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Electric and Diesel – A Match Made in Heaven for the Commercial Operator

Courtesy of Power and Motoryacht

By George L. Petrie

Rising fuel costs are always a concern.  As a result, achieving improvements in fuel economy has become a high priority throughout the marine industry, as it has in many other segments of business. Engine manufacturers have risen to the challenge, delivering sophisticated new power plants that not only burn less fuel but also produce fewer emissions. However, the fact remains that diesel engines will generally run most efficiently when they’re operating near their full rated capacity.

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And that poses a problem for vessels in commercial service. For example, tugboats have big engines to develop lots of towing power but may spend much of their time operating at well below their maximum towing capacity. Or consider that crew and supply boats in the offshore industry must run at high speed to and from oil rigs that may be 100 miles or more from shore, where they then spend several hours idling alongside the rig while supplies and personnel are transferred. And last but not least, there are cruise ships, floating cities with power requirements that vary substantially, depending on whether the ship is in port or underway.

One of the ways that cruise-ship owners, tug-boat operators, and others in the commercial sector have confronted the problem posed by widely varying power requirements is to use diesel-electric propulsion systems. Rather than having a diesel engine drive the propellers directly, a diesel-electric system typically uses multiple diesel-powered gensets to produce electricity, which then powers an electric motor that turns a propeller shaft.

Using the diesel engine to make electricity, then using that electricity to drive an electric motor seems like a woefully inefficient process. And if power demand is constant, a diesel-electric drive can be slightly (about five percent) less efficient. But for circumstances where power demand is widely variable, the versatility of the diesel-electric system can yield big dividends, because gensets can be brought on line or shut down as power demand changes.

Other than efficiency, one of the big advantages of a diesel-electric drive system is that big propulsion engines need not be located in line with the prop shaft; electric motors of the same kilowatt capacity are generally much smaller. This allows the yacht designer more latitude in deciding where to locate the gensets. And, in fact, the propulsion motors do not even have to be located inside the hull. One notable implementation of diesel-electric propulsion is the Azipod, developed by the German firm ABB. The Azipod system utilizes an electric motor mounted externally in a pod beneath the hull. The motor drives a fixed-pitch propeller mounted at one end of the pod, and the entire pod is free to turn 360 degrees. By locating the motors outside of the hull, problems like alignment, noise, and vibration that are associated with a conventional shaft, strut, and bearing drive train are eliminated. Moreover, because the pods can turn, they offer extraordinary maneuverability compared to a normal shaft and rudder system on a boat.

As long as forward thinking shipyards remain open to out of the box power systems, creative solutions can be found.  Northern Lights manufactures a line of fully customizable generator sets, up to 545kW – 60 Hz (475 kW – 50 Hz), ready to respond to the challenge.

Formerly a professor of naval architecture at the University of New Orleans, George Petrie is a professor of naval architecture at the Webb Institute and continues to provide consulting services to small craft designers, ship operators, and other members of the marine industry.


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