Collection: Field Notes - Old Fashioned Seamanship

Series Hub: Traditional Navigation Techniques 

Subject: The lead line — making and marking a lead line, arming the lead with tallow, reading depth and bottom type from the chart, fog navigation by sounding, what the lead tells you before anchoring, and how it connects to the water colour and foreshore skills already covered

How to Make and Use a Lead Line for Depth Sounding — Bottom Type, Fog Navigation, and Approach Work Without Electronics

A lead line gives you two pieces of information from one cast. Depth, and what the seabed is made of. Together, in the shallow tidal waters of the North Sea, Channel, and Baltic, they place you on the chart with a precision no other instrument can match. This is about how to make one, how to mark it, and what it tells you that an echo sounder cannot.

In 1937 a young boy named Cooper went out with his father, skipper of a Lowestoft trawler, on a three-day voyage to the Dogger Bank and back. They consulted no charts on the passage. The compass was the only other instrument aboard. The navigation was done by lead line. Hourly casts on the way out. Ten-minute casts on the trough between the ridges where the flatfish were. Hourly casts on the return. Each bottom sample examined visually. Smelt. Tasted. The seabed of the southern North Sea was as readable to Cooper’s father as a road he had driven a thousand times.

That account, published in the Journal of Navigation in 2010 and documented in detail by Kemp and D’Olier, is the most complete description of lead-line navigation as a working primary system in North Sea waters in the modern period. It is what I think about every time I pick up the lead. The instrument in my hand is the same one Herodotus described Egyptian ships using off the Nile in 440 BC. The technique is the same one that brought a Lowestoft trawler back from the Dogger without a chart.

What I am building, on each passage, is a smaller and more modest version of what Cooper’s father had. A working knowledge of a specific stretch of water that the chart describes in averages and the lead confirms in specifics.

What a lead line is and why it matters

A lead line is a weight on a marked line. The weight has a hollow in its base, the arming cavity, filled with tallow or soft grease before casting. When the weight hits the seabed, the tallow picks up a sample of whatever is there. Fine sand. Coarse shell gravel. Soft mud. Rough broken rock. Or nothing at all, in which case the arming comes up clean and smooth-faced, indicating hard rock or packed stone that nothing sticks to.

The combination of depth and bottom type places you on the chart more precisely than depth alone could. Large areas of the North Sea give ambiguous depth readings. The Dogger Bank shoals from around 18 to 36 metres across a wide area, and a reading in the twenties could place you almost anywhere on it. Add bottom type and the position narrows considerably, in the hands of someone who knows the chart and has used the line in those waters before. That qualification matters. This is a local-knowledge system that compounds over repeated passages and becomes, in time, genuinely precise. The chart provides the framework. The lead confirms and refines it.

Making a lead line

I don’t cast lead. My weight is built on a length of M16 galvanised threaded bar about 25 centimetres long. A nut goes on the bottom end first, then penny washers and M16 nuts alternating, as many as needed to reach the target weight. For inshore work in moderate tidal streams, around 2 to 3 kilograms is enough. For faster water or deeper sounding, stack it heavier. Finish with a nut at the top and bend the last 3 centimetres of bar into a ring with pliers, or drill through the top nut and thread a brass ring through it for the line. The whole assembly takes twenty minutes and can be adjusted by adding or removing washers as conditions require.

If you want something cleaner and are happy to spend more, lead sounding weights are still sold by a handful of nautical suppliers and chandlers, a proper pear-shaped casting with a ring top and arming cavity already formed. They work better in fast tidal streams because the denser material sinks more directly, but the threaded bar version is functional for the waters most small craft work in.

For the arming, the threaded bar assembly has no cavity of its own. A small muslin or cotton pouch a few centimetres across, with a drawstring, packed firmly with tallow and tied to the bottom nut works well. On impact the tallow presses against the seabed through the cloth and picks up the sample. If you want a more traditional arrangement, a short length of wooden dowel with a drilled cavity can be screwed or tied to the base of the assembly and armed in the conventional way.

The line should be hard-laid natural fibre. Hemp or manila are the correct choices and both have been used for lead lines for centuries. Hemp swells slightly when wet, which helps it hold marks securely and adds weight to the hanging line. A tarred hemp line resists rot far better than untarred. For the inshore and coastal passages this series addresses, a line of about 45 metres covers almost all useful sounding depths.

