Speed log- how it measures speed of vessel

Speed log- how it measures speed of vessel

What is a Speed Log?

• In simple terms we can define speed Log as an instrument which we use to measure the speed of the ship and distance travelled by it.

Speed log
Satellite speed log

The speed logs are mainly of three types. Most of them show us the speed through water. Only doppler log and the Satellite speed log show us the speed over water well as speed over ground.

Table of contents (Types of speed logs)

Now we will try to understand their Functions, Principal, Working,  Advantages and Disadvantages :

1: Electromagnetic speed Log:

Electromagnetic speed log has another name called EM log. It measures the speed of the vessel through water.


It works as per faraday’s law and Maxwell equations.


Works on a principal which states that when a conductor (such as water) passes through an electromagnetic field, a voltage is created and 2 the amount of voltage created increases as the speed of the conductor increases.

● Advantages:

A) Have no moving parts.

• B) Less affected by sea growth than Pit Sword

● Disadvantages:

• A) Provides boat/ship speed relative to water not ground. Current affects accuracy.

• B) Salinity and temperature of water effects calibration.

• C) Measurements effected by boundary layers.

Now coming on 2nd I.e.

Pitometer speed log:

It’s referred also as a Pit log. The device is mainly used for to measure the ships speed relative to water. Data from the Pitometer log is usually fed directly into the ship’s navigation system.


It works on the principle that the amount of water absorbed equals the amount of transpired so it measures the amount of water absorbed .

• Advantages:

Pitometer was mainly used during the world war times The logs were directly integrated in to the ship’s navigation system which also helped the ships weaponry system to automatically track the targets.

• Disadvantages:

The disadvantage of this principle is that changes in speed or direction can only be computed with a delay and that derived direction becomes inaccurate when the distance travelled between two position measurements drops below or near the random error of position measurement. 

Doppler speed log

It’s based on the Doppler effect in which the wavelength of moving object appears to shift in relation to observer. This shift cab be converted into speed thereby giving an accurate result.

What is Doppler Effect?

Its the change in frequency or wavelength of a wave for an observer who is moving relative to the wave source.

•It’s principal: It’s actually 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.

In Doppler log an observer is moving with a source of sound towards a reflecting plane then the received frequency.

By measuring the received frequency and knowing the value of transmitted frequency and velocity of sound in seawater. The speed of the vessel can be determined.


A)It’s most accurate.

B) Can measure very low speeds.

C) Can be used for ocean navigation.

Factors effecting it:

A)The performance gets reduced or even lost under certain conditions like:

1: Rough weather.

2: Improper location of sensor.

3: Depths  under the keels if less than 3m.

Impeller speed log

These logs function by attaching an impeller or paddle wheel to the bottom of a hull. They are employed on small yachts and other such vessels.


Impeller rotates as the ship moves through the water. Converts the rotation of impeller into electrical signals .


They all consist of similar design elements they may perform slightly different operations depending upon their intended application.

Satellite speed log

Satellite speed log is a bit different from the earlier mentioned speed logs. The satellite speed log provides us the speed over ground.

The satellite speed log is introduced because of the speed accuracy it provides. No matter which type of weather it is outside, the satellite speed log provides us the accurate speed upto an error of 0.02 kts or 0.2%.

Satellite speed log works on the GPS signals, which enhances the accuracy of the measured speed of the vessel.

Because of the accuracy of the speed over ground provided by the satellite log, this is used for berthing and unberthing operations.

The satellite speed log provides us the fwd and aft speed as well as the athwartship speed of the vessel.