The benefits of diesel-electric propulsion

Would you like your next power voyaging boat (or your present yacht after modification) to be:
 Maneuverable as a jet ski
 More fuel-efficient
 Quieter and smoother running
 Easier and less costly to maintain?
All of these attributes (plus a number of other very worthwhile advantages) can be yours when you equip your boat with the power and propulsion system used on large cruise ships: diesel-electric generators powering vectored thrust propellers.

The pod-mounted azimuthing propellers used on cruise ships deliver thrust in whatever direction is required to achieve the commanded maneuver, allowing even these massive vessels to dock independently. Volvo Penta’s IPS and Cummins MerCruiser Zeus systems achieve the same results in yachts. These vectored thrust propeller systems also significantly improve fuel efficiency, primarily by eliminating the downward thrust vector created by a conventional prop shaft system. Initially installed on high-powered planning hull vessels, IPS and Zeus drives are equally valuable on displacement and semi-displacement hulls, including the recently announced Grand Banks 41 Heritage EU.

A continuously variable transmission
Powering propellers with a diesel-electric power system improves fuel efficiency by eliminating the efficiency detriment created by a fixed-pitch propeller which can be matched to the hull and engine at only one power setting: wide open throttle. The diesel-electric drive optimizes fuel efficiency by operating the engine at the precise speed necessary to provide the required power. The electric prop motors also enhance the maneuvering performance of the pod drives by delivering maximum torque at speeds as low as a few rpm and by reducing control response time by eliminating the clutch and reverse gear required when the props are directly powered by a diesel engine.
A cruise ship’s diesel-electric power system typically devotes 75 percent of its massive power to the hotel, the power-hungry air conditioning systems, laundry, tens of thousands of lights and all of the other power consumers it takes to run a town with a population of perhaps 4,500 people. Power is provided by a number of (often five or six) diesel-electric generators, used in combinations that efficiently meet widely varying power demands and provide redundancy. The power needs of a power voyaging yacht are the inverse of that of the cruise ship: the overwhelming power demand is for propulsion; a modest amount is devoted to serving the vessel’s air conditioning, galley and other electrical loads.
The use of twin engines in displacement power voyaging boats is largely driven by the low speed maneuvering advantage provided by twin screws, plus the redundancy advantage in the event of a fouled prop or engine malfunction. In many boats the engines are larger and deliver more power than is necessary to maintain cruising speeds well in excess of hull speed, even in unfavorable conditions. Some trawler yachts are equipped with sufficient engine power to operate at twice hull speed. Present and predicted future fuel prices may cause a re-thinking of the wisdom of equipping this type of vessel with such powerful engines. In addition to creating greater maintenance expense, twin engines also crowd the typical engine room, impeding service access.

A vessel equipped with a fixed-pitch prop directly driven from an internal combustion engine resembles a car with a single speed gearbox and a fluid clutch that can slip without burning up. The engine-prop combination can be matched only at wide-open throttle, the engine’s maximum rpm. The ability of the prop to convert rpm into thrust, to efficiently absorb engine power, decreases rapidly at all lesser speeds. A prop matched at 3,000 engine rpm will be able to absorb about 65 percent of engine power at 2,600 rpm while the engine can deliver 85 percent power at that speed. Except at wide-open throttle the engine is always operating inefficiently, turning significantly faster than necessary. The diesel-electric drive resembles a car’s multi-speed or constantly variable ratio transmission, allowing the engine to be precisely loaded so that it turns no faster than the speed at which it can deliver the required power.

Three from one
A diesel-electric drive allows a single engine/alternator to provide the electrical power for both prop motors and in addition supply the vessel’s house AC power whenever the main engine is operating. The power system control computer manages the operating speed of the diesel engine for maximum fuel efficiency at any power demand level, eliminating the speed inefficiency created by the fixed-pitch propeller and automatically compensating for varying house power electrical loads. The electric motor powering each prop unit operates independently, at whatever speed and in whatever direction of rotation is needed to meet the commands of the helmsman. Prop speeds can be automatically synchronized or differentially controlled when the vessel is operated in the joystick control mode. Moving a power lever to the neutral position will lock the prop, making it simple to cruise on one prop if desired.
The electrical power from the main engine’s alternator and the power from the yacht’s genset, which in this system is the vessel’s secondary propulsion power system, is delivered to a three-channel power conditioning system that converts the variable frequency AC current from the engine-driven alternator into two independently controlled power feeds for the port and starboard props. The third power channel delivers the 60 (or 50) Hz AC power required for house needs.
The secondary power system provides the AC house power when the main engine is shut down and can also be used to propel the vessel at speeds within its power capability, five to six knots, sufficient for sustained cruising in many speed-restricted waters such as the Intracoastal Waterway with a 20 to 25 hp engine. The ability of the system to deliver constant frequency, constant voltage AC power over a very wide range of engine speeds will be particularly appreciated at night when the electrical load is at a minimum and the engine runs at a speed barely above idle, without concern about carbon buildup or injector fouling.
Power demands will have to be determined for each vessel, however, a typical 42-foot trawler yacht’s main power system might use a 225-hp engine with a 20 to 25-hp secondary engine. The main engine/alternator is compact when compared with a conventional engine/marine gear/prop shaft system. Both diesel engines can be soft mounted to reduce vibration transfer to the hull and since there is no mechanical connection required to the prop drives they can be located wherever desired. The compact power units and space-saving propeller drives will allow more space for accommodations.
In conventional IPS and Zeus drive systems the engine exhaust is discharged though the underhull prop pod, eliminating virtually all exhaust noise and fumes when the boat is underway. It should be possible to use the same system for the main and for the secondary power unit.

Control of the diesel-electric powered vessel will be totally transparent to the helmsman, using a drive-by-wire system that manages both speed and directional control. Conventional power levers (throttles) and a conventional steering wheel can be used at cruise speeds or alternatively the yacht can be controlled using a steer-by-wire, power-by-wire system that places the controls at the ends of the arms of the helmsman’s chair. 

By Ocean Navigator