In the early days, the ship was equipped with a wheel log. By measuring the speed of seawater, the speed of the ship is measured, and then the voyage is obtained by the timing device. Electromagnetic log is widely used in modern ships, and the speed and voyage of ships are measured by electromagnetic induction principle.
Modern log is mainly composed of speed measuring part and indicating part. The speed measuring part is used to detect and amplify the ship speed signal or voyage signal; The indicating part displays the speed or voyage of the ship in mechanical or electrical form, and then displays the voyage or speed in integral or differential way. The working principle and performance of different types of log are as follows.
① Drag the log. The rotor towed at the stern is rotated relative to the ship by water flow, and the accumulated voyage of the ship is displayed on the indicator through the log rope, connecting hammer and balance wheel. This kind of log has poor linearity, large high-speed error, great influence by airflow, inconvenient operation, but reliable performance, and some ships are used as backup log.
② Runner's log. Using the current relative to the ship's navigation, the wheel is driven to rotate, generating electric pulses or mechanical intermittent signals, which are processed by electronic circuits, and the speed and range are given by indicators. This kind of log has good linearity and high sensitivity at low speed, but the mechanical part is easy to wear. Except for boat use, it has been gradually eliminated.
③ Hydraulic logging. Using the dynamic pressure relative to the navigation water flow of the ship, it acts on the diaphragm of the pressure transmission chamber and is converted into mechanical force. With the help of compensation measuring device, the mechanical force is converted into speed, and then the range is given by speed calculating device. This kind of log works reliably, but its linearity is poor, its low-speed error is large, it can't measure the backward speed, its mechanical structure is complex, and it is inconvenient to use, so it is gradually eliminated.
④ Electromagnetic logging. The magnetic field of the electromagnetic sensor installed at the bottom of the ship is cut by current (conductor), which converts the speed of the ship relative to the water surface into induced potential, and then into speed and voyage. Its advantages are good linearity, high sensitivity and measurable receding speed, and it is widely used at present.
⑤ Doppler logging. The Doppler frequency shift between the emitted sound wave and the received underwater reflected wave is used to measure the speed and cumulative voyage of the ship relative to the underwater. This kind of log has good accuracy and high sensitivity, and can measure longitudinal and transverse speeds, but it is expensive. It is mainly used to provide accurate data of longitudinal and lateral movements of giant ships when sailing in narrow waterways, entering and leaving ports and docks. Limited by the depth of action, the Doppler log can only use the water particles in the water layer as the reflection layer when it exceeds hundreds of meters, and become a water log.
⑥ Acoustic correlation logging. Based on the acoustic correlation principle, the time shift of echo information from the same scattering source to two receivers at the bottom of the water is measured, so as to calculate the speed and distance relative to the bottom of the water. This kind of log can measure the backward speed and also be used for sounding. When the water depth exceeds several hundred meters, it also becomes a log relative to water, and it is still being improved.