Cruise prop slip on Seafarer 226

[Hutchy]

Current numbers: 2007 Yamaha 250 with 2:1 gear ratio, Yamaha saltwater series 2, 17 pitch

25.2 MPH @ 3700= 15%
28 MPH @ 4000= 13%
29.3 MPH @ 4100= 11%
36.4 MPH @ 5000=10%
42.8 MPH @ 5900= 10%

For discussion purposes only, the current Grady performance data for the current 228 and 17 pitch prop (with 4.2l 250 and 1.75 gear ratio shows 29.6 MPH @ 2500 RPM. Thats around 10% slip at cruise.

I know 15% isnt terrible, but what should I do to get more efficiency out of the boat at a slower cruise? Trying for good fuel mileage and engine life. I assume drop a pitch and go to a four blade? Top end isnt super important, as I am a huge fan of cruising at as low an RPM as possible. Based on the performance date from Grady on the 228, when you drop to 3000 rpm (coming off plane perhaps), you drop to 21 MPH and 23% slip.

I notice with my numbers, the numbers would imply I should simply cruise faster, at say, 4100 RPM. Its finding the sweet spot between prop efficiency and engine using more fuel running harder I am trying to attain.

Anyone have any input?

I talked to Propgods last week and will have this conversation with them tomorrow. Just wondering what you guys think.
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[ocnslr]

You seem to be using prop slip to measure efficiency, but that fails to consider other factors.

After some tens of thousands of miles on our Islander, with the original single 250hp OX66 and the twin F150s, and a good bit of prop evaluation on each, I will say that your numbers are very good.

IMHO, the best measure of efficiency is MPG, or nmpg. You don’t indicate whether you have NMEA 2K data coming from your engine. If so, your MFD should be able to give you actual MPG, using fuel rate from the engine and GPS speed. You will see this figure vary as you change RPM, engine trim, or trim tab position.

If you don’t have that data feed from your engine, but do have an NMEA 2K bus installed, I would suggest installing the extra network and interface to get this engine data. Much less costly than changing props.

Again, I think your speed vs RPM numbers are very good.