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post #24 of Old 03-17-2013
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Re: Weather helm question

Noob here. I have some limited monohull experience and too much free time.
Nobody seems to bring up weather helm as 3-D problem. When a boat is not heeled there is center of effort fore and aft of the crafts center of lateral effort (jib & main). That's the calculation everybody can figure out and is on the first couple pages of a sailing book. Too much resistance on the jib and the nose points away from the wind, more wind on the mainsail the nose points into the wind. Classic weather vane analogy, once a boat is moving that formula is less worthy. Why is the boat moving forward in the first place?

Think pinwheel or maple seed. You blow on a pinwheel and the “sails” rotate around a fixed point. If you took a pinwheel and removed all but one blade you would have a sailboat that spins around a fixed point, a maple leaf.

Underneath each boat somewhere around the keel is the center of gravity, maybe called something else. This is the fixed point that if you could mount an axel through (there might not even be boat there, think boomerang) it would spin around like a pinwheel with one blade, spinning faster as the wind picked up.

Upright boats don't spin around this point in water because the buoyancy of water in front of center of gravity (the bow half or forward section) prevents the bow from going under and the keel also keeps the boat from flipping end over. Actually modeled this keel-sail system with no hull. The wind energy spent trying to spin the boat is given up in heel, slide, and finally, forward motion. Does your boat seem to want to submarine more often at higher speeds?

When a boat is moving it is due to wind, more wind then boat heels. If a boat’s sail rig is trimmed for max lifting efficiency then as the boat heels it is continuing to generate forward spin. Heel the boat a full 80 degrees or so the craft is almost on its side. The sail still has some lifting force and resistance. If the sail is balanced according to the books, it will neither have weather or lee helm, if the sail rig is trimmed poorly, the craft will just sit there at 80 degrees and slowdown, stalled sails, resisting wind, sliding. If the sails are still lifting, then forward rotation continues, only, the center of gravity of the boat is no longer under the sails, but STILL on the opposite side of the sails by the keel, maybe even close to breaking surface of the water. The boat WANTS to spin around the center of gravity. The farther the center of gravity is from the sails center of effort, the more spinning it must do. Front half of the craft is no longer being rotated down INTO the water; it is being rotated to the horizon, the source of the wind, the air. Adding to the chaos, the keel no longer dampens the boat rotation port and starboard; in terms of resistance it has become nearly invisible, and is doing very little of its original job of keeping a boat going straight and preventing sideways motion, only trying to bring the mast back to an upright position if it is weighted, or give you a platform to stand on to upright it manually. To make matters worse your rudder has now become an elevator, pointing your bow up out of the water or down into the water.

Every point so violently to weather that you actually end up spinning completely around to your original course?
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