Although not always the most powerful sail on a boat, nor even the largest, the mainsail still earns its name. It is an airfoil, or wing, that has the three main properties of angle, shape, and twist. When properly trimmed—with telltales streaming—air flow is laminar, or attached to the canvas, producing lift. If poorly trimmed—with telltales limp or fluttering—airflow stalls or eddies, producing drag.
Visible belly in the mainsail is a combination of draft and twist. Draft position depends on luff tension set with the halyard and cunningham. Draft depth depends on foot tension set with the outhaul. Twist depends on leech tension set by the sheet and vang.
There are four or five ways to control mainsail trim that must be applied in a sequence that varies by point of sail and strength of the wind:
- The mainsheet sets the angle of the boom and, on many boats, also the leech tension.
- The traveler fine tunes the angle between the sail and the wind, changing the angle of attack without much effect on sail shape.
- The outhaul sets the foot tension, which in turn sets the position of the draft.
- The cunningham or the halyard sets the luff tension to move the draft forward or aft.
- The sheet and boom vang control leech tension and therefore twist.
- An adjustable backstay fine tunes the position of the draft.
Tension on the three edges of the sail is what sets the draft position and shape. To trim a mainsail in moderate wind of about 10 knots:
- Steer a steady course.
- Harden up the mainsheet until luffing just stops.
- Use the traveler to adjust the boom angle.
- Adjust twist using the sheet upwind or the vang off the wind.
- Use the outhaul to set the depth of the draft.
- Use the cunningham or halyard to set the position of the draft.
- In higher wind, apply backstay if it's adjustable.
- Constantly make small changes to balance the helm and reduce heeling.
The mainsail should be full, with plenty of draft, in light air or large waves. It should be flat in higher wind or when heading upwind while apparent wind is highest.
Common Mistakes in Mainsail Trim
- Oversheeting the mainsail: Pulling the mainsheet too tight stalls airflow, increases heel, and makes the boat slower. Ease slightly until the top telltale flows. (“When in doubt, let it out.”)
- Using the mainsheet for everything: Use the traveler to control boom position; the mainsheet mainly controls sail shape and power.
- Too much twist: If the top of the sail is too open, power and pointing ability drop.
- Upwind: adjust twist with the mainsheet.
- Offwind: adjust with the vang.
- Ignoring the outhaul: The outhaul controls depth in the lower sail.
- Tight for windy or upwind conditions (flat sail).
- Loose for light wind or downwind conditions (full sail).
- Not using mast bend/backstay: Tightening an adjustable backstay bends the mast, flattens the sail, reduces heeling, and lightens the helm. It is an important depowering tool.
- Ignoring luff wrinkles: Horizontal wrinkles indicate draft too far aft. Use halyard or cunningham tension to move draft forward.
- Ignoring the top telltale: The top telltale is a key trim indicator. When streaming, sail trim is good. When drooping, the sail is oversheeted and beginning to stall. When flickering, while beating upwind, the pointing angle is best.
- Misusing the vang: Offwind, the vang controls twist. Too little vang loses power, Too much over-flattens the sail.
- Set-it-and-forget-it trimming: The wind is always changing, so sail trim must be adjusted continuously.
- Thinking trim only matters to racers: Good trim helps cruisers too by reducing heel, weather helm, autopilot load, rig stress, and passage time.