Concerning the circumstances of oil crisis, it has been a tendency to increase number of vessels, which are operated with reduced power. It is said that the reduced power running of ship will be continued for a long time. So, the study for the reduced power running will become more important and more. On the other hand, the design of ship is based on the normal power running as the most effective condition, to begin with. So, the plant efficiency will be reduced in case of running with reduced power. One of the object of reduced power running is to save fuel oil consumption. Therefore, it is very important to improve the plant efficiency in case of running with reduced power. In this paper, some discussions about the problem in case of reduced power running are made, and the countermeasure i sstudied under the condition of minimum investment.
