A sail-plan is a detailed blueprint of the configuration of sails and accessories on a boat or ship. It demonstrates several possible configurations for different conditions, including stormy seas and average sailing weather. Sail-plans can be kept on board a ship for reference in addition to being filed in an office with other documentation. Generic designs are available as well as custom versions carefully fitted to a specific boat.
There are a number of ways to organize sails, depending on how and where a ship will be used, primary concerns for the shipper, and the type of ship. A sail-plan features an adjustable configuration that will help a ship retain balance even in harsh seas as it moves at optimum speed through the water. The designer considers the design of the hull, the type of cargo carried, and other factors that might cause a ship to capsize or experience other problems while underway. If it is possible to compensate for these issues with the right rigging, the plan will show how to do this.
Each variation in the sail-plan for different conditions is shown separately, and may be depicted from several angles if the rigging is complicated. The sail-plan offers information about sail shape, size, and position, with a drawing done to scale to provide accuracy for the viewer. It may also discuss details of rigging, including spars, lines, and so forth in their suggested positions to show how the sails should be set up. This also provides information about how sailors can adjust the sails in poor weather or rapidly changing conditions where they may need to change the rigging to take advantage of the wind.
In the design of ships for mass production, the sail-plan is typically generic and intended for general use. Sailors can request custom plans if they plan to sail a ship in unusual conditions, or to use it in ways not immediately intended by the manufacturer. For custom boats, the sail design may also be customized, although it is possible to fit a sail-plan intended for boats of a similar size and type. The price of drafting the plan can add to the overall cost of the ship, as can the expense of high quality reproductions for reference and records.
Historic sail-plan information provides details about how people used ships, and what kinds of nautical technology existed in prior eras. Some advanced designs show a deep understanding of the physics of sailing, illustrating that early human cultures developed very advanced seafaring abilities to enable trade and migration. Thanks to the existence of ships on ancient vases and other works of art, it has also been possible to reverse engineer designs to create credible replicas of ancient ships and rigging in the modern era.