Altitudes effect on engine and prop

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Old 01-23-19, 10:45 AM
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Altitudes effect on engine and prop

Hello everyone, this would be my first post for this site. I am currently looking at a Crownline 202 BR with a (hold on to your socks) a factory Mercruiser 502 putting out 420HP. My issue is this, it is located in Colorado and the the guy runs it up at 8150’ altitude, all his numbers are from that sea level. Where I live (Minnesota) it is 1400’ above sea level so what am I going to have to do to this this once it’s at my altitude. It is a EFI so I am not sure the engine will be effected but what about the prop? He says it runs low 60’s a 8000’ so I am guessing I should get a little more out of it here in the land of 10000 lakes. Any input would be appreciated.
 
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Old 01-23-19, 11:07 AM
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With fuel injection the engine should automatically adjust for the change in air pressure. A normally aspirated (no turbo or supercharger) will see about a 3% drop in horsepower for every 1'000 feet in elevation so you might get about 20% more power. With that big a difference in horsepower I'd be prepared to shop for a new prop or have the one on the boat re-pitched. Once you get the boat and can put it on the water you'll be able to tell.

Be mindful of when you do your testing and when you intend to normally use the boat. You can see a 10% or more change in power output from a dry winter day and a hot, humid summers day.
 
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Old 01-23-19, 01:48 PM
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I just talked to the current owner and he said it has a 25 pitch stainless prop on it right now.
 
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Old 01-31-19, 12:36 PM
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Your prop won't be affected. The shaft RPM out of the reduction gear will be the same regardless of altitude. Changing the pitch, diameter and the cupping (if it has cupping) is what will change the RPM of the engine. Since you have EFI, the brain will adjust the fuel/air delivery as needed, the best thing would be to have it recalibrated for your area and go from there. A stainless prop gets you a bit more speed and RPM's due to it being thinner therefore flexing a more than bronze.
 

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Old 01-31-19, 04:08 PM
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Bronze... that tells a bit about what kind of boats you hang around. Small runabouts almost exclusively use stainless steel or aluminum with a few "emergency" limp home props made of plastic.
 
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Old 02-01-19, 10:39 AM
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Right....though I do see bronze in really high speed applications due to the non-flexing properties thereby significantly reducing cavitation and slip. Mostly on very small two or three blade.

In this case, I doubt changing the prop is going to be necessary since it was set up for fresh water and is going to stay in fresh water. If the OP decides they want to go that route, stay away from aluminum for this application. Which that much power going to the shaft, slippage is going to the biggest problem with aluminum then you'll start to see cavitation burn and it's hard on the red-gear.
 
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Old 02-01-19, 10:41 AM
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Yea, you can't forget those sexy surface piercing props.
 
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Old 02-01-19, 01:45 PM
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The past owner said it running low 60 mph at 8000 rpm ? correct ?
Something very wrong here. Anyone else see it. ???
A big block factory 5200 to 5600 RPM . 6K maybe . 8k is way to much And definitely the Wrong pitch Prop.
Note My TRIK small blocks Designed by "Tim M." of Mercury Racing Turn 6200. But not all day.
 
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Old 02-02-19, 03:30 PM
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No, that was 8000 ft above sea level.
 
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