The term eyeball pilotage is generally used to describe the process of conning a boat by direct visual clues, where charted information is incomplete or suspect.

Those clues may be above or below the water and Rachel Sprot, in her article How to navigate any coastline, described the time-honoured method for crossing coral reefs, where a sandy bottom will look darker in the deeper areas, particularly when observed down-sun or through polarised glasses.

In our home waters, around the British Isles and Atlantic France, that technique can be equally valid but needs to take account of local conditions.

Les Écrehou, looking SWW towards Jersey. Credit: Ken Endean

The photo above was taken at Les Écrehou, the small group of islets and rocks to the north east of Jersey.

Sailors from Jersey and Normandy usually arrive at around half ebb, with the tide slightly higher than in the photo.

Some of them, approaching from the left of the picture, like to anchor in the Pool, the mooring area at the top right, and the most popular entry route is via the gully between the two rocks in the middle.

Even when the reef is covered, at a greater height of tide, the submerged gully offers two visual clues: both the deepest channel and the weed-covered rocks look darker than the surrounding sand, although they convey different messages.

Deep sand

A channel through sand is a feature at Hayle, in North Cornwall, where the approach to the drying estuary runs across a wide foreshore but is essentially unmarked, the channel diverging from the old retaining wall (Photo below) and varying its alignment from time to time.

The entrance channel at Hayle diverges from the harbour’s old training wall. Credit: Ken Endean

It is usually visible under water, as a slightly darker area, but only when you are close above it, so finding it is difficult at high tide.

In the absence of swell the outer banks can usually be crossed safely but I always try to identify the line of the channel well before reaching the training wall, where on each side the banks are higher and the water shallower.

Local fishing boats coming and going might demonstrate the best line but most boats from Hayle have shallow draft and simply ignore the channel at High Water.

The best time to observe their track is during the young flood, when they have to depart along the channel in order to make an early start.

And here I must make a comment about local pilotage knowledge.

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Guidance from harbour authorities, shoreside experts and fishing boat skippers may well be very helpful but I never like to rely upon it without forming my own judgement.

Many fishing craft draw less water than yachts and guidance from the shore can let you down.

Some years ago, we anchored off an estuary with a notorious bar and waited for the tide to rise while working out our tactics.

We then telephoned the local Mooring Master to enquire about berthing, but he insisted that we should wait longer, and that he would personally talk us in.

After the tide had risen and covered some of our intended pilotage marks, we were finally invited to make a start, following his instructions, but as we approached the critical point the line went dead.

We later learned that he had dropped his mobile phone in the sea.

At some places no local pilotage advice is available, even if you want it.

The chartlet below show the Kyle of Durness, in the sparsely-inhabited NW corner of Scotland, where we anchored just outside the estuary while waiting for a strong SW wind to moderate and allow us to round Cape Wrath.

Credit: Maxine Heath

There was a chance that an approaching depression would track to the north and cause the wind to veer to NW, in which case the whole of Scotland’s bleak north coast would become a nasty lee shore where the nearest good shelter was 40 miles away.

The only other option would be to retreat into the Kyle and find a drying anchorage near the ferry crossing.

Charts show that the unmarked channel usually runs out towards the NE, but old maps indicate that it can wander occasionally, so we wanted to check it – just in case.

When we rowed ashore and walked uphill for a good view at low tide, we were glad to see that the channel was obeying the chart, with its seaward end close to the eastern shore, as shown in the photo below.

Kyle of Durness, at low tide, with the main channel visible as a blue line across the wide sands. Credit: Ken Endean

In the event, the wind went round to the SE and we did not have to seek shelter, but I’m pretty sure that, knowing the general alignment of the channel, we could have located it under the crystal-clear Atlantic water.

When the sea is silty, a submerged channel will be invisible at high tide but in some places it is helpful to record the scene at Low Water, by a photograph or sketch, and then use the image as a virtual view during the actual passage.

At Norfolk’s Burnham Harbour (see chart below) the only coastal features are sand dunes.

Credit: Maxine Heath

Where the harbour channel runs out through the dunes and across the wide foreshore it develops a dog-legged bend but this is constantly changing and can wander over nearly a mile.

There may be some small channel buoys (plastic containers) but in order to find them on a muddy, foam-streaked sea a visitor must have a pretty good idea of where to find the dog-leg.

