Swell Period

Swell period is the time, in seconds, between successive crests of an ocean swell passing a fixed point or vessel. In marine forecasts it describes the spacing and energy of swell generated by weather systems, often far from your location. A longer swell period usually means faster-moving waves with more energy and greater distance between crests.

For boaters, swell period can matter as much as Wave Height. Long-period swell may be comfortable offshore but dangerous over bars, reefs, or shallow approaches where it steepens and breaks. Short-period swell creates a rougher Sea State, especially with Wind against Current, increasing slamming, fatigue, and reduced speed.

PredictWind wave forecasts show swell period alongside swell direction and height, helping you compare route options, select anchorages, and time coastal entrances. In the Forecast App, web forecasts, Offshore App and GRIB downloads, checking Wave Maps helps identify whether swell trains are lengthening, easing, or becoming less favourable.