The mainsail is the same as any other sail and that is to maintain the correct position of the draft with the correct angle to the wind. The mainsail is more controllable than a genoa sail since the mast can bend the boom has an outhaul, boom vang, and a traveler.
Mainsail sheet tension affects every characteristic of the sail. More than any other control, sheet tension will change significantly with changes in wind speed.
On a reach, ease the sail until it begins to luff and you see backwind along its leading edge. You should trim just enough to stop luffing. Ease in every puff for perfect trim on a reach. If the boat is overpowered and heeling too much in a puff, the sheet should be eased, allowing the sail to luff.
Upwind, as the sail nears the centerline of the boat, the mainsheet begins to pull down, affecting the twist. How far the mainsheet can be trimmed to help the boat sail upwind effectively is a function of wind velocity and boat speed. In more wind, the mainsheet can be trimmed tighter without causing a speed loss and pointing will improve. With less breeze, be careful not to over-trim or the boat will not accelerate. In light air you will need a more open leech. The sheet will be eased from the median setting so that the top batten points about 10 degrees to leeward.
The mainsail will need to be eased further than you think. Don’t be afraid to let the sail out until it is against the shrouds. Make sure the boom vang is tight enough.
The traveler has two functions: It controls the boom angle to the wind and drives the boat. Use the traveler to position the boom to the centerline for maximum power and pointing. This means that in the light wind when the mainsheet is well eased to support acceleration, the traveler car will need to be on the centerline. As the breeze increases and the mainsheet tension increases, the traveler will progressively drop to keep the boom at the center line and de-power the boat
The traveler is great for fine trimming the balance of the boat as velocity increases. When cruising, find the best medium that provides a comfort level and keeps the boat from heeling too far.
Luft Tension – Halyard & Cunningham
Controlled by the halyard, boom downhaul, and Cunningham. Increased luff tension when upwind in the moderate and heavy wind so the draft moves forward, and vice versa. Tension cunningham or halyard so that just a hint of wrinkles appear in the lower third of sail.
The deeper the curvature in the sail shape, means more power. Controlled by the outhaul. Foot tension has the effect of flattening (de-powering) the lower half of the mainsail. When the boat is not heeling too much in light wind, you need to loosen up foot tension, extra depth is desirable.
Mast bend is created by backstay or running backstay tension on a masthead boat. When the backstay or running backstay are tightened, the force pushing the top of the mast towards the deck. The middle of the mast forward pulls the luff from the leech and flattening the sail.
Mast bend controls the depth in the upper of the sail. When you increase mast bend the flat mainsail will create less heel and allowing more control. Flatter mainsail shapes work best when you trying to sail upwind.
The boom vang serves to pull down on the mainsail clew and controlling twist. The twist is the change in the angle of attack from the bottom to the top of the sail. In light winds, you need to use the extra twist, pull the traveler above the centerline in order to trim the mainsail close enough while keeping the upper leech open and ease tension on the mainsheet and boom vang. In moderate winds, leech tension can be increased and twist reduced.
Just as you would with the mainsheet upwind, use enough tension on the boom vang to keep the top batten parallel to the boom. The picture shows how to twist the mainsail where the boom vang too eased.