Today, the frequency converter, by way of the electric motor, controls the propeller rotation speed directly. As a result, the propeller can turn much more slowly. In order to maintain a vessel’s position at sea or to move at very slow speeds, the amount of propulsion needed is sometimes so minimal that it need not be more than the power to adjust the pitch of the propeller blades.
This configuration saves fuel because electric motors operate at high efficiency even at low speeds. The frequency converter controls the speed of the electric motors and propellers. Depending on how much thrust is required, the converter adjusts the frequency and amplitude of the alternating current from the diesel generator.
Depending on the type of ship, a diesel-electric propulsion system consists of four to six diesel generators. The ship’s power demand determines how many of the generators are running. As a result, since they produce only the energy that is needed and are not directly coupled to the speed of the screws, diesel engines can operate at a very high level of efficiency. In ships where the load on the propulsion system changes frequently, the savings provided by this type of hybrid system more than compensate for the loss in efficiency due to converting the mechanical energy produced by the diesel engine into electrical energy.
Compared to purely diesel-powered vessels, diesel-electric ships are quieter and use up to 35% less fuel.
Siemens has been using this technology since 1996. The Skandi Marstein, a supply boat for drilling platforms, was the first vessel with a diesel-electric drive in the North Sea. “That ship was a milestone for us,” says Odd Moen, a Scandinavian ship designer. On a three-day cruise, the Skandi Marstein used 35% less energy than a diesel vessel.”