I'm Brad, a student at SUU in Southern Utah.  we are constructing a Long EZ air craft.  We have the 13b with turbo and Crooks redrive RD-1C.  We want to put a two blade prop on and we are trying to find out what length can be used. What is our limitation and what has worked best for people.  If any one knows what has been used and has worked please help us out.  Does 68" sound reasonable for a two blade prop? Any info would be great. Thanks.  Brad