mule

A mule is a small triangular or trapezoidal sail set on a yacht, typically rigged fore-and-aft between the mast and a boom. It occupies the space that would otherwise be filled by a conventional mainsail or jib, and serves as an auxiliary or experimental sail rather than a primary driving sail. The mule is relatively uncommon in modern racing and cruising, but appears in certain racing classes and in older yacht designs where light-air performance or specific trim conditions warrant its use.

The sail is distinguished by its geometry and rigging arrangement. Rather than being hoisted along the full height of the mast as a mainsail would be, the mule typically extends from a point partway up the mast down to a boom, creating a compact footprint. This configuration allows it to fill specific aerodynamic roles, such as providing drive in very light winds where a conventional sail plan would be too heavy or poorly shaped, or to balance the yacht's center of effort during certain points of sail.

The mule appears primarily in classical yacht designs and in certain one-design classes where sail plans are tightly specified. It may be rigged as a standalone sail or combined with other sails in the inventory. Some racing rules and class specifications explicitly permit or exclude the mule, reflecting its marginal role in modern fleet racing. Its practical advantage lies in delivering shape efficiency at low apparent wind speeds, where a larger, heavier sail would lose aerodynamic advantage.

The term's origin is unclear but may reference the mule's function as a hybrid or utility sail, or its stubborn behavior in certain conditions. Unlike its agricultural namesake, the sailing mule has never achieved widespread adoption, and many modern yacht crews regard the sail as a historical curiosity rather than a working part of the rig. Modern racing and cruising sails have largely superseded the mule through better material science and loft design.