In the constant quest for more speed, boaters are always looking to squeeze the most horsepower from their engines. But when boats operate on high altitude waters like Lake Tahoe, skippers will notice a definite decline in their boat’s performance.

Boat engines, be they two-stroke outboards or four stroke inboards, extract a portion of the potential energy contained within the fuel, and transfer that energy to the propeller. Within the engine, the cylinders contain the combustion process. Air and fuel enter the chamber, are compressed by a piston, and then ignited by a spark.

The rapidly expanding gasses produced during the combustion exert a very high pressure on the piston, forcing it down the cylinder shaft. The downward motion of the pistons is transferred into the rotary motion of the propeller by various shafts, linkages, and gears.

So what does Lake Tahoe have to do with power output? It is a matter of molecules.
At low altitudes, the molecules of any gas, such as air, will be spaced more closely together. This is because the air pressures at low altitude force the molecules together. At higher altitudes, there is less of this squeezing force, so the molecules are more spread out.

Just a few feet in elevation make a difference. Blow a little air into a balloon, and when the balloon is brought down ten feet to the bottom of a swimming pool, it becomes much smaller. The greater pressure at the bottom of the pool has forced all those air molecules closer together.

A boat engine running at sea level packs in a certain number of air (and thus oxygen) molecules into every intake cycle. While the volume of air filling each engine cylinder at Lake Tahoe is the same at sea level, there are less oxygen molecules contained within each cylinder up at 6,225 feet.

With less air molecules available for the combustion process, the engine will produce less power. In addition, carbureted engines tend to run too rich at high altitude, further reducing the efficiency of the burn.

There are a number of countermeasures available that will increase an engine’s efficiency and power at altitude. A ram-air induction or turbo-normalizer system uses a compressor to force more air molecules into the cylinders. While it can increase an engine’s performance at altitude, it will definitely put a large dent in one’s wallet.

Also useful, but still expensive, is to convert an existing carbureted engine to a fuel injection system. It will largely reduce the fuel-rich mixture problems at altitude. Fuel injection also atomizes the fuel more finely and evenly, increasing combustion efficiency.

Another option for a carbureted engine is to re-jet the carburetor to optimize the fuel-air mixture. This works well when a boat will be used exclusively at a given altitude. Stock carburetors are jetted for sea-level use.

Propeller choice should be considered carefully at Lake Tahoe. A propeller with a flatter pitch will give the boat greater ability to achieve hull planing speeds, and to maintain recommended engine rpm’s at wide open throttle. Changing screws does not give an engine more power, but it does alter how available power is transferred to forward thrust.

The immutable laws of physics will continue to conspire against boaters on high altitude waters. But while boat performance at Lake Tahoe may be lower, the enjoyment factor up here is definitely higher.