LOGS

The log is an instrument for measuring the speed and distance traveled by a ship. It is important to note that the speed measured by log is affected by current and tidal streams.

There are 2 main types of log

  1. The towed or patent log – This type of log is usually towed astern of the ship through the water.
  2. The bottom log – This log is fitted on the bottom of the ship usually on the bottom plating. It is more reliable than towed log it can be withdrawn into the ship’s hull when not required(At the end of the voyage when navigating in shallow water)There are 3 main types of bottom log –
    1. Impeller Log – Impeller log as the name itself suggests – the impeller is fitted in a tube and is installed on the bottom plating of the ship. The whole tube can be retracted into the hull when not in use.
      • Working – Impeller rotates as the ship moves through the water. Converts the rotation of impeller into electrical signal. This is how we get distance and speed on bridge.
    2. Pressure tube or Pivot type log – It is also a tube fitted at the bottom and can be retracted back into the hull when not required. The tube has 2 pipes fitted to it, one facing forward called the DYNAMIC TUBE and the other tube facing downwards called the STATIC TUBE.
      • Working – When the ship is stopped and there is no current or tide the pressure in both tubes will be equal to the depth of tube below the waterline. When the ship moves the pressure in the dynamic tube will be higher than the static tube. This difference in pressure is converted into electrical signal and used for distance and speed information on bridge.
    3. The electromagnetic log – This log works on the principle that if any conductor passes through a magnetic field a small EMF will be induced within itself which is proportional to speed of the movement of the conductor. In case of electromagnetic log the conductor is the seawater, the magnetic field is created by a coil in the tube and the induced EMF is measured by two sensors on the side of the tube.
    4. The doppler log-A transducer fitted on the bottom of the ship emits a continuous beam of sound vibration in the water at an angle of about 60 degrees to the keel in the forward direction. The beam is bounced of the sea bed or a layer of water and received back at the transducer. Now the difference in the frequency between the transmitted and received signal is measured and is proportional to the speed of the vessel.
      • When vessel is in shallow water the signal is bounced back from the sea bed giving speed over ground but when in deep water transmitted signal is bouched back form layer of water giving a speed through water.
      • JANNUS CONFIGURATION – Most logs have transducers to measure both fore and aft speeds(ahead and stern) as well as athwarthship transducers to measure speed in the sideways direction (Useful when berthing). Such a transducer which measures both alongship as well as athwartship speed is called JANNUS CONFIGURATION.
Janus-configuration-Doppler-log
Janus configration, image credit myseatime

Doppler log is a important topic so we will go deep inside the topic –

Modes in Doppler log

Doppler logs will have the functionality to choose the tracking mode. The three tracking modes available are

  1. Water (Measure speed through water)
  2. Ground (Measures speed over ground)
  3. Auto (Selects water mode or ground mode automatically as per depth)

IMO Requirements for Doppler log

IMO resolution A.824 (19) as amended by MSC 96(72) gives the details of the performance standards for the Doppler logs fitted on ships.

The few of the main requirements as per this are

  1. The device measuring speed and distance through the water should meet the performance standard in water of depth greater than 3 m beneath the keel
  2. Error in the measured and indicated speed for a digital display should not exceed 2% of the speed of the ship, or 0.2 knots, which is greater. For analogue display the error should not exceed 2.5% of the speed of the ship or 0.25 knots whichever is greater.
  3. The performance of the equipment should be such that it will meet the requirements of performance standards when the ship is rolling up to 10 degrees and pitching up to 5 degrees.

Working principle of dopler log

Principle of Dopler log

Dopplerfrequency

Doppler log is 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.

Limitation of Dopler log

At least 3m depth is required below the keel to get speed through water. If the mode is not changed to ground (or auto) when the depth below keel is less than 3 meters, Doppler log will show errors in the speed.

But the option to change modes are not present in all makes and model of the Doppler log.

Some Doppler log only measures what it is supposed to measure, which is speed through water.

In depths less than 2~3 meters below keel, these Doppler logs displays the GPS speed for which a GPS connection to the Doppler log is required.

Errors of the Doppler log

  1. Error in transducer orientation: The transducers should make a perfect angle of 60° with respect to the keel or else the speed indicated will be inaccurate.
  2. Errors in oscillator frequency: The frequency generated by the oscillator must be accurate and constant, any deviation in the frequency will result in the speed indicated being in error.
  3. Error due to propagation of wave at different temperatures. The velocity of the acoustic wave at the temperature of 16°c and salinity of 3.4% is 1505m/sec, but generally, it is taken as 1500m/sec for calculation.
  4. Errors due to the ship’s motion: during the interval between transmission and reception, the ship may marginally roll or pitch.