The Ocean's Long Memory - Reading Swell

Series Hub: Reading the Sea the Old Fashioned Way

Subject: What swell is, why it persists, how Pacific navigators used it as an all-weather compass, and how to read it on any offshore passage

How to Read Ocean Swell — Direction, Period, and What Long-Memory Wave Trains Tell You About Weather and Position

Swell is wave energy that has outrun the storm that created it, travelling across oceans days after the wind has died. For a navigator without instruments it is the most reliable directional reference available, persisting through overcast and darkness when stars and horizon both disappear. This is about what swell is, how Pacific navigators used it as an all-weather compass, and how much of that translates to North Atlantic sailing.

On the eighth hour of an overcast night passage with no stars and no visible horizon, the Polynesian navigator Tevake stood on the foredeck and felt the swell. He made a perfect landfall on a half-mile gap between two islands.

That account is in David Lewis’s We, the Navigators and it is the cleanest illustration of what trained swell-reading does that I have come across. Tevake was navigating in the Santa Cruz group in conditions where every visual reference had been removed. The sky was closed. No stars. No horizon. After eight hours he placed the vessel in the precise centre of the Forrest Passage between Lomlom and Fenualoa. Forty-five miles of open water without once being able to see where he was going.

The mechanism that made this possible is the most chronically underused observational tool in modern offshore sailing. Lewis is the source for this post. Gooley contributes the physics and the practical exercises. But the big picture, what swell can actually do as a complete navigation system, comes from Lewis, and his account is the most useful thing about waves in print that I know of.

What swell is

Swell is wave energy that has outrun the storm that created it. A depression somewhere in the North Atlantic generates waves in the area of strongest wind. Those waves travel outward in all directions. The longest-wavelength, longest-period waves travel fastest and carry the most energy, covering hundreds or thousands of miles before they dissipate. By the time they arrive at a distant coast, the storm itself may be long past and the local weather entirely settled. The swell continues regardless.

This is the difference from wind waves. Wind waves are local. They exist because wind is blowing on the water right now, and they die within hours of that wind dropping. Swell is historical. It is a record of weather that happened somewhere else, some time ago, encoded in the motion of the water and travelling largely undisturbed through everything on the surface, other waves, other swells, tidal chop, until something solid stops it.

Gooley explains the distinction through period. Ripples have a period of under a second or two. Wind waves fall between two and roughly ten seconds. Swell has a period of ten seconds or more, often substantially more. Large North Atlantic swell commonly has periods of fourteen to eighteen seconds. Counting the time between crests while lying on a bunk on a settled night passage will tell you which regime you are in. A period well above ten seconds means significant swell is present, regardless of what the local wind is doing.

The consequence is that swell is reliable and directional in a way wind waves are not. Wind waves respond to immediate local conditions. Swell direction changes slowly and only as new dominant storms replace old ones. For a navigator without instruments, swell provides a compass reference that persists through overcast, rain, darkness, and shifting local winds. Available continuously. No sky required.

Three swell trains stacking

At any given location in open ocean, two or three separate swell trains commonly run at once from different directions, each the product of a different distant weather system. They coexist and pass through each other without significantly interfering, because swell has enough energy to maintain its character despite the confusion of the surface above it. An experienced navigator feels each swell train as a distinct rhythmic motion in the hull, underneath and separate from the local wind chop.

Lewis documents this multi-swell awareness in detail. Tevake identified three main swell trains in the Santa Cruz group and used them as a combined reference system. Each gave a bearing. The intersection of their directions triangulated position as effectively as multiple star bearings. Carolinian navigators in Micronesia worked with a similar three-swell framework, though the specific swells were entirely different. The Carolines sit on different ocean swell patterns from the Solomons. The principle is identical. The application is entirely local.

