Echo Sounders

Redcar FishTechnical

Courtesy of Redcar Fish

Firstly we will ask the question, why do you require an echo sounder? An Echo Sounder will give you a picture of what lies directly under your boat and it will draw a picture of the contours of the sea bed as you pass over it, this enables you to easily locate reefs, pinnacles of rock, wrecks and other obstructions on the seabed. Different types of sea bed such as rock, sand and mud are easily identified from the picture presented by the sounder.

There are times when it is vital to know the depth of water under your boat if thick fog … descends on you whilst at sea, the intelligent use of your sounder will ensure that you do not run aground on rocks as you try to locate a safe landing place. We will now assume that you are interested in buying an echo sounder and it’s very useful to understand how they work.

Echo sounders, as the name implies, use sound waves, these waves are high-frequency vibrations. Sound travels at different speeds through different substances depending on their density and characteristics, the denser the substance, the faster the sound wave travels. When a sound wave is transmitted into water it travels at a constant speed until it meets another object, if it strikes the sea bed the speed of the sound wave is increased because the sea bed is denser than water.

When this happens, part of the sound wave is reflected back through the water, the greater the change of speed, the bigger the reflected echo. Hard rock, therefore, will send a stronger echo than clay or sand, whilst soft silty mud will send a weaker signal. A good echo sounder will detect a reflected echo of less than one-millionth of the strength of the transmitted signal strength. The sounder transmits a sound wave that travels straight down from the boat using a transducer attached to the boat hull. A transducer acts as a directional loudspeaker and also as a microphone receiver to receive the reflected sound waves. The sounder then measures the time it takes for the reflected sound wave to return and converts this so that the sounder displays it as the distance of the reflecting object from the boat.

There are two main types of signal used for echo sounders;

High-Frequency sets transmit a signal of around 180-220 kHz, and Low-Frequency sets use a signal of around 30-60 kHz.  High and Low-Frequency sets have totally different characteristics, high-frequency sets transmit a signal that is more directional, therefore the ‘beamwidth’, like the beam of light leaving a torch. The beam along which the sound waves travel has not really got sharp side edges, the strongest signals are at its centre and the signal strength progressively decreases towards its edge. You will notice that on high-frequency sets there is often a choice of transducers with different beam angles, the widest is for shallow water, the narrow ones are for deeper water. The reason for this is that by concentrating the signal into a narrow beam it is more powerful and it will, therefore, penetrate to a greater depth, the narrower the angle the greater the sea bed detail, because the area being recorded is smaller.

How is the Picture Displayed on an Echo Sounder

There are two types of presentation of the display, CRT Screen (video) & LCD (liquid crystal display). Each of these has its merits!

CRT Echo Sounders, large sets using this type of display have been in use for many years, modern technology now makes it possible for smaller sets to be produced, these sets have 6 inch CRT screens (measured diagonally). The introduction of the new high-resolution screen has now doubled the picture quality of these sets. These new sets can now have a picture that is made up of 256 x 256 dots (Pixels).

The advantage of this type of set is different strengths of return signals are displayed in different colours, the strongest being red going down to the weakest which are blue, this can help to identify fish type and size for those who use their boats for fishing.

LCD Echo Sounders, some of these sets are designed for the really wet boat, they are totally waterproof, they can be immersed in water to a depth of 2 meters and take no harm. This property makes them ideal for boats that are launched from open beaches, even if your boat is swamped in the surf the sounder should be OK, the divers will also appreciate this type of set for their inflatable.

Output Power

There are two ways in which power output cab be expressed, RMS and Peak to Peak. There is a very big difference which you must remember when buying an echo sounder. An 800-watt peak to peak set would only be rated at 100 watts RMS. If you see a power output quoted that look fantastic, remember that if it is peak to peak, divide it by 8 to get the true RMS power output.

What will I see on my Echo Sounder?

The first thing you will notice is that the picture of the sea bed feeds out from the right-hand side of the screen, so what you see on the right of the screen is what is under the boat now. The picture progresses across the screen to the left, taking a certain amount of time to reach the other side of the screen, so an object that appears on the right of the screen is under your boat now, when it gets to the other side of the screen you would say it was under the boat “X” minutes ago. So if the boat is stood still, the sea bed would be shown as a straight line because the depth is not altering.

If the boat is moving we get a picture of the contours of the sea bed as we pass over them. Say a big peak of rock appears on the right of the screen and you throw a float overboard, the float remains over the peak of rock, you continue to steam away from it. On the screen, the picture of the rock moves across the screen. What you are now seeing on the screen is a cross-section of the sea bed between where you are now (right-hand side of the screen) and the float over the rock which is now on the left-hand side of the screen.

