I think that there is no one universally right answer here but a whole lot of strong opinions when it comes to keel
types. If you are traditional in your view point then you would lean towards a boat with a full length keel
. If you care more about performance and ease of handling then you might lean towards a fin keel.
There are good and bad offshore
boats with all kinds of keels and so buying
a boat with a full length keel will not guarentee that you will end up with a good offshore
boat any more than buying
a boat with a fin keel will guarentee a bad offshore boat. These are at best subjective decisions.
The way that I personally look at this, If your goal is to spend almost all of your time offshore and in really remote
areas of the world, then a full keel boat probably makes sense. If you are going to island hop and perhaps occasionally make longer passages, and you will be traveling in places like the US, Carribean and Europe
, then a properly designed and engineered fin keel boat makes more sense. BUT again that is only my opinion.
The material below is exerpted from an article that I had written for another venue but which might help you as well.
These were the earliest keels and they 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
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 number of ways. The longer the keel, 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 keelboats 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 whise 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. (I could be wrong, there is a resurgence of full keels 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. On most boats under 40-50 feet, there is a tendancy for dynamic directional stability to be more critical to course holding than the directional stability that comes from the a long keels greater longitudinal moment of intertia. Since directional stability as a product of the dynamic balance between the sail plan and underbody is so important to directional stability, in practice many fin keel boats actually hold a course as well as a full keel. In general though you can expect to make more small course adjustments with a fin keel. It is sometimes argued that the lower helm
loads on a fin keeler requires less energy to make each of 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 37 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.
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.
Keels that are not really keels:
Swing keels are ballasted centerboards and drop keels are ballasted daggerboards that are ballasted beyond what it takes to submerge themselves. They are really forms of centerboards. More on them in with centerboards.
Keels that are keels that move.
I said in the introduction
that keels do not move. That used to be true. We now have canting keels, which can be pivoted from side to side. They are best designed to be light fins with heavy bulbs that can be canted to windward increasing the effectiveness of the righting aspects of the keel. Just one problem, a keel canted to windward losses efficiency to prevent leeway so they really need other foils to keep leeway in check. I frankly do not like the idea of a canting keel. I think canting keels are too complex and potentially problematic.
Centerboards are appendages that can be raised and lowered on or near the centerline of the boat. They can rotate up into a trunk or rotate below the boat. Daggerboards are a type of centerboard
that raises vertically or near vertically in a trunk. Swing keels are a type of rotating centerboard that actually contains a substantial portion of the boat’s ballast. They may be housed in a trunk like a Tartan 27 or 34 or hung below the boat like a Catalina
22. In the case of the Tartan 27 or 34 they are more frequently referred to as a Keel/ Centerboard (abbreviated k/cb) A swing keel is intended to act as a fin keel when lowered and allow some sailing in the partially raised position. My biggest problem with swing keels is that most do not have a positive lock down. In an extreme knockdown they can slam up into the hull
greatly reducing the boat’s stability. This is a pretty rare occurance and usually requires big wave action combined with a lot of wind
, but I have experienced it out in the Atlantic.
A drop keel is a daggerboard that actually contains a substantial portion of the boat’s ballast. These are easier to lock down but can be more easily damaged in a grounding. They generally have better shape than a swing keel and can be more robust, but not always are.
Other appendages: (besides the rudders)
keels (or twin keels for our English
friends) are a pair of keels (usually fins these days) that emerge on either side of the boat and angle out. They offer some advantages. If you let the boat dry out the boat can stand on the two keels and wait the next tide. There are dubious theories about increased efficiency since one is vertical like a good leeway resisting foil and one is canted like a good stability inducing foil. With computer modeling there has been greater success in aprpoaching that theory on large bilge
keel boats. While bilge keels do allow shallow draft
though, they extremely difficult to free once aground.
I hope this was helpful although I don't know if it really answers your question since this topic is so subjective.