Join Date: Feb 2000
Location: Annapolis, Md
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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.
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.
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.
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.
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 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.