Grounding is one of many possible accidents that a vessel encounters. This happens when the bottom of the ship touches the seabed. The damage can be minor or it can lead to a severe oil spill.
To avoid this, bridge officers can check the paper charts or ECDIS (for paperless vessels) for the depth and sounding of water. While the vessel is in port, the ship’s crew may use a sounding lead to measure the depth of water.
What Is Echo Sounder?
To get accurate information, an echo sounder is used by officers on watch. It measures the depth of water and more importantly, the under-keel clearance.
The echo sounder transmits sounds from the bottom of the ship to the sea bed. The main components are the transmitter, transducer, receiver, and display unit. Transducers can either be electrostrictive or magnetostrictive.
How Does Echo Sounder Work?
Magnetostrictive transducers use iron, cobalt, and nickel. Nickel is widely used in the industry as it has the greatest change when placed in a magnetic field. Nickel contraction happens when an alternating current is applied. It uses a permanent magnet to compensate for the process. Magnetostriction is effective up to a few hundred kilohertz frequencies.
For electrostriction, the echo sounder uses different types of lead materials and works on compressive or tensional forces. These forces create a change of length or dimension in the material. It is effective up to 1 megahertz frequency.
The display unit can either be a paper recorder, video type, or digital info display.
The echo sounder uses the formula distance=speed x time / 2.
The transmitter creates an electrical pulse which then passes thru the transducer. The transducer which is located at the bottom of the ship converts it to sound waves. The sound waves may either hit the sea bed or anything in between.
For fishing vessels, it is used to find schools of fish underwater. These pulses hit the scales of the fish and reflect it back to the transducer which allows trawlers to find fishing grounds.
Once the transducer receives the back sound waves, it is converted back to current which is then reflected on a digital or video display and in some cases, burned in a paper recorder-type echo sounder.
Echo Sounder Types?
Echo sounders are classified into two types. These are the single-beam echo sounders(SBES) and the multi-beam echo sounder(MBES)
Single-beam echo sounders were initially developed around 80 years ago and were used instrumentally for primary oceanic discoveries and standardization. It is also known as a fathometer or depth sounder. It works on a single sound pulse in a single narrow beam and can only measure one point per echo.
Nowadays, SBES is used for depth measurements, sub-bottom observations, and seabed imaging.
Other SBES are defined by their beam angle and frequency of transmitted sound waves. Water depth capabilities of single-beam echo sounder range from less than one meter to full ocean depth.
The second type of echo sounder is the MBES or the multi-beam echo sounder. It emits a multidirectional beam to obtain information in the affected area. The sound waves received back by the transducers provide detailed information on the depth of water, the shape of the river, lake, and other underwater features.
This information is used by Geological survey companies to accurately generate high-resolution maps of harbors, berthing terminals, lake bottoms, and channels used for navigation.
What Is The Difference Between Echo Sounder And Sonar?
Though both the echo sounder and Sonar are quite both similar, they have some differences.
The echo sounder allows a ship to measure the depth of water and under keel clearance from only under the ship. The echo sounder uses a transducer that is mounted on a fixed position at the keel of the vessel.
The direction principle of an echo sounder is mainly vertical which uses single beams or multiple beams. It is mainly used by merchant ships, trawlers, tug boats, yachts, and similar vessels.
The sonar on the other hand gives a wider picture underwater since it utilizes an adjustable transducer which enables it to scan 360 degrees. It also works on a much stronger frequency and uses a transmitter and receiver which are capable of transmitting and detecting ultrasonic waves.
The sonar is mainly used by military vessels such as warships, submarines, and aircraft carriers.
How To Use Echo Sounder?
Before using the echo sounder, the officer of the watch checks that the equipment is working properly and has been set up. First, be familiar with the mode options of the echo sounder to avoid confusion and possible grounding is prevented.
The normal mode will be the navigation mode. In navigation mode, the echo sounder measures the depth of water from the transducer position to the seabed. At times, it is referred to as the under-keel clearance.
The DBS (draft below the surface) mode is the next available mode. This is the depth of water including the draft of the vessel. Simply add the known draft of the ship and the reading below the surface in navigation mode. There is a draft button to enter the ship’s draft as well.
The history and log data mode allow the officers to see previous readings of the ship as well as recorded data in the echo sounder.
How To Check Accuracy Of Echo Sounder?
Before an accurate reading of the echo sounder can be achieved, it must be set up properly. The initial setting can be done in two stages.
The moment of transmission must be set to the depth of transducers below the waterline.
Next, the speed of the echo sounder stylus is aligned with the sound velocity in seawater.
The most widely used method by industry experts to check echo sounders is the bar check. This is recommended for shallow waters up to depths of about 300 meters, at which a good clear echo can be obtained from the bar.
Though in shallow waters, it is quite difficult to check the accuracy of echo sounders. The weather must be fairly good, with good wind, smooth and calm sea, and tidal streams that are slack.
Since the transducer is very close to the seabed, it is difficult to know when the echo is actually received. The contour of the seabed, strong current, and drift can greatly affect the sound waves wherein readings can be doubtful.
Take note that errors in deep waters are much more compared to depths of 30 meters or less. The speed of sound in shallow water may be greater compared to the sound velocity in deep waters, hence the error will produce depths greater than the actual readings. This must be checked and confirmed to avoid miscalculations during berthing maneuvers or shifting of the vessel.
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