What Waves Know - Understanding the Sea

Collection: Field Notes — Old Fashioned Seamanship

Series Hub: Reading the Sea the Old Fashioned Way

Infographic in pen and watercolour - understanding waves at sea

Subject: Wave behaviour, refraction, reflection, diffraction, and how to read disturbed water ahead

There is a patch of rough water ahead. Not directly ahead — off the headland, half a mile out, where the sea is throwing itself around in a way that has nothing obvious to do with the wind currently blowing. You have two choices. You can aim at it and find out, or you can read it first.

This note is about the second option.

The behaviour of waves as they interact with coastlines, headlands, shoals, and each other is not random. It follows a small set of physical principles that, once understood, make rough water readable at a distance — and navigable accordingly. Most of what follows comes from Tristan Gooley's How to Read Water, which takes wave physics more seriously than almost any sailing book I've come across, explaining it from first principles rather than assuming you already absorbed it somewhere. I've tried to apply that framework to the kind of sailing most readers of this site actually do — coastal passages in British and northern European waters, where headlands, tidal races, and awkward geometry are facts of life.

Waves are not water moving

This sounds obvious but it changes how you think about everything that follows. A wave does not carry water from one place to another. It carries energy. The water itself moves in a small orbital loop — up, forward, down, back — and returns almost exactly to where it started as the wave passes.

Gooley uses the image of shaking out a bedsheet. The wave you see travelling along the sheet is real and visible, but the sheet itself hasn't gone anywhere. At sea, the proof is any piece of floating weed or a seabird sitting on the water: as a wave passes under it, the bird bobs up and forward and back down and slightly back, tracing a small oval. It does not travel with the wave. Only the energy does.

This matters practically because it means that rough water ahead of you is a concentration of energy, not a mass of water moving at you. The water in the disturbed patch you can see off the headland is not coming from somewhere and going somewhere else. It is expressing what the energy arriving there is doing. Once you understand what is causing the energy to concentrate or collide at that spot, you can predict where else it is doing the same thing — and where it isn't.

Ripples, waves, and swell: three different animals

All three are waves in the technical sense, but they behave differently and should be read differently. The dividing line is period — the time between successive crests passing a fixed point.

Ripples have a period of less than a second or two. They are governed by surface tension and disappear within moments of the breeze that created them dying away. They tell you about conditions right now, not a minute ago. In a dinghy, the arrival of a patch of rippled darker water is a warning that a gust is on its way, and it gives you perhaps ten seconds to act.

Waves proper have periods of a few seconds to around ten seconds. They are gravity waves — large enough to have broken free from surface tension — and they persist for a while after their generating wind dies. They tell you what the wind has been doing for the past hour or few hours.

Swell has a period of ten seconds or more, often much more. Atlantic swell arriving on the Irish coast might have a period of fifteen or eighteen seconds and have originated in a storm near Newfoundland three days earlier. It travels thousands of miles without losing its direction and crosses everything else on the surface — wind waves, tidal chop, other swells — essentially uninterrupted. It tells you what the weather has been doing hundreds of miles away, days ago.

This layering matters. In practice you are often reading three sets of information simultaneously: the ripples telling you the immediate wind, the waves telling you the recent local conditions, and the swell underneath both telling you something about the larger meteorological picture. The post on reading swell covers that last layer in detail. This note is mostly concerned with the waves, and what happens to them when they hit things.

Fetch: why Force 5 in the Faroes is not Force 5 in Dover

Before we get to headlands, there is one other concept that changes everything about reading wave height: fetch.

Wave height is not determined by wind strength alone. It is determined by wind strength, the length of time the wind has been blowing, and the distance of open water the wind has blown over — the fetch. All three need to be above a certain threshold for waves to develop, and increasing any of them makes the waves bigger.

Gooley puts this with admirable directness: a Force 5 with hundreds of miles of open Atlantic behind it is a completely different sea from a Force 5 that has blown across a few miles of sheltered water. He notes that a Force 7 in the Faroe Islands area worries him considerably more than a Force 9 in Dover — not because of the latitude but because of the exposure.

The practical implication for coastal sailing in UK and northern European waters is significant. The North Sea is shallow and relatively enclosed in comparison to the Atlantic; in a westerly, a Force 5 off the East Anglian coast will produce short, steep chop because the westerly fetch is limited. Turn the wind to a north-easterly, and suddenly you have fetch all the way from Norway, and the same Force 5 is building a much more uncomfortable sea. The Baltic, for all the chart suggests relatively calm inland waters, can build a nasty steep chop in strong winds simply because it is shallow — waves build quickly but don't have the depth to take on a long, rolling character. My own experience sailing in Finnish waters bore this out: a 25-knot southwesterly had produced a confused, close-spaced chop that was genuinely harder work than bigger Atlantic seas I've sailed in, simply because of the water depth and the geometry of the surrounding land.

