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, a capsize and possible sinking.
Types of sail
- A fore and aft sail is one that, when flat, runs fore and aft. These types of sails are the easiest to manage, because they often do not need to be relaid when the ship changes course.
- A gaff rigged sail is a fore-and-aft sail shaped like a truncated triangle the upper edge of which is made fast to a spar called a gaff. The top of the gaff rigged sail tends to twist away from the wind reducing its efficiency when close-hauled. However, due to the gaff on the top edge of the sail the center of effort is typically lower, somewhat reducing the angle of heel (leaning of the boat caused by wind force on the sails) compared to a similar sized Bermuda rigged sail.
- A square sail is set square to the mast from a yard, a spar running transversely in relation to the hull (athwartships). It is not, as commonly thought, named after the approximate shape of the sail, it is named for the square angle between the sail and the mast. In the olden days design of a square rigger, sailors would have to climb the rigging and walk out on "footropes" under the yard to furl and unfurl the sails. In a modern square rigged design the crew can furl and unfurl its sails by remote control from the deck. Some cruising craft with fore-and-aft sails will carry a small square sail with top and bottom yards that are easily rigged and hauled up from the deck (not requiring climbing the mast); such a sail is used as the only sail when running downwind under storm conditions, as the vessel becomes much easier to handle than under its usual sails, even if they are severely reefed (shortened).
- A lateen sail is a triangle with one or two sides attached to a wooden pole. This is one of the lowest drag (the sailing term is windage) sails, and it is not easy to manage.
- A Bermuda or Marconi sail is a triangular sail with one point going straight up.
- A staysail ("stays'l") is a piece of cloth that has one or two sides attached to a stay, that is, one of the ropes or wires that helps hold the mast in place. A staysail was classically attached to the stay with wooden or steel hoops. Sailors would test the hoops by climbing on them.
- A jib is a staysail that flies in front of the foremost vertical mast.
- A spanker is a gaff-sail flown on the mizzenmast of a tall ship. For ease of handling in large sail plans, these sails would sometimes be split into lower and upper spankers.
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