You may want to check the hoist height before putting an F18 chute on a Hobie 18. F18 mast height is about 28.5' or so and the spinnaker runs only a couple of feet shy of that. I would be a little concerned about doing that to a comptip mast ... but if yours is still aluminum it will probably be OK (as long as the mast height is similar - otherwise you will end up with a masthead spinnaker!).