Lidgard Sails // News

28 March 2012

Trimming the Mainsail with Rod Davis. Part#2

When you bend the mast to flatten the sail, though the effective height of the mast becomes shorter so you have to re-adjust the Cunningham to get to the original luff tension and draft placement. Normally the Cunningham should be pulled on just hard enough to take any horizontal wrinkles or sag out of the luff. It is far better to sail with too little mainsail luff tension than too much. Off the wind you should always ease the Cunningham off.

The leech line is a small line that runs inside the leech fold from the top of the sail to the clew where it exits out of the sail. It is used to control any flutter of the leech between the battens.

The mainsheet is used to control the amount of twist in the leech of the sail. Twist is the amount the sail is turned to leeward at the top. If you sight up under the boom comparing the boom to the top batten you will note the difference in the angle of the two, this is called twist and is measured in degrees. In light or medium wind and smooth water the batten and boom should be parallel or zero degrees twist. In windy conditions or rough water three or four degrees of twist is preferable. Leech tell tails are helpful in deciding how much sheet tension to use. These tell tails are streamers off the leech at each batten and indicate the flow at the back end of the sail. In an untwisted sail the top one or even two should be stalled 50% of the time. Tell tails are stalled when they curl back around the back of the sail. In a twisted condition they will be flowing out straight back from the leech.

The traveller controls the athwartships position of the boom and thus the mainsail. A good traveller can be pulled to weather in light air so the boom can be on centre-line with a minimum of sheet tension. When the sheet is pulled in or when the weather helm becomes too much, the traveller should be let down to leeward. This will help to keep your boat on itâs feet and efficient. Effectively the traveller allows the main to keep the air foil shape while changing the angle of attack.

The boom vang is used to keep the boom down much like a mainsheet. When sailing off the wind particularly reaching, the boom will be out of the range of the traveller. In this case you will be forced to use the mainsheet. The problem is that the first thing that happens when you ease the mainsheet is that the boom goes up, not out. In turn this puts too much twist in the sail. Enter the boom vang: it is used in place of the mainsheet (to control the leech tension and twist) and the mainsheet becomes the traveller controlling the in and out positioning of the boom and sail. The boom vang can also be used up wind to induce bend in the bottom of the mast. By using the vang as the mainsheet, there is a great deal of forward pressure put on the gooseneck. This forward pressure bends the mast flattening the mainsail. This can be a very effective mainsail control but the boat must be set up for it as this technique is hard on the boat equipment.

Good mainsail trim requires good communication with the helmsman. The helmsman can feel through the tiller the amount of load on the helm. If he has too much helm the main is probably over-sheeted (not enough twist) or too full. If he cannot seem to get the boat on the wind or even has lee helm try sheeting the main or making it fuller. The main has to make the boat feel right if the boat is going to be fast. The most important main controls are the sheet and fullness (mast bend and outhaul).