Added Wave Resistance, Part 1: Diffraction

Continuing Phase 2 of our vessel resistance series (Added Weather Resistance), today we dive into the first half of Added Wave Resistance: Diffraction.

When discussing wave drag, the immediate assumption is that a pitching and heaving ship requires more power. While true, that is only half the story.

Before a ship even begins to pitch or heave, it loses massive amounts of energy to Wave Diffraction.

This phenomenon dominates in short, steep sea states where the wavelength is relatively small compared to the ship's length (λ/L is low). In these conditions, the vessel acts as a rigid obstruction. The incoming waves physically strike the hull — particularly the bow — and scatter.

This scattering of wave energy creates a localized dynamic pressure shift. A distinct "pressure wall" forms directly ahead of the bow.

This pressure differential acts as a blunt force, actively opposing the vessel's forward momentum. The energy required to shatter and scatter those incoming waves is stolen directly from the propeller's thrust.

The operational reality here is critical: even in sea states where the ship feels perfectly stable, with minimal heave or pitch, wave diffraction is quietly acting as a massive hydrodynamic brake.

This blunt force resistance means the engine must burn significantly more fuel just to maintain the same speed through the water. Furthermore, added resistance in waves scales non-linearly with significant wave height (H_s) — meaning a seemingly small increase in wave height can cause an exponential spike in fuel consumption due to this diffraction penalty.

To truly optimize a voyage, we cannot just look at severe storms that cause heavy ship motions. We must account for the continuous, invisible penalty of short-wave diffraction hitting the bow.