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finding the correct prop

I have a 19 foot cobia cuddy cruiser powered by a 3.7 l mercruiser stern drive with a alpha 1 outdrive. The WOT on the engine is rated between 4600 and 4800 RPM. I am currently using a 13 inch 4 blade 20 pitch prop. It is not the original prop. It will plane out at about 3000 rpm with only me on board. With 3 or 4 folks on board it take a long time to get to plane. I do not have trim tabs.
Any ideas would be most helpful

Regards Middle Gary
 
WAY too much prop. My 19 footer with a 305 V-8 is about right with a 21 pitch three blade prop (and happier with a 19 pitch). Furthermore, a three blade 21 is equal to a four blade 22-23 prop, so you're WAY over propped.

When in doubt, LESS prop is better than too much; easier plane off (especially with extra bodies aboard) with little loss of cruising speed and mpg. Only those maniacs who run flat out all the time need more pitch!

Jeff
 
Is it the original outdrive?
Is it possible to get the gear ratio and or serial number off the drive?

Most boats are designed for max rpm spec with a 19 pitch
Not all but most.

What is your max rpm at wide open throttle?

The outdrive may have been replaced along the way and you have the wrong gear ratio...

History???
 
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WAY too much prop. My 19 footer with a 305 V-8 is about right with a 21 pitch three blade prop (and happier with a 19 pitch). Furthermore, a three blade 21 is equal to a four blade 22-23 prop, so you're WAY over propped.
I would agree.
As suggested, first see if the drive reduction is correct for your engine.


When in doubt, LESS prop is better than too much;
Agreed!

easier plane off (especially with extra bodies aboard) with little loss of cruising speed and mpg.
Only those maniacs who run flat out all the time need more pitch!
And those are the ones who shorten the life of their engine!

Jeff

To prop correctly:

Engine must be healthy and in good tune.
Drive reduction must be correct for the engine.
Bottom clean and prop in good condition.
Boat loaded as you would intend to load it normally..... people, fuel quantity, gear, etc
Prop must allow the engine to reach the OEM WOT RPM range.

If the engine cannot reach the OEM WOT RPM, it will be over-burdened at all RPM.


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