The Mainsheet

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The mainsheet is one of those controls that serves a dual purpose in that it is used both to pull the mainsail in toward the

An example of a simple mainsail handling system.

An example of a simple mainsail handling system.

All the telltales on the leech of this mainsail are flying indicating that the sail is trimmed perfectly.

boat's centerline, i.e., to control its angle to the wind, as well as to control the amount of tension on the leech of the sail. When a boat is on a reach or run and the boom is no longer directly above the mainsheet, leech tension is controlled by the boom vang or main traveler. When the boom is on or near centerline when sailing to windward, the sheet is the most effective means of controlling this aspect of sail shape. Note that by regulating leech tension, the mainsheet also controls the amount of twist, which is very important to the overall performance of a sail. For example, as soon as the sheet is wound on too much, the leech becomes tight and cups to windward. This not only adds depth to the sail, but can also disrupt the flow of air along both the leeward and windward sides. Easing off the mainsheet will allow the leech to open up and increase twist in the upper portion of the sail. On small boats, especially singlehanded dinghies without backstays like the Laser or Finn, the mainsheet will also affect mast bend. It does this in two ways: first, by pulling back on the top of the mast via the leech; and second, by pushing the middle of the mast forward by loading up the boom. On larger boats, mast bend is adjusted primarily with the backstay and running backstays. But it's still good to keep in mind the effect that mainsheet tension can have on the spar.

The best way to gauge the correct amount of tension is to sight up the sail by standing below the clew and looking up. As you wind the mainsheet in so that the boom is over the center of the boat, you will see the line of the leech go from curved to straight. At first all the telltales will be streaming out behind the sail. This indicates that the wind is flowing off the back end of the sail without interruption, although it's unlikely that the back of the main has started to provide power or lift. Wind the sail tighter and watch what happens. As the leech becomes straighter, the telltales will continue to stream until you overtrim the sail, at which point the top telltale will start to dip behind the mainsail closely followed by the rest of them. When this happens you should ease the sheet until all the telltales start streaming evenly again, indicating a smooth and steady air flow. What's happening here is that at first the wind is just blowing across the sail, since the sail does not yet have an aerodynamic shape and there is no power being generated. As you begin to trim the sail, however, it starts to assume a more aerodynamic shape with the back end coming into play. Specifically, as the back of the main starts to go from curved to straight it begins to provide power. Not only that, the center of effort in the whole sailplan starts to move aft creating weather helm, which forces the water to attach to the underwater appendages and in doing so they start to generate lift. If, however, you wind the sail on too tight, the air toward the leech will begin to stall because it can't exit the sail easily, and with the flow over the sail disrupted it begins to lose drive. You will still have weather helm and the water will still be attached to the underwater appendages, but because the boat speed has slowed they will not provide as much lift, because the faster you sail the more lift they will generate. Eventually, if you do not ease the sheet and start to get the wind flowing across the mainsail again without disruption, you will slow down to the point where you will start to make a lot of leeway. At this point you are no longer going where you want to go. It's a delicate balance, and one of the reasons why the mainsail trimmer needs to work so closely with the helmsman.

In smooth water with between 5 and 10 knots of true wind, having the top batten parallel to the boom is another indicator of a mainsail that is correctly trimmed. As the wind increases the top of the sail will twist off naturally, and that's a good thing. In winds less than 5 knots you will want to ease the mainsheet and vang, if you are using one, to allow the top to twist open a little, cocking the batten slightly to leeward. This allows the wind to exit the sail smoothly and will help keep your boat speed up. When the wind is light it is really susceptible to stalling and as soon as it does you lose any power and lift that might have been generated. In stronger winds it's easy to get the wind to start flowing again because there is that much more wind available, but in light airs be careful, erring on the side of an open leech where the wind can exit the sail easily.

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