|Advanced Mainsail Trim|
They don’t call it your “main” sail for nothing. This is the sail you use all the time; upwind or downwind, light air or heavy air, it has to do it all. And to get the most out of it you have to keep it trimmed at its optimum all the time.
Upwind it does three things. It adds to the overall power your sailplan generates, it has a huge effect on pointing, and it helps balance the boat for different conditions. The added area and power to your sailplan is obvious so let’s take a closer look at the other two. Upwind it is all about pointing. You are limited by how tight you can pull the headsail in by the rig so it is fairly easy to get it trimmed correctly; move the lead fore and aft until the genoa is set so the leech is a few inched of the upper spreader and about the same from being pulled tight against the chainplates. Those adjustments will give you all the pointing you can get out of it. After that it is all about the mainsail and in particular the mainsail leech. Start by having the mainsheet tight enough so that the telltale on the top batten is stalled out at least part of the time. It will be stalled more in under 7 knots of wind, stalled some in medium air, and not flowing all the time until you are in enough wind to be over powered.
Next pull the traveler far enough to weather so that the boom is up right to the centerline of the boat. If you need more pointing you can even pull the boom a few inches to weather of the centerline. On boats with overlapping genoas this is very important and most boats sail at their best in winds between 6 and 12 knots with the boom in this position. To keep the top telltale stalled some of the time requires a LOT of mainsheet tension. Sailing with other people, and watching other boats I sail against, I can tell you most people don’t trim their main tight enough a lot of the time. In general the best rule is “if you’re not pointing as well as you should, tighten the mainsheet a little more”.
As you start to get overpowered, over 11 knots or so, the boat will start to generate more and more weather helm. This is caused by two factors; first the side of you boat is very curved, as you heel over more you put more and more of that curved side in the water and the boat becomes quite asymmetric. The large curve on the leeward side acts like a rudder and want to turn the boat. Second is that as you heel over more the mast is moving further to leeward of the boat. The force generated by the sail plan is pushing the sailplan and the boat forward. When it is centered straight up above the boat the boat goes fairly straight, the more off to the side it gets the more the forward force on the rig is trying to make the boat pivot to weather. Either way the reason you get more weather helm as the wind increases is because the boat is heeled over more. The mainsail is your first weapon in counteracting this. When you are in the upper range of your headsail and are overpowered you decrease the power and the heeling by reducing the force on the mainsail. First you make the sail flatter by tightening the outhaul to reduce camber in the sail, tightening the cunningham or downhaul to keep the draft forward in the sail, and tighten the backstay to further flatten the main and to also increase headstay tension to help keep the genoa from getting too full. To start with the most efficient way to depower the main is to play the traveler, drop it down in the puffs but make sure to pull it right back up in the lulls. You will find that even in 12 to 15 knots of breeze there is a lot of the time when you can still have the boom pulled right up to the centerline.
As the wind increase you should be sailing with the traveler to leeward of the centerline more of the time but it is very important in this range to keep the mainsheet on very tight to keep the leech tight so that you don’t lose pointing. Only when you reach the wind range where you can’t drop the traveler far enough to unload the boat do you start easing the mainsheet. This is where having another person to trim the main becomes really important. The traveler should be cleated so it is a foot or two to leeward of the centerline, just far enough down so that in the lulls you can sail with the mainsheet tight. From here on you will need to be playing the sheet and not the traveler. It is a lot of work and needs to be played continually so you main trimmer better be very strong and be really on top of what you need to keep the boat balanced. In this condition you want the vang set fairly tight so that when you ease the sheet the leech will only twist open a slight bit more.
Reaching you want to make sure the main is never luffing but in general it should be trimmed in even a little tighter than that. Look at the angle between the tack and the clew of the headsail and trim the main so that the boom is almost at that same angle. You should trim it in just past where it stops luffing and should feel more pressure on the sheet and in the boat. Keep the vang tight enough that the top telltale stalls out once in a while. The main since the main leech adds a lot to the weather helm as you get into conditions where you are over powered ease the vang off more and more so that the top of the main twists well open and decrease the helm pressure.