While sonar was originally conceived as a means of finding submarines, its relatively recent development as a leisure aid for fishermen has seen it become not just more sophisticated but also more user-friendly. However, the basic principles remain unchanged.

Garmin black-and-white fishfinder

A basic black-and-white 2D fishfinder like this Garmin model does a very serviceable job.

What is a fishfinder?

As the name ‘echo-sounder’ suggests, it is all about sending and receiving sound waves. As these sound waves travel away from the propagating source on your boat (the transducer), their route sees them expand in a cone shape. If a solid object is encountered within that cone, a signal is bounced back to the device. By measuring the time the sound signals take between their departure and their return, your fishfinder can then calculate the distance and location of the various objects relative to your position and express them by means of a sub-surface picture on a digital display.

The size and type of target encountered can also be determined by the nature of the returning signal. For instance, if the outgoing wave hits a hard, rocky surface, the ‘echo’ will be much stronger than that which returns from a soft, muddy one. In short, if you have the right gadget and you know how to use it, you can get a pretty lucid picture of the wedge of water between your transducer and the seabed. However, as you might expect, the continued development of the product has given us several extra things to think about.

Cone considerations

Raymarine fishfinder illustration

This illustration by Raymarine demonstrates the importance of cone angle.

The angle of the sonar beam is measured in degrees and it’s called the cone angle. A wider angle obviously covers a greater area but (all other things being equal), it does so at the expense of signal strength and therefore depth penetration and image clarity. The cone angle tends to go from less than ten to more than 60 degrees – and while a 20-degree cone is more or less a standard fit for most recreational anglers, fishfinders now commonly emit more than one cone from the same point. In addition to dual and triple beam models, more specialised units also now include side, forward and downward imaging for 3D renderings of the subsurface environment. These high-frequency scanning technologies use a different technique to the traditional cone approach, but the principles remain unchanged. In all cases, an additional beam is designed to help encompass a greater area or a more accurate and lucid picture – and in so doing, it inevitably bumps up the price.

Power and frequency

The power of a modern fishfinder starts from as little as a couple of hundred Watts and goes well beyond 1,000. Again, if you keep the rest of the variables unchanged, then greater power equates to greater depth penetration and faster, sharper image returns. Naturally then, if you’re a serious offshore fisherman, who routinely ventures into deep waters, some extra power is likely to prove useful.

The frequency of a fishfinder’s sonar is also critically related to power and cone angle. Most recreational gear goes from 50 to 200kHz, though much higher frequencies are not uncommon. Higher frequencies give greater short-range accuracy in shallow waters, while lower frequencies are more commonly used for a broader picture in deeper waters. Happily, however, you don’t have to compromise here, because a dual frequency model will buy you the best of both worlds, painting a very serviceable image of the zone inside the cone and any fish that happen to stray there.


The importance of the screen

Inevitably, the size and resolution of display is tied directly to price. From blocky, low-res, black-and-white displays to large, multi-coloured, high-resolution 3D displays that make target recognition and differentiation much easier, your solution will be as much a function of your budget as your fishing ambitions.

However, it will also be a direct function of your fishfinder’s capabilities. After all, there’s no point having a device that can scan sub-surface structures and differentiate between small, moving targets if it’s unable to show them to you in a legible fashion. Even on modest modern fishfinders, therefore, it can be productive to dispense with compact, phone-style 3.5-inch displays and buy the best you can afford. Naturally enough, ‘the best’ is a subjective concept, but in general, you want a screen with plenty of pixels, good colour definition, lucid contrast in daylight and a physical scale of at least five inches. You can then profitably split the screen to generate a tailored and intuitive display that gives you a subsurface picture you can understand at a glance.

Posh but expensive

As you move up the scale, more modern and intuitive aids like down-imaging come into play. This is not merely a downward cone, but a very narrow, high-frequency beam that conducts clean, crisp, high-detail sweeps in order to construct a 3D image to help supplement your traditional 2D picture.

Side-imaging sonar (which uses much the same technique) can generate an even more comprehensive picture, detailing the subsurface region on both sides of your boat as well as below it. However, this essentially places your craft at the top of the screen and shows you a picture of what’s behind you – and that’s where forward-scan and 360-degree units come in. These devices are able to place your vessel in the middle of the screen, enabling you to see the sub-surface picture ahead as well as below, behind and to the side, but they are expensive and (in sonar terms) they are still in their infancy as leisure technologies. As for GPS in combination with sonar, this is now a relatively common feature, enabling you to establish an easy hit-list of favourite fishing spots and combine surface and subsurface pictures to great effect on a single screen.

Humminbird 360 scan – fishfinder technology

Humminbird's 360-degree scan puts the boat at the heart of the picture.

Man versus machine

Plainly, the possibilities have never been so tempting for those with a little extra ambition and a little extra budget. Just be aware that however much you pay for your fishfinder, it is vital that you read the instruction manual properly. These are complex pieces of equipment, so if you want to dial out the superfluous detail, hit the right targets and interpret the results effectively, knowing your equipment remains the single most important step you can take.

For detailed explanations of specific features and applications, the websites of the major industry players are a great place to look:Humminbird; Lowrance; Garmin; Raymarine; Simrad.

For more on electronics, see: Marine electronics: the 10 commandments or Dometic launches refrigerated cup holders.

For more on fishing see xxx.

Written by: Alex Smith
Alex Smith is a journalist, copywriter and magazine editor with a long history in boating and a happy addiction to the water. He’s worked on boats, lived on boats, bought boats, sold boats and – when he’s not actually on board a boat – he can generally be found in his Folkestone office, tapping away at the computer and gazing out to sea.