Long swell

Long swell is low-steepness, long-period ocean waves, typically 14 seconds and longer, that have traveled far from their generating storm and arrive as regular, well-grouped sets. It is the main driver of harbor resonance and of ranging at a berth, because its period can match a basin’s natural oscillation and its energy passes a breakwater entrance with little loss. Moored ships surge, sway, and yaw under it, snapping mooring lines and stopping cargo work even when the visible sea inside the harbor looks calm. A floating breakwater gives little protection against it, since the beam needed scales with the long wavelength.