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Tool used to estimate the speed of a vessel through water
Diagram of a chip log attached to a log-line and reel on a ship
A chip log, also called common log,[1]ship log, or just log, is a navigation tool mariners use to estimate the speed of a vessel through water. The word knot, to mean nautical mile per hour, derives from this measurement method.
All nautical instruments that measure the speed of a ship through water are known as logs.[2] This nomenclature dates back to the days of sail, when sailors attached a piece of lumber (a "log" of wood) to a rope knotted at regular intervals off the stern of a ship. Sailors counted the number of knots that passed through their hands in a given time to determine the ship's speed. Today, sailors and aircraft pilots still express speed in knots.[3]
Model of chip log and associated kit. The reel of log-line is clearly visible. The first knot, marking the first nautical mile is visible on the reel just below the centre. The timing sandglass is in the upper left and the chip log is in the lower left. The small light-coloured wooden pin and plug form a release mechanism for two lines of the bridle. From the Musée de la Marine, Paris.
A chip log consists of a wooden board attached to a line (the log-line). The log-line has a number of knots at uniform intervals. The log-line is wound on a reel so the user can easily pay it out.
Over time, log construction standardized. The shape is a quarter circle, or quadrant with a radius of 5 inches (130 mm) or 6 inches (150 mm),[1] and 0.5 inches (13 mm) thick.[1] The log-line attaches to the board with a bridle of three lines that connect to the quadrant's vertex and the two ends of its arc. To ensure the log submerges and orients correctly in the water, the bottom of the log is weighted with lead.[1] This provides more resistance in the water, and a more accurate and repeatable reading. The bridle attaches in such a way that a strong tug on the log-line makes one or two of the bridle's lines release, enabling a sailor to retrieve the log.
A navigator who needed to know the speed of the vessel had a sailor drop the log over the ship's stern. The log acted as a drogue, remaining roughly in place while the vessel moved away. The sailor let the log-line run out for a fixed time while counting the knots that passed over. The length of log-line passing (the number of knots) determined the reading.
The first known device that measured speed is often claimed to be the Dutchman's log. This invention is attributed to the PortugueseBartolomeu Crescêncio, who designed it in the end of the 15th century or in the beginning of the 16th century.[4] A sailor threw a floating object overboard and used a sandglass to measure the time it took to pass between two points on deck. The first reference to a Dutchman's log is in 1623—later than the ship log.[5] The Dutchman's log could be used with a brass tobacco box, rectangular with rounded ends. This box had tables on it to convert log timing to speed.[6][7]
Mariners have used the log for a long time. The first known description of the device in print is in A Regiment for the Sea by William Bourne, in 1574. Bourne devised a half-minute sandglass for timing.[8] At the time, a mile was reckoned as 5,000 feet, so in 30 seconds at one mile per hour, a ship would travel about 42 feet:
Initially, the log-line was not knotted and sailors measured the length directly on the line. With the introduction of the nautical mile as a standard unit of measure at sea in the 15th century, they began to mark the line at equal intervals proportional to the nautical mile and to the time interval used for measurement. Initially, the markings were simply knots in the line. Later, sailors worked knotted cords into the log-line. Many ships used knots spaced 8 fathoms (48 feet or 14.6 meters) apart, while other ships used the 7-fathom prescription.[9] The time interval needs to be adjusted according to the distance between knots. Substituting 6,000 feet for 1 mile, the above formula yields 28.8 seconds for a distance of 8 fathoms. In fact, 28-second and 14-second glasses used to be common among navigation equipment.[10]
Frequent measurements helped mitigate some of these inaccuracies by averaging out individual errors, and experienced navigators could determine their speed through the water with a fair degree of accuracy. Because a log measures the speed through the water, some errors—especially the effect of currents, the movement of the water itself—could not be corrected. Navigators relied on position fixes to correct for these errors. Modern navigation tools, such as GPS, report speed over ground, and in general do not give the same result as a log when currents are present.
