I had a Hobie 18 for 19 years. The sails were seriously under-developed. Back before spinnakers were common, I cut the top off a full, mylar, Mystere 6.0 mainsail for the Hobie 18 and the infamous "Mystobie" was born (I made a combined class logo for the sail)! I lost only a few square feet of sail area with a new big fathead sail (I even made a cut out in the head board to gain a few more inches of hoist) and the 18 suddenly would beat any 18 or 20 foot sloop rigged boat in light or medium wind boat for boat. It was very noticeably faster.

Try that option before going to all the trouble with a spinnaker. It is worth it because you always use the main so you get the most bang for your buck and you do not have to change anything. Cutting the top off a bigger sail is the way to go. The class sails were always poorly made, too flat, and of dacron material. Shortly before Hobie quit making the boat, they came out with much nicer sails that made the boat a little faster (doncha just love "strict one design"?) The boat has much more speed potential but was hampered with no "engine" development due to class rule restrictions. So this development makes a big difference, is easy to do, easy to handle, and a good value.