For a North Atlantic sailor this translates to something more modest but still useful. A dominant Atlantic groundswell from the southwest or west is usually present in the approaches to the Bay of Biscay and the Western Approaches whenever any significant depression has been active in the mid-Atlantic in the previous few days, which for most of the year is almost always. That swell direction is consistent enough to serve as a course check when you are down below, or when you wake up suddenly because your body has noticed the change.

Feeling swell in the hull

There is a physical technique to swell-reading that is not intuitive to sailors trained on instruments. Visual observation of swell, watching the waves from the cockpit, is useful in moderate swell and poor in confused conditions. What the Pacific navigators described to Lewis was something more proprioceptive. A sensitivity to the vessel’s motion that allowed them to identify swell direction and character from the hull’s behaviour rather than from looking at the water.

Lewis himself attempted this and found it trainable, though humbling. He describes lying in a bunk on a rolling passage and trying to separate the swell motion from the local chop. The slow, long-period roll driven by swell arriving from one direction. The shorter, sharper movement from local wind waves arriving from another. With practice the two become distinguishable. The swell is the steady underlying rhythm. The chop is noise on top.

The starting point for a western sailor is any settled offshore passage where swell is running and local conditions are moderate. Lie flat athwartships, that is, with your feet toward one side of the boat, and attend to the motion. Close your eyes. The long roll from swell arriving on the beam feels different from the pitch of a following swell, which is different again from the corkscrew of a beam swell meeting a quartering wind sea. These become distinguishable once you have stopped trying to look at them and started trying to feel them. Ten minutes on a bunk on a Biscay passage in reasonable conditions teaches more about swell-reading than any amount of theory.

I am being honest about my own limit here. I have done this on Biscay passages and I can reliably identify one swell train at a time, in clean conditions. Identifying two simultaneously, with confidence in the direction of each, is something I have not consistently achieved. The Tevake feat is at a level of skill I have not approached. The first level is available to anyone with a settled offshore night and the willingness to lie still.

Stick charts and swell refraction

Swell does not pass around islands and headlands unchanged. The geometry is in What Waves Know. Swell approaching an island refracts around both sides and converges in a confused sea behind it. Reflected swell bounces back and creates interference patterns.

In the Marshall Islands, Lewis documented a sophisticated use of these interference patterns. Marshallese navigators learned to read the pattern of swell interactions between specific islands as position indicators, encoded in traditional knowledge as stick charts. Physical models of the swell interaction patterns around and between islands, made from coconut frond ribs and shells, with each shell marking an island and the ribs showing the dominant swell paths and refraction lines.

These stick charts, mattang and meddo types, are not navigational instruments used at sea. They are teaching tools, used to explain the pattern before a voyage so that the navigator could recognise it in practice. What they represent is a formalised understanding of how each island’s underwater footprint distorts the passing swell trains. Highly location-specific knowledge that cannot be transferred to unfamiliar waters. The underlying observation skill, recognising that a change in swell character indicates the proximity of land, transfers universally.

For a North Atlantic sailor in practice, a confused or disturbed swell pattern in otherwise open water, where the underlying rhythm suddenly becomes less predictable, indicates that swell is being reflected or refracted by something close enough to affect the pattern. Worth attending to. The chart confirms what is there. The swell gave the first indication.

Swell as a storm warning

Because swell travels faster than the storm that generates it, a building swell under clear skies and without local wind is evidence of a depression working somewhere upwind.

Gooley documents the historical example of Galveston, Texas in September 1900. Locals remarked on unusual, powerful swell hitting the beach under clear conditions the day before the hurricane arrived and killed over six thousand people. The swell had outrun the storm. The sea was providing a day’s advance warning that the sky was not yet showing. This is the most chilling specific instance of swell-as-warning in the literature, and the one I think every modern sailor should know about, because it makes the point in a way that abstract physics does not.

A sustained increase in swell height and period over several hours, with no corresponding deterioration in local conditions, warrants checking the latest synoptic forecast and the barometer. It does not mean a gale is certain. It means one is possible within the range the swell has travelled, which on a North Atlantic timescale is twenty-four to thirty-six hours. In Biscay in particular, where Atlantic swells arrive from depressions tracking northeast of the crossing route, a sudden building of the background swell in otherwise stable conditions is a recognised precursor to a change in the weather.