You can watch fetch effects in miniature on a lake. Stand with the wind behind you and you will see relatively calm water at your feet, with progressively bigger ripples and waves across the width of the lake in front of you. Turn around and the reverse is true: waves at your feet from the full fetch of the lake, calm water on the far bank. This basic map is operating on every stretch of coastal water you sail, at scales that matter.

Refraction: how headlands concentrate wave energy

When waves move from deep water into shallow water, they slow down. The mechanism is not friction against the seabed but rather the constriction of the orbital motion within the wave — once the water depth is less than half the wavelength, the wave's motion gets cramped and it decelerates.

The critical consequence follows from a simple piece of geometry. If the sea floor broadly mirrors the shape of the coastline — deeper in the middle of bays, shallower off headlands — then parts of a wave crest approaching a headland reach shallow water first and slow down, while the parts of the same crest still in deeper water continue at their original speed. The crest bends. It steers itself toward the shallow water, toward the headland.

This has two significant practical effects.

First, wave energy concentrates around headlands. Waves converge on them from both sides, bent by the shallowing water on either flank. The sea off a headland is not just rough because the tidal race is there (though that makes everything worse). It is rough partly because the basic physics of wave propagation in shallow water is directing energy toward precisely that point, from a wide arc of sea. The Long Beach breakwater incident in 1930 illustrates what this focusing can do at an extreme: wave energy was refracted by an underwater hump into a concentrated beam that destroyed a structure that the prevailing sea state, measured offshore, had no business damaging. The oceanographers took seventeen years to work out why.

Second, bays are calmer than headlands by the same mechanism. Waves entering a bay spread out, fanning toward the sides and diluting their energy over a wider area. Any crescent-shaped beach is a product of this effect. The waves have been doing the same thing long enough and consistently enough to move sand into the shape that matches the refraction pattern.

For practical passage planning this means: the rough water you expect to find at a headland will be there for reasons beyond the tidal race. It will be there in moderate conditions even without much of a race. And the shelter in a bay is genuine, not illusory — the geometry is on your side.

Portland Bill on a spring ebb with a westerly swell is everyone's textbook example, and for good reason. But the principle is operating everywhere. Duncansby Head at the northeast corner of Scotland. Rattray Head on the Aberdeenshire coast. Every significant headland on the Brittany coast. In Orkney, where I have had a limited but instructive acquaintance with the waters during a tall ships race from the Orknies to Denmark, the headlands don't just have races — they focus swell from several directions into one unpleasant place at once, and the chart gives only a partial warning of where that place will be.

Reflection: the sea bouncing back at you

The second thing waves do when they meet the coast is bounce. When a wave meets a sufficiently vertical surface in sufficiently deep water, it reflects back out to sea with most of its energy intact — similar to the way light bounces off a mirror.

Where this reflected wave meets the incoming wave, interesting and occasionally alarming things happen. Crests from both directions arrive at the same point simultaneously, stack on top of each other, and produce momentarily a crest twice as high as either wave alone. Troughs do the same in reverse. The resulting pattern is called clapotis — from the French, meaning "lapping" — and in its purest form it creates standing waves that appear to rise and fall in place, rather than travelling anywhere.

In practice, perfect clapotis — where incoming and reflected waves are exactly the same size and meet head-on — is relatively rare. What you see more commonly is the cousin Gooley calls clapotis gaufré, waffled clapotis, which forms where reflected waves are meeting incoming waves at an angle and creating a cross-hatched, confused sea. This is the rough, anarchic water close to a cliff or a harbour wall that seems to have no particular direction and is worse than the conditions further out. It is not random. It is the superimposition of two wave trains from different directions.

The important practical note is that this rough water extends for some distance out from the reflecting surface. The wall or cliff is not the problem zone — the sea immediately in front of it, where the reflected energy is meeting the incoming energy, is the problem zone. Gooley observes that the steeper the obstacle and the deeper the water at its base, the more faithfully the reflection occurs. A gently shelving cliff with a reef at its base will absorb most of the incoming wave energy before it gets a chance to reflect. A sheer granite cliff dropping into forty metres of water reflects almost everything.

Try standing at the top of some cliffs next to a moderate sea, and see that the confused water a couple of boat-lengths from the cliff face will be noticeably worse than the open sea fifty metres further out.

Diffraction: the incomplete shelter problem

The third effect is diffraction. When waves pass the end of a breakwater, a headland, or any obstacle comparable in size to their wavelength, they don't simply continue straight on and leave a shadow of calm water behind the obstacle. They bend around the end of the obstacle and fan into the sheltered area behind it.

Gooley describes this with the analogy of sound travelling around a tree. You can hear someone speaking on the other side of a tree even though you can't see them, because sound waves (with their relatively long wavelength) diffract around the trunk. Light waves are far too short to diffract around a trunk, which is why you cannot see around it.

