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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 09/28/2018 :  06:41:38  Show Profile
quote:
The consensus I've seen is the EFI system automatically leans out the mixture to compensate for the thinner air,

quote:
Does the computer just compensate for altitude without anybody having to think about it?
I’ve even driven up at elevation in the hills outside of Boulder and the car ran like a champ.


The fuel engine management sensors automatically calibrate all altitude conditions in late model fuel injected vehicles unlike older carburetor engines.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


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Dave5041
Former Mainsheet Editor

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USA
3754 Posts
Response Posted - 09/30/2018 :  19:30:57  Show Profile
Engines will not produce as much power at altitude as at sea level. Less dense air means less oxygen. A leaner mixture, automatic on newer fuel injected cars means cleaner, smoother running, but burning less fuel means less energy is produced on each power stroke. Supercharging, including turbocharging, compensates at less than full throttle if you use a heavier foot. Still, 6 - 7 available HP should push to hull speed. I have gotten around 6 - 6.5 on long motoring trips on the GICW at 4.5 -5 kts with variable currents and a Merc Bigfoot 9.9

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle
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Erik Cornelison
Navigator

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USA
194 Posts
Response Posted - 10/01/2018 :  11:06:29  Show Profile
Hey I'm going about 5.5 knots at full throttle with my Big Foot 9.9 long shaft 4 blade prop at 7,000 msl elevation.

Yesterday it was windy for our lake and we were cruising at 6.8-7 knots with just the jib alone. I thought the motor would do a little better.


I'm originally hoped the boat would do max hull speed with the 9.9 motor at the lake, but I think the elevation and summer temps just take away too much power.

Also, although I doubt it, I may not be at full throttle when I give it full throttle, but I'm pretty sure its at full throttle. The motor runs excellent and we love it.



As an example of the power lost for anyone who boats in multiple altitudes, my Zodiac with 50HP Johnson runs:
38MPH at Arizona sea level with a 19 pitch prop @ 5500 RPM
34MPH at 3652 elevation 17 pitch prop 5500 RPM
29MPH 15 pitch prop at 7,000 elevation 5,500 RPM.

Thanks for the replies.
I don't run a full speed, its just used for calculation.

Oh and thanks for the GPH fuel burn replies, just about what I was expecting. Our Lake Powell Utah trip will be probably 150 miles without any chance of getting fuel, so I want enough gas but not too much gas. Can't count on sailing my way back (unless I take off more days off) as the wind isn't steady or predictable in the narrow canyons of Lake Powell Utah, Escalante Canyon area.

Erik



quote:
Originally posted by GaryB

Guys, need to remember he said he was at 7,800 MSL. In a normally aspirated airplane at 7-8,000' the best you will get is approx. 75% of the power you would at sea level. To get 75% you have to use full throttle.

Eric - How fast do you go at full throttle at your altitude?


Sorry Herd... I didn't read your whole post before I posted. Didn't mean to be repetitive guys.
Erik Cornelison
6th Generation Professional Sailor, First Gen Submarine Sailor.
1986 Standard Rig SW. #5234
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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 10/01/2018 :  16:58:58  Show Profile
Here's an easy calculator for HP loss at elevation. A 9.9 at 7000ft would lose 2.08hp resulting in 7.82hp.
http://www.wallaceracing.com/braking-hp.php
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


Edited by - islander on 10/01/2018 17:10:50
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Mark Maxwell
Captain

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USA
329 Posts
Response Posted - 10/01/2018 :  20:45:44  Show Profile
I think you have enough HP, I’m wondering if a 3 blade prop will help your top speed. 4 blades are usually the power thrust you need to get that heavy boat going quickly but on a lake with little to no current maybe a 3 blade will still provide enough thrust to get moving (may not jump out of the hole as quick) and then up to a higher top end?...

Mark-
'Impulse...’
1978 C25 #533 SR/DIN/FIN ~_/)~
Bakersfield, CA.
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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 10/02/2018 :  06:20:50  Show Profile
Lake Powell at 3600ft would result in a 1.07hp loss resulting in 8.83hp.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


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Erik Cornelison
Navigator

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USA
194 Posts
Response Posted - 10/03/2018 :  09:20:54  Show Profile
Remember this calculator doesn't calculate the altitude density, just the basic's. In reality the engine is probably making even less power. But a cool little calculator.

Erik



quote:
Originally posted by islander

Here's an easy calculator for HP loss at elevation. A 9.9 at 7000ft would lose 2.08hp resulting in 7.82hp.
http://www.wallaceracing.com/braking-hp.php
Erik Cornelison
6th Generation Professional Sailor, First Gen Submarine Sailor.
1986 Standard Rig SW. #5234
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.