The ship’s log is basically the “speedometer and odometer” of a vessel. And performs exactly the same function as that of a motor car, i.e. it records the speed of the vessel and the distance travelled. Initially logs were purely mechanical and required no electrical power to operate. They only recorded the distance covered by the ship, whilst the speed was calculated by accurately measuring the time difference between successive readings of the log and dividing the distance covered by the time taken. An example of this type of log was the Trident towed log. Since it was a purely mechanical device, it will not be covered in this module.
It should be noted that when ordering a particular speed to proceed at, use is made of the shafr revolutions to speed table. This table is compiled during the sea trials conducted after a vessel is built when the vessel carries out speed trials over a specially measured and demarcated range. The table provides the number of shaft revolutions required to propel the vessel at every speed within the vessel’s speed range.
Of the electromechanical and electronic logs there are various types:
- The Pitometer log.
- The Chernikeeff log.
- The Electromagnetic log.
- The Doppler log.
- Pitometer Log.
The Pitometer log is a “pressure type” log which measures the difference between the static water pressure at the depth of the log sensor and the pressure caused by the movement of the ship through the water.
An analogue pitometer log.
The pressure sensors are contained in a hollow metal rod with an oval cross section which projects approximately 1 metre below the bottom of the hull. The unit is fitted near the pivoting point of the vessel and can be drawn into the hull, after which the aperture can be closed. This is done when the vessel is due to enter harbour or when proceeding in very shallow water when the log may be damaged.The device which measures the pressure difference converts it to a distance and speed recorder where the results are displayed visually. In the past the processing and display of this data was done using analogue technology. These days it is processed and displayed digitally and can be fed to any other navigational system requiring a log input. An important item to note is that the speed and distance provided by the log is through the water and not over the ground.
- Chernikeeff log.
An analogue Chernikeeff log.
In the case of the Chernikeeff log, an impeller is fitted at the lower end of a similar rod to that of the Pitometer. The rod projects approximately ½ metre below the bottom of the hull. Like the Pitometer, it too can be withdrawn into the hull and the aperture closed.
The rotation of the impeller by the flow of water operates a make-and-break mechanism in the shaft which transmits the impulse electrically to the distance recorder which displays the distance covered. By counting the number of impulses over a predetermined time interval, the speed is determined and displayed. As for the Pitometer log, the processing and display of the data is done digitally these days. The problem with a Chernikeeff log is the fact that it has a moving part, namely the impeller, which is susceptible to jamming by underwater debris.
Modern digital display for the ship’s log.
- Electromagnetic log.
Again the electromagnetic log consists of a metal rod similar to that of the Pitometer, which projects below the bottom of the hull. An iron-core coil is mounted in a glass fibre housing at the bottom end of the rod. The coil is fed with an alternating current and, when moved through the water, produces a voltage in the surrounding water. The voltage, which is proportional to the flux and the relative speed of the rod through the water, is detected by two electrodes on the rod, and fed to a processor which converts it to distance and speed. This is then fed to displays on the bridge and to any navigation system requiring a log input.
An Electromagnetic log.
- Doppler log.
Up to now we have been discussing logs which provide the distance covered and the speed measured through the water. Since this does not take into account the effect of tidal streams and currents, it does not reflect the true movement of the vessel over the ground. In order to determine our position and movement geographically, we need a sensor that can measure movement relative to the sea bed.
By using similar technology to that used by the echo sounder (transmission and reception of sound through water), the Doppler log is able to measure movement of the vessel in relation to the sea bed. In the case of the echo sounder the propagation time of the transmitted sound pulse and its echo is measured and together with the known speed of sound through the water, the depth of water can be measured. In the case of the Doppler log, the difference in the frequency between the transmitted pulse and that of its echo is measured (the Doppler shift) and from this is calculated the speed of the vessel over the ground.Unlike the echo sounder, the Doppler transducer transmits a continuous beam of sound at an angle of 60° to the keel of the ship in a forward direction. A second transducer receives the returning echoes resulting from the multi-path reflections from the sea bed. The difference between the frequency of the transmitted pulse and that of the echoes is measured and the speed of the vessel is determined.
The frequency used by the Doppler log is higher than that of the echo sounder. The reasons for this are:
- The shorter wavelength results in better diffusion of the reflection (multi-path reflections).
- The shorter wavelength results in a smaller beam angle and reduces the physical dimensions of the radiating face of the transducer.
- The radiated power of the sound beam spreads less and makes the echo stronger.
Because of the absorption by particles of the water at a depth of between 200 and 400 metres, the Doppler log only works to about 200 metres. The minimum depth at which it will operate is about ½ metre from the transducer. The velocity range is between 0 and somewhere between 30 and 100 knots in the fore and aft line and between 0 and between 8 and 10 knots athwartships. The frequency range in water is 100 to 600 Khz.