
 
Jeff I was fasinated by your report on keels.. One question I have.. What are you doing online on a saturday? You should be out sailing.. :) I have two reasons.. Argo is still on the hard due to low water in the GSL and I have to work on saturdays..

langousta 06-13-2004 03:05 AM

Wing, fin, or bulb...what are the trade offs?
 
Jeff,

thanks for all of that information! That helped a lot.

I am looking into a purchase of a late model Catalina 34. I don''t own a boat yet but have done a ton of chartering of different boats and effectively, keel designs. I like the idea of a shoal keel vs. a deeper draft because most of our sailing will be in and around Naraganset Bay and gunkholing with the family.

I sail with a couple of friends on the bay now and one has a boat with a 4'' 5" draft the other one with 6'' 8". Both are fin keels or small bulbs. Needless to say the deeper keel was able to point a lot higher to the wind. That being said, anecdotally the captain of the deeper keel boat has told me that he wishes he had less draft due to a number of shallow low-tide places he occasionally likes to go.

So given that I am leaning towards a shallow draft boat, the wing seemed like a nice trade off. More stable than the fins with the lower (and greater) ballast. Also, the C34 I chartered this spring seemed to point fairly well for its shallow draft.

In any event, I didn''t know if this was a general characteristic of wing keels or if the C34 is one of the well-designed models you mentioned.

Anyone have an opinion on that?

Thanks again!

Jeff_H 06-13-2004 06:09 AM

Wing, fin, or bulb...what are the trade offs?
 
First to answer Jim''s question, I was and am on the internet because ''Synergy'' is out of the water for a bottom job and I keep my work list and list of needed tools and supplies on my computer. I was just checking it before leaving for the boat.

To briefly touch on Langousta''s last post, a wing keel can be more stable than a shallow fin, but it generally is not as stable as a deeper fin. The wing keel''s greater stability than a shallow fin comes at a price of needing to be heavier and significantly higher drag than a deeper fin of equal stability.

While I have not spent muh time on Narragansett Bay, I would think that you should be able to get by with a draft of 5''-6" to 6''-0 and that should permit you to use a straight fin rather than live with the compromises of a wing keel. If you really are going to be gunkholing, then a wing keel is a really bad chose due to their tendancy to plant you when you do run aground. I am not much of a fan of the Catalina 34''s but that is a whole ''nother topic.

Jeff

SailorMitch 06-13-2004 06:45 PM

Wing, fin, or bulb...what are the trade offs?
 
Langousta,

This topic also was covered about a month ago in the General Discussion category. I suggest taking a look at those posts, so I won''t repeat all that I said back then. Suffice it to say I''ve sailed wing keeled boats since 1989 (first a Pearson 27 and now a Pearson 33.) Yes, they are a compromise and won''t point as high as a deep fin -- although I am quite impressed with the windward ability of my P-33.

But you can''t beat the shoal draft of a wing if that is what you''re after. You won''t go aground much, if at all, because the shoal draft will let you go places other sailors can''t. I''ve touched bottom exactly twice in all these years and I''ve sailed the width and breadth of the Chesapeake, the home of skinny water.

I have no experience with a Catalina wing but it sounds like you have. Go for it.

langousta 06-15-2004 01:41 PM

Wing, fin, or bulb...what are the trade offs?
 
Great!

thanks for all of the great information. I will check out the other posts that were mentioned as well.

orthomartin 01-09-2007 07:34 AM

wing
 
I have a Catalina 387 (hull 100) with the wing keel. Sails fine but I would NEVER buy another wing keel boat. Stick in on the bottom and wow is it stable!! Can not kedge off. If looking to sail in shallow waters you can count on getting the thing pretty stuck

Cruisingdad 01-09-2007 10:38 AM

Intersting comment. My last 2 boats were wings and I would not sail anything but a wing or a shoal draft (like a full keel, IP).

When you hang up on a wing, it is harder to get off, but there are tricks. My boats were a Catalina 380 and 400 (with wings, others were fins). I hung up on my wing about every freaking day... but I was also able to get into places I could not with a fin. If you were sailing the pacific or other deep waters with no intention of shallow water, you would be crazy not to get a fin. Put a 61/2-7 foot draft in S florida or a lot of the Bahamas (and many other places) and you will be running aground all day long or just wont get in at all.

I will also say that the ease of getting off with a fin can be overstated. It is easier than a wing, but you hard ground a fin in the mud, it creates some kind of a super-glue vacuum and that bad boy flat ain't coming out (not tha a wing would really be that much better, but hopefully you did not run aground with it).

Here are some tricks I have learned:

1) Can you back off? Duh. Don't over-do it. But most of the time I can back her off unless I ran aground hard.

2) Pull the halyard with the dink. If you pull the halyard over with the dink, a lot of times with a good reversse, you get the boat off the suction that it seems to create.

3) Lean out over on the boom. Grab a hold of the boom throw yourself out over the water (Trying not to fall in, of course). Have another person backind down at the same time.

4) Anchor method. Depending on where the deep water is, take your anchor out with the dink, set it, and winch it in while backing down. Sometimes we could push the bow around with the tender and use the windlass. You can use the same method setting her to stern and winching in from the cockpit.

5) Wait till the tide comes in.

6) Call Sea Tow. Sail in shallow water, you get to know them by name and they recognize your voice on the VHF. "Brian, you hung up AGAIN?? Do you even LOOK at your depth finder?" "Depth what? Is That what that thing is? Who needs a depth finder when you got a Sea Tow membership?" They lost money on me. Just remember to be friendly or learn patience. Get to know them and there is no such thing as a hard grounding.

- CD

S/VNirvana 01-09-2007 11:39 AM

My last 2 boats were fin keels and the last a modified fin keel. The fin keel on my Pearson 36 draws 6' and the modified fin keel on my Jeanneau SO49 draws 7'.

The Pearson 36 is up here in CT and the Jeanneau is in the BVI. I wouldn't have a shoal keel up here. If you are going to be in shallow waters most of the time then it would be a neccesity to have a shoal keel/centerboard.

By the way the Pearson 36 is for sale. It's been reduced to $40,000.00

http://www.ablboats.com/details.asp?ListingID=74941


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