The Chip Log - Measuring Speed Over Water
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
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 running out over a boat's stern. The word knot did not come from rope-tying. It came from a piece of weighted wood thrown overboard and the number of knots in the line that passed through a sailor's fingers before a sandglass ran out.
The chip log is among the oldest speed-measuring instruments in practical maritime use, and one of the simplest. It requires a piece of shaped wood, a length of line marked at intervals with knots, and something to time the measurement — originally a sandglass, now a watch. Its accuracy, properly made and used, is sufficient for the dead reckoning described in Dead Reckoning Without Electronics. Its construction requires about an hour and materials costing nothing.
This post covers the history and mechanics of the chip log, how to build one, how to calibrate it, and how to integrate it with the DR plot. It also covers the alternative method that David Lewis documented in We, the Navigators — the foam-timing technique that Pacific navigators and Lewis himself used as a speed estimator without any purpose-built instrument — and why the chip log produces more consistent results than that method for most sailors.
Why speed matters and why it is difficult
Speed through the water is one of the three primary inputs to a dead reckoning 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. The DR 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 through the water varies continuously. Wind shifts, sail changes, wave state, tidal acceleration and deceleration all alter speed. An electronic log averages these variations automatically 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 since the last sample. The chip log makes this sampling objective rather than guesstimated.
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 described in What Moving Water Tells You. This distinction matters, and confusing the two is a classic source of DR error.
The foam-timing method
Before describing the chip log itself it is worth covering the simpler method that Lewis documents from his Pacific voyages, because it requires nothing at all and can be used as an immediate 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 describes 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 results were generally less reliable than the experienced navigators' estimates — which were not produced by any deliberate counting technique at all, but 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 consistently accurate to a level that verified itself at landfall, while his own calculations were sometimes significantly astray. The trained navigator's multisensory estimate outperformed the deliberate counting method in Lewis's experience.
What this suggests is that the foam-timing method is a useful tool for a navigator who is building the skill, because it forces conscious attention to speed at regular intervals. But 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 consists of 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 means higher speed. The number of knots counted equals the speed in knots — provided the line intervals and timing period are correctly proportioned to each other.
The original standardisation, established by the sixteenth and seventeenth century Royal Navy and subsequently adopted internationally, used a timing period of twenty-eight seconds and spaced the knots at forty-seven feet and three inches apart — a distance 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: 6,076 ÷ 3,600 × 28 = 47.3 feet. Each knot that passes through the hand in twenty-eight seconds therefore represents one nautical mile per hour — one knot — of speed. This elegant proportionality is why the word knot means both a speed unit and a physical knot in a rope.
For practical use today a fourteen-second timing period is often more convenient, with knots at half the standard spacing — approximately 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 a chip log
The chip itself is traditionally cut from a flat piece of hardwood in a roughly 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 should 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 — a length paid out before timing begins, to allow the chip to clear the boat's wake and sit in undisturbed water. The stray line is typically 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 counted 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 timing 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 for as many knots as you need. 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 piece of shaped timber works. The requirements are that the line can be coiled neatly and can run freely when deployed. A wooden rolling pin comes to mind.
Calibrating against a known distance
A chip log can be made as precisely as described above and will give results close to the standard calibration. But the most reliable calibration comes from testing against a known distance, because individual variation in chip construction, line weight, and towing characteristics all affect the result slightly.
The simplest calibration method uses a transit distance — a measured stretch of coastline between two identifiable marks, used on a calm day in negligible current with the engine running at a steady throttle. The engine RPM are set to a known speed (referenced to the engine manufacturer's performance figures at that load, or cross-checked against a known distance-to-time from previous passages). Log casts are 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 accordingly.
Alternatively, the Admiralty method uses the local transit range — a measured mile marked by transit posts 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 eliminates the current component. Log casts taken throughout give the chip log's 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: practical routine at sea
On a passage, log casts are taken at intervals throughout each watch — every half hour in settled conditions, more frequently 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 taking 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 count or a watch at the mark, and counting knots to hand until the timing period ends. This is manageable with practice. The watch is started 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 remaining stationary, giving a falsely low speed reading. In these conditions the foam-timing method Lewis describes in We, the Navigators is a useful cross-check — if the chip log is reading three knots and a bow wave foam patch takes two seconds to pass from stem to the 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 resolves this.
The log and the DR plot
The output of the chip log feeds directly into the Dead Reckoning Without Electronics plot as the speed input. A typical hourly DR update uses: course steered from the compass, distance made good 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 bearing check described in What Moving Water Tells You. The result is an estimated position one hour ahead of the last fix.
The chip log makes the speed component of this calculation explicit and recordable. Instead of a navigator's 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 unexpected 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 log casts also 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 some forty-seven feet of rope.
Speed and the Beaufort connection
One additional use of the chip log is worth noting. The Beaufort scale sea-state descriptions in The Beaufort Scale and What It Actually Looks Like describe visible sea conditions that can be read from the cockpit. A chip 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 that is specific to your vessel. Over a season this becomes a reliable shortcut: in Force 4 conditions on this heading this boat typically does four and a half to five and a half knots, so if the log is reading three knots 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 that 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.
David Lewis's account of speed estimation at sea — including the foam-timing method and its comparison with the trained navigator's semi-conscious integration of hull signs — is in We, the Navigators, in the chapter on dead reckoning. His observations on the relative reliability of the counting method versus the experienced navigator's estimate are a useful prompt for any modern sailor learning to use a chip log: the instrument is a starting point, not a ceiling.
The chip log feeds the dead reckoning plot covered in Dead Reckoning Without Electronics. Speed through the water requires the current correction covered 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.
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