i have a 21' lobster boat with a 2008 90hp yamaha four stroke. the current prop is a 13.25x17 3 blade aluminum and most rpms i can get is 4700. i cruise at about 4000 rpms which is about 12 knts. top speed is around 15-16 knts. not sure how much boat weighs but i know its heavy and always has at...
