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A knot log, also called a ship log or just a log, is a navigation tool used by mariners to measure the relative speed of a vessel through water. Traditionally a ship's speed is measured in the unit of the knot, for nautical mile per hour, which was derived from the chip log one of the earliest reliable methods for the direct measurement of a vessel's speed. Today sailors still use the unit of knots to express a ship's speed. The speed of the ship was needed to navigate the ship using dead reckoning, which was standard practice in the days before modern navigation instruments like GPS. It should be realized that a log usually measures a vessel's relative velocity through the water, but its 'speed over ground' its actual velocity over the surface of the Earth, or 'velocity made good' along its course may be more significant for navigational of sailing purposes. For instance a vessel may be sailing through a strong current, or it may be sailing a tacking course, so that its actual rate of forward progress may vary considerably from its speed.

History
All nautical instruments designed to measure the speed of a ship through water are known as logs. This nomenclature dates back to days of sail when sailors 'cast' a piece of wood, a 'log', attached to rope knotted at regular intervals off the stern of a ship. The simplest version, called a chip log, known at least since the fifteenth century, was simply a piece of wood attached to a piece of line knotted at regular intervals that sailors could throw overboard and used together with a sand glass or other timing device, to measure the speed of a ship relative to water. The sailors would count the number of knots that passed through their hands in a given period of time. This information would be recorded at regular intervals on a traverse board and eventually in the ships logbook.

Mechanical logs called Patent or Taffrail logs logs were introduced in the eighteenth century, and self-recording logs in the early nineteenth century. These could also indicate the distance sailed from one time to another. Pitometer logs, and mechanical and electro-mechanical logs came later in the middle of the 20th century. More recently ultrasonic, doppler/sonar, and Global Positioning System (GPS) systems started coming into use.

Dutchman's Log
Probably the oldest method of determining the speed is the so-called Dutchman’s log, its invention is attributed to the Portuguese inventor Bartolomeu Crescêncio near the end of the 15th century.

The Dutchman's log consists of a floating object, the log, which was dropped overboard from the bow of the ship; the time elapsing before it passed the stern was counted off by the navigator and recorded in the log book. The log being considered 'dead in the water' once dropped, speed could be calculated by dividing the length of the vessel by the length of time it took the log to float from the bow to the stern of the vessel. Tables for making this calculation were sometimes inscribed upon brass tobacco tins.

Chip Log
A chip log consists of a wooden board attached to a line (the log-line). The log-line has a number of knots tied in it at uniform spacings. The log-line is wound on a reel to allow it to be paid out easily in use.

The log has been used by mariners for a long time. The first occurrence of a description of the device in print was in A Regiment for the Sea by William Bourne in 1574.

Over time, the log was standardized in construction. The shape is a quarter circle, or quadrant, and the log-line is attached to the board with a bridle of three lines connected to the three corners. In order to ensure that the log submerges and is oriented correctly, the bottom of the log is weighted with lead. This provides for more resistance in the water and a more accurate and repeatable reading of speed. The bridle is attached in such a way that a strong tug on the log-line results in one or two of the bridle's lines releasing, allowing the log to be retrieved with relative ease. When the nautical mile became standardized, the distance was set to 47 feet, 3 inches (14.4 meters) for a standard glass of 28 seconds.

To use the log, a sailor casts the log over the stern of the ship. The log then acts as a drogue and remain roughly in place while the vessel continued to move forward. The log-line was allowed to run out for a fixed period of time normally 28 seconds. The speed of the ship was indicated by the length of log-line passing over the stern during that time.

More modern logs and replacements
Since the origin of the log at the end of the 15th century, new techniques and technologies have emerged that improved upon the reliability and accuracy of the Chip Log.

Patent Logs
Mechanical logs called patent logs or taffrail logs, were developed in the eighteenth century (or earlier) but became practical nineteenth century and replaced the traditional chip log. A patent log consists of a bronze or iron vane or rotor attached to a rope is towed from the stern taffrail of the ship connected to a mechanical register that counts the log's rotations as it is being towed.

Propeller RPM
The speed of a ship is roughly proportional to the rotation rate of the propeller. This approach is most useful when the propeller has constant pitch. It is less useful on ships with propellers that have variable pitch propellers or no propellers (like sailing ships). A variation of this approach was used by submarine's during World War II to determine the speed of targets. The sonar operator would listen to the sound of a target's propeller and would determine its rotation rate. Knowing the propeller rotation rate and visually identifying the type of ship, the target's speed could be estimated.

Pitometer Log
Pitometer logs (also known as pit logs) came into use in the middle of the 20th century. They are used on both surface ships and submarines. They operate similarly to a pitot tube on an aircraft by measuring the differential pressure between the static and dynamic pressure of the water in metered orifices. The dynamic pressure of the seawater is a function of the depth of the water and the speed of the vessel.

Typically, the pitometer has a long tube that penetrates the ship's hull near the keel. The part of the pitometer protruding from the ship is sometimes called a pit sword or rodmeter. This tube usually has two openings: one facing the direction of seawater motion that is used to measure the dynamic pressure of the seawater and one at 90° to the direction of seawater motion that is used to measure the static seawater pressure.

Data from the pitometer log is usually fed directly into the ship's navigation and fire control systems.

Mechanical & Electro-Mechanical Logs
Since the second half of the twentieth century, more modern mechanical and electro-mechanical versions based upon a small impeller or paddle wheel attached (or through) to the bottom of the hull continue to be used especially on smaller yachts.

Impeller Log: A small propeller mounted in the water that passes by the ship hull. The rotation rate of the propeller is proportional to the speed of the ship. This device is similar in principle to an anemometer, which is used to measure wind speed.

Ultrasonic Logs
In recent years ultrasonic speed sensors have become available. These new type use two ultasonic transducers one forward one very slightly aft of the other that send ultrasonic pulses that measure the presence of particles in the water flow past the hull. By calculating the time differential in the passage of a particle from one sensor to the other, the speed of the hull through the water can be calculated directly.

Doppler Logs
Today, some ships are equipped with sonar instruments that can measure the Doppler shift in a sound pulse generated by sonar as it reflects off the ocean bottom. Unlike a most other types of logs, the speed measurements from a doppler instrument may be of ground speed or speed through the water.

GPS
Speed measurement via Global Positioning System (GPS) are now also cheap and easily available to mariners.