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

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