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How To Trim a Mainsail
The objective in close-hauled mainsail trim is to achieve the optimum mix of speed and pointing. We adjust mainsail controls to match our trim to the prevailing conditions and control the overall power and balance of the boat. As with any sail, there are three sources of mainsail power: angle of attack, depth, and twist.
We want to be at full power – neither overpowered nor underpowered; and we want the optimum mix of power.
Angle of attack is the angle of the apparent wind to the sail. Angle of Attack is the first adjustment to make. At zero angle of attack, sails are luffing. As we trim in sails, or bear away from the wind, angle of attack and power increase.
For close-hauled trim in light to moderate winds we trim the main to put the boom on the centerline. When we are overpowered—indicated by too much heel and weather helm—we can reduce power by easing the boom down below the centerline, or by heading up (feathering up) slightly.
Mainsail depth is then set to match the conditions. Depth is the amount of curve in the sail from luff to leech, described as a percentage of the overall distance. A deep sail is sometimes referred to as a full sail, whereas a flat sail is, well, a flat sail. In addition to controlling the amount of depth, we can also control the position of the deepest part. A deep sail is generally more powerful than a flat one.
Initially we trim the main to its designed shape by setting the halyard just tight enough to take out almost all the wrinkles up the luff, and the outhaul just firm enough to take wrinkles out of the foot. From there, we can add depth if the boat needs more punch (for light air or chop), or flatten the sail to reduce power in heavy air.
Twist is the third component of power; it’s the way we describe the change in angle of attack from the bottom to the top of the sail. A sail with lots of twist is open at the upper leech, spilling power aloft. A sail with little twist has a closed upper leech—the upper leech is nearly parallel to the lower leech. Talking about twist can be a little confusing, since reducing twist increases power and pointing ability.
Controlling sail power with mainsail controls
The mainsail has a wide variety of controls. The most important is (surprise!) the mainsheet. Other controls include the backstay, outhaul, traveler, halyard/cunningham, and boom vang. If your boat is missing some of these, then simply use the controls you do have.
The mainsheet impacts every component of mainsail power. Initially, as we trim the mainsheet from a reach to close-hauled, the primary impact is on angle of attack. Once the boom gets close to the centerline, the primary impact of sheet trim is on twist: as the sheet is trimmed in, the leech is tensioned, so twist is reduced. The mainsheet also impacts sail depth, but to a lesser degree.
For best upwind performance, first trim the mainsheet so the upper leech or upper batten of the main is parallel to the boom. The leech telltales will be flowing, with only an occasional stall. From there, experiment—try extra trim for extra pointing, which will stall the upper telltales more than half the time. Then ease a little, and see if you can add a boost of speed without any loss of pointing.
Changing twist with the mainsheet will impact both pointing ability and helm balance. To point higher, reduce twist by sheeting harder. If the boat is slow, overpowered, or difficult to steer, try easing the sheet to add twist.
As we move on to secondary controls, remember to always recheck mainsheet trim after any other adjustment.
After the mainsheet, an adjustable backstay is the second most powerful controller of mainsail shape. Tightening the backstay adds mast bend, which reduces mainsail power. The biggest impact is in the middle and upper portions of the sail.
Adding mast bend flattens the mainsail by increasing the distance from luff to leech, which reduces both power and drag. It also adds twist by shortening the distance from head to clew. When the boat is fully powered, adding mast bend can increase speed by reducing drag—but only when fully powered. When overpowered, adding mast bend increases speed by reducing heel, weather helm, and the drag associated with them.
The outhaul controls depth in the lower portion of the mainsail. Pull the outhaul tighter as the wind builds, and ease it off for extra power in light air or chop. Don’t get carried away—ease just enough to round the foot of the sail.
The traveler positions the boom, changing the angle of attack. Adjust the traveler to keep the boom nearly centered, unless you are overpowered; then ease the traveler down to reduce power and relieve weather helm. Playing the traveler is particularly effective in puffy conditions. Once the overall sail shape is set for the prevailing conditions, play the traveler to quickly dump power in gusts.
If your boat lacks a traveler or has one that’s hard to adjust, ease the mainsheet to reduce power.
Halyard and cunningham
The halyard and cunningham control luff tension and, through it, draft position – that is, the position of the deepest part of the sail. To hold the draft in its designed position (just forward of the middle of the sail), tighten the halyard or cunningham as the wind builds. Use the halyard first, and when at full hoist, use the cunningham.
When you add mast bend the draft will move aft, so add luff tension; ease luff tension as the mast is straightened.
The boom vang is primarily an offwind control. Upwind, take the slack out to help control twist. In light air, be careful to keep the vang eased.
Steering is another way to control mainsail power. Heading up reduces power, while falling off widens the angle of attack, and adds power.
The helm also tells us if we are out of trim. While a slight weather helm is desirable, too much weather helm tells us we ought to reduce power.
With so many controls (and the jib to trim as well), optimizing trim and performance is a complex endeavor. Once you get the sail to its designed shape, you’ll need to experiment to find what shape is best for the prevailing conditions. As you make changes in trim, pay attention to boat speed, pointing, angle of heel and weather helm; the boat will tell you when you’ve found the best balance of power for the conditions.