How CODOG separates economical cruising from a fast dash

Gas turbines are least attractive at idle and slow speeds, so CODOG assigns routine cruising to a diesel engine. When the ship needs to accelerate beyond that economical range, the diesel shuts down and the gas turbine takes over. The word "or" in combined diesel or gas describes a genuine machinery choice because the two engines do not add their power together. This arrangement gives a ship gas-turbine dash performance while avoiding its fuel penalty during the rest of the voyage.