Taffrail Log RotatorMechanical speed logs called patent logs[1] or taffrail logs, operating on physical principles in a manner similar to a car's odometer by towing a vane or rotor from the stern (or taffrail) by a long line, were developed in the eighteenth century (or earlier) but became practical in the nineteenth century and replaced the traditional chip log.[11] Examples of taffrail logs include those made by Thomas Walker & Son. Since the second half of the twentieth century, sailors continue to use more modern mechanical and electro-mechanical versions based upon a small impeller or paddle wheel attached to (or through) the bottom of the hull, especially on smaller yachts.
Most commonly used however (especially on merchant ships) is the electromagnetic log, more reliable as there is no mechanism subject to breakdown.
In recent years ultrasonic speed sensors have become available. These use two ultrasonic transducers—one forward, one aft—that send ultrasonic pulses through the water flowing past the hull. By calculating the time differential in pulse propagation from one sensor to the other, the device calculates the speed of the hull through the water.
Doppler log: Today, a very accurate maritime speed measurement comes from Doppler measurement, either derived acoustically with Doppler-Sonar —or with radio interferometrically by Doppler measurement of satellite signals, such as those from the Global Positioning System (GPS). However, most commercial GPS systems are not configured to operate in this mode.
^URL:"Ciência Viva - Projectos e Materiais". Archived from the original on 2011-09-19. Retrieved 2011-09-06. Agência Nacional para a Cultura Científica, Ciência Viva, Descobertas e Invenções Portuguesas (in Portuguese)
^Timothy J. Runyan, ed. (1987). Ships, Seafaring, and Society: Essays in Maritime History. Wayne State University Press. p. 74. ISBN9780814319918.
^Broelmann, J (1998). Robert Bud and Deborah Warner (ed.). Instruments of Science. An Historical Encyclopedia: Logs. New York, London. pp. 361–363.{{cite book}}: CS1 maint: location missing publisher (link)
The Oxford Companion to Ships and the Sea, Peter Kemp ed., 1976 pp 192–193. ISBN0-586-08308-1
Chip Log pattern on the webpage of the Navy & Marine Living History Association, Inc. Note: the distance given in the materials list for this pattern is 33+1⁄3 feet, but part C of "Construction" uses the modern distance of 47+1⁄4 feet.
The chip log, also known as the common log, is a traditional nautical instrument used to estimate a ship's speed through the water by deploying a weighted wooden drag device attached to a knotted line.[1][2]Consisting of a triangular or quarter-circle shaped wooden "chip" (typically 12 inches on each side, ballasted with lead to ensure it remains stationary in the water) tethered to a long line marked with knots at regular intervals of approximately 47 feet 3 inches (corresponding to one nautical mile when timed over 28 seconds), the device is thrown overboard from the stern of the vessel.[2][3] As the ship moves forward, the line unreels from a handheld reel, and the number of knots that pass through the mariner's hands during a fixed interval—traditionally measured with a 28- or 30-second sandglass—is counted to determine speed in knots, where one knot equals one nautical mile per hour.[4][1] This method, requiring two crew members (one to heave the log and time it, the other to count the knots), provided a practical means of dead reckoningnavigation before mechanical or electronic alternatives became widespread in the 19th and 20th centuries.[5][2]The origins of the chip log trace back to ancient maritime practices, where sailors simply dropped pieces of driftwood or logs overboard and timed their passage past the ship to gauge speed, a rudimentary technique known as the "Dutchman's log" by the 15th century.[4] By the late 16th century, the method evolved into the more precise chip log, with English mathematician William Bourne describing its use in his 1574 book A Regiment for the Sea, incorporating a half-minute sandglass and knotted line spaced to account for a nautical mile of 5,000 feet at the time.[5] Earlier attributions credit Portuguese navigator Bartolomeu Crescêncio in the 15th century with pioneering the knotted line approach, though the term "knot" for speed derives directly from the physical knots on the line.[4][5]Despite its simplicity and widespread use on sailing ships like the Cutty Sark (which achieved speeds over 17 knots), the chip log had limitations, including inaccuracies from line stretch, water currents, and variations in the sandglass timing.[1][5] It remained a standard tool until the early 19th century, when innovations like the taffrail log (a rotating propeller towed astern) and later patent logs improved precision, and by the 20th century, it was largely replaced by mechanical log lines and eventually GPS and Doppler systems.[2] The nautical mile itself was standardized internationally at 1,852 meters (6,076 feet) in 1929, refining the knot as a unit still used today in aviation and maritime contexts.[4][3]
