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Extension of a boat's keel at the back, also a surfboard's fin

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.

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. Fixed fins were introduced to surfboards by surfing pioneer Tom Blake in 1935. Around 1936, Woody Brown independently added a fixed fin to his second surfboard design, which further popularized the feature. The stability and control it allowed revolutionized the sport. Small single aluminum fins first evolved into larger wooden versions, then ones made from fiberglass and carbon fiber. In time, hydrodynamic improvements took place, pioneered by George Downing, who also created the first removable skeg, which was a teak wood skeg in a teak wood box which was supposed to be held in place by the swelling of the wood in water. In modern surfing board design, the conventional set up is to have three fins, with single fins being a minority. While most windsurfing boards are single fin, wave boards now feature some twin fin, tri fin and quad fin designs. Directional kitesurfing boards are usually three fin, with five fin designs being used for improved upwind performance.

A skeg is employed in the type of kayak used on more open water such as the sea. Its purpose and use are rather different from those of the surfing skeg. In the kayak, the amount of exposure of the skeg to the water, and also its effect on the position of the boat's centre of lateral resistance (CLR), is freely adjustable by the crew. The adjustment varies the degree to which the wind affects the boat – that is, the amount of lateral movement the wind can cause by impacting the upper parts of the boat and the crew. If properly configured (e. g., use of street sign aluminum in a narrow box that extends through the hull) they will not flex, and will greatly decrease and counter pitch, roll and yaw, like a centerboard on a sailboat, when the craft is moving. In that sense, the skeg acts as a lifting foil.

Skegs have been used to improve the directional stability of seaplanes. They have been installed on floats and hulls.

The skis on a snowmobile have a metal wear bar on the bottom of them. Many sledders call these "skegs". These skegs help the skis to steer on hard surfaces. These often have carbide embedded in them to reduce wear when driven on non snow surfaces.