Join Date: Feb 2000
Location: Annapolis, Md
Thanked 235 Times in 186 Posts
Rep Power: 10
Wing vs Fin Keel
This is partially exerpted from an article written for another purpose. 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 none of the virtues. 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 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 41 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. 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. There is a very specialized type of bulb keel called a Scheel keel (named for Henry Scheels who was its inventor) that is carefully modeled to reduce drag and maximize lift when used in concert with a shoal draft keel.
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 and are no where near as effective as the advertising hype that surround them.
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. Stright fins were much easier to free, especially when heeled, and the easiest keel to free was the bulb keel.
As others have mentioned, I would either do a shallow bulb, fin or else a centerboard boat where shoal draft is critical.