Drive Ratio and Prop Pitch

Prop Efficiency vs Pitch

I am a member of the The Donzi Registry and have also asked this question there. But I figure the more information I have the better and more informed decision I will make. I will also pass this on to the other members of the Registry to add to their information.

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The Bravo I on the 22' DONZI Classic project I recently acquired is a bit wonkey (a technical term meaning not quite flablunget). So I have been trying to learn as much as I can about drives and drive types before fixing or replacing it. Can someone explain how to choose what ratio drive to run? Bravo I I/O's are available in 1.5:1 and 1.32:1 ratios, and I can get or convert to either. This is my question, everything else being equal is it better to run a 1.5:1 with a high pitch prop or a 1.32:1 with a low pitch prop? For example a 1.5:1 with a 30 pitch or a 1.32:1 with a 26 pitch? This assumes 85 - 87 MPH at 5200 RPM's with a 10% slip.

A DONZI 22' Classic is a deep V, weighs approx. 3700#, is 22'6" in length overall, with a beam of 7'2". I would like the boat to be able to run in the mid to high 80's (mph) and can build the engine with enough power to do this. Other 22" Classics are currently running in this speed range, so there is no question as to whether or not this boat can go this fast.

Thank you.

rayjay

 

Okay, let me ask this a different way. Does prop efficiency go up or down as the pitch increases, or is it more like a Bell Curve where there is a range of pitches that are good and above and below that range the efficiency goes down?

rayjay

 

What RPM's is the engine built for?

 

I will be building this engine to have its peak torque between 4500 and 5000 rpm’s. To keep the o/d together I would like to limit the engine to 5200 to 5500 rpm’s. It will be either 540 or 552 c.i. with an AS&M EFI. My basic question relates to what gears I should get in my out drive: 1.5:1, 1.32:1, or 1.26:1? Is there a pitch, or a range of pitches, that work the best? And then I will gear accordingly.

rayjay