Groundswell: A key element of ocean sailing
Often arriving several days after a storm, or generated by persistent trade winds, the defining feature of groundswell is its long period - often 10 to 20 seconds or more between waves. This distinguishes it from the shorter, choppier windswell typically found near coasts.
A key factor to understand when navigating groundswell is that the longer the period, the more energy the wave carries. Even moderate-height waves with a long period can be far more powerful than they initially appear. When this energy meets shallow water, the waves build in size and intensity, which means that when approaching landfalls, groundswell can turn into substantial breakers as it interacts with the continental shelf or underwater features.
To help you better understand the effects of groundswell on ocean passages and landfalls, here are three factors to look out for:
Reflection: the first sign of landfall
As you approach a landfall after an extended passage, groundswell often provides the first indication of nearing land.
For example, when approaching the Marquesas or other steep sloped oceanic islands, it is common to feel the effects of swell bouncing off the steep cliffs long before the islands themselves come into view.
The process of swell reflection causes waves to rebound off the coastline, creating additional wave patterns that can alter the motion of your vessel. These reflected waves interact with the primary swell, resulting in a steeper pitching effect as the opposing swells collide.
It is not uncommon to feel these interactions well before you sight land, and this effect serves as a useful early warning of an upcoming landfall.
Refraction and swell shadows
While it is often expected that the leeward side of an island will provide shelter from the swell, this assumption does not always hold true.
Although a swell shadow can form immediately downwind of a well-sheltered area, swell refraction can cause waves to bend around headlands and islands, bringing confused seas where the swells collide and resulting in unexpected motion and discomfort to what might seem like a protected anchorage.
This phenomenon is important to keep in mind when planning anchorages, as the effects of refracted swell can make conditions more challenging than initially anticipated.
Swell and currents: The steepening effect
When ground swell interacts with an opposing current, the waves become steeper and more powerful.
This effect is particularly noticeable where strong ocean currents run against incoming swell, such as the Agulhas Current off the coast of South Africa or the Gulf Stream in the North Atlantic. In these instances, the interaction between swell and current can create steep, sometimes breaking waves, making navigation more difficult.
Near land, tidal streams can have a similar effect. When swell encounters a strong tidal flow, particularly near river mouths or coastal inlets, it can quickly steepen, creating larger and more challenging waves.
Anticipating these interactions is crucial when planning your approach to coastal waters or navigating through areas with significant tidal currents such as tide races in the western English Channel that are exposed to Atlantic groundswell.
Always check swell period
In ocean sailing, ground swell is a constant companion and understanding how it behaves is critical to managing your passage. By focusing on the wave period, recognising the impact of landfall interactions, and preparing for the steepening effects of currents, you can better predict and adapt to the conditions you’ll encounter.
To give you more support planning your next ocean passage, Ocean Passages and Landfalls by Andy O’Grady and Rod Heikell offers comprehensive guidance on routes, landfalls, and the swell patterns encountered along the way.