Summer, that time of year boater's so welcome, is actually hard on boats. In fact, your boat most likely runs much better in the spring and fall. The problem is that summer weather conditions–higher temperatures, higher humidity and low barometric pressure–negatively impact engine efficiency. The end result is that engine power is reduced and your boat speed may decrease by as much as two to five miles per hour, with not much to be done for it except wait for cooler, dryer air.

As with many things in life, one problem begets another. In this case, the summer-induced diminished horsepower of your engine means that your propeller, a model of efficiency in the spring, no longer turns within the recommended revolutions per minute (RPM) at full throttle, but rather operates at something less than the recommended figure. This circumstance reduces horsepower and speed even further. Fortunately, switching to a lower pitch propeller during this summer period can provide a boost to the RPM, somewhat offsetting this secondary loss.

For those who are interested in the whys and wherefores of the matter, internal combustion engines are designed to operate using a combination of air and fuel. The thing is, as air gets warmer, it loses some of its density, and less dense air has less oxygen in it. So as you head into the dog days of summer, the optimum air-to-fuel ratio is changed due to reduced oxygen levels, yielding poorer combustion, which in turn leads to reduced engine power.

The bottom line is, if it's hot and humid out and you think your boat is running sluggishly, it probably is. And now you know why.

– Linda Hoff