Historical Development
Origins and Early Methods
The earliest precursors to the chip log emerged from rudimentary techniques for estimating a ship's speed through water, primarily involving the use of floating objects dropped overboard. Known as the "Dutchman's log," this method consisted of heaving a piece of wood or similar buoyant item from the bow and observing or timing its passage past the stern, often using simple counts or early timekeeping devices like dripping water clocks. This approach provided a rough gauge of velocity based on the ship's length and the object's relative movement, though it was prone to errors from currents and wind. The technique is first theoretically discussed in the mid-15th century by German philosopher and mathematician Nicholas of Cusa, who referenced it in his writings on practical mechanics, predating more widespread practical adoption.[6]In medieval European navigation, adaptations of the Dutchman's log incorporated floating markers such as pieces of cork or oar tips trailed behind the vessel to estimate progress over short intervals, allowing sailors to correlate speed with known distances along coastal routes. Arab navigators in the Indian Ocean trade similarly employed floating objects to assess relative motion against the ship, integrating these observations with wind patterns and star sightings for dead reckoning, as noted in 13th- and 14th-century periplus texts that emphasized experiential estimation over precise instrumentation. While some 14th-century European sailing logs mention "log-lines"—plain cords attached to floats without knots—for marking distance run during a fixed time, these lacked standardization and relied on manual timing.[7]The transition to knotted lines began in the late 15th century, with Portuguese navigator Bartolomeu Crescêncio credited with pioneering the approach.[4] This evolved further in the early 16th century through experiments in England and the Netherlands, where mariners refined the Dutchman's log by securing a weighted float (or "chip") to a line marked at regular intervals. English gunner and mathematician William Bourne detailed an early prototype in his 1574 publication A Regiment for the Sea, describing a line with divisions calibrated to a 30-second sand glass for speed calculation, marking a shift toward quantifiable measurement.[8] These innovations addressed the limitations of untethered floats by enabling repeatable distance tracking, laying the groundwork for broader adoption. This method was further standardized in the 1600s with consistent knot spacing and timing protocols.[9]
Standardization and Evolution
The chip log developed in the early 16th century through refinements by English and Dutch mariners building on earlier informal methods. The first printed description of the device appeared in 1574 in the work of William Bourne.[8] Bourne's contributions included the use of a half-minute sandglass for timing. In 1624, mathematician and astronomer Edmund Gunter proposed a nautical mile of 6,000 feet, which influenced later standardizations of knot spacing on the log line, though early variants used intervals around 50 feet calibrated for a 30-second sandglass. The specific 47 feet 3 inches spacing emerged later in the 17th century to align with adjustments for a 28-second sandglass and the evolving nautical mile definition, allowing for direct calculation of speed in nautical miles per hour.[3]In the 18th century, refinements to the chip log included variants like the "Dutchman's log," which utilized simple floating markers and timed observations to enhance usability on smaller vessels or in varying conditions. The Royal Navy used the chip log widely by the 18th century, integrating it into procedures for dead reckoning and position fixing, which greatly supported exploration during the Age of Sail. Notable users included Captain James Cook on his Pacific voyages, where the device aided in charting vast unexplored regions, as well as navigators succeeding Columbus-era explorers in transoceanic routes, enabling more reliable estimates of progress over long distances.[10]By the mid-19th century, the chip log began to be phased out with the advent of mechanical logs, such as the taffrail log with its towed propeller, which offered greater accuracy and reduced human error. The last widespread use in naval service occurred around 1850, after which electromagnetic and other automated systems further supplanted traditional methods.[2]
Design and Components
The Log Chip
