A Simplified Guide to Small Marine Craft Navigation.

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Chapter 16 (v.1) - Measuring Water Depth by Echo Sounder.

Submitted: April 06, 2017

Reads: 94

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Submitted: April 06, 2017

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Measuring Water Depth by Echo Sounder.

 

A more sophisticated method of obtaining soundings is by means of an Echo Sounder, of which there are many different models available for small marine craft, as there are for large commercial ships.

However, regardless of whether an echo-sounder is fitted, a hand Lead and Line should always be carried, since the prudent navigator must regard all electronic devices merely as aids to navigation on account of their dependence on electrical power which can fail, and their complicated circuitry is also liable to fault and failure, usually at the most inopportune moment.

The principle of the echo sounder is based on the velocity of sound in seawater, which is known, the interval of time between transmission of a sound from a vessel's hull and the reception of its echo is proportional to the depth of water under the hull and therefore that depth can be determined. Early echo sounders were sonic but most sets today are supersonic, the transmission taking the form of a series of supersonic impulses. The term supersonic is used here to describe impulses whose vibrations are above the range of audible sound.

Because of the high-speed of sound, which is approximately 4,820 ft. per second in water, and the comparatively short distance it travels between the vessel and the sea bed, the interval of time which must be measured between transmission and reception is exceedingly short, 1/40 second in 10 fathoms. As it is not possible to measure such short times mechanically, the echo sounder uses an electronic time-scale.

The transmitter and receiver used to be mounted separately on the inside of a vessel's hull but the majority of modern models of echo sounder have their transmitters, transducers as they are termed, mounted in a single unit.

The super-sonic impulses emitted by the transmitter are directed downwards from the hull and a percentage of the returning echo impulses from the seabed enter the receiver transducer where they are converted into electrical oscillations, these oscillations are amplified and passed to the display instrument in the navigating space. A factor affecting the performance of echo sounders is the frequency at which the sound pulse is transmitted. Virtually total reflection of the Seabed occurs at about 10kHz; since acoustic absorption and attenuation increases with frequency above this point, it is will be understood that the lowest frequency which will give adequate discrimination will be the best one to employ.

With present low-cost equipment for small craft, the operating frequency is often in the region of 150kHz, usually because of the availability of ceramic transducers of suitable physical dimensions for echo sounders which fall into this region. In consequence, the performance of many small craft sounders is regrettably mediocre.

The electrostrictive principle is used extensively in small craft echo sounders because the transducers are very simple in their construction design, and therefore more economical to manufacture. These units usually have a piezoelectric ceramic disc mounted in a metal or plastic skin fitting. The mechanical vibratory effect is created by the crystalline structure of the material distorting under the influence of an electric field. The reverse of this effect holds good for detecting the returning echo; if the material is subjected to mechanical pressure electricity is generated, hence the name piezoelectricity, pressure electricity.

Commercial ship echo sounders generally employ the magnetostriction principle but although this requires a larger transducer, the advantage of working in the lower attenuation area of frequency, 30-4okHZ, is available. This enables the use of an internal transducer fitting in most types of hull.

It is perfectly feasible to employ an electrostrictive device at lower frequencies, but there seems to be a tendency to adhere to 150kHz because the piezoceramic elements are readily available to equipment manufacturers and the simplicity of construction lends itself to competitiveness in pricing.

Of the vast number of small marine craft echo sounders presently manufactured, the many different types can be grouped together according to the manner in which the information is displayed. Of this grouping, the flashing indicator display is by far the most prevalent in small craft, largely because it is the cheapest.

In this type of echo sounder, the outgoing pulse from the transducer causes a light fixed to a revolving arm to flash, and the returning pulse from the seabed causes it to flash a second time. As the light is revolving, the interval in time between the outward and return pulses is measured by the angular distance between the two flashes. An arm is so set that the first flash occurs when the light passes zero mark on the scale and the graduation opposite the second flash indicates the depth at that instant below the transducer. Earlier models used a neon flashing light but this has been replaced by a light emitting diode, since this gives a more sensitive reading and is easier to read in daylight. Meter or Dial Pointer displays indicate the depth directly against a circular scale with a needle or pointer.

The Flashing indicator sounder has the advantages that it is easy to read and easy to compensate for the depth of keel below the transducer. But the disadvantages are that the meter reads low on craft where engine noise or water turbulence are a problem, any echo failing to return allows the display to move towards maximum depth so that the boat could be resting on the bottom, and the equipment would give an indication of infinite depth.

A Digital Display might appear to be very convenient, but since the circuits are so designed as to pick out the strongest echo and indicate this, but there is little indication of how reliable the reading is. That presents what might be unreliable information in such positive form is a mistake and which could lead the navigator to place unwarranted trust in the reading.

In general, digital displays suffer from the same disadvantages as the meter types and can indicate maximum depth when on or close to the bottom. They also tend to count short when subject to vibration, such as in high-speed craft.

The Paper Recorder type of depth sounder produces a graphic display by means of a rotating stylus on a slowly moving roll of sensitized paper. From the navigator's point of view, this type is by the best because not only does it indicate the actual depth in a manner easy to interpret, but it also shows the trend of whether the water is shoaling or deepening, or whether the vessel has passed over a ridge. Other advantages are that it is linear by comparison in that large echoes show dense black, small echoes show light gray, and mid-water echoes such as fish are detectable in addition to the seabed.

Furthermore, this type of display is always readable irrespective of the ambient light, and the only disadvantages of paper recorders are that they are more expensive and there is the additional running cost of the sensitized paper, but the cost must be weighed against the performance required.

The controls on all types of small marine craft echo sounders are simple, and usually consist of an on-off switch, a gain control used to increase or decrease the sensitivity of the receiver, and a scale switch where more than one depth scale is provided for on the depth sounder.

The provision of a gain control is necessary so that the sensitivity of the receiver can be reduced in shallow water, otherwise the indication of the depth would be swamped by the indication of the zero. By reducing the sensitivity in these conditions the two marks can be distinguished and the depth ascertained. In deep water, the gain control would be set at maximum.

Inconsistencies in performance should not necessarily be taken as an indication that the sounder is faulty, as many external factors influence the behavior of the equipment, such as False Echoes caused either by shoals of fish masking the sea bottom echo or by dense layers of suspended matter in the water, or by sudden changes in temperature or salinity.

Weak Echoes can be caused by water noise interference owing to unsuitable siting of the transducer on the hull or by aeration resulting from rudder movement or from going astern under power. Variable Echoes are caused by good and bad reflecting surfaces on the sea bed and in general, hard sand, coral, chalk and rock are good, but thick mud or silt is bad.

It should be noted that an echo sounder shows the depth below the position of the transducer and that the distance this is fitted below water level must be added to the depth indicated on the display to obtain the correct sounding. An error can also be introduced if the zero transmission indicated on the display is not coincident with the zero on the depth scale. However, the manufacturer’s servicing agent can adjust this, or an allowance could be made for the error when reading the sounding.

On a sailing craft consideration should be given to the fitting of two transducers, one on each side of the keel, because the craft will be heeling steeply when sailing on a tack, and so a single transducer fitted on the windward side of the keel would not be sending impulses directly to the seabed. However, with transducers fitted on either side of the keel, the leeward one will be pointing more or less vertically downwards for use in these circumstances. Handily, switches are available which work under gravity and automatically switch from the windward to the leeward transducer.


© Copyright 2019 Sergeant Walker. All rights reserved.

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