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All I can think is that if the center of pressure of the mainsail is aft of the center of the boat, as soon as you pass through the wind, a tight mainsheet will cause the wind to try to keep the boat from rotating further, instead causing it to weathervane back toward the wind...is that why?
Exactly. You need to have the main sheeted into the centerline of the boat in order to get the boat pointed head to wind. Once your bow has passed head-to wind, you need to begin sheeting out the main so that the boat will continue to bear off on the new tack. Then, as you pick up speed on a close reach, you begin sheeting the main back in and simultaneously steering closer to the wind.
If you just keep the main sheeted in tight, you'll most likely park head-to-wind. This will definintely happen on a uni-rig. On a sloop, if you backwind the jib, you MAY be able to avoid it.
Likewise, if you let the mainsheet out too early, you'll never get your bows to head-to-wind and you'll be stuck on one tack or forced to jibe. This is what can make tacking in heavy air quite difficult. You might have the main traveler out 12" or more to depower, but the problem is that when you go to tack, now the boat won't go head-to-wind.
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I've been grabbing the main sheet just below the boom and walking it over. With one hand on the sheet and one on the tiller, how are you letting out an extra three feet of line? I'm assuming that you're uncleated and you allow the wind to unspool the extra few feet as it fills the sail?
Why are you grabbing the mainsheet below the boom? You should have your hand on the mainsheet where it exits the cleat on the bottom block. A typical tack would have you sitting on the windward side of the boat, uncleat the mainsheet, but don't let it out. Begin turning the boat into the wind. As the bow crosses head to wind, you begin moving to the centerline of the boat and in doing so you automatically let out about 3 feet of mainsheet.
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I'm assuming that you're uncleated and you allow the wind to unspool the extra few feet as it fills the sail?
Partially, also just the mast straightening from easing the mainsheet takes out some mainsheet. On the 17, you actually push the boom up with your back as you cross under to help pull out the sheet.
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