On our last visit, we employed a digital camera as an electronic eyeball (and electronic memory).

After walking from the nearby harbour at Wells, at low tide we strolled down the channel to the sea and then, looking back along the channel towards the dunes (Photo 4), took a compass bearing on a grassy bulge in the dunes that would serve as an aiming point.

Burnham Harbour entrance channel at low tide. Credit: Ken Endean

We also took a photo of that same view.

Burnham Harbour entrance channel at low tide when zoomed in. Credit: Ken Endean

Later, when entering the harbour in messy North Sea swell, we zoomed the camera’s recorded image to help us identify the aiming point (photo above) and then ran in on the compass bearing (photo below).

Entering Burnham Harbour with swell breaking to starboard and the small buoys just visible. Credit: Ken Endean

Dark rocks

As at Les Écrehou, weed-covered submerged rock often stands out clearly as a dark mass against a lighter background of sand.

At Porthallow (photo below) we anchored London Apprentice close to the beach and midway between the flanking reefs, so that she had room to swing.

Anchored on sand between clearly-visible rocks, off Porthallow beach. Credit: Ken Endean

That example may look ‘tight’ but this is a very common technique in places with hard rock geology, where soft sand collects between rock outcrops and the sand offers both reliable holding and the best depths.

However, the situation at Old Grimsby Sound on the Isles of Scilly is more complex (as seen in the photo below).

Old Grimsby Sound, Isles of Scilly, with three boats in different depths of water. Credit: Ken Endean

The small boat on the left is moored on an area of drying sand and the yacht on the right is moored in the main channel through the Sound.

The bottom there is dark weed on rock but not shallow rock; the weed is clinging to bedrock and boulders that have been exposed by the currents scouring away the sand.

There is plenty of depth but in the stones and weed an anchor’s holding could be less secure than on sand.

That’s why the middle boat (ours) is lying just over the boundary between sand and dark weed.

Her anchor is set in the sand but close to its edge, so that she is lying back over the deeper water.

Around the Isles of Scilly and in other places with clear sea water, such as western Brittany and Ireland, I often find myself manoeuvring our boat according to instructions that my wife calls from the bow, as she endeavours to lower our hook on to a clear patch of sand between weedy areas: it’s worth taking the trouble if it ensures good holding and an undisturbed night.

A broader view of pilotage

Eyeball pilotage can be useful for supplementing charted data, and may even be essential where charts are unreliable.

At sandy estuaries with unmarked channels, sailors are exhorted to use the latest charts but those charts are invariably suspect because even the latest editions can be out of date before they are published.

In hard rock areas, the rocks (at least the largest ones) are unlikely to move but the charts show only sparse details of sea bed materials and extra information is useful.

However, eyes are not infallible.

Ken Endean is an inshore pilotage enthusiast who has made a close study of coastal sea conditions around the British Isles

To the right of the Old Grimsby view, just out of the picture, there is a drying lump called Tide Rock, which dries by 1.4m and is covered with the same weed as its surroundings, so it is effectively hidden when submerged.

In the middle distance, at the entrance to the Sound, another concealed menace, Little Kittern, dries by 1.9m.

A few years ago, a charter yacht left her keel on Little Kittern … and her crew did not notice.

Perhaps they were ignoring the chart while straining their eyeballs to search for the invisible obstacle.

Inshore pilotage depends on intelligent use of all available information and equipment.

The human eye just happens to be one of the instruments.

Tactics for eyeball pilotage

If you suspect the charts to be incorrect or inadequate:

• Make a reconnaissance on shore or in your tender to inspect the scene
• At low tide, take note of features such as rocks and bans that will cover on the flood

When the sea water is clear:

• Look for different sea bed colours and try to understand what bottom types correspond to lighter or darker colours
• If possible, have the sun behind you and wear polarising sunglasses
• When anchoring, look for a soft sea bed clear of week and rock

When the sea water is silty:

• At low tide, identify suitable transit marks or objects from which to take compass bearings
• Move with caution, on the rising tide
• On estuaries fringed by tidal marshes, make a move while the marsh edges are exposed to give an indication of the channel line
• Look out for surface ripples where the current may run over a shallow bank

Be your own hydrographic surveyor

• Do tidal curve calculations for predicted heights of tide during your intended move
• Record echo sounder readings to check chartered depths and drying heights.