Speed Log
Speed Log
Speed Log
Speed Log
SDME: Speed and Distance measuring Equipment Log Speeds are affected by current and tidal streams. Speed through the Water measured by:. Patent Log: towed astern of the ship. Bottom Log: can be withdrawn into the ships hull Impeller Log: Impeller fitted in a tube. More accurate than towed log
Speed Log
Pressure tube or Pitot type Log: Dynamic tube and STATIC Tube Difference in pressure converted to electrical signals and proportional to speed. Electromagnetic Log: induced EMF measured by sensors, proportional to ship speed.
Speed Log
Doppler Log: Based on the principle of Doppler shift in frequency measurement i.e apparent change in frequency received when the distance between source and observer is changing due to the motion of either source or observer or both.
Speed Log
Speed Log
Speed Log
In Doppler Log, an observer is moving with a sound towards a reflecting plane, then the received frequency fr= ft (c+v) / (c-v) where fr= received frequency ft= transmitted frequency c= velocity of sound in sea water and v= velocity of the vessel. By measuring fr and knowing the value of ft and c, the speed of the vessel can be determined.
Speed Log
The Doppler system normally measures speed over ground to about 600 feet. After this depth signals may be returned by a dense, colder layer of water located throughout the oceans called the deep scattering layer (DSL). Signals received off the DSL are not as accurate as signals received from bottom reflections but can still be used to provide an indication of speed through the water instead of speed over ground when bottom tracking. Your unit may have a manual or automatic system which will switch from bottom tracking to water tracking at increased depth.