Looking at your boat setup and anticipating the motors will spin 4200 RPM Max, with 2.5:1 gear reduction (as a baseline evaluation criteria).
4200 divided by 2.5 = actual max prop speed ( 1680 RPM )
1680 x 17 (inches of pitch) = inches traveled in one minute ( 28,560 )
A mile has 63,360 inches, if covered in one minute the speed is 60-MPH.
Therefore, setting up the forumula “63,360 is to 60 as 28,560 is to x” provides a theoretical speed of 27 MPH with the 17” prop. Reduce that by 10 or 15% due to slippage and the speed is (in theory) in the 23 to 24 mph range.
Obviously if you start with the 17” prop and you can easily rev to redline (and beyond) then you know you’re underpropped. It will be mighty interesting to see! Personally I would (without having benefit of my weight/horsepower/speed calculations at the moment) tend to agree with Mark Weller, that it looks like the props are a little on the small side. I’ll look into my calcs when I get to where I can use them, and I’ll update this point of view.
Right off hand, I'd think the boat would be in the true 30-mph range with a weight of 12 kips and having that kind of power. My old 35' Sea Skiff was a 12,000 pound boat, and it ran 30-mph with a pair of 210-hp 327F motors. Sorry, I can't remember the props or gearing. However, the weight and power factor being able to achieve 30-mph indicates the props for your boat at 15 x 17 do seem too small.
Regards,
Paul
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