Motor Performance At Altitude Question

[keninaz]

I am looking at an early 1970s Boston Whaler, a 14' model.

But the guy is tell me that recently at 6500' with 4 people on board the boat with a 35 HP motor (about the same year) would not do 10 MPH.

He said he suspected the carb needed adjustment but then said he was told that he needed to change prop pitches at altitude which sounds strange to me.

Any ideas?

[J Francho]

What?

 

Go find a reputable boat mechanic and have it checked out after a test drive.  If it's hinkey, walk away.  There's enough that can go wrong on a good boat - you don't need to start with a bad one.

[keninaz]

That is what I am leaning towards at this point as it doesn't make any sense to me.

Maybe I will just keep looking for another boat.

[J Francho]

Test drive it, if there's something wrong...you'll know right away.  If the seller wont do a test drive, forget it.

[Logan S]

Motors do lose performance at altitude. We have a lake in western MD that is about 2500' above sea level where you can see a decrease in performance. Some guys switch out to a lower pitched prop to help with hole shot...I don't, but I do have a slower hole shot up there.

What he was describing is not inaccurate, but I would still get that motor checked out...

[Glenn]

It's common with carburated outboards to have performance issues at higher altitudes.  Changing props used to be common practice, but has waned since the introduction of fuel injected models in the late '90s.

 

That said, if it can't push the boat more than 10mph at WOT, then something's wrong.  Even at 35 horsepower, it should do better than that.

[keninaz]

Yes, as the nearest place is some 45 miles from where the boat is located that I could try it I will just pass on it.

[Raul]

If 4 people can row faster than a 35 HP then there´s something really wrong with that 35 HP.

 

I bet my 12 ft tub runs faster than 10 MPH with me, my nephew, my compadre, all our stuff and it´s powered by 6.5 HP; and we are at 1,600 mts ( 4,800 ft )  above sea level.

[Nick B]

Several years ago I moved out to New Mexico and took my boat from sea level to 5000 ft elevation. There was a slight loss in power maybe 2-4mph but not much. I did make some adjustments to my carb to adjust the rich/lean on it. When I moved to Arkansas I only adjusted the rich/lean again and got the 2-4 mph back.

[Heron]

Is it 35 horsepower? Or 35 gerbilpower?

 

But yes, I concur, what he is describing is not entirely inaccurate.  The performance of engines that rely on a fuel/air mixture are effected by the changing air density, relative to altitude above sea level. 

 

However, only 10mph?  Altutude aint the only problem with that particular motor.  Sounds like a lemon. 

[Raul]

Or 35 gerbilpower?

 

OMG, that's too funny !

[WRB]

At 6500' altitude you loose 25% of your horse power and must use smaller jets in carbs and 25% less pitch than at sea level. The engine is basically a 28 hp over propped and too rich fuel mixture when at high altitude. Put 4 adults in a 14' Whaler running poorly and over propped it may not be able to plane.

Tom