New prop - 91 Johnson 70 HP

[ggrimm01]

Ok asking the experts. The OEM prop was chewed up pretty good. So I bought a new prop. The new prop is a 13.25 x17. I am running 5100 RPM @WOT and just me in the boat. With the old prop I would run 5700 RPM. I do not know what the pitch was on the old prop. Looked everywhere for marking. I am fine with the new prop. However I want to be within the corrent RPM range for my boat.

 

[steelespike]

If you get us the wot gps speed we can tell better if your rpm is accurate.If accurate it is within the rpm rating.

 

[ggrimm01]

WOT speed with new prop is 33.4mph @ 5100 rpm. WOT with old prop is 28.1 @ 5700 RPM. Both speeds are the GPS speeds.

 

[steelespike]

Using a 2.42 gear ratio the 17 calculates with a really low slip about 2% really not possible.
It's possible the tach is not accurate, reading low.Perhaps you can borrow a shop tach.
Did you look under the prop nut for the old prop size? Your rpm range is 4500 to 5500.
HP is developed at 5500. If your happy with the performance with the 17 I think it will deliver slightly more speed
than a lower pitch.