Terminology » Other Wireless » Electromagnetic Log (EM Log): Advantages and Disadvantages

Electromagnetic Log (EM Log): Advantages and Disadvantages

electromagnetic log em log ship speed marine engineering measurement

This page explores the pros and cons of using an Electromagnetic Log (EM Log). It highlights the benefits and drawbacks of this technology used for measuring a ship’s speed.

What is an Electromagnetic Log (EM Log)?

Introduction:

  • An EM Log is a device used in marine engineering to measure a ship’s speed through the water.
  • It operates based on Faraday’s Law of Induction and Maxwell’s Equations.

Electromagnetic Log, EM Log

  • As illustrated, current is passed through the coil of the flow sensor, creating an electromagnetic (EM) field.
  • When water (a conductor) flows through this EM field, a voltage is generated.
  • The magnitude of the voltage is directly proportional to the speed of the water (the conductor).
  • Consequently, the EM Log measures the ship’s speed through the water by detecting this induced voltage.

Benefits or Advantages of Electromagnetic Log (EM Log)

Here are the advantages of using an EM Log:

  • No Moving Parts: It doesn’t have any moving components, which reduces wear and tear and the need for maintenance.
  • Less Affected by Sea Growth: Compared to pit swords (another type of speed log), EM Logs are less susceptible to fouling from marine organisms.
  • Modern Design: Modern sensor designs don’t protrude from the bottom of the ship, reducing drag and potential damage.
  • Compact Size: Modern EM Logs are smaller, making them easier to install and integrate into ship systems.
  • High Accuracy at Low Speeds: EM Logs provide more accurate speed measurements, especially at lower ship speeds.

Drawbacks or Disadvantages of Electromagnetic Log (EM Log)

Here are the disadvantages of using an EM Log:

  • Affected by Water Conditions: Water salinity and temperature can affect the calibration of the EM Log, leading to inaccurate measurements. Regular calibration is necessary to mitigate this.
  • Measures Speed Relative to Water: It measures the speed of the ship relative to the surrounding water, not relative to the seabed. This is a crucial distinction in areas with strong currents.
  • Influence of Boundary Layer: The boundary layer, where the water speed is reduced due to friction near the hull, can influence measurements. This slowdown of water near the hull needs to be accounted for.