Okay,,,,,,,,,,,,,,,here we go with a 2.0:1 gear reduction tranny, compared to the 2.5:1

Quite interesting, indeed.

With a 2.0:1, the numbers work out as so........

Assume 4200 max RPM. With 2.0:1 that would make the props spinning at 2100 RPM. At at that speed, 17" props x 2100 RPM will push the boat forward by 35,700 inches.

That works out to 33.8 MPH, less the 10% slip reduction that puts the boat right on 30.42 MPH. Therefore, if the transmission is indeed, a 2.0:1, then the 15X17 would be one heck of a good starting point. If it is 2.5:1 gear reduction, it's way under-propped.


If by chance, the same boat is running a 2.5:1 gear reduction transmission, then a 21" prop is the desired starting point for the following reasons.

Assume 4200 RPM max engine speed divided by 2.5 = 1680

Take that times 21" of prop pitch = 35,280 inches moved forward in the course of one minute. Take that number and multiply it times 60 = 2,116,800, and then divide it by 63,360 inches in a mile = 33.4 MPH. Remove 3 MPH due to slippage and you're right on top of the 30-mph speed range.

In PRACTICE, we'll see if the boat actually weighs in at less than 12,000 pounds, or if the power is a bit more or less, and if the gear ratios are as noted. In any case, this should bracket the prop izes down significantly.

A 15x19 is a bit oversquare. A 16x19 sounds a bit better just from looking at the numbers, and this is probably why the prop shop commented accordingly. A 18 x 21 or say, 19 x 21 sounds pretty good on the other hand with the 2.5.

GOOD LUCK, it sounds like FUN !!

Regards, Paul