Picture Presentation on the display

There are various ways in which the area of water under your boat can be displayed on the screen.

Fixed Scales, on sets of this type the top of the display shows the surface of the sea, and you choose a scale that is deeper than the sea bed so that the sea bed is displayed near the bottom of the display, for example, the sea is 18 meters deep, use depth scale 0-20 meters.

Fixed Scales with Options, using fixed scales in deep water the presentation of the sea bed picture becomes diminished because the whole area between the surface and the sea bed has to be shown on the limited size of the display, by making the top of the display a point some way below the surface the sea bed image is increased in size.

Shift Control, with this control you select a bandwidth of say 20 feet in depth, you then “shift” this band so that the sea bed is included in it, as the depth alters you “shift” the area that you are covering to keep the sea bed area on the display.

Auto-Shift, Bottom Track, Multiple Zoom Ranges. These phrases mean that an expanded view of the sea bed area is shown on the screen, as the water gets deeper or shallower the depth scales automatically alter.

There are two ways this can be done, when the sea bed gets too high or low on the screen the depth scales alter, this causes a step in the picture.

On most modern sets the picture feeds out from about one inch up on the right-hand side of the set, as the picture moves across the screen the whole picture moves up or down giving a continuous sea bed picture and the depth scales alter accordingly. Programmable Depth Display Range (manual). This function allows you to select the depth that you want displaying at the top of the display, you then select the depth that you want at the bottom of the display, so that you get the exact degree of expansion that you require.

Zoom Control, there are various interpretations of this phrase, the commonest means the bottom half or quarter of the scale is expanded to cover the whole of the display area, so 0-40 ft would be turned to 20-40 ft and then to 30-40 ft, this in effect doubles the size of the sea bed image with each press of the zoom key. On some sets you can zoom up or down, with these you can zoom in to a band of water of say 20 ft at any depth, this is done by halving the displayed picture from either the top or bottom.

Zoom can also mean that a bar is displayed opposite the normal picture, this indicates the area that can be zoomed, this band can then be moved by the zoom up or down keys. The expanded picture can be displayed beside or under the normal picture or full screen.

Decision time

If you are going to use it in an inflatable you would choose a totally waterproof set, whereas the owner of a luxury cruiser can choose any set for his carpeted dry wheelhouse. Don’t forget, waterproof is waterproof. Water-resistant is not waterproof!

Just a note about storing your electronics. If you use a sealed waterproof container (like a Tupperware container with a sealed lid) to transport your electronics to and from the boat. Do not keep the equipment in it for storage. Every time you use the equipment it picks up particles of salt, these attract water, so when you get home remove the equipment from the container, wipe it with a dry cloth and store it on a shelf in a dry well-ventilated place.

A set may be watertight, but remember that the pins and sockets in the plugs, when un-plugged from the set, and the power and transducer cables plugs will corrode if left wet, so protect them!

The size of your chosen set may be of some importance, the brochure will give the dimensions if in doubt make a cardboard box the same size as the set and try it in the boat to see what it looks like. Some sets are designed to stand on a shelf in the boat, others can be fixed to a vertical surface.

Quality, of the set!

How do you want the picture displayed? You should look at the number of dots or pixels used to form the picture. Here again, the maker of the set can fool you! One set has 2,500 pixels, another set has 200 x 128 pixels, which would you choose? 2,500 is the number of pixels vertically multiplied by the number horizontally, so this set is a 50 x 50 pixel = 2,500. Whereas 200 x 128 is 25,600 pixels, ten times better!  You could also see it expressed as so many pixels per square inch. So get the calculator out and check it.

What power output will I require?

At sea in up to 200 ft of water, you should look for a set with at least 75 watts RMS ( 600 watts peak to peak ) power output and use a transducer with a beamwidth of around 20 degrees. In even deeper water you should consider a narrower, eight-degree beamwidth transducer.

What functions do I need?

The more automatic functions you have the easier it is to use the set. Auto Range, Bottom Track, Multiple Zoom Ranges or Auto-Shift are very good because you just leave the set to run automatically.

Power Supply

Although power output is big, the power requirements of the set are small because it is discharged in minute bursts of power.  If you do not have a power supply in the boat, use a spill-proof 12volt rechargeable car or motorcycle battery. Choose a battery with good Amp-Hours. A-H divided by power requirement = hours running. Choice of the transducer and the way it is fitted are of vital importance in getting the best results from your echo sounder. In fitting the transducer you are looking for a location where there is clear water free from air bubbles.


Only purchase from authorised sales outlets, who have the full support of the factory to repair and/or replace any product that fails within the warranty period. When purchasing equipment, ask to see the warranty card showing the manufacturers appointed dealer(s). The longer the warranty period the better.