A 1960 US Air Force airframe maintenance manual sends mechanics to Waterline 50 to repair a supersonic jet. The episode explains why. A ship’s hull is all compound curves, so naval architects draw waterlines, horizontal slices parallel to the water’s surface, like contour lines on an inverted topographic map, letting builders read the hull’s exact shape at every depth on a two dimensional lines plan. One waterline is also law: the load line or Plimsoll line marks how deep a ship may legally be loaded, with separate marks for tropical fresh, fresh, summer, and winter North Atlantic water, since warm fresh water is less dense than cold salt water and a loaded ship rides lower in it.

Waterline length also caps the speed of displacement hulls. A ship rides between its bow wave and stern wave, and as it speeds up the stern wave slides aft until the stern drops and the bow climbs its own wave. Longer waterlines raise that hull speed, and racing sailboats are shaped so that heeling lengthens the waterline. Early aircraft makers hired shipbuilders, who brought lofting and waterlines with them.

• Lofting takes its name from the lofts above shipyards where lines were drawn full size on the floor.
• Archimedes’ principle means a ship sinks until it displaces its own weight in water.
• Adding engine power to a displacement hull at hull speed mostly makes a bigger wave.
• Aircraft waterlines are measured in inches from a baseline designated waterline zero.
• When waterline zero sits at the nose cone tip, parts below it such as landing gear get negative waterline numbers.