Keels

A keel can mean either of two parts; a structural element, or a hydrodynamic element; these parts overlap.

Structural keels

A structural keel is a large beam which runs the of a ship is built around. The keel runs in the middle of the ship, from the bow to the stern, and serves as the or spine of the structure, providing the major source of structural strength of the hull. The keel is generally the first part of a ship's hull to be constructed, and laying the keel, or placing the keel in the cradle in which the ship will be built, is often a momentous event in a ship's construction--so much so that the event is often marked with a ceremony, and the term has entered the language as a phrase meaning the beginning of any significant undertaking.

Hydrodynamic keels

A hydrodynamic keel is a structure on the bottom of the hull that is used to give the vessel greater directional control and in many vessels contains a good portion of the balast. In traditional boatbuilding, this is provided by the structural keel, which projects from the bottom of the hull along most or all of its length. In sailing hulls, the keel helps the hull to move forward, rather than slipping to the side.

Sailboat Keels

 

Yacht with fin keel

In sailboats, keels use the forward motion of the boat to generate the force to counter the lateral force from the sails. Sailboats have much larger keels than non sailing hulls. Keels are different from centerboards and other types of foils in that keels also hold ballast to stabilize the vessel. Keels may be fixed, or non-moveable, or they may retract to allow sailing in shallower waters. Retracting keels may pivot (a swing keel) or slide upwards to retract, and are usually retracted with a winch due to the ballast. Since the keel provides far more stability when lowered than when retracted (due to the greater moment arm involved) the amount of sail carried is generally reduced when sailing with the keel retracted.

There are several types of fixed keels including: full keels, fin keels, winged keels, bulb keels, and twin keels or bilge keels among other designs.

Non-fixed keels are known as canting keels or Swing keels. These are found on racing yachts. They provide much more righting moment for a lot less weight, as the keel moves out to the windward-side of the vessel. The perpendicular distance from weight to pivot is increased, therefore a larger righting moment is produced.

Winged keel

A winged keel is a sailboat keel, usually of high aspect ratio, that uses a nearly horizontal foil, the "wing", at the bottom to provide additional performance. The horizontal foil serves two purposes: it acts as an winglet on the foil, effectively doubling the aspect ratio, and they produce additional lift, but downwards. Because the yacht is heeled over when sailing upwind the leeward foil is closer to vertical, and provides additional side force hence making the boat sail upwind more efficiently. The windward winglet is closer to horizontal and hence produces a force directly downward, which gives a small benefit to the vessel's stability. Winged keels are generally found on high performance sailboats, if they are not prohibited by class rules. They are only of benefit for yachts sailing upwind where stability and the ability to produce side force are important. Downwind the extra skin friction drag is a hindrance. The increased stability afforded by the winged keel, due mainly to the extra lead in the wings producing a very low centre of gravity. The wings were angled downwards at about 20 degrees, which, since they were lifting downwards, acted as a dihedral. Most winged keels also incorporate a significant amount of ballast where the wings join the keel. This feature makes these winged keels also a type of bulb keel.

Bulb keel

 

A bulb keel is a keel, usually made with a high aspect ratio foil, that contains a ballast-filled bulb at the bottom, usually teardrop shaped. The purpose of the bulb keel is to place the ballast as low as possible, therefore gaining the maximum possible amount of leverage and thus the most righting moment. A example of a class of boats that use a bulb keel is the International 110 racing class, which uses a 300 pound cast iron bulb keel on a vessel whose minimum racing weight is 910 pounds.

Since bulb keels work best on long, thin keels or daggerboards, they are generally not used on sailboats intended for shallow waters, but are most often found on offshore racing craft. Daggerboards built like bulb keels are often referred to as "lifting keels", and they can be retracted into the vessel to reduce the draft, and to allow the boat to be loaded onto a trailer. Lifting keels are also more likely to be found on craft built for speed, as a winch or a set of pulleys is required to provide purchase to lift the heavy keel.

Some vessels with lifting keels go so far as to allow the keel to be canted side to side, allowing the bulb to be placed to windward to reduce heel in high winds; this type of keel is called a canting keel. It is also possible to shape the bulb into a horizontal foil, called a winged keel.

Twin keel

 

Twin Keels or Bilge Keels are two keels that emerge at an angle from the hull of a sail boat at the bilges. The angle allows the boat to have a shallower draft.

Canting keel

A canting keel is a form of sailing ballast, suspended from a rigid canting strut beneath the boat, which can be swung to windward of a boat under sail, in order to counteract the heeling force of the sail. The canting keel must be able to pivot to either port or starboard, depending on the current tack.

Purpose and history

The traditional keel serves two functions: lateral resistance, and keeping the boat upright by counteracting the heeling force of the wind. The traditional fin keel, pointing straight down from the boat, provides no righting moment when the boat is level. The heeling force of the wind on the sails is therefore not counteracted until the boat has heeled over by a certain amount, moving the fixed keel to windward of the centerline. The purpose of the canting keel is to allow the boat to develop righting moment when level, by swinging the keel to windward independent of the boat's angle of heel.

With the canting keel handling the ballast functions, lateral resistance and steering can be managed separately with a foil (or pair of foils fore and aft). This allows for much quicker maneuverability than traditional keelboats, with about half the weight usually required for ballast.

Problems

The current canting keel technology is far from perfect. At least three of the seven boats in the 2006 Volvo Ocean Challenge, one of the first major long term races allowing canting keels, have had problems with the keels. One area in particular, the plates sealing the opening through which the keel passes, are prone to leaks. Unlike a centerboard or daggerboard trunk, the opening for a canting keel must allow significant lateral motion, which requires sliding seals.