The log chip serves as the primary drag element in the chip log, a historical nautical instrument for estimating a ship's speed through water by resisting forward motion when deployed astern. Developed around 1600, it functions by remaining relatively stationary in the water, allowing the attached knotted line to pay out at a rate proportional to the vessel's velocity.[11]The chip's design emphasizes hydrodynamic stability and drag, typically taking the form of a quarter-circle or pie-shaped wooden board with a radius of approximately 6 inches (12-inch diameter) and about 1 inch in thickness. This quadrant shape positions the flat face perpendicular to the water flow, creating resistance that anchors it against the ship's progress without being towed forward. To achieve this orientation, the curved edge incorporates lead weights, often melted and poured into several shallow holes (typically 5–7, each about 7/8 to 1 inch in diameter) along the arc, ensuring the board floats upright and maintains consistent drag.[11][2]Historical variations in the chip's form reflect early experimentation, with simpler wooden pieces giving way to the more efficient quarter-circle shape, which became the standard design for practical use across naval and merchant vessels. The wood selected was generally dense and buoyant enough to support repeated submersion, prioritizing shape over specific species for effective performance. A temporary peg or bridle attachment secures the chip to the line during measurement, facilitating quick deployment and retrieval to support ongoing speed readings.[11][2]
The Knotted Line and Reel
The knotted line, also known as the log-line, was typically constructed from loosely laid, flexible, untarred hemp rope to ensure it could be easily paid out and wound back without excessive friction or stiffness.[12] This material allowed for a total length of approximately 150 fathoms (about 900 feet), sufficient to measure speeds over extended periods without running out during a single trial.[12] Knots were tied at precise intervals of 47 feet 3 inches, equivalent to one-sixtieth of a nautical mile, which established the nautical unit of speed known as the "knot."[12] These markings consisted of small pieces of red cloth, leather tags, or short lengths of fish line secured in place, often using simple knots like reef knots for durability and visibility against the rope.[12] A stray line of about 10 fathoms preceded the first knot, marked by a red flag or bunting to allow the chip to stabilize in the water before measurements began.[12]The reel, essential for controlling the line's deployment and retrieval, was a wooden hand-held device typically 2 to 3 feet long and 5 to 8 inches in diameter, featuring an axle with cross-handles for easy rotation, often forming a T-shaped or pegged structure.[12] This design enabled a sailor, sometimes secured by a harness to prevent falling overboard, to manage the payout smoothly while the ship moved forward, minimizing drag on the line itself.[12] The reel was mounted on a rail or held aloft to keep the line clear of the ship's wake, ensuring accurate unwinding without snags.[1]Calibration of the knotted line was critical to maintain precision, as hemp could shrink when wet or stretch under load, potentially introducing errors.[12] Before use, the line was stretched under controlled tension—often by hanging weights or pulling it taut—to simulate deployment conditions and account for shrinkage, with knots then marked using a specialized tool called a line glass or measuring frame for exact spacing.[12] Periodic checks involved laying the line out straight on deck and remeasuring intervals against a standard, aiming to keep overall errors within 1-2% through adjustments to knot positions or line replacement if wear was evident.[12] This process, drawn from established nautical practices, ensured reliable distance measurement over repeated uses.[12]
The Timing Device
The timing device in the chip log system was a marine sandglass, an hourglass consisting of two pear-shaped glass bulbs joined by a narrow neck, containing a precise quantity of fine sand calibrated to flow from one bulb to the other in 28 seconds. This half-minute glass was typically encased in a protective frame, often of brass with markings such as "28 SEC" stamped on the ends, or sometimes wood to shield it from shipboard conditions. Variations included 14-second quarter-minute glasses for finer readings and earlier 30-second models used before standardization.[13][14][12]The sandglass was calibrated to run for exactly 28 seconds to align with the log-line's knot spacing, usually set at 47 feet 3 inches (approximately 14.4 meters), ensuring that at a speed of one knot, precisely one knot mark would pass during the timed interval. This duration represented an evolution from the initial 30-second glasses paired with intervals of about 42 feet, as described by William Bourne in 1574; later, as the nautical mile was lengthened to 6,080 feet, intervals were adjusted to about 51 feet for continued use of 30-second glasses. By the early 18th century, the 28-second standard had become widespread in British and American navies to better approximate the nautical mile of 6,080 feet and reduce measurement error to around 1.5 percent. The first documented use of such a timed log-line system dates to 1574, described by mathematician William Bourne, though the precise 28-second calibration emerged later as nautical measurements refined.