Know Your Diesel’s Rating

Modern computer-controlled marine diesels are a marvel of engine technology, and like all technologies, some basic understanding is helpful.

Onboard computers make it possible for most modern marine diesels to start quickly and easily without smoke, rattle or clatter. Older, non-computer-controlled diesels, like many that are powering older used vessels, were not machined to the fine tolerances of current engines. Their fuel pumps and injectors dumped fuel into the cylinders until the engine fired. They smoked and rattled a lot on startup, and continued to do so until the engine warmed up.

Those older diesels had, basically, two duty cycles: continuous and intermittent. If a vessel, such as a small tug, had to have power applied pretty much all the time it was operating, it was generally run at 20 percent less than top rpm. Many of those older engines had mechanical rev limiters installed to ensure they did not overrun their rating.

The engines on a boat we tested recently made full use of the computer. They had five different ratings, M 1 through M 5. Most engine manufacturers have a similar rating system.

M 1 is the continuous rating, and an engine can run continuously, at full power, for 24 hours a day between recommended service intervals. Such engines generally operate more than 3,000 hours per year (as displayed on the engine hourmeter) and can be found in long-haul tugs and tow boats, fish and shrimp trawlers, and displacement-hull fishing boats of more than 60 feet. At this rating, the engines we tested produced 425 hp at 1800 revs.

An M 2-rated engine will likely be found in long-range ferries, larger passenger vessels, displacement-hull fishing vessels of less than 60 feet, short-range tugs and tow boats. Such engines can operate continuously at full power for no more than 16 hours during a 24-hour period. Thereafter, they have to be slowed to cruising speed — at least 200 rpm less than the rated top rpm. After eight hours at cruising speed, they can once again be operated at full power. At this rating, our tested engines’ speed was limited to 1900 revs and power output to 500 hp.

The M 3 rating is geared toward vessels that require full power for no more than four hours in any 12-hour period. The remaining time must be at cruising speed. Vessels likely to use engines at the M 3 rating include coastal fishing vessels, offshore crew boats, research vessels and short-range ferries. Such vessels operate up to about 2,000 hours a year, and our test engines produced 575 hp at 2000 revs.

M 4-rated engines are for vessels requiring full power for no more than one hour out of every 12, and these would be vessels that operate up to 800 hours per year. After the one hour at full power, the engine must operate a cruising speed. Vessels using the M 4 rating will include inshore crew boats, charter fishing vessels, dive boats, pilot boats and planing-hull commercial fishing boats. At this rating during our test, the computer limited top engine speed to 2100 revs, which produced 650 hp.

Engines rated for M 5 use find homes almost exclusively in recreational boats. Engines with this rating can run at WOT for no more than 30 minutes before they have to throttle back to cruising speed for the remainder of the eight-hour period. However, after that eight-hour period, the engine is not supposed to be operated for the remaining 16 hours of the 24-hour day. Top engine rpm for this rating was 2200, and the engines produced 750 hp.

It is clear that with the various computer chips available for the same basic engine, a knowledgeable naval architect or marine engineer will be very helpful in selecting the right reduction gear and prop to make effective use of the engine selected for any given boat. The architect will also consider engine load factors.

While the selection of the right engine for the right hull is more complicated than with the older diesels, the efficiency of the modern engines is far greater than of diesels of the past. Experience has shown that modern engines burn about 25 percent of the fuel of an older engine of the same power. Servicing the newer engines is also much easier, and with proper service and use, the new engines will be able to operate twice as long between major overhauls as the older ones. If a boater wants to change his engine rating, a service rep comes aboard and reprograms the engine computer.

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