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Electric and Diesel – a Match Made in Heaven

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.


And that poses a problem for recreational as well as commercial craft. Many yachts are fitted with engines capable of delivering speeds of 20, 30, 40 knots or more, yet their owners cruise at far slower speeds most of the time. While they’ll certainly burn less fuel at reduced speed, those big engines are not performing with maximum efficiency at very low load factors.

The problem can be even more acute 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.

One of the early adaptors of Azipod propulsion in the megayacht arena is the German builder Lurssen. It had a client who demanded that his yacht (originally named Air but now named Ice) be as clean and environmentally friendly as possible, with low emissions, minimum vibration, and sound levels like that of a cat on plush carpet. (The yacht’s emissions standards are so stringent, the yard proudly touts her as its response to the Kyoto agreement.)

To meet these criteria the yacht was fitted with eight diesel gensets, mounted in pairs on elastically supported foundations located in four separate rooms. The yacht’s power-management system is designed to maintain the gensets operating as close as possible to 95 percent of rated capacity, their peak efficiency. Moreover, the diesel in each genset is fitted with a “soot” cleaning system (not feasible for large main-propulsion engines) that continually cleans the exhaust gas.

Because the Azipod has no struts or shafts, it operates in largely undisturbed water, so it creates much smaller pressure pulses on the hull as the propeller turns. That, coupled with the absence of bearings and gearboxes, makes for much less noise and vibration in the hull. After running the yacht for more than 18 months, the captain and the chief engineer have found that the system is easy to maintain, and they are quite satisfied with its performance.

The system on Benetti ‘s recently delivered Ambrosia III is similar, but with only two main gensets powering a pair of 1,070-kW Azipod propulsion units. Complementing the main gensets is a pair of smaller  units located in a soundproof compartment; the smaller units are used for station keeping and for leisurely cruising at speeds of up to about 9 knots.

As long as forward thinking shipyards like Lurssen and Benetti 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.