Marking the line

The traditional system uses different materials at different depths so the marks can be read by feel in the dark without counting. It has been broadly consistent since the eighteenth century and is built around the fathom, six feet, roughly 1.8 metres.

Two fathoms (3.6 metres) is marked with two strips of leather. Three fathoms (5.5 metres) with three strips of leather. Five fathoms (9 metres) with white bunting or a white rag. Seven fathoms (12.8 metres) with red bunting or a red rag. Ten fathoms (18.3 metres) with a piece of leather with a hole punched through it. Thirteen fathoms (23.8 metres) with blue serge or blue rag. Fifteen fathoms (27.4 metres) repeats the white bunting from the five mark. Seventeen fathoms (31 metres) repeats the red bunting. Twenty fathoms (36.6 metres) is a cord with two knots.

The depths between the marks are the deeps. Reported as such. “By the deep, eight metres” means somewhere between the 5.5m and 9m marks, felt but not precisely measured.

For a fully metric system, different coloured natural twine whipped into the lay of the line at each metre works well. One colour for even metres, another for odd, with a leather tab at five-metre intervals and a distinctive double tab at ten. I made mine this way using red-dyed cotton twine for odd metres and undyed for even, with a small leather slip stitched on at 5, 10, 15, and 20 metres. It took an afternoon and has held through repeated use without slipping.

Arming the lead

Pack the tallow pouch firmly before each cast, without air pockets. When the weight strikes the seabed, the impact presses the soft material against whatever is there. On retrieval, the arming carries the sample.

Fine grey-brown mud coats the arming smoothly and leaves a thin greasy film. Coarse sand shows as distinct grains. Shell gravel produces fragments of shell. Clay comes up as a small waxy plug. Chalk produces a white paste. Rock produces no impression at all. The arming comes up clean.

Cooper’s father went further than visual inspection. He smelt and tasted each sample. The organic material in silt and clay produces a sulphurous smell that distinguishes it from sand. The shell content has a marine character distinct from inorganic gravel. The chalk south of the Dogger has a different character from the igneous and sedimentary mix to the north. Smell and taste are the highest-resolution version of arming interpretation. Requiring accumulated local knowledge to use, but the principle is available to any sailor who pays enough attention over enough passages through the same waters.

One thing I have found in estuaries with active tidal deposition. There is often a thin skin of fluid surface mud over firmer material below, and the arming sample reflects the surface only. The character can change between one cast and the next if you are working across a transition. This is useful rather than confusing. The transition itself is a position.

Reading the chart

Charts use a standardised set of abbreviations for seabed type. S for sand. M for mud. Oz for ooze. Sh for shells. G for gravel. Cy for clay. Co for coral. R for rock. These are often combined. fS for fine sand, cS for coarse, mS for muddy sand, sM for sandy mud. Gooley notes in How to Read Water that this shorthand, a few scattered letters on a chart, reveals something that matters enormously to any vessel wanting to anchor, and explains many of the subtle shifts in water colour that the eye picks up before the chart is consulted.

The first English rutter’s description of the Channel approaches, specific bottom types at specific depths off specific headlands, was the fifteenth century equivalent of these chart abbreviations encoded as a verbal tradition before print existed. The tradition that Kemp and D’Olier trace from Herodotus to Cooper runs directly through those chart letters. What the chart abbreviation shows in one or two characters, the arming confirms or contradicts in the sample on the end of the line.

The transition between one bottom type and another often follows a depth change on the chart as the character shifts with the gradient. In the approaches to the East Anglian rivers, the Ore, the Deben, the Alde, the transition from fine mud in the deep channel to coarser sand on the upper slope often coincides with a contour line. Knowing where that line runs gives the lead two simultaneous confirmations. Depth and arming together.

Fog navigation by depth profile and contour

In the North Sea, the Channel, or any tidal estuary in reduced visibility, the lead line provides a continuous navigational track that does not depend on visual observation, star sights, GPS, or anything else that fog can deny. The technique is to take regular soundings at intervals through the passage, plotting each depth on the chart and tracing the depth profile along the ground.

On a compass course in fog, take a sounding every five minutes. Plot each one against the chart. Note when the depth changes, in which direction, and by how much. Cross-reference with the expected tidal state. Chart Datum depths assume approximate low water, so actual depths will be chart depth plus the tide height at the time of sounding.

