The only plans I know of are Lindahl Mk II or III. Try searching. If you can't find them, let me know and I will try to find someone who has a set.

The 18 sq is a box rule class 18 ft waterline and 18 sq meters of sail. I have forgotten about the width but normally they do not go over ~12.

The nacra 18 sq was an early 18 sq and a 1 design. The first version used 5.2 hulls and the 2nd version used 5.5 hulls.

There are 3 catagories in 18 sq.

Cat 1 - unlimited
Cat 2 Nacra 18 sq
Cat 3 Nacra 5.5 uni

Very few 18 sq have been built in the last 10 years. The ones that have been built, have been very very fast. They have about the same upwind and downwind speed (hullspeed).

Keep in mind these boats are top quality with carbon or carbon wood hulls; carbon mast and beams; top quality rudders and boards and custom sails.

Good Luck