I've been told different

[Dadilator]

With all variables other than altitude being the same, ie boat, weight, trim angle, prop, weather, humidity etc… If @ 5400rpm my speed is 50mph at sea level wouldn’t my speed be the same at 5000ft if I were able to achieve 5400rpms?

[joedog]

Engine performance is irrelevant if the (fixed pitch) propeller is turning the same RPM. Of course normally aspirated engines suffer an open throttle manifold pressure decrement of about 1" of mercury absolute pressure (about 0.5 psi) per thousand feet rise in elevation. They also suffer decreases in performance due to rises in ambient air temperature, and this effect can be significant. Increases in humidity also adversely affect performance. But fixed pitch propeller RPM is the ultimate test (if the propeller is not changed, and if it is operating normally). The suggestion that lower ambient air pressure can cause an increase in cavitation is interesting, but I think it is ultimately unsupportable. Cavitation occurs when local pressure is lower than the vapor pressure of water. The vapor pressure of water at 20 deg C is 2.3 kilopascals, or .3 psi. A propeller operating at 8 inches depth experiences a static pressure of atmospheric (nominally 15 psi) plus about .3 psi hydrostatic pressure (about 15 psi in 33 feet). At 2000' the atmospheric pressure is about 14 psi and the hydrostatic is the same, given equal water density. To achieve cavitation the pressure must drop from the 14=15 psi range to the .3 psi range. I would think that any minor differences between 14 and 15 psi atmospheric would be obscured by the large (hydrodynamic) drops necessary to achieve .3 psi locally.
Hope that helps.

Just messing with you!
Welcome to the site.

To be alittle more helpful although the junk above is true.
A general rule-of-thumb regarding horsepower vs elevation is that you lose three percent of your horsepower for every 1000 feet gain in elevation. This applies to normally aspirated (not super-charged or turbo-charged) engines.
I think speed loss due to the elevation change is understandable when considering the above. I think the lose of horse power would also result in lose of speed. But I really don't know.


You lose speed at 5000 elevation with an internal combustion engine.
ALWAYS. My opinion only.

[nofearengineer]

An outboard motor has no slip between the crankshaft and the prop shaft; there is no clutch or torque converter like in a car transmission. Therefore, a certain rpm on the crankshaft will always produce the same relative speed, no matter what other conditions are.

However, I am skeptical that you could produce the same power at the same rpm at two elevations 5000 ft apart. You'd have to be playing some major games with your air/fuel ratio. Even if possible, it would likely be very bad for the motor, and the EPA would probably be chasing you all over the lake for the black, sooty exhaust you'd be spewing out.

A tradeoff would be to slightly raise the gear ratio between the prop shaft and crankshaft, and live with the loss of torque and throttle response.

[joedog]

I have a house on Lake Washington- 60ft above sea level.
I have a house on Lake Pend Oreille- 2063ft above sea level
Both fresh water.
Same conditions/load: I lose 3 mph at Lake Pend Oreille
The air is less dense. I lose rpm.I lose speed.Every year.
All the time.It's not a mystery.
Your situation sez: Same water/conditions/load/SAME RPM.
You HAVE to be going the SAME SPEED.

Above stolen from a boating forum.

[lilmule]

Same for winter one will rpm more and lose summer,alt makes the amount of air taken in be less for any given hp,one at sea level will if tuned for there lose about 3 mph at 3,000 or more.Take same motor to Albuquerque, New Mexico,lose about ten mph or more.Rough est about 1 mph per 1000 ft.

[Dadilator]

Originally Posted by nofearengineer

However, I am skeptical that you could produce the same power at the same rpm at two elevations 5000 ft apart. You'd have to be playing some major games with your air/fuel ratio. Even if possible, it would likely be very bad for the motor, and the EPA would probably be chasing you all over the lake for the black, sooty exhaust you'd be spewing out.

A tradeoff would be to slightly raise the gear ratio between the prop shaft and crankshaft, and live with the loss of torque and throttle response.

Thank you nofearengineer, I currently run at 5000ft WOT=54-5600rpm's 60mph. I was talking to a marine mechanic that was telling me I'd go faster in Texas because of the lower altitude running the same rpm's with the same prop and that didn't make since to me. That is where my question came from.

[Brudd]

He was half right. You'll go faster but it will be because you'll be able to achieve higher max RPM. Unless like someone stated you have a rev limiter at 5400.

[Brudd]

Originally Posted by nofearengineer

However, I am skeptical that you could produce the same power at the same rpm at two elevations 5000 ft apart. You'd have to be playing some major games with your air/fuel ratio. Even if possible, it would likely be very bad for the motor, and the EPA would probably be chasing you all over the lake for the black, sooty exhaust you'd be spewing out.

True enough but not what he asked. He's already turning that at 5000 ft so it's to a question of if he can achieve the same performance at altitude but will he go the same speed if all other factors remain the same.

