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tow winches, and sundry dc equipment. The switchboard also supplies ac for the ship's normal service, auxiliary loads, and lighting. Ken Robbins, president of Marine Propulsion Inc., Hammond, La., relates the arrangement to an electrical power station for a small community. "You can draw as much power out of it as you put into it," he says, "and can send that power wherever it's needed." The advantages of diesel-electric drive impact three interrelated areas of prime consequence to vessel owners: perfor- mance, operating cost, and payload capacity. Performance-related benefits focus around the superiority of dc traction motors for high-torque maneuvering and dynamic positioning. The motors are capable of providing full torque at all times, even at zero rpm, and furnish dynamic (or regenerative) braking capa- bility. Propeller reversals happen within five to 10 seconds by reversing the polarity of the dc power, thus eliminat- ing the need for a reverse gear. With dc traction motors rpm can be controlled all the way to zero, whereas diesel rpm cannot be reduced below idle speed. Moreover, dc motors are indif- ferent to extended periods of lightly loaded low-speed operation — a condi- tion hostile to direct-diesel fuel econo- my and engine life. During high-speed operation, a dc motor can exceed rated rpm by 50 to 100 percent — safely and harmlessly — sim- ply by reducing the field current. Horse- power holds steady at maximum output, although torque output above rated rpm decreases proportionately as revolu- tions-per-minute climb. Other diesel-electric operating benefits help lower the vessel's overall cost of operation. The engines, for instance, never have to lug or strain against the governor; they're always humming along at rated rpm under 60 percent load or more — a condition promoting com- ponent longevity and maintenance sav- ings. The use of engine-generator sets permits adding-on or shutting-down engines as demand fluctuates, ensuring the efficient use of available horsepower, economical fuel consumption, and mini- mized wear. "A good analogy would be several air compressors supplying a large reservoir serving several air motors operating at variable outputs," suggests DeJong. "If you need more air, engage another com- pressor. If you need less, disconnect a compressor or two." With horsepower requirements tied to electrical generation rather than pro- pelling and maneuvering the vessel, smaller diesels can be specified. "Main- taining top vessel speed with two large engines running at 60-percent load is not April, 2000 nearly as cost-effective as running two smaller engines at full load," Robbins says. "Yet that's precisely what can hap- pen with direct-diesel, should maneu- vering performance dictate engine size." The cost-efficiencies with electric drive go even further. For example: • No dedicated engine (and associated fuel-delivery, starting, cooling, lubricat- ing, and exhaust equipment) for each driven load (thrusters, main screws, winches, pumps, etc.) — just standard, direct-coupled dc motors powered from the ship's service generators; • no clutches or reverse gearing to maintain or repair. Additionally, the engine-generator sets come skid-mounted for level installation on resilient pads that minimize noise and vibration and make for a quieter vessel. There's no aligning engines with gear- ing, since there's no gearing in the first place, and thus no mulling over tilt angles when complying with American Bureau of Shipping (ABS) trim stan- dards. These latter benefits introduce the sub- ject of design flexibility and how diesel- electric drive can favorably impact cargo space. PLASMA 12 x 12 50% Stronger, 20% Less Expensive, And It's Repairable. 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