[13][15][12]In practice, a second crew member held the sandglass steady or secured it in a wooden stand near the log-reel operator to monitor the timing without interference from the ship's motion. To initiate measurement, the glass was turned on its side and then upright exactly when the chip was fully trailed astern, allowing the sand to flow evenly; upon completion, it was swiftly inverted with a smooth motion to prevent air bubbles from forming in the neck, which could impede the sand's descent and skew the interval. This device complemented the line payout by providing the fixed temporal benchmark for counting knots.[2][12]Historically, the shift to the 28-second glass by the 1700s addressed inaccuracies in earlier designs, but environmental factors remained a challenge; in humid or damp conditions, the sand could clump, causing the glass to run slower and underestimate speed by several percent, necessitating regular calibration against a pendulum or watch when available. Such errors were common on long voyages, prompting sailors to store glasses in dry conditions and inspect them frequently.[12]
Operation and Measurement
Deployment Procedure
The deployment of the chip log, known as "heaving the log," was a coordinated process typically performed at the stern of the vessel on the lee side to minimize wind interference. Preparation involved ensuring the log chip—a flat, quarter-circle wooden board weighted with lead to float upright—was securely attached to the log-line via a bridle, and the line was properly wound on a wooden reel for smooth payout. The half-minute glass, calibrated to run for 28 or 30 seconds, was checked for accuracy, as was the log-line for any stretching or wear that could affect measurements.[16][12]The procedure began with an officer of the watch throwing the log chip overboard from the taffrail, allowing about 10 fathoms of stray line to run out to clear the ship's wake and prevent turbulence from influencing the reading. Once the stray line was expended, the officer called "Turn," prompting the glass-holder to start the half-minute glass and respond "Done." The seaman holding the reel then paid out the line hand-over-hand or allowed it to unreel freely, counting the knots and stricks (divisions between knots) as they passed, while ensuring the line veered out quickly to avoid the log "coming home" due to insufficient momentum. When the sandglass ran out, the glass-holder called "Stop," and the reel was immediately halted to mark the endpoint; the log was then hauled in by tugging the line to release a peg in the bridle, flipping the chip to skim across the water for easier retrieval.[16][12][2]Crew roles were distinctly assigned to maintain precision and safety: the officer directed the deployment, issued commands, and often recorded the results; the glass-holder managed the timing device exclusively; and the log-man or seaman controlled the reel, preventing tangles or burns from the line running too hot through the hands. This process was typically conducted hourly during a voyage, or at watch changes, to log speed variations and contribute to the ship's daily reckoning, with adjustments made if heavy seas or shifting winds altered conditions midway through the interval.[16][12]Safety protocols emphasized avoiding line entanglements and personal injury, with commands like "log-haul" signaling the retrieval phase to alert nearby crew. For smaller vessels such as sloops, adaptations included using shorter or lighter lines to suit the scale, while larger ships employed standard 150-fathom lines; in all cases, the procedure required at least two to three crew members to execute efficiently without risking overboard falls or equipment loss.[16][2]
Speed Determination
The speed of a vessel using a chip log is determined by counting the number of knots that pass over the rail during a fixed time interval, typically measured with a 28-second sand glass, where the count directly corresponds to the speed in knots, or nautical miles per hour.[17][18] This basic formula, Speed (knots) = Number of knots passed / Time intervals (adjusted for the glass duration), assumes ideal conditions where one knot counted in 28 seconds equates to 1 nautical mile per hour.[1]The derivation of this measurement stems from the precise calibration of the log line's knot spacing at 47 feet 3 inches (approximately 14.4 meters), which represents the distance a vessel travels in 28 seconds at a speed of 1 knot.[17][19] A nautical mile is standardized at about 6,076 feet, so the spacing is roughly 1/128.57 of a nautical mile; divided by the 28-second interval (or 28/3,600 of an hour), this yields exactly 1 nautical mile per hour per knot counted, ensuring the count provides a direct reading without further adjustment.[17] This calibration evolved from earlier approximations, such as 30-second glasses with slightly longer spacings, but the 28-second version became standard for greater accuracy in the 18th and 19th centuries.[17][18]In practice, the officer records the speed in the logbook simply as the number of knots counted, such as "5 knots" to denote 5 nautical miles per hour, often averaging multiple hauls—typically three or more—to improve reliability and account for minor variations in deployment.[1] These entries formed the basis of daily navigational logs, contributing to dead reckoning calculations.The term "knot" as a unit of speed originates directly from the knotted log line of the chip log, where the counted knots represented increments of nautical miles per hour; this usage was formalized in 19th-century navigation tables and standards, such as those adopting the nautical mile at 6,080.20 feet in the United States in 1880.[17]
