Proper Prop sizing???

[Dobb15]

Hey just wondering what size of prop should be on my 1973 fiberglass 21.5ft starcraft with a 165 mercruiser straight six and model I outdrive, it currently has a 4 blade prop on it but not sure of the size???

 

[Bt Doctur]

Re: Proper Prop sizing???

The one that allows the motor to develop it rated rpm band 4400-4600 .
prop selection is a black art. change pitch, diameter , cupped vs uncupped,raked,clipped blades, and the end result is how does the boat perform for you.May use a 17P for tubes, 19P for cruise, 21P to a bit more speed ,it`s a give-take question.