The tachometer, or tach for short, is the instrument that measures the revolutions per minute (RPM) of your boat's engine and displays the numbers on either a calibrated analog dial or a digital display. When you're trying to assess your engine's performance, this is the instrument you will consult.

For optimum performance, your boat's engines should run at the recommended wide-open-throttle (WOT) operating range, which is expressed as a specific horsepower at a specific number of revolutions per minutes (RPM). Understand that this is a range, not a single number.

Your tachometer, then, tells you whether or not you are within the recommended WOT range, or if you are running at levels that are too high or too low. If the RPMs are too high, you need to look at the possibility you're over-trimmed. If not, consider a higher pitch prop. Conversely, if the RPMs are too low, you may be under-trimmed, or your engine height or prop pitch may be too high.

You can also use your tachometer to establish a speed curve for your boat. What you want to do, here, is to make several timed runs at different RPM levels, which you will record. The end result is a simple chart that details your speed through the water at various engine settings.

The bottom line is that you want your boat to run at the recommended WOT. Once you are certain your boat is correctly propped for that to happen, any unexplained deviations (explained changes include changes in climate, elevation or load) should alert you to the possibility of potential problems. These are the kind of problems you want to catch early, and your tachometer is the tool to help you do just that.

– Linda Hoff