How to interpret wave patterns for comfortable sailing

Which way is the tide flowing, and how do we tell?

We could look at a tidal diamond on a chart, or at the arrows in a tidal atlas, but these sources are not infallible, particularly near irregular coastlines where the streams may be deflected or even reverse direction.

It’s often helpful to go back to basics and simply look at the sea, beginning with the water around moored objects, although this may pose more questions when sailing in waves.

For instance, in photo 1 (above), the tide past the buoy is obviously rushing from right to left, but the moored yacht against the far shore is in a current flowing from left to right.

This is where things get interesting, because that photo also shows that the ‘texture’ of the water surface changes between the buoy and the yacht.

In the foreground, there is a fairly uniform carpet of small waves but near the yacht the surface has smooth patches, caused by turbulence.

That area of water is an inshore eddy that is flowing counter to the main stream.

It is unlikely to be shown by a chart’s tidal diamond, or even in a tidal atlas, but could be very helpful to any skipper who wants to cheat the tide.

For a closer view of a similar effect, photo 2 was taken on Loch Linnhe, while beating through the Corran Narrows against the tide.

Corran Point is close to starboard, behind the jib, and the flood tide is flowing clear of the point, leaving an inshore area of turbulence that is not flowing anywhere in particular.

The boundary between the two is quite distinct, with a thin line of foam, and the yacht is being sailed through the turbulence to avoid the adverse current.

Tide plus wind

Under windier conditions turbulence would be masked by larger waves, but changes in currents will also change the wave shapes.

In photo 3, we are beating past Land’s End, towards Cape Cornwall, riding on the first of the north-going tide.

The wind is only at the top end of Force 3, and yet the sea seems to be covered by white horses and the waves look brittle, too short and steep for that strength of breeze: a classic case of wind-against-tide.

On weaker currents, that effect would be less noticeable.

Even so, a change in wave form should be obvious at a boundary between two currents.

Photo 4A was taken on the seaward side of Hurst Spit (see Diagram 1).

In the distance, the Solent ebb was pouring out past the fort on Hurst Point.

Part of that tidal stream was running out through the North Channel, past the photographer, but it had begun to swing away as a developing inshore eddy flowed back along the beach, towards the fort.

That eddy was clearly visible as an area of slightly larger waves, but it is less obvious in a still photograph because the eye and brain normally take account of movement in the water when distinguishing between different wave patterns, and a still picture omits that information.

That is a problem when writing about waves, because printed photos do not capture all their character.

However, for Photo 4B the camera programme was set to ‘Dramatic Tone’, which creates a lurid image but also accentuates wave patterns so that the outer edge of the eddy is crisply defined, much as it appears to the naked eye.

For a yacht heading into the Solent, with a crew who can read the surface clues, it should be possible to make progress against the main tide by short-tacking within that eddy until close to the fort, when the curve of the spit might allow her to hold starboard tack on a close fetch past the point – potentially a valuable tactic when racing.

In both examples, off Land’s End and at Hurst, the wind is blowing against the current and that influences the waves in several ways.

First, it increases the apparent wind speed over the water, although in most instances this will only amount to a few knots – less than one wind force.

Continues below…

It also increases the apparent wave fetch, because waves that are moving against a contrary current will take longer to reach anywhere and this allows them to grow larger before they arrive.

But a third phenomenon often has a greater influence: when waves that are rolling upstream meet an adverse current they are immediately foreshortened as the crests bunch closer together.

That compresses the wave energy into a smaller distance, so the waves also grow higher and the combination of shorter wavelength and greater height makes them much steeper.

Coming to a halt

Shortening the wavelength means the waves move more slowly through the water, and if they run up against a very strong contrary current this may be sufficient to completely halt the movement of wave energy.

The wave crests themselves do not stop still but become very steep and then fade away or break – generally a bit of both – leaving calmer water upstream and creating a typical overfall pattern as in photo 5.

There the tide is flowing from right to left, into the estuary of the River Bandon, the wind is blowing against the tide and the overfall has appeared where the current pours over a submerged bank.

The overfall pattern indicates the tidal stream direction and marks the bank, which is why the fishing boat has been able to ignore the buoy and steer outside the navigation channel.

Waves can also be stopped when they are moving downstream on a current.

Most of us will be familiar with wave reflection when waves rebound from harbour walls, typically producing a ‘spiky’ water surface as in photo 6, where the reflected waves are smacking against the incoming waves so that the crests appear to be jumping up and down.

The same phenomenon, known as clapotic waves, can occur when waves and current are moving in the same direction but abruptly run into a different current.

In photo 7, we are back off Hurst Point, but sailing into the Solent with the flood tide and a following wind (see Diagram 2), and the little jumpy waves are just like those in photo 6.

Two flood streams are converging here and if we swing the camera to the left, towards the fort on Hurst Point, in photo 8 we can also see an abrupt meeting of currents.

A vigorous tidal eddy, its surface smoothed by turbulence, is surging out from behind the point and the incoming waves, carried on the strong flood, are being stopped and reflected at the edge of this eddy.

Therefore, if we meet clapotic waves when sailing downwind, we are probably about to run into slack water or an adverse current.

When the waves are large – either in rough weather or when they arrive as ocean swell – overfalls and clapotic effects are capable of generating particularly violent sea states, but there are some important differences.

