High-Altitude Boating: Where Did the Power Go?

All but two of highest lakes in the United States are in Colorado. The highest, for information’s sake, is Lake Muriel in Washington, but it’s more like a superheated puddle in the crater of Mount Rainier, so there likely aren’t many boaters on it.

Grand Lake in Colorado is at an elevation of over 8,000 feet. Boaters there can expect a 24-percent decrease in engine power compared to performance at sea level, because of the reduced oxygen at that altitude.

Even if you don’t live in Washington or Colorado, your family might enjoy traveling to the mountains on vacation. If you’re a boater, you probably take your boat with you, and that’s when you realize your boat can barely pull up one skier, let alone two. Call it mechanical altitude sickness, or whatever suits you, but the reason your boat doesn’t run as well in the mountains is the same reason you run out of steam on a ski run long before you normally would back home. Quite simply, there’s less oxygen in the air, and oxygen is essential to the controlled explosions inside your engine.

“The rule of thumb, for us, for fuel-injected engines, is approximately a three-percent power loss per thousand feet,” said Robert Grantham, product director for MerCruiser in Stillwater, Okla. “That’s typically what you’ll find on a normally aspirated four-stroke engine.”

So if you’re 7,000 feet up, which isn’t that high, figure about a 20-percent loss in power. That means your 200-horse engine is now a 160-horse engine.

For fuel-injected engines it’s less of an issue, because input from barometric pressure and manifold absolute pressure sensors helps compensate for the decreased oxygen: The fuel injection system feeds in less fuel, so it doesn’t create another problem by fouling spark plugs. If your boat has carburetor, well, it’s more of an issue.

“At higher altitudes, they go rich because there’s not as much oxygen in the air and you’re still giving it the same amount of fuel that you would give it as if you had calibrated it at sea level,” Grantham said.

The only real fix for a carbureted engine is to rejet the carburetor—which sounds like fun while you’re on vacation doesn’t it? But even then it will still be down on power. In terms of a course of action, here’s what all this technical gobbledygook means.

If you have a fuel-injected engine, consider taking along a “high altitude propeller.” It will be a bit lower in pitch, but it will make your boat a little more happy while you’re in the air up there. If you have a carbureted engine, a high-altitude prop is still a good idea, as is a spare set of spark plugs.

Air sickness bags not necessary.

Brett Becker


  1. Chad says:

    I have a 2007 Stratos 285 XL Pro with a Yamaha V Max 150hp. I fish lakes all over Colorado and fish Lake San Cristobal all the time, but this was the first time I put my Stratos on Lake San Cristobal. Elevation 9,003ft above sea. The first day all 6 spark plugs were fouling out and soaking with oil. Need some information on this problem, do I need to run a different Mass Air Flow Sensor? Is there a programmer for High Altitude? I had to clean off my plugs after ever 4hours of trolling. Please if anyone has some information on why the plugs got oil soaked so fast or if there is a fix to this problem. I also fish All over the southern states and this problem never happens, so I really am thinking it has to do with the ECU not adjusting to the high Altitude.

  2. Fred Daynes says:

    Back in the day when I was tuning my own Carburetor on a two stroke MAC 101, the process was to run the engine rich then lean out until the oil just started to dissappear and was burning clean. I imagine that this is much like what your experiencing here, Running rich.
    With California emissions at hand and leading the nation, much of the abilities to self adjust carbs or even the ECU have been made difficult. In the 1980’same we would get new vehicles that would need adjusting to bring the dwell in line that would require drilling out government regulated lockouts of the adjustment screws to clear such problems. Think of finger size freeze plugs found on the carburator.
    If your plugs are fouling out that fast, then your not fully burning up the fuel. Your running rich with the oil to gas ratio. If your jetting a oil mix then adjust this down. If your running a premix, then figure on adding a little less oil to clear your own problem. There are always several different ways to attack the problem. But the ultimate goal is to have a clean burn. You could also run a hotter plug. The longer the inside element is from the center core the hotter it will run and thus a 4 stroke burner engine will clean it’s self up better and prevent fouling.
    Good luck

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