A keel balances a monohull in the water. The keel is a large flat shape with a aerodynamic leading edge. They come in six general variations: full, fin, bulb, wing, centerboard, and canting.
Full
A full keel by definition runs the length of at least 50% of the hull. The forward edge curves vertically while the aft edge often connects to a rudder. The main advantage of a full keel are safe grounding and directional stability.
Safe grounding means a full keel is safer when you lie on hard ground. Whether along the coastline or in travel lift slings, the full keel provides a strong, stable balancing point for the hull. A full keel boat will not tip on its bow or stern. When you ground then, the boat will lay on her side in shoal water and take less damage. In a travel lift, the full keel is easier to sling with the longer keel line. Blocked up on the hard, the full keel provides a line of balance. Be it an unintentional or intentional grounding, a full keel has more stability.
A full keeled boat points well when in the water. It has good directional stability. Put the boat in a compass direction and with a properly designed full keel is likely to stay pointing in that compass direction. The full keel has longer waterline length so controls the flowing water more than shorter keels. The water rushes by for a longer distance and smoother flow. Turbulence is less likely to generate forces to twist the boat. The full keeled boat is less likely than a fin to fall off because this smoother flow. The boat has greater directional stability than a fin.
Fin
By a fin, I refer to a simple deep keel that’s length is less than 50% of the hull. The fin is flat and sharp edged, shaped like a shark fin. A fin keel is fast. The fin keel has less wetted surface area than fuller keels and drafts more. The deeper draft makes the boat sail great. In general, the deeper a sailboat drafts, the faster she is. For racing and performance cruising, the fin keel is king. Reading threads in Sailnet, you’ll see Jeff_H as a great proponent. His articulate posts are worth a postmodernist’s read.
Bulb
The bulb is a shoal draft fin keel. Basically, you saw off a deep fin keel and attach a torpedo shaped bulb of lead to the keel bottom. This shallower keel is a compromise between the performance of a fin but the realities of cruising in the Bahamas, Cheasapeake Bay, and other shoal water holes. Often makers these days produce shoal and deep versions of their designs. Hylas, Valiant, and Tayana come to mind. If you plan to sail in shoal waters, they recommend a bulb keel but otherwise you will enjoy the deep fin keel. As a side benefit if you do ground on a soft bottom and sink in, the bulb keel is the easiest to free. The bulb does not stick way down into or catch the muck like other designs. The bulb at the bottom plops out easily.
Wing
The wing is another shoal alternative to the deep fin. Instead of one bulb at the keel bottom, the wing has two bulbs laterally offset and connected via lead airplane wings. Or the wing is a thick foil of lead without bulbs. The wing has better performance than the bulb because she reduces tip vortex turbulence. The draft can be even less. Because the two bulbs are offset they do not mess with the leading keel edge and generate turbulence like a simple bulb keel. As a drawback, the wing is the most difficult to free if you slide into muck. The wings have a way of gripping down into the bottom. Wing keels are seen on Catalinas and Irwins.
Centerboard
Another shoal idea is the centerboard. The centerboard keel has a base keel with a dagger than rotates downward. When the water is deep, you stick the centerboard down. When shoal, you sail centerboard up. You get the performance of a deep keel and the manuverability of the shoal – a perfect idea, right? The drawback is maintenance with the centerboard. As with any moving part, problems arise. The centerboard has to be maintained.
Canting
The latest and oddest in performance is the canting keel. A canting keel mounts on a hinge and when the boat heels, racers hydrolically rotate the keel to windward. Rotating the keel away from your heeling direction, generates force which both rights the boat and propels her forward. The downside is the maintenance and complexity associatied with the canting keel. It is the future for racing but not perfected yet. I saw the 65 Windship, Procyon, of the Harken brokers which has a canting keel (and bipod mast to boot). “How do you like the whole canting keel thing?” a fellow broker asked. “It’s great. Oh boy is she fast.” The owner loves the canting keel but of course there are downsides of maintenence and the inherent risk of failure with any complex system. Every sailboat decision is a trade-off.
Conclusion
The main types of keels are the full, deep fin, bulb, wing, centerboard, and canting. The full is the oldest and slowest while the canting is the newest and most complex. In general, the bulb, wing, and centerboard are compromises on a deep fin to allow for shoal water cruising.