How Sailboat Keels Work
In simple terms, the keel of a sailboat acts as a water foil to keep the boat on a straight course. The protruding keel of a sailboat serves the same function as a centerboard or a leeboard with additional structural integrity.
But why do sailboats need a long keel when powerboats use a flat keel? Sailboats don't always navigate with the wind directly behind them. If they did, there would be no need for an extended keel. In other words, sailboats can sail in almost any direction to the wind because the keel helps convert angled force into forward motion. Without the keel, strong side winds would simply push the boat sideways or flip it over.
Keels also provide additional stability and allow the boat to heel over without capsizing. Long, heavy keels reduce rolling in rough weather, and thin, sporty keels reduce friction and increase speed in lighter winds. Next, we'll cover the most common types of sailboat keels and their uses.
Sailboat Keel Types
Designers and engineers have perfected the design of sailboat keels over the years. Keel types vary between boats based on size and the intended purpose of the boat. Fixed keels, which are used on medium and large sailboats, include full keels, modified full keels, fin keels, bilge keels, wing keels, and bulb keels.
Full Keel
Traditional sailboats often utilize a full (or 'deep') keel. This long and heavy keel flows with the shape of the vessel and extends far below the bilge level of the hull. Full keels run the length of the boat, and they're typically deepest at the stern of the vessel. The rudder often runs deep and matches the length of the keel.
Deep keel sailboats are often heavier and displace more water than other boats of similar length. They often have a very deep draft as well, making shallow-water cruising off-limits in many cases. That said, deep keels offer an enormous amount of stability and seaworthiness. Full keels offer the most comfort in rough weather.
Deep displacement hulls are excellent in heavy weather. They're also quite stable at extreme angles, as the added weight and depth of the hull make knockdowns less likely. However, some full keel designs have a tendency to heel violently to a certain angle, which makes some sailors uncomfortable. The 'sweet spot' angle of a full-keel boat is often a bit more dramatic than some would prefer.
Modified Full Keel
The modified full keel is a slightly modernized version of the old-fashioned displacement hull. It runs long and deep like a standard full keel but stops short of the bow and stern. Modified full keels are common, and they likely produce less drag than traditional deep keels.
One benefit of the modified full keel is a reduction in heeling. These vessels offer good all-around performance, but they're not designed for shallow water. Like the full keel, the rudder runs deep along the length of the modified keel. Modified keelboats are quite comfortable at sea, but nothing beats a full keel.
Fin Keel
Fin keels are quite common on modern boats, and they range in size in depth dramatically. Fin keels are generally mounted from the center of a round-bottom sailboat and extend several feet below the base of the hull. At their base, fin keels are usually no longer than 1/3 of the vessel's waterline length.
Fin keels generally offer superior windward performance compared to full keels. They can be designed to reduce heeling, which can improve occupant comfort in some weather conditions. Fin keels are often made of steel, filled with lead, or weighted down in some way. This additional ballast improves stability and balances the boat.
Speed is another major advantage of the fin keel design. Fin keel sailboats achieve incredible speeds thanks to reduced drag. Racing boats often employ fin keels, but the benefits aren't limited to competition craft. Adding a few knots of speed on a long voyage can shave days off your arrival time, which is why fin keel sailboats are popular for ocean crossings.
Fin keels often run as deep or deeper than full and modified full keels. Sailors need to keep this in mind, as running aground with a fin keel can damage more than just the bottom of the boat. Fin keels are designed for coastal and bluewater cruising. And while fin keels are suitable for bluewater sailing, they can be less comfortable than full keels in inclement weather.
Skeg Rudder Fin Keel
Some fin keels feature a skeg rudder. This type of rudder runs deep and often matches the depth of the keel. But unlike a full keel, a skeg rudder is often separated from the keel itself by a gap. Sometimes, the keel re-emerges right before the rudder. This offers additional protection for the rudder.
Spade Rudder Fin Keel
The fastest fin keel design utilizes a spade rudder and a long, thin knife-like keel. A fin keel with a spade rudder has the least amount of drag and therefore outpaces all other keel designs virtually. However, speed comes at a cost. Windward performance suffers, and so does rough water comfort.
Additionally, sailboats with these types of fin keel have no extra rudder protection. Any contact with the bottom can shear the rudder clean off. Rough water and debris are also quite unwelcome and sometimes pose a serious threat. That said, these boats are superior for racing and traveling on the clock.
Bulb Keel
A bulb keel can be considered a type of fin keel, but this design deserves its own category. Bulb keels are excellent for shoal-draft boats, as they reduce draft without sacrificing a significant amount of performance.
Bulb keels are shortened fin keels with a torpedo-shaped bulb at the base. The bulb increases the surface area of the fin without significantly increasing its depth and also stores the required lead ballast to maintain stability.
Additionally, bulb keels are less vulnerable to grounding than traditional fin keels. For one, they're shallower and less likely to touch the bottom in the first place. But unlike fin keels and full keels, they're blunt—that means they won't cut into the seafloor like a blade, and they're easy to free from the muck.
Wing Keel
The concept behind the wing keel is the same as the bulb keel, but the execution is a little bit different. Wing keels are short fin keels with horizontal outcroppings at the base. From the front or rear, wing keels look like an upside-down 'T' mounted to the base of the boat. They're great for shallow water, and they offer reasonable stability and windward performance.
Bilge Keel
Bilge keels are indeed multiple keels. The bilge keel is perhaps the most brilliant shoal draft adaptation for fixed-keel designs. Instead of a single long keel in the center, bilge keel sailboats feature two short keel fins on either side of the hull. They're mounted at opposite angles, reducing draft and coming into vertical alignment when the boat heels.
Twin-keel boat performance varies widely, but these vessels can be impressively seaworthy. Grounding isn't the end of the world, as bilge keels allow the boat to rest flat on the mud without digging in. With twin bilge keels, there's no need to sweat it when the tide goes out.
