Swell

Swell consists of long-period, regular wave trains that have propagated away from the storm system that generated them, often travelling thousands of miles across open ocean before reaching a coastline or ship. Unlike wind sea, which is choppy and irregular, swell has a smooth, undulating character with a longer period — typically eight seconds or more — and can persist long after the storm has dissipated. Swell direction is frequently different from the local wind direction, creating combined and cross sea conditions that are more demanding for ship-handling. For an officer on watch, swell is relevant to the vessel's motion, the comfort and safety of the crew, and the effective operation of loading and discharging equipment at anchorage. A vessel in a beam swell rolls heavily and unpredictably; cargo operations at exposed anchorages may need to be suspended when swell exceeds safe limits. Masters planning a coast passage must consider whether a distant storm system is generating swell that will affect conditions in the departure or destination area even if local weather is benign. Swell is reported in synoptic weather observations using the WMO coding system, specifying direction, period, and height separately from wind sea. NAVTEX broadcasts and weather routing advisories routinely include swell forecasts. Officers should understand that long-period swell from the Southern Ocean can affect vessels operating off the west coasts of Africa and South America even in the absence of local storm activity, creating conditions unfamiliar to crews accustomed to northern hemisphere seas.