Skeg

A skeg (or skegg or skag) is a sternward extension of the keel of boats and ships which have a rudder mounted on the centre line. The term also applies to the lowest point on an outboard motor or the outdrive of an inboard/outboard. In more recent years, the name has been used for a fin on a surfboard which improves directional stability and to a movable fin on a kayak which adjusts the boat's centre of lateral resistance (it moves the center of resistance relative to the center of effort). The term is also often used for the fin on water skis in the U.S. It has been used for the vertical fin on seaplane hulls and floats. The wear-bar on the bottom of snowmobile ski may also be called a skeg.

The word originates in the Scandinavian word for beard; in Old Norse, skegg. In Icelandic the word remains skegg, in modern Norwegian Bokmål and Nynorsk, it appears as skjegg, in Swedish, it is skägg and in Danish, skæg. The Norwegian pronunciation of the letter combination skj is as in the English sh. The word is related to the English shaggy. It also appears in the English place name Skegness - 'beard point', from the way in which a series of tombolos forms, towards the nearby Gibraltar Point. Here, the English pronunciation reflects a probable Danish origin, which pronounces the sk letter combination as an English speaker would expect.

Where a vessel's rudder is mounted on the centre-line, it is usual to hang it on gudgeons and pintles, the latter being upright pins and the former, rings to fit round them. Together, they form a hinge. This naturally leaves a small gap between the sternpost and the rudder, into which stray items like kelp and rope can catch, causing drag and threatening the security of the vessel's steering. In ships such as Mary Rose, the skeg is a very small feature; a tapered extension of the keel below the leading edge of the rudder. This somewhat beard-like sternward extension of the keel is the basic skeg. Subsequently, the lowest pintle was commonly mounted below the rudder on a metal extension of the keel. This helped further stabilize and protect the rudder and the name skeg was transferred to it. It used to be relatively small until screw propellers were introduced, when it had to reach below the screw and became a proportionately larger feature protecting both screw and rudder from damage.

On wooden vessels, the skeg may be protected from worm damage by the addition of a bug shoe, or a "a length of hardened material, such as ironbark, placed on the sternward keel extension (skeg) to protect from shipworm damage."

In more modern installations, with more than one screw, a fitting supports each propeller shaft just ahead of its screw. This is usually called a shaft bracket but the part of it which extends below the shaft bearing to protect the lower part of the propeller is also a skeg. Similarly, the protective projection of the drive casing, below the rotational axis of the propeller of an outboard motor is another form of the skeg.

Where a yacht is designed with a fin keel, it will normally also have a skeg-mounted rudder.

A skeg on rowing shells is the fin attached to the keel of the shell that helps stabilize it and maintain a straight course. The rudder attaches to the skeg and is steered by cables attached to it. In select sweeping boats, typically fours and eights, a coxswain will control the rudder, while in sculling boats and some sweeping boats, especially pairs, the rudder will be controlled with toe-steering. A skeg typically consists of a flat piece of metal or plastic. Some crews, like Rutgers Crew, use polished wooden skegs that break off upon impact with debris in order to protect potential damage to the hull of the shell.

In surfing, windsurfing, and kitesurfing, skegs, usually known as "fins", are attached toward the tail of the board to improve directional stability and control through foot-steering. Fins allow the rider to control the board's direction by varying their side-to-side weight distribution.