High Altitude Boating

[Sea Rider]

If in the need to use a inflatable at a high altitude lake 4,000/5,000 meters above sea level, where the engines losses about 12% of it's horsepower each 1,000 meters, would I need to have at hand a lower or higher size pitch compared to the factory prop delivered to compensate this high altitude loss. It's a Yamaha, 2 strokes 8 HP engine, don't know the pitch size, any ideas, suggestions ?

As the engine will be used from sea level to the stated previous high altitudes, would be too complicated to change carb jets, adjustments, each time the engine will be used. So would like to play just with prop pitches if applies.

Happy Boating

 

[j_martin]

Re: High Altitude Boating

If in the need to use a inflatable at a high altitude lake 4,000/5,000 meters above sea level, where the engines losses about 12% of it's horsepower each 1,000 meters, would I need to have at hand a lower or higher size pitch compared to the factory prop delivered to compensate this high altitude loss. It's a Yamaha, 2 strokes 8 HP engine, don't know the pitch size, any ideas, suggestions ?

As the engine will be used from sea level to the stated previous high altitudes, would be too complicated to change carb jets, adjustments, each time the engine will be used. So would like to play just with prop pitches if applies.

Happy Boating

You would need a lower pitch prop. It may not be worth the effort unless the engine is laboring to the max at sea level now.

I don't know how hard the main jets are to get at on that engine, but if I am set up right, I can swap all 12 main and idle jets in a merc v6 in about ten minutes. For that much altitude, it might be worth the effort if you don't have to take too much off the engine to get to them.

hope it helps

 

[fishrdan]

Re: High Altitude Boating

4-5000 meters,,, wow that's like 13-16000' At those altitudes I think you are going to need to down-size the jets so the engine will run properly.

I've run at 9000' (about 3000 meters) and have needed to change the jet(s) in my outboards, if not they were sluggish, doggy, had no power, hard to start and even if it started hard to keep running. Even with the jet change and the outboard running good I would say my outboard was down 50% on power.

I didn't change my prop, that would have helped the outboard too and something I'm doing on my next trip up that high.

 

[Sea Rider]

Re: High Altitude Boating

4-5000 meters,,, wow that's like 13-16000' At those altitudes I think you are going to need to down-size the jets so the engine will run properly.

I've run at 9000' (about 3000 meters) and have needed to change the jet(s) in my outboards, if not they were sluggish, doggy, had no power, hard to start and even if it started hard to keep running. Even with the jet change and the outboard running good I would say my outboard was down 50% on power.

I didn't change my prop, that would have helped the outboard too and something I'm doing on my next trip up that high.

Hello

The issue about modifying engine carbs for high altitude uses is that once on 0 water level will need to put back parts changed for high altitude. Would be too much work, will need to stripe down the whole carb to change jets, that's why opting to play with pitch a bit, so not to row, at that altitude, get's too tired, so 2 issues:

1.Would need to change for a smaller or bigger pitch assuming the engine has a intermediate factory propeller and has lost 50/60% of it's rated HP.

2-Would need a smaller or bigger size jet to perform better ?

One thing know for sure, at high altutudes you don't need high octane gasolines, are useless, the least octane will perform best do to resctricted air intake. According to manuals you need a jet modification starting 1,000 meters above sea level to compensate power loss.

Any ideas, comments, recommendations are welcome.

Happy Boating

 

[Titanium48]

Re: High Altitude Boating

You'll be lucky if the engine runs at all at that altitude with a sea level carburettor setup. At 5000 m the atmospheric pressure is barely over half of sea level, so the engine will be getting nearly twice as much fuel as it can burn. Even it it does run, it will foul plugs quickly. If you really have to use the same outboard at such widely different elevations and can't rejet the carb in between, you will probably need an intermediate altitude setup. Set it up for 2000 m and it should run (but not well) at 5000 m and at 0 m. You might also try running a full high altitude setup and using the choke to give it more fuel and less air at low altitude.

 

[fishrdan]

Re: High Altitude Boating

The high altitude outboard jet's I have purchased have been noted as 2500-5000', 5000-7000', 9000' and above,,, or something like that. The different jets were noted in the parts breakdown for the outboard with which altitude they should be used. See if Yamaha lists high altitude jets for your outboard.

To jet for altitude you need to lean down the engine, so a smaller jet. The decrease is supposed to be something like .001" per 2000' of elevation. My 140HP I/O had 60's (.060) jets for sea level, so I put in 55's (.055) for 9000+' elevation.

You are correct that the jets have to be changed back and forth if you are going to be using the outboard between sea level and high altitude. Kind of a pain swapping them, but at the altitude your going the engine might be useless without the jet swap.

On the prop you want less pitch, so if you have a 15 pitch, then look a 13 or a 12,,, or ?. I've only re-propped 1 boat for elevation so I can't really give you good advice on how much pitch to drop for your elevation. Hopefully someone else with more experience will chime in...

 

[Sea Rider]

Re: High Altitude Boating

Thanks guys for the tech input, sounds that high altitude boating has big tech issues attached to it, will check with Yamaha about high altitude 5 K jets and lower pitch popellers for 8 HP engine, seems that big rainbow trout will have to wait for us longer "Thumbs-Up"

Nobody has ever sibbed at that altitude down here, we plan to take a 3.20 mtr air floor sib + Yamaha 8 HP or Mariner/Tohatsu 5 HP to be the first, as a comment wrote Tohatsu tech department asking same high elevetion issues, their answer: Above 3 K cannot tell, Mount Fuji is less than 3,000 meters high Hope Brunswick will have a better high altitude tech opinion & experience.

Last 2 issues:

1-Which carb elements do you need to change: Main Jet or Slow Jet, or both, as an example Tohatsu 5 HP main jet has 78, slow jet has 52, but same 4 HP brother has: 78 main jet & 55 slow jet. Any ideas ?

2-Would you recommend to take the Yam 08 to high/alt once the engine has gone throught at least first 2/3 hours break-in process from total 10 hour period recommended using double gas/oil mix ratio.

Happy Boating