A ship floats as one body, yet designers split its personality. The hull is treated as a submarine, the superstructure as a skyscraper. The split grows from physics, not habit.
Below the waterline, hydrostatic pressure and wave loading dominate, so the hull is sized like a pressure vessel in reverse. Naval architects work with buoyancy distribution, bending moments and shear forces along the longitudinal axis, using concepts close to fluid statics and Euler–Bernoulli beam theory. The aim is to control stress concentrations when the ship hogs and sags over waves, much as a submerged shell must resist external pressure without buckling.
Above the main deck, the rules change. Wind load, center of gravity and aeroelastic response become critical, echoing skyscraper design. Engineers track modal frequencies and dynamic amplification factors to keep the superstructure from resonating with waves or wind, a problem familiar from tall buildings and bridges. The mass is smaller, the lever arm longer, so stability and serviceability criteria resemble high-rise codes more than submarine standards. One floating hull therefore hosts two different structural logics, separated by the waterline but linked by the same steel.
