Ed-

With the straight spreaders when your mast gets "loaded" it bends only sideways and the spreaders simply limit the amount of this sideways bend (between where the diamonds attach). With a sloop as the mast bends sideways it bends into the "slot" betweent the main and jib "choking off" the wind that is able to go between them and you lose power (only comes from wind passing around the sails). With the raked spreaders, as the mast loads up NOW the spreaders not only limit the sideways bend, but because they are raked behind the middle of the mast, forming a "triangle" in essence, as the forces on the diamond wires increase they tend to try to force the spreader forward- forcing the mast to bend fore and aft- Downhaul also helps here since it helps to apply a force between the top and bottom of the mast which also tends to "bend" the mast in a fore and aft curve- So now, as the mast loads up, instead of bending into the "slot" it bends forward, keeping the "slot" open and allowing the wind to pass around the sails more effectively.

In a "prebend" condition the mast is not really allowed to bend sideways (very much anyway) at all so the slot is more constant (ie even in light air where there would be only sideways bend of a non-prebent mast the prebent one is not bent sideways). With the raked spreaders you will use less diamond "looseness" to keep the sail powered up and out of the slot in lighter air and then allow the diamonds to force the mast to bend fore and aft, along with downhaul, to keep the sail flattened as you need to depower. The other "advantage" of prebend is you can then utilize the top of the mast (usually in conjunction w/ a squaretop main) to "bend off" in gusts by keeping the mast relatively straight fore and aft and reducing the wall thickness/ "roundness" of the mast section (ala the wingmasts on the "A" cats, Taipans, F 18HT's, etc.)-



Kirt