Ocean waves diffract around headlands and breakwaters in exactly this way. The energy fans out into the "shadow" zone, progressively diminishing the further into the shelter you go but never disappearing entirely. This is why an anchorage described as "sheltered from the west by a headland" will still have some swell from the west — it will be much reduced compared to the exposed side, but it will not be zero. The further around the headland you anchor, the more the diffracted wave has had to travel and the more dilute it is. But "behind the headland" is not "perfectly flat."

Gooley tells the story of his own wedding anniversary sail along the lee side of the Isle of Wight in a Force 7 southerly. He knew theoretically that refraction and diffraction would bring swells around both ends of the island before he'd finished passing it — and was vindicated, to his wife's considerable displeasure and the subsequent sale of the boat. One of those nautical tales that is funny in hindsight and instructive immediately.

Shallowing water: what the sea floor does to waves on approach

As waves enter progressively shallower water approaching a coast, they steepen before they break. The orbital motion of the water within the wave gets cramped by the rising seabed, the crest narrows and heightens, the trough broadens and flattens. The wave face becomes steeper and the wave slows down, bunching up. Gooley notes this creates a detectable change in the quality of the sea — a "treacly" character, a heaviness — that experienced sailors notice before the breaking point.

In areas of scattered rocks, reefs, or offshore sandbanks — the Thames Estuary approaches, the Goodwin Sands area, the East Anglian offshore banks — any unexpected change in wave character in what should be open water is worth attending to immediately. The wave is telling you the depth has changed. An Arab navigator of the fifteenth century, Ibn Majid, whose work Gooley quotes, noted a patch of unexpectedly choppy water in otherwise calm conditions and mentally filed it as a shoal. He returned to the same spot years later to find an island had formed. The water had been telling him the truth years before the chart caught up.

When waves finally break, they do so differently depending on the seabed gradient. A gentle slope produces spilling breakers — the crest tumbles forward in foam without ever forming a dramatic shape. A steeper gradient produces plunging breakers — the classic, photogenic arch of water where the crest overtakes the base and the wave pitches forward. Very steep beaches produce surging breakers where the wave runs up the beach without quite completing its break.

The type of breaker visible on an approach to a beach or harbour tells you something useful about the approach — steep plunging breakers in the entrance suggest a bar with a sharp gradient; the gentler spilling foam of a shelving beach is a different proposition. This is navigational information available without any instrument at all.

Surf beat: the rhythm of sets

Waves arrive in groups. A set of larger waves — typically five to ten — arrives at the shore, there is a period of relative calm, and then another set. This oscillation in wave height at the shoreline is called surf beat, and it has a period of several minutes.

The mechanism is that waves generated by a storm are not uniform in size. The larger waves within a group travel slightly faster than the smaller ones, so by the time a storm's output reaches a distant coast it has partially sorted itself: larger waves arrive together, smaller waves arrive together, creating pulses. The timing of sets is not perfectly predictable, but it is consistent enough that watching a surf or harbour entrance for ten minutes before attempting entry in marginal conditions tells you considerably more than a single glance.

Reading it all together: the approach to disturbed water

Back to the rough patch ahead, off the headland. Armed with the above, the questions you are now asking are:

Where is the swell coming from, and does refraction around this headland put the focus point in my path? If the headland has a shallow bank extending from its tip, the refraction focus will be further out to sea than the headland itself.

Is there a cliff or wall here that could be producing a reflected wave train? If so, the rough water will extend some way offshore.

Is there an island or headland providing "shelter" that might actually be delivering diffracted swell into the lee?

Has the wave character changed in the last mile — steeper, heavier, shorter period — that might indicate a shoal even where the chart shows nothing?

And is there a timing rhythm to the disturbance? Sets arriving and passing with a few minutes of relative calm between them are manageable at a different moment to a continuous, confused sea.

None of this replaces a chart. None of it replaces tidal atlas work, pilot book notes, or prudent weather assessment — all of which belong in passage planning alongside the observational toolkit. What it adds is the ability to read in real time what the chart describes in averages and generalities. The chart shows you where the headland is. The sea shows you what it is doing today, in this swell, with this wind.

The observational habit is not difficult to develop. It requires only that you watch with the right questions in mind, and that you build a small vocabulary of causes to match the effects you see. Tristan Gooley's How to Read Water provides that vocabulary more completely than anything else I've found — it is patient with the physics, specific about the signs, and takes the reader seriously enough to explain why rather than just what. It is worth having on the shelf before any serious coastal passage.

Next in the series: Beyond the Blue — Water Colour 101 — what depth, sediment, temperature fronts, and tidal boundaries look like from the surface. Or start from the beginning with reading tidal currents from the surface, which covers the other major force shaping coastal sea state.

The full series index is at Reading the Sea the Old Fashioned Way.