An outhaul is a line which is part of the running rigging of a sailboat, used to extend a sail and control the shape of the curve of the foot of the sail. It runs from the clew (the back corner of the sail) to the end of the boom. The line is pulled taut to the appropriate tension to provide the desired shape to the foot and then secured to a cleat on the boom.
The details vary, but the two most common methods are:
- A knot, usually a bowline, is tied to a grommet provided for the purpose in the clew of the sail, then fed directly to the cleat.
- The line is attached to the boom, lead through the same grommet, and thence to a cleat; this system provides a factor of two mechanical advantage over the previous one.
The outhaul, besides simply holding the sail out, is an effective sail shape control. Tightening or slackening the outhaul can flatten or fill out the sail, shift the draft forward or aft, change leech and foot tension, and increase or decrease camber.
The outhaul pulls on the clew at an angle, exerting force to tighten both the foot and the leech of the sail, and when the foot of the sail is not attached to the boom with a bolt rope, the outhaul may also be the only thing keeping the foot of the sail down on the end of the boom. When the sail is reefed, the reefing line becomes, in effect, the outhaul, and the reefing cringle becomes the new clew.