This paper describes adjustable power transmission systems from main diesel engine to the shaft generator. Shaft generators are devices that convert mechanical into electrical energy. They are powered by a main diesel engine. All construction features are highlighted and examples of shaft generator exploitation are outlined. Advantages of using shaft generators are mentioned, the most important advantage is the fact that they do not pollute the environment, they are cheap and simple to maintain, contribute to reducing fuel consumption in navigation in a way while the shaft generator is online and generating electrical power diesel generators can be switched off and put in a standby mode and since in nowadays diesel generators in the navigation are consuming heavy fuel oil by shutting them off we are not only saving fuel, the EEDI (Energy Efficiency Design Index) off the ship also becomes better. Disadvantages of using shaft generators are mentioned, the main problem with use of these devices is that they are sensitive to big and sudden changes in the speed of generator shaft because the frequency depends on the speed, and, due to the fact that electrical network requires constant frequency and is sensitive to sudden changes of the frequency (for the most majority of cases) electric power from the shaft generators can not be generated at all while the ship is in the port.