I started with an H18 about 4 years ago, and just purchased a used F18 (C2) last December. The C2 is for travel racing with CRAM (and maybe Sarasota this fall - keeping my fingers crossed), and I keep the H18 stick-up on a perch on a nearby medium sized lake (800 acres).
Anyway, I love them both, but for different reasons. H18 is perfect for single, double, triple, or more handed rec sailing. Its relatively simple, quick and easy to set up and more-or-less bullet proof. The C2 is for racing. Plain and simple.
Upwind, the new boats point much higher and get a bit more speed (maybe a knot or 2 more) as their foils and sails are much more efficient.
On a beam reach, the huge jib on the H18 seems like a big gennaker and if its not as fast as an F18, its only by a little. That boat was built for reaching, and a beam reach in 15+ is a ton of fun - especially when the lee bow gets just a little too low...
Downwind with the chute up on an F18 is like Damon says: Unbelievable. Probably 2X plus faster. To answer the question in your terms it is like getting passed on the highway. You're doing 70 and F18 passes you at 100+.
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Jeff R
'88 H18 "Jolly Mon"
'10 C2 USA1193
NE IN / SE MI
cramsailing.com--