I looked at the parts diagram on Hobiecat.com, it's not obvious. I was hoping to see an 18SX-specific mast or comptip, but no dice.

Per the class rules (hobieclass.com), Hobie 18SX Rule 14 b): "The [spinnaker] sail cannot be attached to the mast above 23 ft. 11-1/2 in. (7.30 meters) measured from the bottom of the mast extrusion."

I believe that the only difference between an 18 mast and an 18SX mast is the comptip (the 18SX is longer). Matt Miller or the guys at Surf City might chime in here, but I'm very certain that there was an upgrade kit at one time to upgrade a standard 18 to an 18SX, and it included the wings, sails, rigging, and longer comptip.

If this is true, measure your current 18 mast, find the place where 23 ft. 11-1/2 in from the bottom puts you, and maybe go a little lower. I'm fairly sure they are rigged from the comptip.

Hope this helps.

Mike