Originally Posted by scully
The mast , after pre-bend, should adapt the luff curve of the sail and is not usually straight when you tension the backstay. Having a straight mast would usually induce a lot of "shape" in the sail which may not be suitable in most conditions as the wind may detach from the leeward side of the main. This is especially so at the top of the sail.
Well, yes on a"normally" cut sail. If the sail luff is cut for a straight mast, then its shape would be best on a straight mast. My reference to "putting a lot of curvature" was referring to overbend.
However, I have not seen an adequate explanation of prebend lately and it is different than the normal use of "prebend, particullarly in full size boats. Since masts are not infinitely stiff and IOMs are fractionally rigged, in order to get the forestay tension you want you must tension the backstay. The forstay attachment acts as a fulcrum so the top of the mast bends to maintain the tension. The prebend is a way to increase the forestay tension and decrease the bend by "preloading" the mast.
The prebend is forward bend to give additional force to the forestay.