What is a Speed Log?
Speed Log is one of the most important marine equipment that accurately controls and records your ship's speed and travel distance. Speed logs, too known as transport logs, chip logs, or common logs, degree the speed of a vessel. The speed is decided by water streaming by the body (water reference speed) or to the seabed (ground reference speed). Such equipment is referred to as a log because of the out-of-date activity of using wood logs to distinguish how quickly a vessel moves. During the sailing, the crewmen would toss logs overboard after joining them to a rope with ties at fixing intervals.
The total number of knots that passed by over a specific amount of time gauged the ship's speed. This method led to the introduction of the knot as the unit of measure for vessel speed (1 knot=1 nautical mile per hour). The design of the instruments with estimating the speed was standard. A part of lead attached to the bottom of the log amends water drag, resulting in higher reading accuracy.
Ultrasonic speed detectors are devices that measure the velocity by means of pulses transmitted through water in close proximity to the hull. Using the Doppler shift from sonar or by radio GPS measurements of satellite signals, better accuracy can be obtained. The SOLAS Regulation requires vessels of over 50,000 GT and passenger ships of 300 GT or more that have already been constructed after July 1, 2014, to be fitted with a reference device stating both water velocity and ground speed.
Types of speed logs
To sum things up, a speed log is a tool that we use to calculate our ship's speed and distance traveled. The speed logs are mainly of three types. Most have shown us how fast water is moving. The speed above water as well as over the ground is recorded by a Doppler and satellite speed log, respectively. Modern seafarers rely on a variety of technical means for determining the vessel's speed, such as:
- Electromagnetic (EM)
- GPS Global Position Satellite GPS
Electromagnetic speed log
These devices use a coil in an electromagnetic sensor, which is also known as an electromagnetic log. The coil is stimulated by an alternating current, creating a magnetosphere around the sensor. An electric field is produced as the ship moves along the water's surface, and it will be perpendicular to the gravitation field. Sensor electrodes detect the signal representing ship speed and transmit it to a preamplifier which decodes that signal into the digital mode for display. This method is easy to use and does not require any relocation of parts. Nevertheless, the calibration is affected by water salinity and temperature. Because of friction, the water flow tends to slow close to the hull and makes measurements more difficult. Disadvantages of Electromagnetic speed Log:
A) Furnishes boat or ship speed relative to water, not ground. The current affects efficiency.
B) Salinity and temperature of water effects standardization.
C) Measurements affected by boundary layers.
Pitometer speed log
Pitometer, also known as a pit log, uses differential pressure to determine the speed of an object in the water. It's operating by putting in a Pitot tube that has an opening at the bottom of the water. When the tube is immobile, it continues to be pressurized by continuous static pressure. The tube will create a dynamic water pressure that can vary on the basis of speed as it begins to move past the water. To make a correct determination of the dynamic pressure for speed calculation, the effect of static pressure shall be removed. To do so, a new tube with the equivalent static pressure will be set close to the existing one. The second tube does not have any dynamic pressure generated by movement in the water.
The velocity of the object moved shall be determined by the difference in true dynamic and static pressure readings. The disadvantage of this principle is that, if the distance traveled between two position measurements drops below or near the random error of the position measurement, the change in speed or direction can only be calculated with a delay and the resulting direction is incorrect.
Doppler speed log
Doppler transducers are used for measuring the velocity of moving objects by frequency shift effects. The transducer, which uses two transmission windows, creates high-frequency sound pulses at both ends of the vessel. Two scanner windows situated within the transducer are configured to detect reflected pulses. The time delay associated with the transmission of signals confirms that acoustic waves have been received from unresonant water around the object. Processing of the signal information estimates the vessel's fore and aft speed vector. The information is then transmitted in a digital form via the preamplifier.
What's the Doppler effect? For an observer who's moving closer to a source of the wave, it changes the frequency or wavelength of that wave. It is, in fact, based on the principle of Doppler shift in frequency measurement, i.e. the apparent change in the frequency received when the distance between the source and the observer changes due to the motion of either source or observer. In Doppler log an observer is moving with a source of sound towards a reflecting plane then the received frequency. By measuring the frequency obtained and by knowing the value of transmitted frequencies and sound velocity in seawater. The speed at which the vessel is moving can be determined. Under certain conditions, the performance shall be reduced or in some cases eliminated:
1: We're in bad weather.
2: The sensor's location is wrong.
3: If depth is less than 3m, under the keels.
Impeller speed log
The logs may function by using an impeller or a steering wheel mounted to the bottom of the hull. They're hired on small yachts and similar types of ships. When the ship passes in the water, the impeller is rotating. It switches the impeller's rotation to an electromagnetic signal. All of them are essentially the same design elements, which can be used for a variety of operations depending on their purpose.
GPS Global Position Satellite
To measure the speed of objects in both longitudinal and transversal directions, sensors are able to access GPS satellite signals. The heading velocity, attitude, and trajectory shall be calculated by a GPS receiver with integrated frequency gyros and an antenna. In relation to the vessel's size and direction of movement in the water, the heading shall be taken into account. The system shall process the information provided by the satellites and shall integrate the velocity data with the longitudinal and transverse ground speeds of the vessel. The difference between vessel translational and rotational motion is helped by a set of speed vectors coupled with data relating to the velocity turn. It is necessary to specify in this way the transverse acceleration for both bow and stern, as shown on the vessel's docking display.
The speed log for the satellite is different from that which has been reported previously. The satellite's speed log shows us how fast we're going on the ground. In order to give an accurate indication of satellite speed, the Satellite Speed Log is introduced. No matter which type of weather it is outside, the satellite speed log provides us the accurate speed up to an error of 0.02 kts or 0.2%.
Speed log usage history
A specially shaped wood chip had been affixed to the length of line cut on a hand wheel during the days of sails. A knot at 47 feet 3 inches from the chip, two knots at 95 feet 6 inches, and so forth were used to mark this line. The wood chip has been thrown overboard, and a 28 sec's glass of sand was taken to measure when the line had run. Ships' speed in knots was indicated by the number of knots that had elapsed in 28 seconds. There was another method used by the Dutch. A piece of wood (log) was thrown into the water from the forward part of a ship. The sailor with a glass of sand noted at the end part how long it took his ship to move an object float log. The ship's speed has been calculated based on time and the known distance of both points in the vessel. Let's assume that the southerly current is 3 knots. (From North to South as Current always flows towards) This means the water is flowing at a speed of 3 knots from north to south.