Can I Keep These Props?

Here is what I have now: 160hp diesel 4000 rpm engine with a 1:1.92 ratio, 2072 rpms at the prop WOT, with a top speed of 21.8 mph

I want to move to 315 hp 5000 rpm gas engines. Torque will be roughly the same. My rough calcs say I would need a 1:1.2 ratio to load this engine properly with the existing props.

This puts the WOT prop rpms at around 4000. Is this too fast? Any risk of cavitation or other issues with prop performance? I am limited in diameter to about 17 for tip clearance because of the prop pockets on this boat.

 

Your rough calcs are way out of proportion. With a diameter limit of 17" you should have a prop shaft speed of something like 2790 rpm; ie a gear ratio of about 1.8:1.

If you insist on a shaft speed of 4000 rpm, you would end up with a prop dia of 13.5". Whether that is ok or not is impossible to say without knowledge of the boat, weight, shape aso.

 

Hmm I guess I don't follow. All things being equal, the only change is the HP and the gearing. I am keeping the hull and running gear the same if possible. I am basically doubling the HP and increasing the rpm WOT by 20%. What is the proper gearing to take advantage of existing running gear?

 

Today your shaft is rotating 2083 rpm at 4000 engine revs and consuming 160 hp. You propose a 5000 rpm engine with gearing 1:1.2, making a shaft speed of 4167 rpm. That is a doubling of shaft speed, not a 20 % increase.

Since propeller power is roughly proportional to shaft rpm^3, your setup will call for 8 times the original power, ie 8*160 = 1280 hp. So, back to the calculator again!

 

I think I've got it now... Used the boat diesel calculator. With1.51 gears I should see a top rpm of 4200 with the same props.