When the waves break in overfalls they curl over in an upstream direction and break heavily.

Clapotic waves, on the other hand, are inclined to fling their crests directly upwards.

While a big crest might then dump itself on a yacht it is potentially less dangerous because it is less likely to roll the boat over.

Even so, it could be unpleasant.

A few years ago we were beating west from Cherbourg and taking advantage of a west-flowing counter-current off Omonville while big waves rolled past in the opposite direction on the main east-going tide.

Where those waves collided with the edge of the counter-current the clapotic crests were huge and one of them swamped our cockpit, with some water penetrating the cabin.

If we try that route again, we’ll remember to keep the hatch firmly shut!

Bending the waves

Both overfalls and clapotic waves have a useful side-effect, in that they concentrate wave energy in one place so that other patches of sea become calmer – such as the area upstream of an overfall.

Oblique interaction between waves and adverse currents produces a similar result, even if the waves are not stopped.

Where waves meet a tongue of contrary current, as in Diagram 3, they bend into the current and then move across it diagonally.

When they reach slack water on the far side, they can travel forward more quickly but that causes the crests to turn back into the current, so that they are effectively trapped and continue to roll upstream, while more waves alongside the current may swing on to it.

The tongue of current therefore absorbs wave energy and becomes rougher while the water beside it becomes calmer, as in photo 9.

This phenomenon makes the edge of the current more prominent, which is helpful if you either want to stay in it, or to stay out of it and sail on the calmer water.

There is also a valuable breakwater effect.

On a Scottish cruise, we called in at the small port of Girvan and moored in the harbour basin, which is separated from the river channel by a training wall.

A stiff north-westerly wind was sending waves straight into the harbour and London Apprentice moved uneasily against the pontoon, fenders squeaking.

Later, while we were eating supper, we noticed that the motion had ceased and the water in the basin was almost flat.

It was an hour after high water, the tide was running out of the river channel, and the incoming waves were now locked into that current, rolling straight up the river without agitating the basin (Diagram 4).

Photographs could not show the whole scene and to illustrate the phenomenon I needed a scale model, which we later found at Lynmouth.

Here the small harbour is very similar to Girvan, with a basin separated from a river channel by a training wall, and Photo 10, taken from the high edge of Exmoor while the tide was ebbing, shows most of the incoming waves rolling up the river channel, against the current, while the harbour basin is relatively undisturbed.

I have again used ‘dramatic tone’ to emphasise the wave patterns, including a distinct band of smooth water close alongside the current.

Similar clearly defined current boundaries are often seen on estuaries and even on the open sea, giving keen sailors more precise information than they would find in a tidal atlas.

The breakwater effect is also important for cruisers, because it provides a temporary period of calm.

Within the Channel Islands, popular beach anchorages experience Atlantic swell that wraps around the islands but they often become quieter when the tidal races are flowing towards the south-west, against the incoming swell.

The strong currents between the islands appear to be absorbing some of the wave energy because the waves at the shore decrease but then increase again when the streams reverse.

Crews who have landed by tender on an open beach may be surprised by rougher conditions if they attempt to relaunch after the tide has turned.

Select your sea state

A knowledge of surface wave patterns can be used in various ways.

Small waves that change shape to make currents visible can indicate a favourable stream for a fast passage.

With larger waves, if we have made ourselves familiar with the tidal streams it is possible to predict areas of rough water and avoid them.

In some places we’ll even find calmer water that is flowing the way we want to go.

Speed or comfort, or both: the sea sometimes allows us a choice.

Which waves could be dangerous?

This article has focused on the visible interactions between waves and currents, which help us to make best use of tidal streams.

Those interactions sometimes affect safety, by creating very rough water, and a lot depends on the wavelengths.

In complex currents, short waves often make a fuss without being dangerous, while tranquil ocean swells can become surprisingly violent, although very long waves may grow more impressive without turning nasty.

Rather than attempt to illustrate those points by going for a sail with my camera in seriously rough conditions, I should like to use a couple more photos in which the waves can serve as small-scale models.

Photo 11 was taken while racing through Jack Sound with a strong spring tide.

This is a place where pilot books promise chaos and doom unless the passage is made at slack water, but although the surface could be described as ‘seething’, there is nothing in that picture that resembles a threat.

In photo 12 we are running against the stream, through a small tidal race off Peterhead that doesn’t seem to merit a mention in any pilot book.

The waves in this picture are larger and the yacht is being flung about vigorously, spilling wind from the sails, yet only the tips of some waves are breaking.

Just to add interest, those waves are overtaking the boat and rolling towards the horizon in a determined fashion but up ahead we can see a wide expanse of calm water, which actually owes its presence to the tidal race.

In practice, for a wave to be really dangerous, it needs to be both big and breaking heavily.

Further Reading on wave patterns

Interaction of Water Waves and Currents, D H Peregrine

Prof. Howell Peregrine was a master of the subject of wave patterns and this 1976 paper is for maths enthusiasts. Available from libraries and some websites.

Coastal Turmoil: Winds, waves and tidal races by Ken Endean (Adlard Coles Nautical, £16.99). Explains the principles of wave patterns, but with hardly any maths.

Buy Coastal Turmoil: Winds, waves and tidal races at Amazon (UK)

Buy Coastal Turmoil: Winds, waves and tidal races at Amazon (US)

 

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