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Pitometer logs (also known as pit logs) are devices used to measure a ship's speed relative to the water. They are used on both surface ships and submarines. Data from the pitometer log is usually fed directly into the ship's navigation system. ==History== All nautical instruments designed to measure the speed of a ship through water are known as logs. 〔 (【引用サイトリンク】 url = https://navy.rotc.psu.edu/NROTC%20Classes/203/notes/Lesson05/lesson05.doc )〕 This nomenclature dates back to days of sail when sailors tossed a log attached to rope knotted at regular intervals off the stern of a ship. The sailors would count the number of knots that passed through their hands in a given period of time. 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. During World War II, pitometer logs were often interfaced directly into warship fire control systems. This interface was necessary to allow gunnery and torpedo fire control systems to automatically track targets. While the pitometer log is very commonly used today, there are a number of other logs that are also in use. These logs include: : * 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. A variation of this approach is known as a patent log, which is towed from the stern of a ship. The patent log includes a mechanical register that counts the log's rotations as it is being towed. The patent log was invented in 1688 by the English instrument maker Humphry Cole. The patent log is also known as a screw log or taffrail log.〔 (【引用サイトリンク】 url = http://www.arcticcorsair.f9.co.uk/corsair/in-depth/taffrail_log.htm ) 〕 : * 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 submarines 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. : * GPS measurements yield only a position, and neither speed nor direction of movement. However, most GPS receivers can automatically derive velocity and direction from two or more position measurements. The disadvantage of this principle is that changes in speed or direction can only be computed with a delay, and that derived direction becomes inaccurate when the distance travelled between two position measurements drops below or near the random error of position measurement. To counter this effect, more advanced navigation systems use additional sensors like a compass or an inertial navigation system to complement GPS. : * Measurement of Doppler Shift: Some ships are equipped with sonar instruments that can measure the Doppler shift in a sound pulse generated by the ship's motion as it reflects off the ocean bottom. Unlike a pitometer, the speed measurements from a doppler instrument are of ground speed. : * Correlation Velocity Log (CVL): A CVL consists of a sound transmitter and a number of receivers. These very expensive devices measure the ship's ground speed by performing correlation processing between the signals received by the different receivers at different times. This correlation can be related to the distance moved by the ship.〔 (【引用サイトリンク】 url = http://www.marine-acoustics.co.uk/Technical%20Papers/hsluust1999.pdf )〕 CVL's are used primarily by ships that need very precise measurement of low speeds, which a pitometer has difficulty measuring accurately. The pitometer log was patented〔 〕 in 1899 by Edward Smith Cole.〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「pitometer log」の詳細全文を読む スポンサード リンク
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