This is incredibly slow for a 5ft prop. I am not sure why you are getting best results with such a low cut in speed unless your prop is much lower tsr than you think. Certainly you are more in the speed range of the 8ft prop and whatever happens the large prop will have far more power. I am not sure that such a tiny alternator is going to hold down an 8ft prop so you will need to make every attempt to make it furl as early as possible. In low winds you will be way ahead but you will need to watch it in high winds.