What is a Sailboat Mainsail?

What is a Sailboat Mainsail? | Life of Sailing

Last Updated by

Daniel Wade

June 15, 2022

A sailboat mainsail is a primary sail used for propulsion. The mainsail is also usually the largest sail on the boat.

The mainsail on a typical sailboat is located on the aft section of the mainmast. This means it 'points' towards the stern of the vessel. The mainsail is usually the largest in terms of sail area, but sometimes other sails (like the spinnaker) are larger.


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What does a Sailboat Mainsail Do?

The mainsail catches the wind and propels the boat forward. In essence, it captures the force of the wind and transfers it to the boat. The keel (or centerboard), rudder, and mainsheet control how the mainsail moves the boat.

What does a Sailboat Mainsail Look Like?

The mainsail is a tall piece of fabric with at least three sides. Mainsails are generally white and located 'behind' the mast and towards the cockpit of the boat. When stowed, the mainsail is often wrapped around the boom and covered by a protective cloth case.

Sailboat Mainsail Types

When most people think of a mainsail, they picture a tall triangular white canvas with a right angle. This representation is accurate to most vessels, but not all mainsails are shaped like triangles.

On a typical cruising sloop, you'll find one of two kinds of mainsails. The most common type of mainsail is tall, triangular, and somewhat narrow lengthwise. The triangular shape belongs to the Bermuda rig, which is a popular and stout sail plan.

Some vessels have a four-sided mainsail that increases in height towards the end of the canvas. This is called a Gaff rig, and it's especially common on older and traditional vessels. Due to its greater area-to-height ratio, the mainsail on a Gaff-rigged vessel can use a shorter mast.

Both Bermuda and Gaff boats have a boom at the base of the sail. However, Gaff-rigged sailboats also utilize a smaller spar at the top of the mast, effectively 'enclosing' three of the four sides of the mainsail.

Parts of a Sailboat Mainsail

Complete sailboat mainsails are a single piece of fabric, but they're made up of many individual parts. Mainsail parts and nomenclature varies between rigs. Here are the primary parts of a common Bermuda-rigged sailboat mainsail, listed from boom to head.


The mainsail tack on a boat is the bottom corner of the sail. The tack is located at the base of the only right angle, which lies above the point where the boom and mast intersect.


The foot of a mainsail runs lengthwise along the bottom of the canvas. The boom lies directly underneath the foot of the sail and extends just a touch further than the foot.


The clew is located opposite of the tack on the other end of the sail. Like the tack, the clew resides on the bottom of the sail and directly above the boom.


The luff is the forward vertical section of the sail that stretches from the clew to the head (top). The luff runs along the mast and sometimes feeds into the mast via sliding guides.


Battens are reinforced canvas straps that run lengthwise across the sail at regular intervals. You'll notice these cross-sections immediately, especially when the sun is behind the sail. Battens help the mainsail retain its shape and strengthen it.

Reefing points

Reefing points are located between battens. The mainsail usually has several sets of reefing points, which consist of a grommet and a small piece of reinforcing material. Reefing points allow you to shorten the sail and tie the excess to the boom. Reefing is useful during high winds when less sail area is necessary.

Tell Tales

Telltales, which are often located near reefing points, are small, lightweight strips of material that help you determine which direction the wind is blowing. Telltales are distributed up and down the sail, and there's usually a few on each side.


The leech is the long side of the mainsail opposite to the luff. The leech, which is located on the after part of the canvas, is the longest side of the sail. This is true on Gaff and Bermuda-rigged vessels.


The head is the top corner of the mainsail. This is the point where the main halyard attaches to the sail and hoists it up to the top of the mast. The head of the sail is the highest point of the luff and the smallest angle on a Bermuda mainsail.

Gaff Mainsail Parts and Differences

The mainsail on a gaff-rigged vessel has many of the same parts as the triangular Bermuda mainsail, but there are some slight differences. Unlike the triangular sail, a Gaff mainsail has an additional side that connects to a spar.

The fourth side of a Gaff mainsail is located on the top of the sail. The leech of a Gaff mainsail extends far above the luff and beyond the head. The tall side of the sail is called the "peak," and it's located directly across from and above the head. The side of the sail between the head and the peak is called the "gaff." The spar runs along the gaff and acts as a second boom.

Mainsail Materials

Sailcloth technology has come a long way, especially in the last century or so. Originally, sailboat mainsails were made of an organic cloth such as canvas. Non-synthetic sailcloth is rare but still available, and it's often made of a mixture of hemp, cotton, and other materials.

Synthetic sailcloth is much more common on modern boats. Most modern mainsails are made of a synthetic material called Dacron. This polyester-based material is strong, lightweight, and long-lasting.

How to Hoist a Mainsail

Hoisting the mainsail brings it to the top of the mast. When the mainsail is up, the boat can catch the wind and begin to move. Before hoisting the mainsail, make sure the mainsail is connected to the boom and the mast at all the proper points, including the sliding sail slugs on the mast. Here are the steps to hoisting the mainsail.

Attach the Head Shackle

Once the sail is hanging on the mast and the boom, attach the head shackle to the grommet (clew) at the top corner of the sail.

Loosen the Mainsheet

As a precaution, loosen the mainsheet so the boom can swing freely. This prevents damage or unexpected movement when the mainsail catches the wind.

Find (and Hoist) the Main Halyard

The main halyard is connected to the head shackle. Take this line and pull it down, or attach it to a winch if it's too difficult. Continue pulling or winching the main halyard until the luff of the mainsail is tight.

Tie Off the Main Halyard

Find a cleat and tie off the end of the main halyard. This will prevent the sail from loosening while underway. Be sure to organize the leftover halyard slack, as you don't want it getting tangled with any other part of the rigging.

Reefing the Mainsail

Once underway, you're likely to encounter a situation where you have too much sail up. When the wind picks up, you'll need to reef (or shorten) the sail and reduce its area. Here are the steps required to reef the mainsail.

Find the Correct Direction

Maneuver the vessel to a close-hauled or close-reaching position. That means the bow of the boat should be roughly 30-45 degrees to the wind.

Loosen the Main Halyard

Loosen the main halyard and free up some slack to work with. Also, release the mainsheet vang.

Reef the Sail

With the main halyard released, pull the mainsail down to the desired reef point. Tighten the reef tack and secure the excess sail to the boom.

Hoist the Sail

Hoist the sail again and secure the main halyard as it was before. Make sure the luff of the sail is tight. Now you're set to resume your course and trim the mainsail as necessary.

What is a Sailboat Mainsail?
Daniel Wade

Daniel Wade

I've personally had thousands of questions about sailing and sailboats over the years. As I learn and experience sailing, and the community, I share the answers that work and make sense to me, here on Life of Sailing.

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