Sail-plan

A sail-plan is a set of drawings, usually prepared by a naval architect. It shows the various combinations of sail proposed for a sailing ship.

The combinations shown in a sail-plan almost always include three configurations:

• A light air sail plan. Over most of the Earth, most of the time, the wind force is Force 1 or less. Thus a sail plan should include a set of huge, lightweight sails that will keep the ship underway in light breezes.
• A working sail plan. This is the set of sails that are changed rapidly in variable conditions. They are much stronger than the light air sails, but still lightweight. An economical sail in this set will include several sets of reefing ties, so the area of the sail can be reduced in a stronger wind.
• A storm sail plan. This is the set of very small, very rugged sails flown in a gale, to keep the vessel under way and in control.

In all sail plans, the architect attempts to balance the force of the sails against the drag of the underwater keel in such a way that the vessel naturally points into the wind. In this way, if control is lost, the vessel will avoid broaching (turning edge-to-the wind), and being beaten by breaking waves. Broaching always causes uncomfortable motion, and in a storm, the breaking waves can destroy a lightly built boat. The architect also tries to balance the wind force on each sail plan against a range of loads and ballast. The calculation assures that the sail will not knock the vessel sideways with its mast in the water, capsizing and perhaps sinking it.