High altitude Prop

12jeep

Cadet
Joined
Jun 11, 2012
Messages
6
I have a 1985 Bayliner with a 5.0 eng. The prop is a 14 X 21. What would be the best prop for running at 6500" altitude. I don't want to rejet the carb because I usually run it at sea level.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Does 21 make full rpm at normal altitude? Figuring about a 20% loss of power I'm guessing a 17" should work.
If it can make 4800 rpm speed would be about 42.
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
I posted earlier but my reply didn't post.... Anywho,,, I thought the same thing SS suggested, 17P prop.

But, that would depend on your current engine RPM, speed and elevation. If your current WOT RPM is low, you may need a 15P prop.

Need numbers.......
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Go for a wot spin with lightly loaded boat on flat calm, no wind waters. Check achieved max wot numbers, compare them to min-max factory stated parameters for that engine. Will know how much underpitched you are, play with prop pitches to achieve near max wot rpm numbers. When at sea level again must remove prop or engine will be over reving. You can lose as much as 15% power every 1K meters of elevation.

Happy Boating
 
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