A fractional rig on a sailing vessel consists of a foresail

Foresail

A foresail is one of a few different types of sail set on the foremost mast of a sailing vessel:* A fore and aft sail set on the foremast of a schooner or similar vessel....

, such as a jib

Jib

A jib is a triangular staysail set ahead of the foremast of a sailing vessel. Its tack is fixed to the bowsprit, to the bow, or to the deck between the bowsprit and the foremost mast...

 or genoa sail

Genoa (sail)

The genoa or jenny was originally referred to as the 'overlapping jib' or the Genoa jib, being named after the city of Genoa as explained below. It is a type of large jib or staysail used on bermuda rigged craft that overlaps the main sail, sometimes eliminating it. It is used on single-masted...

, that does not reach all the way to the top of the mast.

In the picture to the right, the forestay that secures the mast at the front of the boat is attached to the mast at a lower point and the fore sail (jib or genoa) is rigged to this stay. The mast is farther forward on the boat than on a masthead rig

Masthead rig

A masthead rig on a sailing vessel consists of a forestay and backstay both attached at the top of the mast.The Bermuda rig can be split into two groups: the masthead rig and the fractional rig. The masthead rig has larger and more headsails, and a smaller mainsail, compared to the fractional...

 and so it has a larger main sail. Masthead rigs are most common on larger keel boats or cruisers.

A fractional rig is typically used on dinghy sailing boats and racing oriented keel boats, such as the J/24

J/24

The J/24 is an International One-Design keelboat class as defined by the International Sailing Federation. The J/24 is a one design class created to fulfill the diverse needs of recreational sailors such as cruising, one design racing, day sailing and handicap racing.The J/24 is the world's most...

.

Fractional rigs were introduced on race boats in order to allow more controllability of the surface of the main sail and also less drag during tacking. According to one manufacturer, "a key to making fast boats easier to sail than slow boats is the fractional rig".

Usually a fractional rig features a larger main sail and a smaller non-overlapping jib. This configuration is optimized for up-wind sailing efficiency. For downwind sailing, a larger jib is more desirable but in the case of many high-performance fractional rig sailboats, the smaller jib is substituted by a spinnaker

Spinnaker

A spinnaker is a special type of sail that is designed specifically for sailing off the wind from a reaching course to a downwind, i.e. with the wind 90°–180° off the bow. The spinnaker fills with wind and balloons out in front of the boat when it is deployed, called flying. It is constructed of...

 or gennaker

Gennaker

A gennaker is a sail that was developed around 1990. Used when sailing downwind, it is a cross between a genoa and a spinnaker. It is asymmetric like a genoa, but the gennaker is not attached to the forestay like a jib or genoa. The gennaker is rigged like a spinnaker but the tack is fastened to...

. On many modern skiffs and race dinghies, the jib is relatively small compared to the size of the main and it is normally left in place when the spinnaker or gennaker is used due to this type of jib's minimal aerodynamic interference.

A major advantage of the fractional rig, especially the '3/4' rig (where the fore stay goes 3/4 the way up the mast), is that the jib can be taken in without the boat rounding up into the wind like a weather vane. With the masthead sloop, both the main and the jib must be reefed, when shortening sail, to keep this from happening.

Since jibs need considerable fore stay tension to set properly and the amount of tension needed increases with wind speed, being able to drop the jib first in windy conditions puts less strain on the rig and hull. Fractional rigs are used on lightly built multi-hulls for this reason.