So there I am, shooting my mouth off about diesel engine maintenance and longevity, and this guy on the Hatteras motor yacht next to me says, "Hey, what's the big deal? All you're saying is the harder you run an engine, the faster it wears out and the slower you run it the longer it lasts. Any bozo can see that.”

Naturally, I was wounded by the remark, but I managed to reply calmly, "No, that's not what I'm saying." And the exchange initiated a discussion that saw the sun set and my waiting supper turn to tepid mush, and me realizing I’ll never learn to keep my big mouth shut.

Anyway, be that as it may, there are several really important points to be made about the issue. 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. Here's why.

Power in response to engine load

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.

Meaningful relationship

Perhaps even more importantly, since fuel bum varies with load, total fuel consumption is a better indicator of accumulated engine wear than engine hours. As a senior applications engineer at Caterpillar once told me, "If you run a 10,000-hour engine only under light loads, independent of rpm, it will last a heck of a lot longer than 10,000 hours before needing an overhaul." Given that fuel burn is closely linked to torque and horsepower production, and therefore heat and stress, he pointed out further that, "The most practical field indicator for determining accumulated engine wear is the total, accumulated amount of fuel burned."

Indeed, today, most major marine engine manufacturers including Caterpillar, have programs for scheduled maintenance that are based on accumulated fuel consumption, as the preferred alternative to engine hours. The principle is straightforward. The higher the engine loading, the more fuel you put through your engine in a given period of running time (engine hours), and therefore the more rapidly the engine will wear. The converse is that a generally lightly loaded engine will wear less during the same period of running time. All of which is a very strong argument for basing maintenance intervals on accumulated fuel burn, rather than engine hours.

Hours of horsepower

An engine's fuel curve depends on its rating, which relates to specific load and duty conditions. The precise nomenclature chosen varies somewhat from manufacturer to manufacturer, but some common rating designations are: 1) continuous, 2) intermittent, and 3) maximum intermittent duty. An engine rated for continuous duty can be operated at its maximum rating for its entire designed life, i.e., TBO or time between major overhauls. Continuous duty horsepower ratings are generally applied to commercial applications and are typically lower than other ratings. A rating for intermittent duty allows for more horsepower to be drawn out of a given engine, but not on a continuous basis. Yacht engines rated for maximum intermittent duty typically offer the highest horsepower for a given engine, but do so only for very brief intervals, in between which they must be operated at intermittent or continuous duty levels in order to achieve their rated TBO.

Maximum intermittent duty rated diesel engines often have the shortest projected TBO, followed by intermittent duty rated engines, and then continuous duty rated engines, for which will be projected the highest TBOs. When some manufacturers rate for intermittent or maximum duty, they limit engine hours at higher load levels to ensure their engines last the projected TBO. Others, however, say that their intermittent or maximum duty engines will last for the same number of hours as one rated for continuous duty, provided the engine's load history conforms to the specifications for the rating. Check with the manufacturer of your engines or prospective engines to see which policy applies in your case. And be sure to ask about the manufacturer’s preferred method for determining regular maintenance intervals.