Waves as Loads: A Bridge to Structural Design

In still water the ship's weight curve (how mass is distributed along its length) and its buoyancy curve (how the displaced-volume support is distributed) are roughly matched, and the small leftover mismatch produces a calm-water bending moment. This lesson is about what happens when a wave passes through.

A wave is, mechanically, a redistribution of water. Where there is a crest the waterline is locally higher, so the hull there is supported by more buoyancy; where there is a trough the waterline is locally lower, so the hull there loses buoyancy. The wave does not change the ship's weight — it changes the buoyancy curve superimposed on the still-water one. That altered buoyancy curve bends the hull girder.

• Crest amidships, troughs at the ends — buoyancy piles up in the middle and is lost at the ends, pushing the midships up relative to the ends. This is a hogging wave condition.
• Trough amidships, crests at the ends — buoyancy is lost amidships and piles up at the ends, letting the middle sag relative to the ends. This is a sagging wave condition.

As a wave train moves past a hull, the buoyancy curve shifts continuously, so the hull rocks between hogging and sagging in a cycle — the very effect the next lesson's simulator will let you watch.

For a first structural estimate the wave is usually treated as a fixed, static shape held against the hull rather than as a moving disturbance — a quasi-static assumption that captures the peak bending the girder must survive without needing the full dynamic seakeeping response (Module 11). Because the crest-amidships and trough-amidships positions produce bending in opposite directions, a designer checks both the hogging and the sagging extreme: they load the deck and bottom in opposite senses (one puts the deck in tension, the other in compression), and the larger of the two governs the longitudinal strength the hull girder must be built to carry.