LOG means it’s a gauge to measure speed and sailing distance of the vessel.
The speed and distance signal generated by SPEED LOG are provided to machine(RADAR, AUTO PLOT, VDR etc) to have a major impact on navigation of vessel. 
Speed Log emit ultrasonic waves by transducer installed in vessel and measure speed of vessel using reflection ECHO from the ocean floor

Benefits with a solution from consilium

• A minimum of hull penetrations
• Fulfils the new SOLAS 2014 MSC.334(90)
• Small transducers
• Cost efficient solutions
• Easy to install
• No divers needed during retrofit
• Low maintenance
• High quality
• High accuracy by correlation principle
• Non reliant on easily interfered satellite systems
• Flexible retrofit solutions
• 24/7 worldwide support
• 

• Benefits

  • Simpler and cheaper installation
  • Only ONE hull penetration
  • Only ONE transducer for spare part handling
  • Only ONE cable to lay to the transducer
  • Only ONE gate valve needed