The Keel

A keel is a dual purpose hydrofoil, firstly it counteracts any heeling movement, secondly it minimizes any sideways drift. There are boats without keels where each task is performed seperatly. The most noticable are the catamarans and the trimrarans where "form stability"* mostly replaces gravitational stability and leeward drift is reduced by boards** and the use of narrow hulls. To undestand form stability we must first understand buoyancy, its law dictates that any object immersed in fluid or gas will come under a "floating" force equal to the amount of fluid/gas it displaces. Form stability is achived when an object is shaped so that when it is heeled on either side it will develop more boyancy on the lee side, causing a larger force and resulting in the object righting itself.

Initial (static) Stability can be provided by two different forces, boyancy and gravity. Every boat uses a comination of these two, mono hulls depend on a lead keel and a wide bow*** while multi hulls get gravitational and boyancy stability from the large righting arm created by spacing the hulls apart. The great thing about multi-hulls is that they have a very large righting arm even when heeled only slightly, the downside to this is that as the boat heels the righting arm is actually reduced. On a mono-hull the righting arm increases untill you reach 90° by which time it is reduced down to where the boat would rather float upside-down. If you look at the stability pictures on the right the righting arm is drawn out in green, notice the differences and picture the righting arm at other angles of heel. The danger of using only form stability is quite apparent with multi-hulls, once they have capsized it is almost impossible to righten them due to the inverted form stability.

The third kind of stability is the driving force for modern keel development, by shaping the keel like a wing it can generate huge amounts of righting force when the boat is heeled. The Bernoulli effect affects liquids and gas equally and just like with ruddes and sails it can be put into use in keels. Like aerobatic planes a keel is a symetrial wing, only by altering the angle of attack can lift (force) be generated.**** When the angle at which the waterflow hits the wing is altered both deflection and the Bernoulli effect create a huge force. The trick with keels is to automatically alter the "angle of attack" as the boat heals. The great thing is that the faster the boat moves through the water the more downforce is created and all this without adding extra weight. On fixed keel boats the only way to alter the angle of attack is to design the hull in a "wedge" shape. The best way to visualize this is to imagine a wedge shaped piece of wood, attach a steel keel in the shape of a wing to the center as detailed in the drawing on the right. Now roll the wedge onto it's side, If you imagine that the flow of water is paralell to the surface the wedge lies on you will notice that the keel is now at an angle.

Modern mono hulls use a mixture of all three forces, the racers take each to extreeme, long narrow wings with lead filled bulbs give maximum dynamic stability along with great gravitational stability. Wide bow sections not only give more form stability but also increase the angle of attack for the keel. The danger of wide (full) bows became all to apparent during the 1979 Fastnet race when a large portion of the fleet was capsized and never recovered.

The last task performed by a keel is to minimize the drift of a boat, this is one of the most controversial design aspect. Some people will swear by full keels*****, they might limit the drift but this is offset by a huge wetted area and by the increased pointing ability****** the drift can be counteracted.

Keels come in many shapes and forms, each has an advantages and disadvantages. Draft is the largest concern, a deep keel is very beneficial for crossing the Atlantic Ocean but limits access to shallow water. The second concern is one of strenght, a deep fin keel with a bulb is naturally much weaker than a full keel or a moderate keel. In the end it is a compromise depending on what kind of sailing the boat will do, for racing fin keels and bulbs are the way to go. For Cruisers something more moderate is the solution. Full keels might sound great for strength and draft but they create a small righting arm and are not wing shaped, this must be offset by greater ballast.

*Form stability is a result of distributed boyancy, see picture.

** Centreboards | Daggerboards, and Leeboards

*** Referring to a crosscut of the The wider the bow of a mono-hull the more form-stability.

**** For more information on lift, aerodynamics and hydrodynamics see the Sails section and the Rudder section

***** Full keels are keels which run 60% or more of the length of the boat.

****** Pointing ability is a measurement of how close to the wind a boat can sail, how steeply upwind it can go.