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Figure 11.3

When you wind the backstay on, you not only bend the mast, you move the top of the mast aft a few degrees. This movement will in turn have an effect on the clew height of the headsail.

Looking up the back of the mainsail on this multihull reveals a flat shape perfect for a boat that sails at high speeds.

Looking up the back of the mainsail on this multihull reveals a flat shape perfect for a boat that sails at high speeds.

Rule No. 4 - The part of the boat under water has a lot to do with how you trim your sails.

Rule No. 5 - The back end of your mainsail works with your keel to provide lift when sailing to windward.

With regard to rule No. 1, momentum has a lot to do with sail trim. Therefore, if you have an old-fashioned, long-keel, heavy, sea-kindly yacht, you can rely on inertia to keep you moving through the water once you have been able to get it up to speed. Subtle differences in wind speed and sail adjustment have little effect, and you do not need to be trimming constantly. On the other hand, a light, responsive, fin-keel boat will not be able to keep up speed if the wind drops or if it hits a wave and slows down. Those kinds of boats require a lot of "changing gears" to keep them moving well, and these gear changes are the result of the way you power up and depower your sails.

Rule No. 2 states that you need more powerful sails to move heavier boats, with power in this case being a function of draft, or the relative depth of a sail. Heavy boats need powerful sails to get them moving, whereas lighter boats can use flatter and less powerful sails to achieve the same results. In addition, flatter sails will allow them to point closer to the wind. Multihulls also have flats sails because they generate so much apparent wind. This difference in chord depth is important to understand, not only as it relates to the weight of a boat, but also as it relates to how boats accelerate. At slow speeds all boats need deeper, more powerful sails to get them moving: As they accelerate, however, the sails need to be flattened. Deep sails generate more power because of this depth, but as speed picks up you need to flatten the sail so that the wind does not become unattached as it tries to bend its way around the deep curve in the sail. Again, that's why multi-hull sails are generally flatter. They quickly generate apparent wind, and if the sails are not flat the air flow will become separated.

Rule No. 3 is an old standard. Most sail trimmers have their sails overtrimmed. It's just one of those things. People tend to keep winding sails on harder, feeling that tight sheets must translate into more power and speed when in fact that loss of power is a result of forgetting to ease the sheet when the wind drops. As a result the sails are wound on too hard and it's like putting on the brakes. Ease the sheet, let the sail fabric relax and the sail will resume its designed shape. Let the top twist open a little and nine times out of ten your speed will pick up. At times you need to have your mainsail and headsails trimmed on tight to narrow the angle of attack of the sailplan, for example, when you are squeezing up to make a turning mark or try-

ing to get above another boat. But most boats can't sustain these close angles and tight sails for very long, and you will soon have to ease out the sails out and resume a more normal course. If you find yourself overtrimming your sails to stop them flapping, there may be a problem with the design or the way you have them set. If you can't get your telltales all streaming together, the sail might need recut-ting.

Rules 4 and 5 go hand in hand. Most sailors do not give much thought to how they need to trim their sails to take advantage of what's below the water in terms of appendages. But in fact the relationship is a crucial one. Good foils can provide as much lift as good sails, assuming the sail trimmer allows them to function properly. The key is that there needs to be water flowing past the keel and rudder for them to provide lift, and the faster the water flows, the more lift is created. If the sail trimmer has the boat stalled because the sails are overtrimmed or trimmed improperly, there will not be any lift from the underwater appendages, and the boat will slip sideways. Of course, it's a delicate balance to get the right amount of lateral pressure exerted on the keel and rudder to get lift, but the potential results are such that it's worth the effort. A good example of the relationship between keel and sails can be seen when you are sailing to windward, so try this. Sail hard on the wind with your sails sheeted on tight and watch the boat track through the water. Now let your mainsheet out and watch what happens. Chances are the boat speed will not only pick up (rule No. 3 coming into effect) but the added speed will also be turned into lift from your keel and the result will be a better, faster track to windward. Be aware, however, that if you ease the mainsail out too much you will lose lift, and while the increase in speed will help your foils, the trade-off will not be an overall advantage and the result will be a loss in efficiency.

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