A sail plan is a drawing of a sailing craft, viewed from the side, depicting its sails, the spars that carry them and some of the rigging that supports the rig. By extension, "sail plan" describes the arrangement of sails on a craft. A sailing craft may be waterborne (a ship or boat), an iceboat, or a sail-powered land vehicle.
Depending on the level of detail, a sail plan can be a visual inventory of the suit of sails that a sailing craft has, or it may be part of a construction drawing. The sail plan may provide the basis for calculating the center of effort on a sailing craft, necessary to compare with the center of resistance from the hull in the water or the wheels or runners on hard surfaces. Such a calculation involves the area of each sail and its geometric center, referenced from a specific point.
Considerations for a sail inventory in a yacht include the type of sailing (cruising, racing, passage-making, etc.) and the weather conditions anticipated. An assessment starts with a sail plan that depicts each kind of sail under consideration. The sail plan becomes a guide for which sails to use under the anticipated weather conditions, while under way. Sail names encompass fore-and-aft rigs, square rigs, and rigs that encompass both types.
A cutter-rigged yacht, intended for off-shore sailing might have a sail inventory that includes: a mainsail, a roller furling genoa, and a working staysail for most wind conditions, and, for strong winds, a storm staysail and trysail. Sails for lighter winds would include a spinnaker, a drifter, and a mainsail with lighter sail cloth.
Each sail has a separate set of considerations within the plan, for example with a performance sloop one may consider the following about its suit of sails:
A square-rigged sailing vessel carries both fore-and-aft sails, the jibs, staysails and mizzen sail, and square sails. Their naming conventions are:
In the late 19th and early 20th centuries, choosing a sail plan for a displacement watercraft stemmed from the size and tonnage of the vessel, its purpose (working vessel, cargo vessel or yacht) and the anticipated winds in the region where it was expected to sail. In that period, sail plans might start from smallest to largest boat or ship in a hierarchy of sailing rigs:
Working boats and coastal freighters
Ocean-going merchant vessels
The following sail plans are at various scales.