Language: jargon: sailing
Mainsail: Naut.
n.- The primary sail on the main mast.
- The lower course on any mast of a square-rigged ship. e.g. "Reef the fore mainsail!"
Salty pronunciation: mains'l
On a square-rigged ship, the mainsail is the lower sail on a yard on any mast, and called by its mast's name. e.g. Fore mainsail, mainsail, mizzen mainsail. Multi-mast rigs will usually have one mast called the main, usually the tallest, which carries the ship's mainsail.
On a single-masted rig, the largest sail is usually obvious, and simply called the main since the mast doesn't need to be named. On a fore-and-aft rig, the main might be hung from a yard (lug rig, lateen rig) overlapping the mast, or spread aft of the mast using a boom (bermudan, wishbone), a combination of boom and another spar (gaff, boomed sprit), a separate spar (sprit), or other specialty spars (crabclaw rig, batwing sail).
The mainsail is, generally, the largest and/or most powerful sail in most rigs. Even if the genoa is actually larger in sail area, it might not be as powerful due to the slot effect when working upwind. Because the mainsail is so powerful, much of the working of the rig revolves around it. Although it is usually the last sail struck, the main is often the first to be reefed.
The mainsail may be equipped with reefs, or might be furling, or could even have panels which can be removed called bonnets or drabblers. The purpose is to reduce the sail area in a blow, which helps the boat resist heeling, slow down, and even de-stress the rigging.
References:- Marino, Emiliano; The Sailmaker's Apprentice: A guide for the self-reliant sailor; International Marine; © 1994 International Marine/Ragged Mountan Press; ISBN 0-07-157980-X