altitude

[Apadilla]

when going from sea level to high altitude do you increase or decrease the diameter of prop.thanks for imfo.

 

[QC]

Re: altitude

Generally you leave diameter alone and decrease pitch. For all intents and purposes you should leave diameter out of your deliberations. If you are talking a real situation vs. theoretical, read the stickies at the top of this section. That info will help take this discussion further. What do you have today and at what altitude do you use it?

 

[steelespike]

Re: altitude

I believe you lose about 3% of power for each 1000 ft above sea level.
And thats if it is jetted right for the altitude.
If you expect to be boating exclusively at higher altitudes with a carburatered motor you should consider
rejetting for the altitude.

 

[Philster]

Re: altitude

You should be prepared to drop pitch.

I would say you can get an idea in advance of what the likelihood is. If you're running at the very top of the engine's operating RPM now at wide open throttle (WOT), the given drop-off in performance from altitude might only drop you into the middle of the engines max RPM range, so you don't need to drop pitch in that case.

However, even if your educated guesses would indicate that you won't need to drop in pitch, you should still be prepared to drop pitch in case any other less-predictable variables arise:

>More humid
>hotter than expected.
>Or, for whatever reason, your engine just happens to lose too much oomph at higher elevations. Engines, carbs and electronics are fickle things.

So, how does she run now? Worth knowing as you might be able to make an educated guess that you'll be ok. Regardless, if it were me, I'd still be prepared to step down in pitch.

How prepared (1-2 steps down ? ) depends on how she runs now to redline at WOT and how much of a change you are making in elevation, temp, humidity, etc.

 

[fishrdan]

Re: altitude

High altitude, how high?

As the others have said, more or less diameter is not a consideration for going up in altitude, decreasing pitch what you need to look into. Though, as pitch decreases, diameter usually goes up.