Also noteable is that if this is a refit of an existing boat - sheeting the spinnaker to the rear beam puts a good bit of direct stress to whatever attaches the beam to the hull. I know of two Nacra 6.0's that broke beam bolts due to the additional loads induced by sheeting the spinnaker to the rear beam. You are much better off if you can figure out how to attach the hardware to the deck - somewhere around the shroud is a great place. The sailmaker can also tell you at what angles the spin was designed to be sheeted (you probably already know all of this - if so, I appologize).