A Sunfish would suit you fine. Of course, there are hundreds of other types of boats with masts shorter than 18'. Go search the lake shore and see what's lying around and hasn't been used in some time. If you find something that intrigues you, make a house call. Also, talk to other sailors on the lake and make your needs known. If there is a club, go there and chat. Advertize in the local paper. Search the usual sources, Craig's list, E-Bay etc.