Accuracy and Limitations
Sources of Error
The log line, typically made from materials like hemp, was prone to stretch under tension and shrinkage when wet or dried, altering the effective spacing between knots and introducing measurement errors in distanceestimation.[12] Knot slippage could occur if the knots loosened over time or due to improper tying, further disrupting the calibrated intervals and compounding inaccuracies in speed calculation.[12]The performance of the chip itself was susceptible to drift caused by uneven currents or wave action, which could cause it to veer off course rather than trail steadily astern, leading to overestimation or underestimation of the ship's speed through the water. Improper weighting of the chip exacerbated this issue, as insufficient lead might allow it to ride waves instead of maintaining submersion. In rough seas, the chip often "came home" toward the ship, necessitating allowances of up to 10% per mile to account for the reduced payout of the line.[12]Timing variances arose primarily from the sandglass, whose sand flow was inconsistent due to temperature fluctuations and humidity, potentially causing deviations of 1-3 seconds from the intended 28- or 30-second interval and resulting in speed errors of 1.5% with a precise 28-second glass but up to 5% with a less accurate 30-second one.[12][20] Human reaction time in starting and stopping the glass introduced additional variability, often on the order of fractions of a second per measurement.[12]Environmental factors such as ship leeway from wind pressure on the hull, windage pulling on the trailing line, and turbulence in the ship's wake interfered with the line's free payout, distorting the relative motion between the chip and the vessel. These effects, combined with variable currents and waves, led to cumulative errors that could accumulate to 10-15% over extended voyages, particularly when repeated measurements were integrated into dead reckoning.[12][21]
Navigational Adjustments
Navigators employed calibration routines to maintain the accuracy of the chip log throughout a voyage. Prior to departure, the log line underwent stretching to counteract natural contraction from drying, with knots re-tied at precise intervals—typically 51 feet for a 30-second glass—to align with the nautical mile standard of approximately 6,080 feet.[12] Periodic checks involved comparing log readings against dead reckoning estimates derived from course and time, allowing adjustments for cumulative errors over daily runs.[12]Empirical corrections addressed environmental influences on measurements. In windy conditions or head seas, where the log chip might be pulled forward by waves, navigators added an allowance of about 1 mile in every 10 miles traveled to compensate for underestimation of distance.[12] To mitigate variability, multiple readings were taken during heaving—often three or more—and averaged to smooth out inconsistencies from line sag or irregular timing.[12] If the sand glass ran short (e.g., 28 seconds instead of 30), the knot interval was empirically shortened by 3-4 feet to preserve proportionality in speed calculations.[12]Logbook practices integrated chip log data with contextual observations for reliable dead reckoning. Entries were made hourly, recording knots and fathoms run alongside notes on sea state, wind direction and force, and any deviations like damp weather affecting glass timing.[12][10] Cross-verification occurred by transferring speed and course data to a traverse board, a pegged wooden disk updated every half-hour and reconciled at watch's end against logbook totals to detect discrepancies early.[10][5]Historical examples illustrate formalized approaches to these adjustments. In the 18th and 19th centuries, navigational manuals like Nathaniel Bowditch's 1802 American Practical Navigator detailed routines such as line calibration and environmental allowances, emphasizing training for officers in estimating errors from sea state or glass faults to refine dead reckoning positions. These practices were taught through apprenticeships and texts, ensuring navigators could apply judgment-based tweaks for practical accuracy.[12]
Modern Developments and Replacements
Intermediate Innovations
