Truth about Boat Sounders

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How deep do you need to see the bottom?

You can buy boat sounders today that can probe the depths to thousands of metres, but realistically, how deep do you really need to go? If you are an average recreational fisho in the harbours and bays around New Zealand or Australia, anything over 50m is probably overkill. For the more serious fisho, something that reads accurately to 100m may be more suitable. Anything deeper is just showing off.

Determine how deep you want to see the fish and the bottom by checking the specifications of the equipment and regard the quoted depth as a maximum. There are several factors that will reduce this but take about half this depth for fishfinding ability as a rough guide. Also, remember that although you may be in 1000m of water, you may only want to see fish down to 100m and don’t care what the bottom looks like.

What alarms do you need in a boat sounder?

There are different alarms for different jobs. A surface or shallow alarm is the most common and will find shallow ground, reefs and pinnacles.

It is set from the surface down. To find deep water or holes, you will need a “deep” alarm, one that is set from the bottom up. A “window” alarm is used for specific depth areas such as 5m, 10m, 20m, 30m, etc. The above alarms usually also detect fish but generally do not know the difference between fish and bottom.

A separate fish alarm can be useful to give you an audible warning of the presence of fish. Test the audible level of an alarm to make sure you can hear it over engine or wind noise.

What does a transducer actually do?

The physical device inside a transducer that creates the sound wave is a piezoceramic disc called the element. The element, when voltage is applied, vibrates – it distorts and reforms its shape in very rapid succession. This vibration occurs at a specific frequency and creates
compression waves, or acoustic energy – sound waves. These waves travel outward from the element in a vaguely cone-shaped pattern and encounter targets along the way.

As this acoustic energy encounters targets such as fish or bottom structure, some of the beam will be attenuated (absorbed by the target), some will be reflected back at the transducer as an echo and some will be scattered. As the reflected echoes strike the transducer they cause a minuscule distortion in the shape of the crystal. This distortion of the crystal creates a small fluctuation of voltage, which can be detected and processed by the fishfinder. The end result is an image on your display.

By measuring the time from when the sound wave is generated to when the return echo is received, we can learn the depth at which a target is encountered. The strength of the reflected echo can tell us about the size and density of the target. Some transducers are referred to as single element transducers. This means that they contain a single piezoceramic disc that vibrates alternately at 50kHz and 200kHz, utilizing both operating frequencies. Furuno offers a wide range of single-element transducers that are very popular and carry a low price tag.

When greater performance is desired or required, multiple element transducers are available that can significantly enhance the performance and sensitivity of your fishfinder. A multiple-element transducer is one in which separate elements vibrate individually at their respective frequencies. Some high-end models utilize seven, nine or even fifteen 50kHz elements along with a large-diameter 200kHz element. The dedicated 200kHz element offers enhanced sensitivity in shallow water, while the greater surface area of the 50kHz array will receive echoes from deeper water with much more clarity and detail.

Frequency refers to the number of sound waves that radiate from a transducer each second. Sound waves are made up of highpressure and low-pressure pulses traveling through a given medium. The wavelength of sound is defined as the distance between two successive high-pressure pulses or two successive low-pressure pulses. For example, when an electrical pulse is applied to a 200kHz transducer the element vibrates at a frequency of 200,000 cycles per second – that is, 200,000 individual sound waves are transmitted from the element each second. Short-wavelength, high-frequency transducers produce sharp, crisp images on the fishfinder display.

Why use two frequencies?

For recreational and sportfishing applications, the 50/200kHz pairing of frequencies offers an ideal balance of both shallow- and deep-water performance. The 200kHz frequency produces sharp, crisp images in shallow water while 50kHz allows you to “see” much deeper.
Some frequency pairings are more suited to an individual application than others, and for this reason Furuno has always offered the commercial and serious sport fisherman a choice when it comes to selecting frequency pairs for a commercial fishfinder. These include 28, 38, 50, 88, 107 and 200kHz.

Note: If you buy a dual frequency fishfinder you must make sure that the transducer is also dual frequency. Some higher end models may transmit at frequencies other than 200kHz and 50kHz.


The beamwidth of a transducer is a numerical value that describes the effective angle of the sound wave. This value is defined as the total angle between the points at which the acoustic energy has been reduced to half its peak value, commonly referred to will be able to “see.”
As the frequency increases the beamwidth will become narrower, similar to focusing the beam of a flashlight. As you adjust the lens, the beam of light focuses and covers a smaller area, delivering more energy on-target.

This is more a function of the design of the transducer rather than an inherent property of frequency. Remember that the lower frequency wavelengths “see” deeper in the water column than higher frequency wavelengths, and so a boost in power is not always necessary to detect fish in deeper water. The lower in frequency that you go, the deeper the echo sounder will see for the same amount of power. You can also increase the fishfinders detection range in all frequencies by using a narrower beam transducer. A narrow beam delivers more energy on-target, resulting in stronger echoes, improved target resolution, and the ability to “see” in deeper water.

Which is better – wide or narrow beam?

