Outboard performance at high altitude

[DocBob]

I currently run a 17.5 ft bass boat with a 150 outboard (Mariner) running a three blade stainless steel prop (unfortunately I don’t know the size or pitch, it was on the boat when I bought it).<br />I will be moving to Colorado, and will be boating on a lake at about 7000ft above sea level.<br />I can currently able to get about 55-60mph at sea level, but was told I will need a different prop at high altitude. Generally, is this true? If so, what will be my loss of performance if I keep this prop?<br />Will I likely need to change jets as well?

 

[LubeDude]

Re: Outboard performance at high altitude

I believe you "WILL" have to rejet. The stock jets are tuned to about 2500'. You are going to run very rich at 7000' You will need to drop at least two sizes per 2,500' on your primary jets and open up you Idle jets the same. 12 X about $3.50 per jet.<br /><br />You can try doing it with just a prop change, but I think it will be to no avail.<br /><br />Youre probably in the neighborhood of a 14.25 X 25P on your prop now, and you will have to go to at least a 23P and probably a 21 to get up on plain at that elevation. Youre going to load up a lot though, and I think you should go with the jet change.

 

[walleyehed]

Re: Outboard performance at high altitude

At 4000ft, I turned a 15 pitch at 6000....now at 610ft, I can turn a 19 pitch the same RPM.<br />On J/E, we drop .002 at 4000ft, and advance timing 1-2 degs. At Loveland (6000) they usually go .005 smaller, and about 3 degs. advance.<br />Each engine is different on mains, but I'd say start at .003 smaller mains, about .005 bigger on Idle if it's bleed air, or .002 smaller if they are idle mixture equipped.<br />At Dillon, (9000Ft) we run 6" less pitch, .006-.007 smaller on mains, and at times, have to remove the bleed air orfices completely.<br />Before the optical ignition came about, we were advancing timing 7-8 degs at 9000ft.