By Whitten Hall
Regular oil changes, proper fuel filtering, and the like notwithstanding, the most effective way to derive maximum longevity from your boat’s engine is to operate it within its specified horsepower range. Contrary to common belief, this cannot be guaranteed by simply by operating the engine below its maximum rated rpm, for example, by running an engine with a maximum rated 2300 rpm at 1800 rpm. The fact is, you can overload an engine, and consequently increase its rate of wear, at just about any rpm.
An engine develops power in response to load. At any time, your engine may be producing more or less horsepower than its rating specifies. The more horsepower the engine produces, the more internal heat and stress it produces. Since these factors contribute to wear, increased horsepower (or, more accurately for our purposes, increased torque) means shorter engine life. Therefore, the key to maximizing engine life is to keep torque and horsepower production at or below the maximum levels specified on your engine’s rating curve. How do you do that, you ask, since you can’t measure horsepower without a dynamometer (a bench- or floor-mounted resistance brake)? Or can you?
Horsepower is a measure of work accomplished, and is the product of torque and engine rpm. Other factors held constant, torque is produced by combustive force in an engine’s cylinders acting through its pistons, connecting rods and attached crankshaft. Combustive force is determined by the quantity of fuel burned, which in tum depends on the throttle setting. For instance, if you need full throttle to reach 1800 rpm on an engine with a rated maximum rpm of 2100, that engine is likely developing more horsepower than it is rated to produce at that point on its rpm curve. So, that engine is likely wearing out as a faster rate than the engine manufacturer anticipates or judges acceptable. You can draw the same conclusion if your engine fails in operation to be able to achieve its maximum rated rpm or takes an excessively long time to do so.
A good way to keep tabs on torque and horsepower production is to monitor fuel consumption. By comparing actual fuel burn with that charted on an engine’s rating curve, you can judge whether the engine is being overloaded or not. If your engines are expensive units, this may be a good reason to install fuel flow metering, if you don’t already have electronic monitoring that provides the needed information. Moreover, understanding what percentage of your engines capacity for power production you are actually using gives you a good indication of a number of other key items, for example, how well your reduction gears and props match the engine’s rated power curve and, therefore, whether you can expect any improvement with adjustment to such factors.
Whitten Hall, DT&F Yacht Sales