[Dadilator]

It seems to me a straight forward calculation of rpm’s and pitch, (all other variables being the same).
If 3000rpm’s renders 30mph at sea level wouldn’t 3000rpm’s render the same speed at 1000, 2000, 3000, 4000, etc. feet of elevation?

[lilmule]

You wont get 3000rpm at alt,if you got that at sea level you will get less rpms-then less speed.A motor that does 6000 rpms at sea level would do about 5600 -5700 at 3000 ft,at a guess.The higher one goes the thinner the air,you or your body wont function as well as alt increases,at 10000 most people want more oxygen.
And other than a difference in density of water yes same speed for same rpms just wont get the same rpms,your starving the motor of needed oxygen the higher you go.

[Brudd]

You missed the point that he is already at 5000 ft and turning 5400 rpm. The question is not will the engine achieve the same max rpm but will he see the same speed at the same rpm.

[lilmule]

Didnt miss anything he cannot turn same rpms at each alt,unless retuned at each and an oxy bottle attached,if the tach is saying so and a loss of speed which would be correct replace the tach as its incorrect.Not possible,unless lab conditions and an apparatus that operates like a catalytic converter weighing 1/2 ton.
If an award of a million dollars was offered and a sluice to the top ,doubtfull anyone could claim the prize by climbing pikes peak,with a standard mfg outboard in a bass boat,as mfg.
Of course if he added 20 hp to it or more with each increase in alt,could achieve same rpms and same speed ,but its a loss of hp so never will.With this loss eventually one has almost no hp at the highest a motor will operate,even airplanes have cielings,like 18,000 ft or 20,000 for the older like ww2 ones requiring the operator to wear oxy beyond 10k,with oxy added via boost or it wouldnt run at 18,000,no boost or bottles normally in bass boats.Race boats sometimes run nitrous and or oxy on funny bottles,not a life lengthening event for the motor.
As mfg if one took a boat and motor from fla to col,it would lose speed and rpms -no way around it.

[Brudd]

He didn't ask if he would turn the same rpm at each altitude. He asked if the speed would be the same when the rpm was the same.

[nofearengineer]

Brudd, you have serious reading comprehension issues. My answer was 100% to the point of the man's question.

[lilmule]

Of course if same rpms same speed or close only water density would make a difference but will never achieve it,if tack and speedometer or gps is saying all is the same replace as inaccurate.
Agree with nofear

[Brudd]

Originally Posted by nofearengineer

Brudd, you have serious reading comprehension issues. My answer was 100% to the point of the man's question.

The first part was accurate but the part I quoted answered a question he did not ask. That part correctly answered the question "can I achieve the same power output at 5400 ft that I do at sea level?" and you gave reasonable advise on what to do to compensate for being at higher altitude. But he's already stated that he achieves 5400 rpm at 5000 ft. So, he's not asking what will happen when I go from sea level to over a mile high. Even though in the initial question it was stated in a converse fashion, he's actually asking if I go 50 mph while turning 5400 rpm at 5000 ft will I still go 50 mph at sea level while still turning 5400 rpm? We'd all agree he can probably go faster and turn a higher RPM at sea level but again, he didn't ask that. My reading comprehension is just fine.

[Tavery5]

"What's the airspeed velocity of an unladen swallow?"

[nofearengineer]

Originally Posted by Brudd

The first part was accurate but the part I quoted answered a question he did not ask. That part correctly answered the question "can I achieve the same power output at 5400 ft that I do at sea level?" and you gave reasonable advise on what to do to compensate for being at higher altitude. But he's already stated that he achieves 5400 rpm at 5000 ft. So, he's not asking what will happen when I go from sea level to over a mile high. Even though in the initial question it was stated in a converse fashion, he's actually asking if I go 50 mph while turning 5400 rpm at 5000 ft will I still go 50 mph at sea level while still turning 5400 rpm? We'd all agree he can probably go faster and turn a higher RPM at sea level but again, he didn't ask that. My reading comprehension is just fine.

You're really picking a fight you can not win.
The OP used the word "if". This would imply some uncertainty whether it could be achieved. I expressed my own doubts about whether it could be achieved.

Hey guys! Did you check the new site rules update?!?! We're not allowed to offer any more information in our responses that isn't strictly requested by the thread starter!

[Brudd]

That is simply how he posed the question. He did later confirm tht he does actually achieve that performance at altitude. You can post anything you want but you then also have to accept that others can too. I didnt tell anyone not to post just that they missed a few details in the question. I'm not picking a fight with anyone and it unfortunate that this is how you perceive viewpoints that are not in agreement with yours.

[lilmule]

Uh whatever hes getting at alt,would be a tad more rpms and speed at lower elevation,doesnt mean he cant do that speed at alt means he can do more lower.And if from lower going higher it becomes less.Has nothing to do with viewpoints simple physics.