UK Sailmakers' Encyclopedia of Sails

5.1 - Mainsail Trim

Mainsails must perform over a wide variety of sailing conditions. This required versatility is achieved by adjustment and trimming.

The mainsail can be adjusted to vary the amount and location of draft, and trimmed to control the shape of the leech and its angle to the wind. While bewildering verbiage is sometimes used to describe this process, we are doing only three things to the sail:

1. Adjusting the tension on the three edges.
2. Adjusting the shape of the leading edge if mast bend is possible.
3. Trimming the boom in and out.

Here’s a brief explanation of terms dealing with mainsails:

DRAFT: The amount of curvature in the sail. Sometimes called depth, draft is measured along a straight line running between the leech and the luff.
DRAFT LOCATION: The point where the draft is the greatest, measured along a straight line running between the leech and the luff.
LEECH SHAPE: The straightness or curve of the leech.

The mechanics of attaining proper mainsail characteristics vary according to class rules, rating rules, and personal preference. The basics of control are:

LUFF TENSION: Controlled by the halyard, Cunningham, and boom downhaul if the boat is equipped with one.
FOOT TENSION: Controlled by the outhaul and flattening reef.
LEECH TENSION: Controlled by the mainsheet and traveler upwind, and by the boom vang off the wind. The leech line is used primarily to remove flutter from the very edge of the sail.
MAST BEND: Controlled by various combinations of the backstay, babystay and running backstays. Blocks of wood or hard rubber can also be used to chock the mast where it goes through the deck to control bend.
TRIM: Controlled by the mainsheet and the traveler. Although closely interrelated, each control has a distinct effect on the mainsail’s characteristics. It is instructive and fun to work the controls and observe the effects.


Increased luff tension moves the draft forward.
Decreased luff tension moves the draft aft.


1. Increase halyard tension until the headboard reaches the upper black band.
2. Pull down the main boom downhaul until the lower black band is reached.
3. Put tension on the Cunningham.


Increased foot tension removes draft from the sail.
Decreased foot tension adds draft to the sail.


1. Tighten the outhaul. Note: The effects of foot tension are most pronounced in the lower third of the sail.


Increased leech tension straightens the leech and cups the sail.
Decreased leech tension eases the leech and twists the sail.


1. Trim mainsheet harder when sailing on the wind.
2. Tension boom vang when sailing off the wind.
3. Tighten leech line to control leech flutter.


1. Ease tension on mainsheet and boom vang. When beating in light winds, you’ll need to pull the traveler above the center line in order to trim the mainsail close enough while keeping the upper leech open.
2. Ease the leech cord.
3. In very light air, reduce the effect of the weight of the boom by tightening the topping lift.


Bending the mast decreases the draft in the sail, it flattens the sail. Removing mast bend adds draft to the sail.


1. Tighten backstay.
2. Tighten baby or midstay, or forward lowers.


1. Ease backstay and/or tighten headstay
2. Ease babystay and/or forward lowers.
3. Tighten running backstays and/or after lowers.

Before working on sail shape and trim, check these points:

1. Battens should be straight with the flexible end forward and the back end snug against the pocket. The most flexible batten should be in the top pocket.
2. Telltales should be installed on the leech near the top two battens. Additional telltales midway between the luff and the leech are also useful.
3. Check mast tune with the main and jib set. Some bend aft is desirable, while there should be no bend to the side.

Variations in wind velocity, wind direction, and sea state require the mainsail to be very adjustable. Experimenting will help you get the best results on your own boat, but these general principles should be kept in mind.

1. Sailing upwind requires a flatter sail than reaching and running.
2. Rough water requires a fuller setting than smooth water.
3. Light winds require a fuller setting than strong winds.