concept

Added mass

The quantity of fluid a body must drag along as it accelerates, which shows up as though the body were heavier than its actual mass. Changing course means accelerating not just the body itself but the air or water it disturbs. For a dense object in air the effect is negligible; for a hollow, enormous, light one — a balloon, an airship, a floating organism — the added mass can rival or exceed the body's own. This is why very large buoyant bodies answer the helm over minutes rather than seconds, and why every turn has to be commanded far in advance.