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Innovative developments and trends in conventional propulsion technology for naval ships, especially frigates, corvettes and submarines, are advancing more and more. It is becoming apparent that electric drive systems will become more and more popular.

A decisive criterion for the selection of propulsion systems for naval ships is their power density as well as ease of maintenance and the total cost of ownership (TCO). The space requirements on naval ships are generally tight. Therefore, the space requirements of the propulsion system must always be taken into account in the ship design.

Medium-speed diesel engines with an output of approx. 26 MW have a power-to-weight ratio of 8 to 20 t / MW, for high-speed diesel engines with approx. 9 MW it is approx. 3 to 5 t / MW. In contrast, gas turbines with outputs of 20 to 40 MW have a power to weight ratio of less than 1 t / MW. In addition to the high power density of a gas turbine, its ease of maintenance and its ability to provide high performance at very short notice are also important. As a result, gas turbines are more often found as propulsion systems on naval ships than on merchant ships. The advantages and disadvantages of drives such as gas turbines, diesel engines or electric motors must be carefully weighed against each other. For example, the extremely high specific fuel consumption of a gas turbine in the low partial load range must be taken into account.

The class 125 frigates have the CODELAG propulsion system; here the type ship, the frigate "Baden-Württemberg" (Photo: PIZ / M)

Navy ships with a drive requirement of up to 25 MW usually have a drive system consisting of two or four high-speed diesel engines (Combined Diesel and Diesel, CODAD). If the power requirement exceeds 25 MW, the drive system is expanded with a gas turbine, which leads to drive systems with different combinations of gas turbines and diesel engines, such as CODOG (Combined Diesel or Gas) or CODAG (Combined Diesel and Gas). This achieves higher performance within the given restrictions in terms of installation space and displacement. Here, the combined drive systems enable the individual drive components to be used in such a way that their advantages can be used to the maximum. Class 123 frigates, among others, are equipped with the CODOG system. In the case of CODAG concepts, the version with two diesel engines and a gas turbine is the standard version today, with the torque being divided between two propeller shafts via a so-called cross connect gear and the respective port / starboard main gear. The class 124 frigates were fitted with this version for the first time worldwide. The driving profile of these frigates enables 80 percent of the operating hours per year to be operated with just one driving diesel. This has advantages in terms of fuel consumption and the maintenance of the diesel and thus lowers the TCO costs.

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