Ed,



The rotation of ypour sails has to do with having the right shape, and timing of your controls. i.e. items like closing off the slot will keep your mast counter rotated.



The spreader rake is a method of providing prebend in the mast to match your sail. Diamond tension is what you use to keep the mast stiff for a fuller pocket. Combining the 2 allows you to have a stiffer or looser mast with the same prebend by adjusting the speader rake.





Matt