Log - a device designed to measure the speed of a vessel [1] .
In ancient times, a manual or sectorial lag was used (and is still used on small ships) as a lag. It is a triangular-shaped board (sector) with a rope tied to it ( tench , laglin) and a load. On the line at the same distance from each other, knots are tied. The board is thrown over the stern and the number of nodes that went overboard in a certain time (usually 15 seconds or 1 minute) is recounted. From here the measurement of the speed of the vessel in knots began ; 1 knot is numerically equal to 1 nautical mile per hour.
The principle of operation of modern devices is based on measuring the pressure of water, or sonar of the seabed. The most common lags are Doppler (using the Doppler effect), induction and correlation.
Content
Lags and how they work
Relative Logs
Currently, induction, hydrodynamic and radio-Doppler logs are used on ships of the marine transport fleet, which measure speed relative to water.
Induction Logs
Their action is based on the property of electromagnetic induction. According to this property, when a conductor moves in a magnetic field, e is induced in the conductor. d.s., proportional to the speed of its movement.
Using a special magnet, a magnetic field is created under the bottom of the vessel. The volume of water under the bottom, which is affected by the magnetic field of the lag, can be considered as a set of elementary conductors of electric current, in which e. d.s .: the value of such e. d.s allows you to judge the speed of the vessel.
The induction logs IEL-2 and IEL-2M operated on ships of the Navy are constructed in the same way: they measure only the longitudinal component of the relative speed; There are no parts protruding beyond the hull. Currently, only the IEL-2M log is currently being manufactured. Log IEL-2 was discontinued in 1980. Log IEL-2M can be installed on all marine vessels, including icebreakers and hydrofoils.
Recommendations for use are as follows. With fouling of the hull of the vessel, the lags IEL-2 and IEL-2M begin to give underestimated readings.
A filter averaging their readings is included in the IEL-2 and IEL-2M lag circuits. Therefore, when measuring the ship’s speed, the lag captures this change with some delay.
Hydrodynamic Logs
The principle of operation is based on measuring the hydrodynamic pressure created by the high-speed pressure of the incoming water flow during the movement of the vessel.
Correction of the hydrodynamic lag, as a rule, is unstable. The main reasons for its changes during sailing are the drift of the vessel, trim, fouling, hulling and changing the navigation area.
It is not possible to calculate the change in the lag correction from the influence of the first three reasons.
Absolute Logs
Absolute are logs that measure the speed of the vessel relative to the ground. The absolute lags currently developed are hydroacoustic and are divided into Doppler and correlation ones.
Hydroacoustic Doppler Logs (GDL)
The principle of the GDL is to measure the Doppler frequency shift of the high-frequency sonar signal sent from the vessel and reflected from the bottom surface.
The resulting information is the longitudinal and transverse components of ground speed. GDL allows you to measure them with an error of up to 0.1%. The resolution of high-precision GDL is 0.01–0.02 knots.
When installing an additional two-beam antenna, the GDL allows you to control the movement of the bow and stern relative to the ground, which facilitates the management of a large-tonnage vessel when navigating through canals, in narrow places and when performing mooring operations.
Most existing GDLs provide absolute speed measurements at depths below the keel of up to 200-300 m. At large depths, the lag stops working or goes into the relative velocity measurement mode, that is, it starts working from a certain layer of water as a relative lag.
GDL antennas do not protrude beyond the hull. To ensure their replacement without docking the vessel, they are installed in klinketami.
Sources of GDL error can be: Doppler frequency measurement error; changing the angles of inclination of the rays of the antenna; the presence of the vertical component of the speed of the vessel. The total error for these reasons in modern lags does not exceed 0.5%.
Correlation Logs
The principle of operation of the hydroacoustic correlation lag (GCR) is to measure the time shift between identical acoustic signals received by two ship antennas, spaced a certain distance, received ~ - ~ from a moving vessel ~ - ~ signal reflected from the ground. At depths of up to 200 m, the GCR measures the speed relative to the ground and at the same time indicates the depth under the keel. At great depths, he automatically switches to work relative to water. The advantages of the GCR with respect to the GDL are the independence of the readings on the speed of sound propagation in water and more reliable operation on pitching.
Notes
- ↑ Stanyukovich K. M. Glossary of marine terms found in stories .
Literature
- Krasnov V.N. History of navigational technology: The origin and development of technical means of shipbuilding / Institute of the History of Natural Science and Technology. S. I. Vavilova RAS .. - M .: Nauka , 2001 .-- 312 p. - 420 copies. - ISBN 5-02-013119-9 . (in per.)
- Aksyutin L.R. , Cooper V.M. Ermolaev G.G. and other Reference captain of the sea. - M .: Transport , 1988 .-- S. 246.