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.