Waves in Soundings

On a continental shelf wavelength shortens and the waves steepen. Closer to land the colour of the sea may change and rubbish may appear in the water.

If you are approaching a steep-to coast with no beach to absorb the energy of breaking waves, then when a swell hits the coast it is reflected back out to sea (see Figure 20.3 and Figure 20.4). This creates a cross swell which may be detectable some distance offshore. Before acting on this you must be sure that there is no other possible source for the cross swell and answer the question why, if you are within a handful of miles of a cliffy coast, you have not seen land.

On sand and shingle beaches waves hit the beach at an angle but their backwash runs straight down the beach taking the beach material with it. These beaches absorb the force of breaking seas and there is less chance of detecting reflected waves out at sea.

When swells approach an isolated obstruction such as an island, they try to wrap themselves around it (Figure 20.5). The first sign is the reflected swell from the up-swell

This shows what happens if there are vertical cliffs plunging into deep water. If there is a shelving beach the incoming swell curves round trying to hit the shore at right angles. The further offshore the shelving extends the more the incoming swell swings round.

A beach absorbs the energy of the incoming swell. The gentler the slope of the beach and the further offshore it extends the more energy it absorbs. When this happens there is often a line of offshore breakers and the reflected swell will peter out close inshore. The depth of water will affect how far the reflected swell goes offshore. It will go further at high tide than low especially if there is a wave cut shelf. (Figure 20.4)

20.3 Relected Waves

Diagrams always show refracting waves swinging round like guardsmen on parade. In the real world refraction is largely controlled by the shape and composition of the bottom and waves can wander about like drunks after the bar has closed. This may not be apparent from seawards. There will little or no reflection from this type of beach and in thick weather the first sign of approaching land may be the sudden appearance of swell in an otherwise calm sea or an abrupt shortening of wavelength and increase in wave height if there is a sea running. You may be very close to land when this happens and safety lies in heading back out to sea immediately.

20.3b Wave Refraction 170

20.4 Wave Cut Shelf

20.5 Island Swell side of the island creating a cross sea. Then the swell curves round the island and meets itself to create another cross sea just beyond the lee of the island. The range at which these often subtle signs can be picked up varies with the swell and the size of the island. A large island will have a greater effect than a small island. The original swell may be confused by cross swells from other sources. You could be looking at a babel of waves that make no sense at all.


Very few seabirds live at sea. Most nest on land and fly out in the morning to their fishing grounds and fly home at the end of the working day. Their flight paths are a good indication of where land lies. Flocks of birds feeding at sea are probably a sign that land is within a maximum of 50 nautical miles or a fishing boat within 50 metres.


During the day, land heats up faster than sea. The warm air over the land rises and is replaced by moist air drawn in from the sea, which in turn is warmed by the land and rises. The rising air cools and the moisture it picked up over the sea chills out as a cloud.

If there is no wind then this cloud sits over the land. On windless days the warm air will continue to rise after it has lost its moisture. This can split the cloud into two and then it sits, like bushy eyebrows, either side of the island.

If a wind is blowing, the top of the cloud may stream downwind. Sometimes the entire cloud may be blown downwind, only to be replaced by a new cloud forming over the island.

Sometimes the base of the cloud reflects the colour of the land below, taking on a greenish tinge if the land is wooded, or a light bluish-green tinge if there is a lagoon.

Clouds are visible at great distances. Divide them into High Clouds, Middle Clouds and Low Clouds (Figure 20.6) and look for:

• Stationary clouds. Many books have pictures showing a single cloud sitting over a solitary island. Real life is complicated by the presence of other, very similar looking clouds that have nothing to do with the island. These move with the wind. The island cloud is the one standing still.

• 'Eyebrows' either side of an out-of-sight island.

• Clouds with their tops streaming downwind when every other cloud moves downwind in its entirety. This is caused by the stationary cloud continuously reforming above the island as its top blows away.

Cloud Level



Cloud Type

High Clouds

Above 20,000 ft (6100 m)

Mainly ice crystals

Cirrus, Cirrocumulus, Cirrostratus

Middle Clouds

6500-20000 ft (2000-6100 m)

Mostly water droplets although higher regions may contain ice particles

Altocumulus, Altostratus,

Low Clouds

Up to 6500 ft (2000 m)

Entirely water droplets.

Stratocumulus, Nimbostratus,Cumulus, Cumulosnimbus


• Clouds always forming in the same place. The wind may be strong enough to blow the island cloud away entirely but another forms in its place. It should be obvious but if there are other clouds around it is only noticeable if you watch the clouds carefully.

• Clouds whose base is a different colour to other clouds. While this is a good indication of land it is really just a sign that something other than deep water is reflecting light into the cloud base. This 'something else' could be a reef.


Contrails high in the sky can be a useful guide to direction. Trans-oceanic aircraft tend to follow great circle routes and their final destination may not be in exactly the direction they are pointing, but they do fly between landmasses.

If you are making an island landfall then aircraft landing on the island can point the way, especially at night when their lights are visible from 30 or more miles away as they circle the sky like stars gone walkabout.

Commercial Radio Stations

A simple transistor radio can not only warn of approaching land, it tells you where it is. When you begin receiving programmes, rotate the set horizontally so that even with the

20.7 A Transistor Radio

volume at full almost nothing is heard. The ferrite rod antenna in the set is now pointing at the transmitter, which is almost certainly on land (see Figure 20.7).

Long wave signals can be picked up at about 400 nautical miles, medium wave at about 100 nautical miles and VHF (Fm) signals around 30 nautical miles. Signal strength should increase as you approach land. The easiest way to determine this is to keep the volume setting constant and listen for the sound growing louder. If the signal strength decreases then either the batteries are flat or you are sailing in the wrong direction. This works. Japanese pilots tuned into Radio Honolulu to find Pearl Harbor.

Shore Lights

Like the cloud base reflecting the colour of the land underneath, shore lights also light up the cloud base and are visible from many miles offshore. The bigger the settlement the bigger the area of cloud lit up. If there are no clouds the loom of shore lights can be seen from miles offshore. Be careful. Shore lights may be some distance inland. Do not approach too closely until you are sure where the sea stops and the land begins.


As you near land there is a better-than-even chance that you will cross one or more shipping lanes. As their position is known, this can confirm your DR. Offshore from most major harbours, particularly if they are upriver, there are large-ship anchorages, which provide another excellent position check. In some areas such as the North Sea you will find oil or gas platforms. These carry numbers and their position appears in pilots and almanacs.

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