How to navigate using waves

When voyages of discovery were being made 500 years ago and navigators had to make landfalls without any charts, being able to navigate using waves was a valuable technique, writes Dag Pike.

Indeed many of the ancient sea-going cultures also relied on waves for navigation.

The waves breaking on the rocks or reefs of an island would often be the first sign of land, but there are also more subtle and varied ways of reading the waves too.

Learning to interpret them is an art rather than a science, but is one that will give you an invaluable sense of your surroundings, and will alert you to danger when something doesn’t feel right.

Electronic navigation has taken over our lives as navigators, even before GPS in the days of Loran C and Decca.

Satellite navigation is more accurate than ever, and with the advent of satellite receivers that can derive a position from several separate satellite constellations, they also have redundancy built in.

A chartplotter screen gives the impression of indubitable accuracy, but datum shifts or, more likely, inaccurate or out-of-date chart data can throw a spanner in the works.

Add in shifting coastlines, river mouths and sandbanks, and it would be foolish to rely solely on what the little glowing screen tells you about your location.

This is why learning to navigate using waves is such a vital skill to have in your armoury.

Spotting shoal patches

If there is shallow water and there is a swell running then you will almost certainly see the waves breaking on the shallow patches.

The problem of course is, what comprises a shallow patch?

Much will depend on the size of the waves but you can be fairly sure that there will be breaking waves in any depth of less than 2.5m (8ft) when there are ocean waves rolling into the shore and these will allow you to identify the shallows.

Think about making an approach to a shallow entrance with a shifting seabed in an onshore wind, such as the Deben on the East Coast, Chichester Harbour bar, or Caernarfon Bar in North Wales.

The charts and buoys may tell you one thing, but if there is breaking water across the entrance ahead, it would be foolish to plough on regardless, and better to stand off.

Patches of broken water, however, may confirm where the shallows are and reveal where the channel has shifted to.

Finding deeper channels

You can use the same technique in narrow channels where there are shoal patches or rocks that can be a danger.

Trying to sail through the narrow Jack Sound on the west coast of Wales can shorten the distance when heading north or south but the deep water channel between the rocks is just 200m wide and the rocks are mainly underwater.

SatNav positions should be accurate enough to position you in the middle of the channel but you can supplement this by actually being able to ‘see’ the rocks by the disturbance on the surface.

The current runs strongly through the channel and this creates patches of white water around the rock areas so you aim between the breaking waves to find the deeper water.

It can be an exciting trip but being able to ‘see’ the rocks helps to give you confidence.

You can use a similar technique to pick out the inside passage around Portland Bill, even in poor visibility.

The first visible warning you are likely to get is when you start seeing breaking waves ahead.

Now you know that you are entering the area where the converging tides are meeting and you will need to take steps to counteract the drift in order to follow a course close into the headland.

Once you are in that inside passage you can see the safe areas because they are the ones without breaking waves.

Approaching land

There are a number of wave characteristics that may denote proximity to land.

Wave reflections, where incoming waves hit a vertical or near vertical face and get reflected back, can create quite dangerous clapotic waves.

You can get this phenomenon on the west coasts of the Orkney Islands where the cliffs go straight down into the depths so the cliffs reflect the incoming Atlantic swells for up to a mile offshore and they are a good warning about your distance off in poor visibility.

You can get the same type of reflected waves off the breakwaters of Dover harbour where there can be a nasty bit of sea on the west side as you approach the Western Entrance.

Much as with spotting shoal patches or rocks by breaking water, waves will often become shorter and steeper the closer to shore you are.

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There are two factors that may give rise to this.

There tend to be stronger tides closer to land, along coastlines and the waves these create – either breaking crests and short waves wind over tide, or flattening the sea off when wind with tide – can be sign of land.

Secondly, shallow water slows the underside of the wave, shortening the wave length and making the wave faces steeper.

Anyone who has sailed in the North Sea will know how uncomfortable this chop can be, but it is a good indication that the seabed isn’t far away.

As you move into deeper water the motion should ease, though the change can be very gradual.

Working the tide

More dramatic can be the changes in the waves that you see when the tide turns.

When the wind is with the tide the wavelength is increased and there is less tendency for them to break in fresh winds.

You are much more likely to see breaking seas when the wind is against the tide and in places where the tide runs strongly.

The change can be significant, making you think that it might be time to run for shelter.

You certainly need to be aware of the cause and effect of wind against the tide when planning a cruise.

Of course it can also make life more difficult because if you are using a favourable wind the tide will be setting you back and to a certain extent you can judge this by the format of the waves you can see and perhaps seek a weaker tidal stream closer inshore.

Again the stronger and weaker tide areas should be visible by the steepness and anger of the waves.

Using radar to navigate using waves

Reading the waves is a valid way of navigating in the daytime but at night there are still possibilities if you have a radar.

Waves tend to start breaking when they have a gradient of around 18° and the more vertical face of these breaking waves will make them a better radar target.

This means that areas of breaking waves should show up on the radar display.

This is a technique that those ancient explorers did not have but it may help you to get a better ride if you know where the breaking seas are, though be aware that radar will more easily ‘see’ wave faces heading directly for your boat.

Steering by the waves

As anyone who has steered a yacht by hand for any length of time will know, holding a course when not heading towards a fixed object can be tricky, as the compass card swings and wobbles around the lubber line.

Using the wave pattern to judge your course can give you a useful reference point outside the boat.

Whilst not enormously accurate, waves offer a visual representation of the wind direction, and keeping the boat at roughly the same angle to each wave ridge can help you orientate yourself within your environment, and get your head out of the boat rather than staring only at the instruments.

Changing course

Sailors of old would reckon that every seventh wave in a wave train was always the biggest.

If you need to manoeuvre the boat, such as tacking or gybing, and it will take more than one wave to complete, watch the waves until you have an idea of the pattern of larger and smaller waves coming towards you, then wait until the ‘seventh’ wave has passed, then take your chance in the smaller waves that follow.

Predicting weather

It isn’t easy to predict the weather from reading the waves, but knowing what the weather is doing can help inform what waves to expect.

Most notably, as a low pressure system approaches, it will bring a front with it.

As the front passes, the wind will veer by 30° or more.

Two separate wave patterns will then collide, leaving you with confused seas and potential for much higher than average peaks.

You may also get some warning of a storm approaching by the arrival of a long, fast-moving swell ahead of the wind picking up.

This is caused by the large waves created at the centre of the storm overtaking the smaller waves ahead of them and travelling faster than the weather system.

If this is the case then the chances are it’s time for you and your crew to hurriedly seek shelter.

Problems reading waves

There are two caveats to all of this.

Firstly, there is no easy way to measure waves, other than the time period between peaks and a guesstimation of their height.

Wave height can also vary within the same train, and you will often have more than one wave pattern interacting at the same time, giving rise to waves much higher than the average.

Any changes in wave pattern can be difficult to identify when you navigate using waves.

There are also a large number of variables affecting the surface of the water, so attributing any change to one cause is not a precise science.

Secondly, you can see very little of the seastate at night, and without visual clues it can be hard to make sense of the boat’s motion.

However, a good sailor will have an eye for what a big wave is and it is this ‘sense’ that is vital to good seamanship.

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