Block coefficient — how boxy a ship's hull really is

The block coefficient is a single ratio that describes a hull's fullness: the volume of water the hull actually displaces, divided by the volume of an imaginary box drawn around its length, beam and draft. A perfect brick would score 1.0; a needle-fine racing hull approaches 0.35. It is one of the first numbers a naval architect fixes, because it encodes the ship's entire purpose.

Fullness is cargo. A supertanker or bulk carrier wants to move the maximum weight for the minimum length, so its hull is close to a floating box — block coefficients of 0.80 to 0.85 are normal, the reason these ships look so slab-sided. A container ship trades some fullness for speed and sits around 0.60 to 0.70. A fast ferry or warship, which must slip through the water quickly, drops to 0.45 or below.

The physics behind the trade-off is wave-making. A full, blunt hull pushes a bigger bow wave and burns dramatically more fuel to go fast — which is exactly why the slow-steaming era after 2008 suited the boxy bulk carriers and punished the finer, faster container ships, and why the bulbous bow exists to cancel some of that wave-making penalty.