From the helmsman's standpoint the Siemens diesel electric propulsion system is totally transparent, functioning identically to a conventional power control system, with some welcome and useful advantages. In a twin-prop installation, two conventional-appearing single-lever electronic power controls send commands to the drive control system computer (DICO in Siemens' terms). Digital messages from the DICO manage the diesel engine's speed governor and control the current flow from the propulsion converters to the prop-drive motors. The power curve of the engine is programmed into the control computer and used to set engine rpm to the speed that provides the required power with optimum fuel efficiency. It is generally unnecessary for the helmsman to monitor engine rpm. Placing a prop control in neutral allows the prop to freewheel, with no risk of damaging the gearbox, a benefit when the vessel is propelled by the other prop. There are no restrictions on rapid movement of the control from ahead to astern. Moving both controls beyond a preselected prop shaft speed, usually about 400 rpm, will automatically and very precisely synchronize the propellers. In fact, the precision with which the system "knows" the relative rotational position of each prop shaft appears to be sufficient to allow future addition of a prop-phase relationship control. Vibration and noise can be minimized with careful control of the relative position of the blades on one prop with those on the other prop. This technique is common on many multiengine aircraft and is called prop phasing.
In a diesel electric system, the engine can be mounted virtually anywhere in the hull and in whatever orientation is required to achieve optimum use of space. With no need to provide a mechanical power-transfer connection to the hull, the engine mounts can be chosen for optimum vibration isolation. If desired, the engine can be totally enclosed in a sound shield, providing superior sound attenuation without the need to insulate an entire compartment. The incorporation of thrust bearings on the prop shafts ensures precise alignment with the "P" bracket and the prop-drive gearbox, reducing wear on the cutlass bearing and shaft noise. The small size of the prop-drive motor and gearbox also makes it possible to locate the props at the most advantageous position on the hull-a great advantage.
A typical Siemens single-engine twin-prop diesel electric propulsion system is comprised of a permanent magnet three-phase alternator powered by the diesel engine through a speed-increasing gearbox, two short-circuit-proof solid state power inverter modules, two 3-phase permanent magnet prop-drive motors, associated prop-drive gearboxes, a control computer and a conventional-appearing helm control station. System components are surprisingly compact and lightweight. The 145 kW (194 hp) alternator suitable to power a 48-foot trawler yacht is 10 inches square, 22 inches long and weighs 264 pounds. Each of the 114 hp drive motors is similar in size and weight. The 40 x 23 x 7 inch inverter modules are typically bulkhead mounted. The spiral bevel gearboxes used to connect the alternator to the driving engine and the drive motors to the propellers are 98 percent efficient and have a medium duty life expectancy of 25,000 hours. Gearbox power capacities range from 150 to 600 hp, meaning that they can be coupled to multiple alternators or motors to meet the power requirements of larger vessels. All of the system's power-handling components, alternators, motors, gearboxes and power inverters are cooled with a circulating flow of water/glycol. The system is also continuously monitored, by using temperature sensors built into the alternators, motors, gearboxes and inverters.
Operating efficiency is an important part of the evaluation of any power delivery system. Although the alternators, inverters, motors and gearboxes used in the Siemens system operate efficiently, the total energy loss through the system will typically exceed the losses imposed by a conventional marine gear-coupled drive. However, the efficiency loss is to a large degree offset by the ability of the electronic control to precisely match the engine's power curve to the vessel's propellers. In a conventional direct mechanical prop-drive system, the fixed pitch propeller can be matched to the engine and the hull at only one point, maximum engine rpm. At all other engine speeds the engine turns faster than is necessary to produce the required power. The diesel electric drive matches the prop load to the engine, producing a result similar to that achieved with use of a controllable pitch prop. Using one large engine to power two propellers can provide an efficiency advantage compared with use of two engines providing the same total power. Depending on the positioning of the engine, power loss due to exhaust backpressure may be less than what can be achieved in a conventional installation. The option of drawing the vessel's AC house power from the propulsion system when under way, eliminating the need to run the vessel's genset, can improve fuel efficiency. Overall, the fuel economy of the diesel electric drive will be close to or as good as what can be achieved using a direct-drive system.
In many installations the cost of a single-engine twin-prop diesel electric power system will be no more than a conventional twin-engine direct-mechanical drive installation. Properly integrated into the design of the vessel, the diesel electric drive's many attributes will likely make it the preferred propulsion system for a growing number of yachtsmen.