A related technique, documented by Kemp and D’Olier from both Erskine Childers’ account of the Frisian Islands and from a modern sailor following the ten-metre contour along the Dutch coast from Den Helder to Ijmuiden. Following a specific depth contour as a track. Where a depth contour runs roughly parallel to the coast, as it does along much of the East Anglian coast and the Dutch Wadden shore, following it keeps you at a known distance from danger in conditions where you cannot see the shore. The lead is cast repeatedly. Each reading either confirms you are on the contour or indicates drift toward shallower or deeper water. Childers’ character Davies navigated foggy Frisian channels at night this way with nothing but a boathook marked in feet. That same channel is still there.

A depth sounder does this faster. A lead line does it without batteries, without transducers, and without a display that can fail. And it produces the bottom-type information the echo sounder cannot give. Which is what allowed Cooper’s father to confirm his position not just by depth but by the specific character of the seabed material under the Dogger Bank.

What the lead tells you before anchoring

The arming sample is particularly useful before anchoring. Fine mud holds well but can have a skin of fluid surface mud over firmer material below. Coarse sand with shell gravel is generally reliable holding ground but indicates an environment where tidal scour may shift the bottom after extreme springs. Clay holds excellently. Rock holds not at all. Weed, especially dense Zostera or kelp beds, can produce a good initial set followed by dragging once the anchor has worked through to what lies beneath.

A sounding twenty metres short of the intended anchor position, followed by one at the position, followed by one twenty metres beyond, gives a picture of the immediate seabed gradient that the chart may not show in enough detail. A sudden shoaling in those last twenty metres is worth knowing before the anchor goes down rather than after.

This connects directly to the foreshore reading in What the Foreshore Tells You. A fine sand beach at low water will have a corresponding bottom in the anchorage nearby that the lead can confirm. A coarse shingle foreshore means energetic conditions and poorer holding on the offshore side of the bar. Lead line and foreshore observation together give a complete picture that either alone gives only partially.

The lead and water colour

Gooley’s account in How to Read Water of how Pacific navigators used bottom colour to identify safe channels through coral, picking a darker blue channel through turquoise shallows by eye, is an extreme but clear version of a relationship between visible water colour, depth, and seabed type that applies in every tidal water. The chart abbreviations predict what colour the water will show in given depths and light conditions. The lead confirms what the eye suggests. The water colour post covers the optics in full. White sand in shallow water produces turquoise. Dark mud in deeper water produces olive-grey. Kelp beds produce a distinctive dark brownish-green.

On an East Anglian bar approach on a falling tide, the colour of the water changes with depth and bottom type in a sequence an experienced local reads as reliably as any instrument. Where the bar has shifted after a winter storm, the colour and the lead together tell the story the chart, surveyed months or years earlier, may not.

Combining the lead with dead reckoning

The lead line integrates into the dead reckoning plot in Dead Reckoning Without Electronics as a position-fixing tool rather than a course or speed input. A sequence of soundings at known intervals, plotted against the estimated DR position, confirms or corrects the DR in the same way a visual cross-bearing would. But without requiring visibility of any landmark.

On a Channel fog passage, the DR plot predicts when you should cross a particular depth contour. If the lead finds it earlier than expected, you are set in one direction. If later, in the other. The correction can be applied to the plot without any visual observation at all. This is not a theoretical capability. It is the system that Cooper’s father used. That Erskine Childers described. That the Lowestoft trawlermen practised as routine. That the navigators in the Bayeux Tapestry were already using when Harold Godwin’s fleet approached the French coast in 1064.

The Dead Reckoning trainer lets you practise integrating soundings into a DR plot on dry land before the conditions matter. Useful for working through example passages from a pilot book in a winter evening.

The fisherman who said depth and bottom was enough was not offering an anecdote. He was describing a system verified by two and a half thousand years of professional use in exactly the waters where GPS is now the primary instrument.

References

Gooley, T. (2016). How to Read Water. . Sceptre

Kemp, J. and D'Olier, B. (2016). Early Navigation in the North Sea — The Use of the Lead and Line and Other Navigation Methods. The Journal of Navigation, 69(4), 673–697.

Lewis, D. (1994). We, the Navigators: The Ancient Art of Landfinding in the Pacific. University of Hawai'i Press. 

Mainwaring, H. (1644). The Seaman's Dictionary. Reproduced in The Life and Works of Sir Henry Mainwaring, Vol. II. Navy Records Society, London, 1922.

Taylor, E.G.R. (1957). The Haven Finding Art. Abelard-Schuman, New York.

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