Sea margin and engine margin
How the propeller design point becomes the specified MCR: sea margin of about 15 percent, engine margin of 10 to 15 percent, and what happens when they are eroded.
Sea margin and engine margin are the two allowances that separate a ship’s calm-water propeller design point from the specified maximum continuous rating of its main engine. They are why an engine is never sized for the trial condition.
Sea margin, traditionally about 15 percent of propeller design power, covers the added resistance of weather, waves, current and accumulated hull and propeller fouling in service. Engine margin, a further 10 or 15 percent, keeps the normal continuous service rating below the specified MCR so the engine is not run continuously at its ceiling. Together they place the service rating near 90 percent of specified MCR.
The margins matter to slow steaming in two directions. A heavily derated or power-limited engine has less reserve for heavy weather because part of the margin has been given away, and a ship whose sea margin has been consumed by fouling is already operating closer to its ceiling than the design intended. The full article will cover how each margin is selected, how they interact with the light running margin, and how they are eroded over a docking cycle. See slow steaming , propeller light running margin and the engine layout diagram .