The intermediate innovations in nautical speed measurement during the 19th and early 20th centuries introduced mechanical towed devices that automated distance and speed recording, bridging the gap between the manual chip log and fully electronic systems. These patent log variants replaced the intermittent heaving of the chip log with continuous operation via impellers or rotors trailed behind the vessel, transmitting data to onboard registers.Early developments included Edward Massey's 1802 screw or rotatory log, which featured a vaned impeller towed astern to drive a mechanical counter, entering general use by the 1830s. This was followed by refinements such as Thomas Walker's 1861 Harpoon log, which incorporated a flexible shaft to more reliably convey rotations from the underwater rotator to the ship's register, minimizing friction losses. Self-registering models further reduced manual intervention; for instance, the Cherub log, patented by Thomas F. Walker in 1878, used a streamlined rotor design where 900 revolutions corresponded to one nautical mile, allowing unattended accumulation of mileage on a dial.[5][22]The taffrail log, popularized in the 1860s, mounted the register directly on the stern rail (taffrail) for easy access. Invented by Truman Hotchkiss and patented in 1864, it employed a multi-bladed propeller towed via a braided catgut line, with rotations mechanically geared to dials indicating speed and distance traveled. This configuration eliminated the need for periodic retrievals, providing persistent data during voyages.[23][24]Compared to the chip log, these devices offered continuous readings without disrupting ship operations, requiring far less crew effort beyond initial deployment and occasional maintenance. Accuracy improved over the chip log's variable estimates—often erroneous above 6 knots due to line sag—achieving reliable measurements when the line was tensioned and free of marine growth, though errors from fouling or stretching could still occur.By the 1880s, taffrail and patent logs were standard in merchant fleets, valued for their practicality on steam and sailing vessels alike. Despite vulnerabilities to biofouling and line wear, their adoption persisted into the early 20th century until supplanted by electronic alternatives post-World War I.
Contemporary Alternatives
Electromagnetic logs emerged in the mid-20th century as one of the first electronic alternatives to mechanical speed measurement devices, utilizing the principle of electromagnetic induction to detect the ship's speed through water. Hull-mounted sensors, typically consisting of electrodes and a coil, generate a magnetic field that interacts with the conductive seawater; as the vessel moves, this induces a voltage proportional to the speed relative to the water, which is then amplified and displayed. These systems provide accuracies ranging from 0.1 to 1 knot, depending on installation and environmental factors, offering reliable performance without moving parts exposed to fouling.[25][26]Doppler and acoustic logs, introduced in the 1960s and widely adopted by the 1970s, employ sound waves to measure speed over the ground by detecting the Doppler shift in echoes from the seabed or water particles. Transducers mounted on the hull transmit acoustic pulses at oblique angles, and the frequency shift in the returned signals calculates both longitudinal and transverse velocities, enabling true motion data essential for collision avoidance. These logs became integral to Automatic Radar Plotting Aid (ARPA) systems, providing input for automated target tracking and enhancing navigational safety on commercial vessels. Accuracy typically reaches within 0.1 knot over suitable depths up to 200 meters, though performance degrades in shallow or turbid waters.[25]From the 1990s onward, the integration of Global Positioning System (GPS) with Inertial Navigation Systems (INS) revolutionized ship speed measurement, delivering satellite-derived velocities over the ground with accuracies better than 0.1 knot and eliminating dependencies on water currents or hull fouling. GPS computes speed from successive position fixes, while INS uses gyroscopes and accelerometers for continuous dead reckoning, with Kalman filtering in hybrid setups correcting drift errors for sub-meter precision in dynamic conditions. This combination addresses limitations of water-relative measurements by providing absolute motion data, now standard in integrated bridge systems.[27][28]In contemporary maritime practice, while electronic logs dominate, the traditional chip log persists in historical reenactments and low-technology sailing vessels for educational or authenticity purposes, such as in naval living history demonstrations. International Maritime Organization (IMO) regulations under SOLAS Chapter V mandate speed and distance logs on ships of 300 gross tonnage and above, requiring independent power supplies for through-water and over-ground measurements to ensure redundancy against system failures.[2][29]
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