75hp prop advice

I am fairly confident that i have my 1976 75hp johnson running quite well. It is on a ~15 foot tri hull fibre glass boat. I pulled it on the scale today and got a wheight of 1700 lbs. (boat trailer was on scale still hooked to pickup that was not on scale) this is the combined mass of trailer and boat. (does this wheight seems realistic?)

prop is currently a 13 1/4" by 17". i can only get about 4600 rpm once on plain, with two, 200 lbs guys on board and 20 L of fuel. any suggestions for a different prop? was thinking of moving down to a 13" pitch, as i plan to have a max of 5 people on board, OR maybe pull someone on a tube. i am not concerned with top speed, just getting out of the whole easy. any advice is appreciated.

Re: 75hp prop advice

I'm going through this myself right now, but without knowing what my current prop is (no #'s on it anywhere! ???) So I'm going to start with a 13-1/4x17" and go from there. I have an old 15' fiberglass tri-hull with an old 80hp motor and an old banged up prop. Supposedly weighs around 825lbs without the motor.

From what I've read, going down a size on the prop pitch will increase your rpms by about 200. So dropping to a 13" pitch, in theory, ought to get you 800 more rpms. One thing to find out is your recommended max rpm range for your engine, and make your judgement from there. They say to shoot closer to the higher end of the range. I found out my recommended max is 5000-5800rpm. I'm running 5000rpm wide open throttle with 4 people on board right now, so wanted to go down a step or two on the pitch myself to get in the middle of that range.

Re: 75hp prop advice

Thanks, and the recomended WOT for mine is between 5500 and 5800. i wasnt sure how much rpm gain there would be by dropping 1 inch in pitch. Now i am maybe even considering an 11 pitch...

Re: 75hp prop advice

Not that I'm an expert on the subject, but dropping to an 11" pitch seems a bit drastic to me. I probably wouldn't stray far from a 15". A 13" at most. But who am I anyway?

Let's see what others have to say. I'm curious myself.

Re: 75hp prop advice

your right, 11 might be to durastic... a motor like mine came standard with a 15" pitch... might be able to find someone in my area witha prop i could try before i buy.