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Old 17 July 2013, 21:59   #1
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High altitude physics lesson

So I'm on vacation at 6200' with my boat and family. I am seeing the 3% per 1000' feet if elevation power loss on my carbeutated 90hp Honda. This isn't something I had considered until we got here and tried to go tubing with the kids. After reading up its technically running rich but doing ok there. Not much stalling or other problems. Just lacking a little go.

I guess physics isn't just a good idea, it's the law.

Jason
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Old 18 July 2013, 00:58   #2
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You can compensate lacking the little go by going at least 1 pitch down to compensate rpm loss at that altitude.

Happy Boating
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Old 18 July 2013, 10:19   #3
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And watch the tube pressure on the drive up. Be prepared to top up on the way down (or live with the flapping tubes.)

6200 feet; Tahoe?

jky
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Old 18 July 2013, 11:36   #4
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I could barely plane 3 people (nothing else) at Yellowstone Lake. Normally I can take 4 divers and about 900 lbs of scuba gear. There's just not enough O2!
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Old 18 July 2013, 20:19   #5
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I could barely plane 3 people (nothing else) at Yellowstone Lake. Normally I can take 4 divers and about 900 lbs of scuba gear. There's just not enough O2!
Use a plain old oxygen bottle instead of Nos for a little extra Oomp
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Old 18 July 2013, 23:28   #6
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Some engine manufacturers sells high altitude carb kits, comes in 1K Mt increments along less prop pitches to compensate power loss, low octane fuels must also be used. Sib/rib tubes must be tied facing each other specially when coming down to sea level to avoid excessive tube slack.

Happy Boating.
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Old 18 July 2013, 23:47   #7
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I thought this might be a bit of over exaggeration, but I checked an air pressure vs altitude chart and you will be a little below half an atmosphere at that altitude. About 7 psi. So your tubes would have been under a lot of extra force on the climb up and if you bled them to compensate at altitude they will be very underinflated by the time you come back down. I didn't think the altitude effect would be that pronounced. Good to know.
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Old 19 July 2013, 00:21   #8
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For instance if you would like to take a sib or rib inflated at sea level to 3 psi to 2K Mt height you will blow tubes on it's way up. Last year we inflated a 420 sib to 3.5 psi at a height of 3.5 K meter, when reached sea level, was just 1 psi.

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Old 19 July 2013, 00:56   #9
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Originally Posted by Luv2Fly View Post
I thought this might be a bit of over exaggeration, but I checked an air pressure vs altitude chart and you will be a little below half an atmosphere at that altitude. About 7 psi. So your tubes would have been under a lot of extra force on the climb up and if you bled them to compensate at altitude they will be very underinflated by the time you come back down. I didn't think the altitude effect would be that pronounced. Good to know.
Not quite that bad, half an atmosphere is at 6000 metres, the OP's at 6000 feet. Pilots in unpressurised aircraft don't go on oxygen until 10000 feet, if they were down to 7 psi at 6000 they'd be blacking out all over the shop. 7 psi is more like 20000 feet which is IIRC three quarters of the way up Everest ( now that'd be a cool - probably literally - place to go ribbing! Except your engine would'nt go...)

Per the chart below 6000 feet is 0.8 atmospheres or 3 psi down from sea level. Still means I guess if the tube was inflated to 2 psi at sea level it'd be something like ( not sure if the maths is as simple as this) 5 psi over at 6000 feet. Still worth thinking about if you ever found yourself towing over an alpine pass or something, never mind actually boating on a high altitude lake!


Air Pressure and Altitude above Sea Level
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Old 19 July 2013, 03:53   #10
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Some engine manufacturers sells high altitude carb kits, comes in 1K Mt increments along less prop pitches to compensate power loss, low octane fuels must also be used. Sib/rib tubes must be tied facing each other specially when coming down to sea level to avoid excessive tube slack.

Happy Boating.
The octane thing is not really an issue. Only the other way around. An engine won't be stifled by too high a rating fuel, it just won't use it. However if too low a rating is used it won't operate at maximum efficiency.
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