Fine-tuning fuel consumption

Editor’s note: Long-distance voyaging by power boat is a trend that is coming of age. A perfect example of this is the recent Nordhavn Atlantic Rally. In the summer of 2004, 18 power voyaging boats made the passage from Ft. Lauderdale to Bermuda, to the Azores to Gibraltar. Given this watershed event in the growing annals of power voyaging, we are starting a Power Voyaging column that will explore issues of prime importance to those setting out on passages accompanied by the reassuring rumble of a big diesel.

In this first installment, Contributing Editor Chuck Husick explains how engine performance data can be used to fine-tune efficiency and improve fuel consumption.

The performance characteristics of virtually every type of internal combustion engine can be portrayed by a set of interrelated curves. For the reciprocating engines used in power voyaging boats, a set of four or five curves is usually sufficient and can tell us what we need to know to operate the engine properly and efficiently. This time we’ll concentrate on one of those curves, the specific fuel consumption (SFC) curve.

The accompanying figure presents a typical SFC curve for a typical 360-hp turbocharged marine diesel engine, a Yanmar 6LYA-STE (this engine is the standard powerplant in the Nordhavn 35 power voyager). The curve portrays the engine’s specific fuel consumption, the amount of fuel it will consume per horsepower-hour while delivering maximum power across the rpm range. The curve is plotted against the normal useful engine operating speeds from 1,600 to 3,300 (maximum allowable for this engine).

The shape of the SFC curve illustrates the changing efficiency with which the engine converts the chemical energy in diesel fuel into mechanical energy. Efficiency at low speeds &mdash for example, 1,700 rpm &mdash is less than at the engine’s most efficient operating speed, 2,700 rpm. The decrease in efficiency at low speed reflects the fact that at lower speeds, the engine is developing less power while the parasitic losses in the engine remain fairly high.

The parasitic losses represent a form of operating overhead; these losses are incurred when the engine is running. They include the power needed to operate the seawater cooling pump, the freshwater cooling loop circulating pump, the mechanical fuel pump and injection pump, the alternator’s cooling fan (a load that exists even when there is no electrical load on the alternator), and the various frictional losses in the engine.

As the engine speed increases, these necessary and unavoidable losses diminish in relation to the total amount of power being produced, and they reach a minimum at about 2,700 rpm. The SFC increases as engine speed increases beyond 2,700 rpm. This increase in SFC (a decrease in efficiency) results from a combination of factors, including the decreasing efficiency of the air-intake system and the greater difficulty in expelling spent combustion gases from the cylinders. The engine behaves much as we do when running hard, our breathing becomes more difficult and oxygenation of our blood less efficient.

Therefore, running this particular engine at speeds close to 2,700 rpm will produce the biggest bang for the buck and should give you the best fuel consumption.

However, it is important to note that the SFC numbers apply when the engine is fully loaded at each rpm. As we will see, our fixed-pitch prop won’t allow us to load the engine that way. If we select the correct propeller for our boat, the engine will be under maximum load only at maximum rpm. The SFC is stated in both grams of fuel per kilowatt-hour (0.746 kw = 1 hp) and in grams per horsepower-hour. The average fuel consumption of a marine diesel engine is about 0.375 lbs of diesel fuel per horsepower-hour.

Although running the boat’s engine at or near the speed that yields the minimum SFC is desirable from a fuel consumption standpoint, engine speed will be determined primarily by the voyaging conditions, wind, sea state, schedule and, in some boats, allowable noise level. Cruise engine speeds in the range of 500 rpm below to 300 rpm above the minimum SFC point will likely produce the best overall performance and economy.

In an ideal situation, a displacement-hull vessel would run at or near hull speed with the engine developing about 65 percent of maximum power, about 400 rpm below maximum (84 percent of maximum rpm). However, many trawler-type boats have substantial power available (given the desire of many owners to run at a speed well in excess of displacement hull speed &mdash 1.3 x square root of waterline length in feet). With these vessels it may be necessary to run the engine at no more than 25 to 30 percent of maximum power (65 percent of maximum rpm) to achieve a comfortable ride in other than a flat sea.

While operation at this rpm will be slightly less efficient from an SFC standpoint, the load on the engine will still be high enough to avoid deposit buildup that can accompany continuous running at very low rpm and low power loadings.

In future power voyaging columns we’ll look at other useful engine curve data and what these curves can tell you about the use of the engine in your power voyaging boat.

By Ocean Navigator