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Old Jan 04, 2013, 06:54 AM
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Bozeman, Montana, United States
Joined Aug 2003
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Dan, the spiral bracing used by the grain ships, like Pamir, was the opposite of what you are suggesting. Namely, the highest yards of grain ships were braced *less sharp* than the lowest yards. The highest yards were braced closer to the run position.

This is the theoretical way to do it, following the maxim "the wind is fairer aloft." That is, on a beat, the wind aloft is not as foul as the wind near the sea. This is due to friction slowing the wind at the sea surface. This friction, coupled with the movement of the hull, yields an apparent wind that spirals from foul to fairer as you go up, away from the sea surface.

Perfect alignment of the yard to get the sail to fill and generated maximum lift thus requires that you leave the yards at the top of the mast closer to square (perpendicular to the hull).

I don't know if one could measure the difference in boat speed for a model using theoretically correct spiral bracing. I do know the "fairer aloft rule" exists for models, though; just look at the twist of a Marconi rig mainsail leach if the boom vang is not snugged: the sail at the top of the mast is not aligned with the boom, yet the sail is not luffing at either the boom or the head. In real boats, letting the sail leach twist (by slacking the boom vang) is used for de-powering the main in strong winds. But really, it could just as easily be called "upping the drive of the sail to make the boat go faster" :-)

There may still be operational advantages for a model using "reverse spiral", like you are suggesting. For one thing, if the wind heads you, the top sails will start to luff before the bottom sails, giving you a heads-up that you need to let the boat fall off a bit. If you use correct spiral bracing, on the other hand, you could get simultaneous luffing for the whole mast, making it more likely that you'd get caught aback.
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