Hello Tinchu,
The propulsion layout of some SSEx ships is a direct drive from a main engine to a fixed pitch propeller. Like any other internal combustion engine with or without clutches, the crosshead diesel engines have a minimum rotational speed, 20 - 35% of their maximum r/min. Direct driven engines are therefore stopped and restarted ahead or astern as needed. Given time, the ship speed will eventually settle at a deadslow speed around or slightly less than 20 - 35% of the maximum ship speed, if the engine(eers) allow it.
In SSEx, the indication displayed is not the power but the shaft rpm, and if relevant, also the pitch ratio in % of the controllable pitch propellers.
Regards,
Luc