Swell as a course reference

The most immediately practical application for a coastal and offshore sailor is simpler than any of the above. Using the swell as a course reference check when other references are unavailable.

On an overcast night in the Channel or Western Approaches, with no stars visible and no sun to track, the swell direction provides an independent check on the compass heading. If the prevailing Atlantic swell has been running from the west-southwest for the last twelve hours and you can feel it on the starboard beam, and you are on a course that should be putting it there, that is confirming information. If you find it on the port quarter when the compass says nothing has changed, something has changed.

This is not precise navigation. A swell confirmation check gives you direction to within perhaps twenty degrees on a good day. It does not give you position. As a cross-check on whether your compass and your sense of direction are in agreement, it works reliably on the open sea in conditions where other checks are temporarily unavailable.

Lewis and his Pacific navigators were doing something more precise. They used swell not just as a direction check but as a primary navigation system, encoding swell patterns into star-path-equivalent knowledge for specific passages. That level of skill requires years of accumulated experience in specific waters. The simpler skill, recognising the swell’s direction and using it as a rough compass, is available to any sailor who has spent a few passages consciously attending to the motion of the boat.

Training the skill

Gooley suggests a simple observational exercise for developing swell-reading. Pick something floating on the surface, weed, a piece of foam, anything stable, and watch it through a full swell cycle. You will see it trace a small oval. Lifted forward slightly as the crest approaches. Raised to the highest point at the crest. Pushed slightly back as the crest passes. Lowered back to the trough. That oval motion is the wave’s orbital geometry, and the direction of the long axis of the oval tells you the direction of swell travel. In regular swell this is unambiguous. In mixed swells it becomes a matter of identifying the dominant rhythm from the composite motion, which is exactly the skill Tevake and his contemporaries were applying at scale.

The starting point is any day with clear swell running. Standing in the bow of a ferry crossing the Channel. Sitting on the foredeck of a passage-making yacht. Lying on a bunk. Twenty minutes of conscious attention to the motion, with the question in mind, which direction is this swell running from and how certain am I, is the beginning of an observational practice that compounds into something useful over a season.

The honest position is that swell-reading at the level Lewis describes is the most powerful unmediated navigation technique in the literature, and the one I am furthest from. The Pacific navigators were applying decades of accumulated calibration to specific waters. Tevake’s overcast landfall is not something a North Sea sailor can replicate by reading a chapter. What you can do, and what is worth doing on any offshore passage where conditions permit, is the first step. Lie down. Close your eyes. Feel which direction the dominant motion is coming from. Note it. Check it against the compass. Do this on enough passages and a foundation builds.

Most modern sailors have lost this sense without ever knowing they had it. I am not above that group. The work of recovering it is slow and offshore-dependent, and there is no shortcut.

If you want somewhere to start with the orbital-motion observation in sheltered water, the Hithe Finder is a community register of slipways, hards, and beaches for small boats. Even a tidal river in a steady fresh breeze gives a small clean wave train to study a piece of floating weed against, which is the cheapest version of the same exercise.

References

Lewis, D. (1994). We, the Navigators: The Ancient Art of Landfinding in the Pacific, 2nd edition. University of Hawaii Press. The fullest account of practical swell navigation anywhere in print, including the Tevake overcast landfall, the Carolinian three-swell framework, the Marshall Islands stick charts, and Lewis’s own attempts to learn swell-reading from practising navigators. The single most important book in this whole series for me.

Gooley, T. (2016). How to Read Water: Clues and Patterns from Puddles to the Sea. Sceptre. The physics of wave types, swell formation, the relationship between swell period and storm approach, and the floating-object orbital-motion exercise.

Series index at Reading the Sea the Old Fashioned Way.

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