High altitude physics lesson

Bigtalljv · 18 July 2013, 02:59

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

Locozodiac · 18 July 2013, 05:58

You can compensate lacking the little go by going at least 1 pitch down to compensate rpm loss at that altitude.

Happy Boating

jyasaki · 18 July 2013, 15:19

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

captnjack · 18 July 2013, 16:36

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!

Hightower · 19 July 2013, 01:19

Quote:

Originally Posted by captnjack

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

Locozodiac · 19 July 2013, 04:28

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.

Luv2Fly · 19 July 2013, 04:47

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.

Locozodiac · 19 July 2013, 05:21

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.

Happy Boating.

Smithyyy · 19 July 2013, 05:56

Quote:

Originally Posted by Luv2Fly

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

mister p · 19 July 2013, 08:53

Quote:

Originally Posted by Locozodiac

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.

9D280 · 19 July 2013, 08:58

Quote:

Originally Posted by Locozodiac

Some engine manufacturers sells high altitude carb kits,

And if it's an oldschool enough Johnsorude it has the mixture controls on the front panel for easy adjustment!

Do the new fuel injected machines auto compensate?

Luv2Fly · 19 July 2013, 13:34

Oops my mistake, the chart I checked must have been meters and not feet as it showed half an atmosphere at 6000! I was both surprised and tired.

simsy · 19 July 2013, 14:00

Yep, 4x4 petrol non turbo car I once hired in the swiss alps had practicaly no performance left at 2800m. Road bike in Spain (118mph in second gear) felt hopeless on top of one mountain road there.... (would still go quick, but felt really flat)...

Bigtalljv · 19 July 2013, 16:49

Yes. I deflated the tubes several times climbing up the mountains and will have to stop several times to inflate them on the way down or they get too floppy. I made this same trip last year but not for so long. We also have various temperature changes to deal with. Driving around California I can see 50F temperature swings. That tends to make the tubes change dramatically also.

As for power we are getting by. We are only here for another day so I didn't change props and its only stalled once or twice. I saw that even the super charged Verados suffer from power loss but only about 1/3 of normally aspirated.

Jason

ian parkes · 19 July 2013, 17:34

Do the carbs have slow running screws , they normally work on the fuel side on 4 strokes so half a turn or so in would improve the bottom end running and first part of the throttle . Quick fix and easy to put back to as it was .

Bigtalljv · 19 July 2013, 18:11

It's just not worth messing with. Only one more day on the lake and that will be at idle for the most part pacing swimmers across the lake.

captnjack · 19 July 2013, 18:19

Quote:

Originally Posted by 9D280

Do the new fuel injected machines auto compensate?

Yes but there's just not enough oxygen to create the rated horsepower. My 60hp was about ~44hp at 7800ft elevation. The worst part was that it was still propped for 60hp so waaay over-propped and only reached about 4500 rpm instead of 5900 at sea level.

Smithyyy · 20 July 2013, 00:20

Quote:

Originally Posted by Luv2Fly

Oops my mistake, the chart I checked must have been meters and not feet as it showed half an atmosphere at 6000! I was both surprised and tired.

Easily done!