Something like a Hobie Wave would work on a lake like that. On a Formula20+spinnaker, you will be continuously tacking and jibing. An old Hobie 14 would be the right length, but they turn so poorly (unless you use perfect technique) that you might not enjoy it-- this is common with older, shorter boats. Newer, shorter (16feet and below) boats tend to turn easily even if the skill level is not perfect. Also, check the depth-- if it is shallow, get a boat without daggerboards and make sure the boat has good reputation for rudders that kick up (eg, hobie is generally good, but old mystere have a poor reputation)