Prop pitch and diameter

[miamicaro]

I just bought a 95 DONZI 255 Medallion 25 footer with a 5.7L mercruiser 350 magnum with an alpha outdrive. I replaced the original corroded 14.5 x 17 prop with a 14.5 x 19 pitch mich wheel. Don't know how the original performed, original owner said it was sluggish. Unable to reach rpm's with new prop, only able to reach 3500 with two persons onboard (450 pounds). I'm looking to achieve good overall performance. Can someone advise the proper pitch and diameter, Thanks.

[Rootsy]

is the engine performing up to snuff? are you using your power trim to get the hull aired out? is the old prop aluminum and the new prop stainless?

figure for each inch of pitch you increase you'll lose 150 - 200 rpm... same for every 1/4 inch of diameter increase "ROUGHLY"

at the current setup you'll need to drop to like 13 or so inches of pitch to get that motor where it needs to be... i'd make sure the motor is performing to it's full potential first...

JR

[Sam]

Ditto on what Jamie said. I would begin by checking the compression in each of the cylinders as a min and from there start looking at the basics. Even if it is a prop issue it would be nice to know the health of the motor. Just curious, was the boat surveyed.

Sam

[miamicaro]

The motor has been tuned. I tried another prop today (14.5 x 17 3 blade) I got 4000 rpms wot, but still a bit short. I was told maybe a 15 1/4X15 may be best. any suggestions.




is the engine performing up to snuff? are you using your power trim to get the hull aired out? is the old prop aluminum and the new prop stainless?

figure for each inch of pitch you increase you'll lose 150 - 200 rpm... same for every 1/4 inch of diameter increase "ROUGHLY"

at the current setup you'll need to drop to like 13 or so inches of pitch to get that motor where it needs to be... i'd make sure the motor is performing to it's full potential first...

JR

[RvR]

You could use the Boat Speed and Prop Slip calculators at http://www.go-fast.com/ to get an idea of what the performance should be. If you know your weight and HP you can calculate a theoretical top speed drive using the speed calculator.

Then enter your gear ratio, pitch and RPM and calculate a theoretical RPM. Then assume some reasonable slip, say 12% and calculate an “actual speed” Adjust the prop pitch until the program calculates a “actual” top speed that matches the speed calculated by the Boat Speed calculator.

These results will probably be optimistic but still better than guessing.