how does prop diameter effect engine rpm?

[lime4x4]

Trying to find out why a 15.25X15 3 blade prop gives me a max speed of 32 mph and 4100 wot while a 16X13 3 blade only gives me a max speed of 19 at 4500 wot

 

[Silvertip]

Re: how does prop diameter effect engine rpm?

Perhaps a spun hub on the 13P or you are experiencing "blow out".

 

[lime4x4]

Re: how does prop diameter effect engine rpm?

It's a new prop and did mark it and it didn't spin. What is blow out?

 

[sauce88888]

Re: how does prop diameter effect engine rpm?

The 13" pitch is the reason for the higher rpms and lower speed.

 

[Silvertip]

Re: how does prop diameter effect engine rpm?

2 inches of pitch does not cut MPH nerly in half. Something is wrong with the numbers. Blow out is when the prop loses its bite on the water. If the hub is not slipping, then it just doesn't compute that a 2-inch drop in pitch would lose 13 MPH. The 400 RPM increase in rpm is consistent with the pitch change but the speed difference is not. Are you using a pitot type speedo, GPS, or a paddle wheel speedo on a fish finder for speed measurements?

 

[lime4x4]

Re: how does prop diameter effect engine rpm?

paddle wheel and gps. Supernova has been on my boat a few times and we have tried many props. So far the best one is a 15.25X15 atleast the boat eventually planes with that prop. So i thought i'd go lower in pitch to get the rpm's up. Which it did just lost forward motion after 19 mph. I have just over 10 hours on a new upper drive unit on this boat so the bearings shouldn't be that tight anymore. Currently boat planes around 26 mph at 3800 rpm's. Could there be something funky with the lower unit??