Fractional rig

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A fractional rig on a sailing vessel consists of a foresail, such as a jib or genoa sail, that does not reach all the way to the top of the mast.

The forestay is a wire that secures the mast to the front of the boat. With a fractional rig, the forestay is attached between about 1/8 and 1/4 of the length of the mast lower down, rather than being attached to the top of the mast as in a masthead rig. The foresail (jib or genoa) is then rigged to this stay. The mast is farther forward on the boat than on a masthead rig and so it has a larger mainsail. Masthead rigs are most common on larger keelboats or cruisers. A fractional rig is typically used on sailing dinghies and racing oriented keelboats, such as the J/24.

Fractional rigs were introduced on race boats in order to allow more controllability of the surface of the mainsail and also less drag when sailing upwind. According to one manufacturer, "a key to making fast boats easier to sail than slow boats is the 'fractional rig'".^[1]

Fractional rigs tend to have a larger mainsail and a smaller headsail which is often a non-overlapping jib. This configuration is optimized for upwind sailing efficiency. For downwind sailing without a spinnaker, the larger mainsail of a fractional rig has a significant advantage over a masthead rig but a disadvantage when spinnakers are flown. Many newer high-performance fractional rig sailboats, fly the spinnaker or the gennaker from the masthead. 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.

Another 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 inducing excessive weather helm which could cause the boat to lose control and “round up” into the wind. With the masthead sloop, both the main and the jib must be reduced in size evenly when shortening sail. This is typically accomplished by reefing the main and either rolling up a portion of the headsail on a furler, or changing to a smaller headsail.

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