I have a 18' Campion w/ aluminum 3 blade 21p x 14.25'' and @ WOT I get 4400 rpm ( 3800-4200 suggested) at a top speed of 48 mph. If I were to go to a SS 4 blade how much pitch would I need to drop to and still keep my 4400 rpm? I'm hoping a 4 blade will give me better fuel economy at cruise!