The Writer’s Guide to Sails
The sails are the engine of every sailing vessel, since they are the primary source of propulsion. Humans have been catching the wind with sails for over 5,000 years. Although there have been advancements in materials and complexity of rigging, the concept remains the same.
I covered the basics of sails in my The Writer’s Guide to the Technology of Ships. However, today I am going into more detail.
As always, magic is the exception to the rule. Because magic.
There are two types of rigs. Square rigs are perpendicular to the keel of the vessel and are usually suspended from a yard attached to a mast. This type of sail is the most efficient at harnessing wind coming from behind the ship, and a vessel that mainly use this type of rig is known as a square-rigger. 
A ship with sails parallel to the keel is using a fore-and-aft rig. Vessels with this type of rig are described as fore-and-aft rigged. 
There are four principle places where sails can be attached to a vessel.
Stay – A stay is a piece of standing rigging that runs along the center of a ship. It helps to stabilize a mast.  They run fore and aft in direction.
Mast – Sails with one edge against a mast are triangular or gaff-rigged. Both have booms at the bottom of the sail, which can swing from side to side to angle the sail to catch the wind. With triangular sails, the top of the sheet is hoisted up to the top of the mast. A gaff-rigged sail has a gaff at the top. A gaff is a smaller boom, usually of wood, with a ring, known as a throat, through which the mast runs. The throat slides up and down the mast when raised and lowered. The other side of the gaff is called the peak and can be raised semi-independent of the throat.
Spar – A spar is a horizontal piece attached to a mast. Suspending sails from a spar is the most common method used throughout history.
Halyard – A halyard is a line used to hoist a sail. Spinnakers are the most common type of sail suspended primarily from a halyard.
Sails can come in a variety of shapes, including square, triangular, and quadrilateral. I covered the main sail shapes in my The Writer’s Guide to the Technology of Ships.
A variety of materials have been used to make sails throughout history. The first sails were probably animal skins or woven reed mats. However, cloth sails appeared in Egyptian art dating back to 3,300 BC. Early sailcloth was woven from flax, which was later replaced by cotton.  Most modern sailboats used laminated poly fibers.
Sails made before the modern period were constructed of multiple panels since it was impossible to weave fabric big enough to create a sail of a single piece. The panel could be horizontal or vertical. Modern sails used radial panels to better stand up to the stress. 
As I mentioned in my The Writer’s Guide: Introduction to Sailing Ships, running rigging is used to raise, lower, and adjust the sails to best catch the wind. This is known as trimming. When a sail is gathered against a boom or spar, it is known as furling. Sails are furled when they are not in use or when the weather is bad, and the winds are unpredictable.
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Copyright © 2022 Rebecca Shedd. All rights reserved.
 Keegan, John (1989). The Price of Admiralty. New York: Viking. p. 280. ISBN 0-670-81416-4.  Knight, Austin Melvin (1910). Modern seamanship. New York: D. Van Nostrand. pp. 507–532.  Keegan, John (1989). The Price of Admiralty. New York: Viking. p. 280. ISBN 0-670-81416-4.  https://www.britannica.com/technology/sail-watercraft-part  Hancock, Brian; Knox-Johnson, Robin (2003). Maximum Sail Power: The Complete Guide to Sails, Sail Technology, and Performance. Nomad Press. pp. 288. ISBN 978-1-61930-427-7. sail panel cut.