Bigger is not necessarily better. A transducer with a wide cone angle scans more water as your boat moves along and can find fish faster, but this advantage an also work against you. The wide cone may cover two or three important rock formations on the bottom and lump their readings together, making it impossible to see just the one with the fish next to it. In open water, a wide cone angle can cover dozens of fish under and around the boat, instead of just those directly below. It’s difficult to determine the best depth to fish, or where the fish are located in relation to you boat.

A narrow cone zeroes in on fish and can detect small details on structure features that fish may relate to. Focusing the transducer’s power into a narrow beam also concentrates the sound output, enabling it to reach greater depths. The principle is much the same as comparing a spotlight with a floodlight. A major disadvantage to a narrow cone angle is that it scans such a small amount of water as the boat moves along that it takes a long time to find these important targets.

Some makes automatically manipulate receiver sensitivity and echo filtering to provide wider coverage in shallow water and narrow, more detailed coverage in deeper water. Add the advantage of multiple transducer beams that look to the side as well as straight down, and it’s easy to see where the fish are in relation to the boat.

Traditional Pulsed Energy vs. Sweeping Frequencies

If you haven’t paid attention lately, there’s surprising new technology that is transforming both radar and sonar for the recreational boater. Old-school devices (the style we’ve been used to over the past 30 years and much more) have operated using similar principles, both above and below the water’s surface. Radar and sonar both transmit a short, powerful, single-frequency burst of energy. In radar, units since World War II utilize a microwave-emitting device called a klystron or magnetron; sonar transducers vibrate a hunk of ceramic material at high frequencies.

Both blast out a “main bang” or “tone burst” and wait for a return echo, then the software inside the display translates this reflected energy into a picture. Meanwhile, over several decades, military radar and sonar has evolved down new pathways, moving away from fixed-frequencies and tone bursts. Finally, some of this previously secret and exotic technology is now filtering down to commercial fisherman and weekend boaters.

Get ready for frequency-sweeping pulse compression technology, commonly called by the acronym CHIRP (Compressed High Intensity Radar Pulse). Instead of transmitting only 200 or 50kHz, for example, CHIRPing devices transmit a signal that sweeps linearly upward (from 40 to 75kHz, 130 to 210kHz, or other frequency ranges).

Traditional “tone burst” fishfinders have always demanded a trade-off between pulse length and target resolution, and no targets can be resolved unless they’re larger than the physical wavelength of the pulse. If your fishfinder transmits a pulse that is 500 microseconds
in duration, it has a pulse length of about 27″. So fish less than 27″ apart will look like a single large mass, and fish less than that distance from the bottom will be hard to detect, appearing to be attached to the sea bed. Longer pulse lengths (like 50kHz) transmit more energy into the water and improve  deep-water penetration, except doubling the pulse length reduces target discrimination by half, likewise reducing detail and discrimination.

So lower frequency delivers more power and depth, but less resolution. With CHIRP fishfinders, this trade-off disappears. CHIRP fishfinders transmit less peak power than a conventional fishfinder, but their wide-band, frequency modulated pulses (130-210kHz, for example) can be very long in duration and put 10-50 times more energy into the water. Using digital pattern matching and signal processing, CHIRP devices achieve unprecedented resolution and target detection. Your ability to resolve individual fish, or separate fish from bottom structure, is now a matter of inches, instead of several feet with traditional fishfinders. See individual fish in groups, instead of a single mass.

Depth ranges of 10,000 feet are standard with these broadband devices, which include black box sounder modules from Garmin (GSD 26), Simrad (BSM-2) and Raymarine (CP450C). These boat sounders feature dual-transceivers that allow for simultaneous and independent dual transducer operation, which allows complete customization, as you can CHIRP or dial each transducer into specific frequencies. CHIRP devices can transmit simultaneously on high and low frequencies. The lower frequency gives greater depth penetration, and it requires less power than a higher frequency signal so it generates less noise.

The result is a “whisper into the water” that locates the fish without disturbing them. The higher frequency signal gives even finer detail at shallow to mid-water depths. You can select from a range of frequencies to best suit the conditions in which you are fishing.

Pixels and Power. Why are they so important in a boat Sounder?

The word pixel is an abbreviation for “picture element”. A liquid crystal display is really a chessboard- like grid of tiny dots (pixels) that darken or colour individually when electricity is applied to them. A fishfinder’s computer forms the picture on its screen by darkening or colouring selected pixels and leaving others blank. The number of pixels on a unit’s screen determines how much detail it can show.

Remember that pixels are arranged, chess- board-style, in vertical columns and horizontal rows. The more pixels a screen has in each vertical column, the less depth each pixel represents and the closer objects like fish can be to each other and the bottom and still be separately displayed. The word used to describe this is resolution – the greater the number of pixels, the greater the resolution. If a screen has 100 pixels in each column and you search for fish in a 0-15m depth range, each pixel represents 150mm of depth. A picture made with 150mm building blocks isn’t going to have a lot of detail. With 240 pixels in each vertical column, each pixel represents 62.5mm of depth.

