Speed Log & Doppler Log


Speed log is the bridge equipment used to find out the speed of the ship and the distance traveled through the water. This instrument is used to find the dead reckoning position or provide input to other equipment such as Radar and ARPA. The speed logs were initially electrical ones and later became electronic navigation aids

Speed Log

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1. Speed Over Ground is the speed of ship over ground taking into account the effect of leeway and current.

2. Speed Over Water is the ship’s speed concerning the sea surface without considering leeway and current.

2:- EM Log (Electromagnetic Speed log)

EM Log

EM log works on the principle of electromagnetic induction. According to Faraday’s principle of ‘ electromagnetic induction, when a conductor in sea seawater passes through an electromagnetic field, a voltage is created and this voltage is converted to calculate the ship’s speed.

 As the amount of water passing through the magnetic field increases, the speed of water also increases as both are directly proportional to each other.

If no water is flowing or moving through the electromagnetic field i.e., when a ship is not moving there is no electromagnetic induction taking place and the speed indication will be zero. As the ship moves, the magnitude of induction increases and the indicated speed also increases.

There is a soft iron cylinder over which a wire is coiled to make a solenoid. This construction is situated at the keel of the ship and acts as a transducer and when water passes through this cylinder as the ship moves this current-voltage gets induced. With the help of a processor the value of voltage is converted and calculated to the ship’s speed and the speed over ground is displayed on the bridge.

3:- Doppler Log

Doppler Log

Doppler log works on the principle of Doppler Effect which states that there is a shift in frequency between transmitted sound signals and received sound signals caused by the motion of a vessel over the sea bottom or layer of water.

The system consists of a transducer, processor, and display unit.

There are transducers connected to the keel of the vessel at an angle α. The first transducers send signals of a certain frequency toward the sea bed which moves through the water and gets reflected at the sea bed or layer of seawater. The second transducers receive these waves the shift of frequency is observed and calculated to give us the speed over ground or water. 

3.1:- Advantages of Doppler Log

1. The waves which are reflected from the seabed show the ground over speed.

2. The waves that are reflected from the layers of seawater the speed shown will be speed over water. 

Doppler log provides both speed over ground as well as speed over water whereas other speed log only provides only speed over water.

4:- Conclusion:

The speed log is a very important instrument on board the vessel, providing real-time data on the vessel’s speed through water. The speed log plays a vital role in ensuring safe navigation, optimizing fuel efficiency, meeting scheduling requirements, monitoring performance, and complying with regulations. Its accurate and reliable data is indispensable for efficient and effective ship operations. 

Frequently Asked Questions

Q1:- What is a Doppler speed log?

Doppler log works on the principle of the Doppler Effect. It is used to find out the speed of ship over ground as well as water

Q2:- What is ship RPM? 

RPM stands for revolution per minute. In the context of naval ships, you might use RPM to indicate how fast any rotating equipment is turning.

Q3:- What are the types of speed logs?

The type of speed depends on the size and type of the vessel. Doppler speed log, electromagnetic log, and satellite speed log are three common shipborne speed measurement tools.

Q4:- What is the advantage of the Doppler log?

The advantage of a Doppler log is that it provides both speed over ground as well as speed over water whereas other speed log only provides speed over water.

Q5:- What is the difference between an EM log and a Doppler log?

There are mainly two differences between the Doppler log and the em log

1 Doppler log works on the principle of the Doppler effect whereas EM log works on the principle of electromagnetic induction 

2 Doppler log provides both speed over ground as well as speed over water whereas EM log only tells you speed over ground.

Q6:- What is a speed log?

Speed log is the bridge equipment used to find out the speed of the ship and distance traveled through the water. 

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