The class legal 8:1 cunningham lets you crank up the class legal dacron sail enough to sound like a nice snare drum. That's what we ran on our 18.
IMHO, 8:1 is not enough DH purchase to make playing it from the wire a reality. I've raced all the Hobie classes (except 17) and think you're kidding yourself pretending to go upwind like a Tornado, N20 or F18. The 8:1, and less, allowed by Hobie rules is not enough purchase to make the DH anymore effective than a set-it and forget it function a on "class legal" Hobie boat.
Different strokes for different folks, I guess. We race the 18 fairly extensively in one of the more active divisions. 6:1 purchase is all my (female) crew needs to play the downhaul from the wire in 95% of the conditions we sail. If we actually need it bottomed out, she will hand it to me to finish the job if need be, but usually she doesn't need to.
Fine tuning the downhaul on the Hobie 18 may not be quite as critical as on some of the modern boats, but it certainly makes a difference and is by no means a set-and-forget adjustment.
sm
