Pitch and diameter change for speed/efficiency

[briwill]

I just bought a Reinell 196FR with a OMC Cobra 5.0. Right now it has a new 15x17pitch prop but WOT is approaching 5000rpms at around 40mph. I'd like to get my rpms down in the 4200range at 45-50mph. I'm finding good deals on SS 14.5x19 props. Will loosing .5 inch diameter cancel out the increase in pitch? Should I go with a 21"? Also, will trim tabs help me get on plane sooner or should I go with a hydrofoil? Thanks.

 

[Silvertip]

Re: Pitch and diameter change for speed/efficiency

If you check various prop specs, you will see that as pitch increases, diameter descreases slightly. No problem there. I don't know the gear ratio in your outdrive so I used 1.7 in a prop calculator. A 21P prop would deliver about 46.3 MPH at 4400 RPM with 10% slip. Those are theoreticals and will depend on the prop design factors. If the gear ratio is 1.5 the speed would be well into the 50's IF the engine can pull that much pitch with a 1.5 ratio. The calculator shows you currently have 15% slip which is a bit high. There are folks here with a heck of a lot more prop testing experience than I have so stay tuned.

 

[WillyBWright]

Re: Pitch and diameter change for speed/efficiency

Tabs are far better than whale tails. No Competition!