Service margin

The service margin, also called the sea margin, is the power added to the calm-water trial requirement so the ship holds service speed in real conditions: hull roughening and fouling over the docking cycle, wind and waves, and shallow or confined water. It is applied as a percentage uplift on delivered or shaft power, commonly 15 percent for unrestricted ocean service and higher on rough routes, lower on sheltered ones. The margin is distinct from the engine-rating margin between the service point and maximum continuous rating. Sizing it too low risks a ship that cannot keep schedule once the hull fouls; too high wastes installed power and capital.