You are right. I just put in an old Merc tach and hooked it up and saturday my friend is going to loan me a 15.5 x 17 3 blade aluminum prop that he has on his v-20 center console also with a merc 150..He gets around 42 and has bottom paint also. He thinks I should get 4 MPH more with the bigger prop. I'll let you know. Thanks Frank