Collection: Field Notes - Old Fashioned Seamanship

Series Hub: Traditional Navigation Techniques

Subject: The chip log — the origin of the word "knot", how the instrument works, how to make and use one, calibrating against a known distance, and how it feeds the dead reckoning plot

How to Build and Use a Chip Log — Measuring Speed Through the Water Without Electronics

The word knot, as a unit of speed at sea, came from a piece of weighted wood thrown overboard and the number of knots in a line that ran through a sailor’s fingers before a sandglass emptied. The instrument is called a chip log. It takes about an hour to make, costs nothing, and produces a speed input good enough to feed the dead reckoning plot. This is how it works and how to calibrate one against a known distance.

Every time a sailor says their boat is doing six knots, they are using a unit of speed that was invented to count pieces of knotted string. The origin of the word is exactly that mundane. No metaphor, no rope-tying, no nautical mystery. A piece of weighted wood gets thrown from the stern. The line attached to it runs out astern as the boat moves forward. The line is marked at intervals with actual knots, and someone times how many knots pass through their hands before a sandglass runs out. The arithmetic produces a speed in knots per hour, shortened in normal usage to just knots, and from there into every weather forecast since.

The chip log is one of the oldest speed-measuring instruments still in practical use, and the simplest. It needs a piece of shaped wood, a length of line marked at intervals, and something to time the measurement. Originally a sandglass. Now a watch. Its accuracy, made properly and used properly, is good enough to feed the dead reckoning plot in Dead Reckoning Without Electronics. Its construction takes about an hour and costs nothing.

Why speed matters and why it is hard

Speed through the water is one of the three primary inputs to a DR plot, alongside course steered and elapsed time. The calculation is straightforward. Distance equals speed multiplied by time. A boat averaging four knots for six hours has covered twenty-four miles through the water. The plot advances by twenty-four miles along the course steered, current and leeway corrections are applied, and an estimated position results.

The difficulty is that speed varies continuously. Wind shifts, sail changes, wave state, tidal acceleration and deceleration all alter it. An electronic log averages these variations and displays a continuous readout. Without it, the navigator has to sample speed at intervals and apply judgement about what average is representative of the period. The chip log makes that sampling objective rather than guessed.

The related complication is distinguishing speed through the water from speed over the ground. A boat doing five knots through the water in a two-knot fair current is making seven knots over the ground. In a foul current of the same strength it is making three. The chip log, like an electronic water log, measures speed through the water only. The current component is handled separately in the DR plot, using tidal atlases and the back-transit technique in What Moving Water Tells You. Confusing the two is a classic source of DR error and one that ruins more plots than people admit to.

The foam-timing method first

Before the chip log itself, there is a simpler method that requires nothing at all and can be used as an immediate cross-check at any moment.

Lewis notes in We, the Navigators that a useful approximate speed can be obtained by timing how long a boat takes to pass a stationary patch of foam or bubbles on the water surface. The relationship he gives is three-fifths of the boat’s waterline length in feet, divided by the time in seconds, gives speed in knots. For a thirty-foot waterline, three-fifths of thirty is eighteen. If the foam takes three seconds to pass from bow to stern, eighteen divided by three gives six knots.

Lewis used this method himself on his instrument-free passages with Hipour and Tevake, and reports that his own results were generally less reliable than the experienced navigators’ estimates. Those estimates were not produced by any deliberate counting technique. They were produced by a semi-conscious integration of hull sound, spray character, wind pressure, wake turbulence, and accumulated experience with a specific vessel. He describes Hipour’s distance estimates as accurate to the level that landfalls verified themselves. His own calculations were sometimes significantly astray. The trained navigator’s multisensory estimate outperformed the deliberate counting method on the same passage, in the same conditions.

The foam-timing method is most useful as a tool for the navigator who is building the skill, because it forces conscious attention to speed at regular intervals. For a sailor in unfamiliar conditions on an unfamiliar boat, without the accumulated experience that Hipour had with his canoe, a more objective instrument produces more consistent inputs to the DR plot. That instrument is the chip log.

How the chip log works

The chip log has three components. The chip itself, a small wooden board weighted to float nearly vertical. A log line marked with knots at regular intervals. And a timing mechanism.

The chip is thrown from the stern and the line allowed to run free. The chip’s drag in the water holds it relatively stationary while the boat moves away from it, paying out line behind. An observer counts how many knots pass through their hands in a fixed time period. More knots in the same time, higher speed. The number of knots counted equals the speed in knots, provided the line intervals and the timing period are correctly proportioned to each other.

The original standardisation, established by the sixteenth and seventeenth century Royal Navy and adopted internationally afterwards, used a timing period of twenty-eight seconds and spaced the knots at forty-seven feet and three inches apart. The distance is derived from the ratio of the nautical mile (6,076 feet) to the hour (3,600 seconds), scaled to the twenty-eight-second glass. The arithmetic is 6,076 divided by 3,600 multiplied by 28, which gives 47.3 feet. Each knot that passes through the hand in twenty-eight seconds represents one nautical mile per hour. One knot. The elegant proportionality is why the word knot means both a speed and a physical knot in a rope.

For modern use a fourteen-second timing period is often more convenient, with knots at half the standard spacing, around twenty-three and a half feet. Two knots counted in fourteen seconds equals two knots of speed. The smaller line interval makes the log more responsive to slower speeds, which is usually where small yacht speeds live.

Making one

The chip itself is traditionally cut from a flat piece of hardwood in a quadrant shape, a quarter-circle similar to a sector of pie, perhaps fifteen to twenty centimetres along the straight edges. The curved edge is weighted with lead sheet or fishing weights to make the log float with the straight edges vertical. Three bridle lines attach at the three corners and join to a single point a short distance from the chip, where the main log line connects. The weighting needs to be enough to hold the chip nearly upright in the water without being so heavy that it submerges.

