Changing prop size question

[Siv]

Last night my prop got bashed up so I need a new one. Motor is a 1981 70hp Johnson on an 18ft fiberglass and the current prop is a 13-1/4 x 17 aluminum. I am currently happy with the performance. I don't know my RPM's because I don't have a tach but the boat gets up around 42kph / 22 knots with a couple people in it which is fine for me as it's just a fishing boat.

I was going to buy the same size because I don't have any performance problems but I found someone selling a brand new 13x19 aluminum prop locally for $50 which seems like a great deal.

What sort of difference would switching to this prop make for my boat would it be positive or negative on my performance?

Thanks

 

[steelespike]

Re: Changing prop size question

It is very difficult to make a prop selection without at least your present max rpm
to go with the speed, 22 with 2 people seems slow to me which may mean your below your rated rpm. Over time not good for the motor.Wastes gas and produces
poor performance, struggling to stay planed at reduced power poor hole shot.
Guessing the 19 will perform poorly and take you further out of your rpm range.
Stick with the 17 its worked so far even though to me performsance seems below the norm.