It’s like comparing a picture drawn with a sharp pencil to one drawn with a dull pencil stub. You’ll see smaller fish more accurately portrayed, fish that are closer together and fish closer to the bottom. Small changes in the bottom contour and subtle changes in the structure spring into view, giving you better information to plug into the fishing equation. The number of pixels in each horizontal row determines how long information stays on the screen before it scrolls off. This is especially important for units to show side display of different kinds of information.

The output power of a fishfinder’s transmitter can be stated in watts RMS, or in peak watts or in peak to peak watts (P-P). The three terms represent different ways to measure output power. To compare units that advertise one rating and not the other, first understand that the true measure of output power is RMS watts and the other two measures are inflated marketing hype. To convert P-P to RMS, simply divide by 2 x 2 (i.e. divide by 2.83). To convert peak to RMS, simply divide by 2 (i.e. divide by 1.414).

Power is important for a couple of reasons. First, the more power you have, the deeper you can reach. Second, and more important to most fishos, the more power you have, the better your signal-to-noise ratio. In simple terms, the more powerful your sound pulse, the easier it is to separate echoes returning from fish and structure from all the other noises your transducer ‘hears’. The following is a guideline for the maximum depths that tranducers of various powers are good for:

100W   120m
200W   150m
300W   180m
600W  240m  – 360m
1kW  550m  – 760m
2kW  760m  – 1200m

Is the size of the screen that important?

In a word – YES. The larger the screen, the easier it is to see important details, providing that the screen has an adequate pixel count. A large screen offers more room for sidefinding displays, navigational displays and zoom displays, without squashing the regular sonar view down so small that it becomes nearly useless. That’s usually why combination units today like the new breed of fishfinder/ GPS/plotters have larger screens to view all the information clearly.

Are all the extra functions necessary?

The extra functions such as boat speed can be added at little extra cost, with the different transducer required for it adding to the price. It is cheaper to incorporate it in the boat sounder than to buy a separate speedo. A distance log is sometimes also added to the boat sounder function and would be helpful for navigation if you don’t have a GPS. Water temperature is useful when looking for warm spots or currents, although it can only read temperature near the surface as the sensor is incorporated in the depth transducer.

However today’s ‘boat sounders’ can offer a lot more than they used to with MFDs (multifunction displays) that turn your simple fishfinder into a very important piece of navigational equipment. MFDs do it all, from radar to GPS plotting and even link in with your entertainment and engine functions. You would be surprised how inexpensive some of the latest units are and they also come in some reasonably compact units from as little as 6” screens. However, as already mentioned previously, the bigger the screen the better.

Where should I mount the boat sounder?

If you have a trailer boat then space may be limited, but there is usually somewhere that will be suitable. Bigger boats in the 10m plus category don’t have a space problem and there’s often enough real-estate to mount two or even three screens. Many of today’s trailer boats have split dashboards that have been specifically designed to take larger boat sounders and plotters. If however you don’t have the space for a flush- mounted unit, then bracket-mount it, in a place where it will be easy to read. Take account of the action of direct sunlight on the screen as a quality black and white LCD sounder needs direct sunlight to get a clear reading. Most of the boat sounders today however are colour and have exceptional daylight viewing capabilities. Also, try to mount the unit in a place that isn’t going to be subjected to constant water as even the so-called ‘waterproof’ units do leak from time to time.

What transducer should I choose and what about position?

The transducer type will depend on the kind of boat you wish to fit the unit to. However, the stronger the output of the transducer the better in most cases. Pacific PowerBoat’s Buccaneer 685 Exess has a 1000-watt transducer mounted on the transom and it works amazingly well. Most trailerboats, be they timber, alloy or GRP have transom mounted versions and they are in fact the most critical ones to get right. Cruisers will traditionally have a through-hull, which is also favoured on some of the larger trailer boats.

Their proper location will depend a lot on the hull shape and what works for one will not always work for another. A transducer must be placed where no turbulence is present if you want it to work effectively, especially at speed. The signals will not travel through air bubbles. If you can, it is better to fit a through-hull unit which will be ahead of any turbulence. Another method is an ‘in-hull’ transducer which is mounted inside the boat with the aid of a ‘wet box’ or glued directly to the hull.

However it is important to discuss transducer type and placement when you purchase a unit as there are a number of types and models which all work a little differently. It is recommended that before you talk to a retailer you bone up on transducers by reading some manufacturers’ websites. One good one is the Furuno USA website where there is downloadable pdf file called “Furuno Transducer Guide”.

Spoiled for Choice

There are dozens of echosounders/fishfinders available and prices from under $350 for a really basic fishfinder that shows bottom shape and fish presence, to more than the cost of a complete-rig small runabout for top-of- the-range multifunction displays that have the added value of radar, plotters, GPS and a myriad of other functions.

Remember that the most important function of any sounder is to give you an immediate and easy-to understand readout of the bottom depth to keep you off the rocks. Everything else is just fish for the table!

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