A short length of line connects the bridle point to the log line proper. The first few metres of log line are the stray line, paid out before timing begins to let the chip clear the boat’s wake and sit in undisturbed water. The stray line is marked with a coloured rag or whipping. Timing begins when this mark runs off the reel. After the timing period, the line is checked at the point reached and the number of knots gives the speed.

The log line itself can be any braided line of manageable diameter, marked at the appropriate intervals with small loops of twine or whippings worked into the line. If using a fourteen-second period, mark the first knot at the stray-line end at twenty-three and a half feet, then every twenty-three and a half feet thereafter. The maximum likely speed of the vessel determines how many knots are required. A seven-knot boat needs at least eight marks to avoid running out of line in a fast-timed cast.

The reel is a simple wooden frame on which the log line is coiled. Any shaped piece of timber works. The requirements are that the line can be coiled neatly and that it runs freely when deployed. A wooden rolling pin works.

Calibrating against a known distance

A chip log made as described will give results close to the standard calibration. The most reliable calibration is testing against a known distance, because individual variation in chip construction, line weight, and towing characteristics all affect the result slightly.

The simplest method uses a transit distance, a measured stretch of coastline between two identifiable marks, on a calm day in negligible current with the engine running at a steady throttle. Engine RPM set to a known speed, cross-checked against a known distance-to-time from previous passages. Log casts taken every few minutes throughout the transit. If the log is reading consistently higher or lower than the known speed, the knot intervals can be adjusted.

The Admiralty alternative is the local transit range. A measured mile marked by transit posts, present on many stretches of UK coastline. A steady run through the measured mile at a consistent engine setting, timed with a watch, gives exact speed over ground for that run. The average of runs in both directions cancels the current. Log casts taken throughout give the chip log reading against which to calibrate.

Once calibrated for a specific boat and chip configuration, the log can be checked against the calibration marks at intervals and the knot spacing adjusted if the chip’s drag changes over time as wood swells, dries, or acquires marine growth.

Using the log on a passage

On a passage, log casts are taken at intervals throughout each watch. Every half hour in settled conditions, more often if wind or sea state is changing noticeably. The speed is logged with the time, and at each DR plot advance the average of the readings since the last plot is used as the speed for that period.

The mechanics of a cast require two people most easily. One to throw the chip and pay out the line, one to time and count. Solo it is done by throwing the chip, watching the stray-line end, starting a watch at the mark, and counting knots to hand until the timing period ends. This is manageable with practice. The watch starts the moment the stray-line mark runs off the reel. Fourteen or twenty-eight seconds later the line is gripped, and the number of knots counted from the chip end gives the speed.

In rough conditions with a steep following sea the chip may be pulled forward by wave action rather than holding stationary, giving a falsely low reading. In these conditions the foam-timing method is a useful cross-check. If the log is reading three knots and a bow wave foam patch takes two seconds to pass from stem to cockpit on a twenty-five-foot waterline, the foam timing gives fifteen divided by two, which is seven and a half knots, suggesting the chip is not holding properly. Adjusting the weight or bridle length usually fixes it.

The log and the DR plot

The chip log output feeds directly into the Dead Reckoning Without Electronics plot as the speed input. An hourly update uses course steered from the compass, distance from the log (speed averaged over the hour multiplied by one hour), a leeway correction, and a current correction from the tidal atlas or from the back-transit check in What Moving Water Tells You. The result is an estimated position one hour ahead of the last fix. The dead reckoning training app lets you practise the calculation off the water until it becomes automatic.

The chip log makes the speed component explicit and recordable. Instead of an impressionistic estimate, or worse, no speed input at all and a rough mental average, the log produces a figure that was measured, can be checked against subsequent figures, and can be compared with the expected boat speed for the conditions. An unexpectedly low reading flags the possibility of a foul current or a fouled bottom. An unexpectedly high reading flags a fair current or a stronger breeze than the barometer suggested.

Over a long passage, the regular rhythm of casts maintains navigational discipline. The act of deploying the log every half hour forces the navigator to attend to speed consistently rather than in an occasional and easily forgotten way. Hipour’s accuracy in dead reckoning was the product of exactly this kind of sustained, regular attention, though his instrument was his accumulated feel for the canoe rather than a chip and a line. For the modern sailor without that accumulated experience, the chip log provides the same regularity at the cost of a little effort and forty-seven feet of rope.

The Beaufort cross-calibration

One additional use of the chip log is worth flagging. The Beaufort scale sea-state descriptions in The Beaufort Scale and What It Actually Looks Like describe visible sea conditions readable from the cockpit. A log reading at the same time as a Beaufort observation, noting that a Force 4 is producing five knots on this boat on this point of sailing, builds a calibrated sense of what each force means in terms of boat performance specific to your vessel. Over a season this becomes a reliable shortcut. In Force 4 on this heading this boat typically does four and a half to five and a half knots. So if the log is reading three something else is limiting speed, and if it is reading seven something in the environment has changed.

This integration of direct measurement with observed conditions is, in miniature, the same synthesis Lewis documents in the Pacific navigators. Multiple independent signals, each checked against the others, producing a navigational picture more reliable than any single input alone.

The chip log feeds the dead reckoning plot covered in Dead Reckoning Without Electronics. Speed through the water requires the current correction in What Moving Water Tells You before it produces a distance over the ground. The lead line, which provides depth and bottom type as a complementary DR input, is in the next post: The Lead Line — Depth Sounding. The full series index is at Traditional Navigation Techniques.

References

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

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