Take a close look at your jib, if it's the standard 5.8 jib I'll bet it is way too big for a self tacker. Just rig it and hold the clew at the mast, if it overlaps the mast to the rear, it will have to be recut to work on a self tacker. The jib clew must be shorter than the distance from the tack point to the self tacker track, plus the clew shackle and block.

You should get up close to a Nacra F18 with a tape measurer and check the distance they are have, jib tack to jib self tacker track, then check yours, to about 2" infront of your mast on the main beam.

Most of the earlier (pre Inter 18/20) Nacra's had big footed jibs that overlaped the mast, won't work on a self tacker.

Of course you could just buy the F18 jib as well. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />