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. But I would like to see experimental or clinical data.

Hope this helps.
Just messing around with ya, sorry.
My understanding is,
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. Decrease in horsepower should equate to loss of speed too.
Now I've never traveled great elevation changes with boat myself so I really DON'T know.

Almost forgot, welcome to the site Dadilator!

[bamabassman]

this is new to me. and i do NOT know the answer. maybe nofearengineer can help? this is MATH and he is a wizard at math. or maybe the capt may know. interesting really. i wanna know the answer now.

[Dadilator]

Originally Posted by joedog

My understanding is,
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. Decrease in horsepower should equate to loss of speed too.
Now I've never traveled great elevation changes with boat myself so I really DON'T know.

Almost forgot, welcome to the site Dadilator!

I agree with you in the fact that higher elevation causes decreased HP But even with the decreased HP, if I can achieve the same RPM's, wouldn't my speed be the same?

And thank you, Glade to be here!

[Brudd]

If you flying an airplane I'd say you speed would be less do to the thinner air at that altitude resulting in lower propeller efficiency. With water I think, but don't know, that the density of water will not have changed that much with the change in altitude. If my assumption is true you should expect the same speed under conditions. If others are suggesting otherwise I'd guess you've got some shade tree engineers in play here. They've heard you'll have poorer performance at altitude but probably don't understand why. Truth is you won't be able to achieve the same max RPM so you won't get the same top speed. You start throwing in words like variables and other ideas related to scientific method and your going to lose people pretty quick. They just know what they know without understanding why so they are not equipped to discuss it in this manner.

[Dadilator]

Originally Posted by Brudd

Truth is you won't be able to achieve the same max RPM so you won't get the same top speed.

I'm achieving 5400 now at 5000ft and I'm sure It'll be easier at sea level. Some one told me I'd go faster at sea level, (at the same 5400rpm), just because of more HP. I beg to differ and posted the thread to get backend support

[lilmule]

You wont go the same 5400 rpm with 5000 ft of elevation difference unless you have a limiter that low,other wise an rpm increase for each 100 ft due to additional air available,somewhere comes into play however since it was tuned at higher wont be able to use it all,but still an increase in both rpm and speed or a major increase in slip factor.
Normally its a sea level motor in reference to hp loss when taken to higher elevation ,not the reverse.You would still gain some bringing it down but if staying would need a tuneup at that altitude to use more of whats available.So you will gain going down some and definately lose going up.,from 5000 ft.