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Determine new prop pitch for added HP

chollyjohn

Regular Contributor
Some may know in this Forum I

Some may know in this Forum I have been searching for a boat. 270DA by Sea Ray. The original engine was a 7.4L (330) HP with Bravo II outdrive. Recommended prop is 17.75 x 21. The engine is now changed to a 7.4LMPI (365HP) and same outdrive being used. Outdrive ration is 2:00:1. 7.4L std has max RPM of 4600 while the Magnum can run at top of 5000RPM. My question is how can you calculate for just the HP difference so as NOT to overrev. One way is to buy several props; but costly. Any comments anyone?
 
"Ayuh,....

Run it,+ See


"Ayuh,....

Run it,+ See where the rpms fall,...
Then go from There....."
 
"If the originally recommeded

"If the originally recommeded prop was a 21 pitch @ 4600 rpms then just with the simple rule of thumb @ 5000 rpms you would move up to a 23 pitch and the motor would operate in it's range.

In general 1" of pitch reduces rpms by 200. So 400 "extra" rpms = 2" - assuming that your gear ratio remains the same and the wide open operating range of the new motor is the same..

But Bondo is correct - the only sure fire way is to run a prop and see what you get on the tach - over revvin' for a moment won't hurt it - or alternately if you get to max rpm and it feels like there is lot's left, you know you can move up a few inches of pitch from whatever you have on it."
 
No prop expert here but if the

No prop expert here but if the current one is in good condition and stainless steel try it out first to see where you stand. A good prop guy can tune a ss prop up to 2 degrees and playing with the amount of cup will alter the w.o.t. rpm also.
 
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