Defender Marine Outfitters logo
Useful Information about FishFinders

For our full line of Fishfinders, click here

What things should I consider when looking for a new echo sounder?
How does an echo sounder work?
How will my echo sounder differentiate between echoes of different strengths?
What is "white line"?
What is a transducer?

Should I get an LCD or a CRT echo sounder?
Why are some echo sounders dual frequency?
Can I interface my echo sounder with other marine electronics equipment?




Transom Mount Transducers
Thru-Hull Transducers
"Shoot Thru" Transducers
Speed & Temperature Sensors



Q: What can an echo sounder do for me?
A: An echo sounder can help you with the following:

  1. Locating where schools of fish are hiding around a structure and their depths.
  2. Locating shipwrecks, reefs, pinnacles, sunken trees and other objects between the hull of the vessel and the bottom.
  3. Determining bottom contours and composition to recognize soft mud, gravel, sand and rock.
  4. Determining the water depth and locating hazards for navigation purposes.
  5. Determining the location of areas where the temperature of the water changes dramatically.
  6. Determining what species of fish are being shown on the echo sounder display. (An experienced operator can determine what species of fish are being displayed by their different shapes and the size of the air bladders in the fish, a trait that shows up well on high quality echo sounders. Other factors such as water temp, water depth and schooling characteristics will help determine fish species.)


Q: What things should I consider when looking for a new echo sounder?
A: Choosing which echo sounder is right for you can sometimes be confusing. Knowing some basic information about your vessel and what your needs are will help this process. First you need to determine how you plan on using your echo sounder. For example, do you plan on using it mostly for fishing, navigation or a combination of the two. It will also help if you know approximately the depth of water where you will be using the sounder. This will help you and your dealer determine the power output level needed on your new sounder.

Once you determine how it will be used, you need to decide where the sounder will be installed. Where you mount the sounder plays an important role in the type of display you should get. Sunlight washing out the display may not be a concern if it is mounted in a covered area, but if it is mounted in direct sunlight you will probably want a high contrast LCD display that will not wash out. Also, if it is mounted in the open, you will want a sounder that has a waterproof case so that the internal circuitry will not get damaged from water spray.

Determine what type of power input will be use when the sounder is installed. Sounders are available in a variety of power inputs, including 12VDC, 24VDC, 32VDC, 110VAC and 220VAC. Finally, you should consider where you want to mount the transducer for optimum performance and minimal interference.


Q: How does an echo sounder work?
A: Echo sounders determine the distance between its transducer and underwater objects such as fish or seabed and show the results on the display. An ultrasonic wave transmitted through water travels at a nearly constant speed of 4800 feet (1500 meters) per second. When a sound wave strikes an underwater object such as fish or sea bottom, part of the sound is reflected back toward the source. The depth to the object can be determined by calculating the time difference between the transmission of a sound wave and the reception of the reflected sound. The sounder will then display this return as one of up to 16 different colors (color sounder) or a different level of gray scaling (monochrome sounder) depending on the strength of the returned signal.


Q: How will my echo sounder differentiate between echoes of different strengths?
A: A color echo sounder will use a different color for any one of up to 16 different signal strengths. This will make your B returns show up as red and your weaker colors as green or blue. Monochrome sounders will use different levels of gray to show the different levels of signal strength. B returns such as a hard bottom will show up as a very dark color and items such as debris in the water or concentrations of plankton will appear as a cloudy gray.


Q: What is "white line"?
A: Some color sounders have a feature called "white line," which allows an operator to replace any color or strength level with the color white. The color white tends to stand out from the rest of the colors on a sounder display, which will aid an operator who is looking for a specific level of echo return, such as bait or individual fish.


Q: What is a transducer?
A: The major function of the transducer is to convert electrical energy from the transmitter into sound/mechanical energy. The transducer then picks up the sound after it reflects off of the object. The transducer is one of the most important parts of an echo sounder system, and it is sometimes the most neglected. The transducer can be compared to the speakers of a stereo system. A high quality speaker placed in the ideal location will deliver the best results. The same is true with transducers.


Q: Should I get an LCD or a CRT echo sounder?
A: There are two display types to choose from, CRT (Cathode Ray Tube) or LCD (Liquid Crystal Display.) Each type has different advantages that need to be considered when choosing an echo sounder. A CRT is similar to a television screen. It has high contrast in normal to low light, allowing for bright and clear targets to be shown on the display. Bright sunlight will tend to make the CRT display fade. CRT echo sounders work best in an environment that is either covered or out of direct sunlight. With echo sounders, color units allow you to discriminate easier between small differences in signal returns.


Q: Why are some echo sounders dual frequency?
A: When acoustic energy travels through the water, the frequency of that energy determines how deep it will travel and also the level of definition you can expect to see after it has bounced off of an object below. The lower frequencies, (50 kHz) provide better ground discrimination and also detect fish better in deeper water. The higher frequencies, (200 kHz) will detect fish such as mackerel and squid with no swim bladders easier and usually provide better discrimination between schools of fish and individual large fish. Having a dual frequency echo sounder gives you the advantage of having a deep penetrating lower frequency and a high definition higher frequency.


Q: Can I interface my echo sounder with other marine electronics equipment?
A: Yes. All echo sounders will accept electronics navigation information from GPS or LORAN. In addition, your echo sounder will output depth, and in some cases, temperature to any device that will accept it. (Optional interconnect cables may be required.) This will allow you to display your depth information where your GPS/Plotter is, as well as displaying your LAT/LON at the location where your sounder is.



The Angler's Options
Boating anglers have several options when outfitting their sportfishing platform with a fish finder. A color CRT, mono LCD or color LCD fish finder offers boating anglers a form of X-Ray vision, where you can actually "see" the vast underwater world that rests beneath your sportfishing rig. But there are two challenges that come with this territory, the first of which is to have the knowledge and skill to be able to tweak your fish finder so that it paints a precise picture of what's going on down below. The second hurdle is to be able to interpret this picture of the bottom and then take the appropriate action steps to leverage your sportfishing opportunities.

The Harder the Echo, the Hotter the Color
A color fish finder that offers eight colors, usually range these from light to dark, depending on the echo return, starting off with soft colors like white, light blue, medium blue, dark blue, green, yellow, orange and finally red. Schools or small pods of baitfish are usually in the blue-to-green family.

Denser concentrations of larger forage fish might even approach yellow hues of color. Game fish, depending on their size, concentrations and depth in the water table, usually are represented by yellow, orange or red colors on the display. Bottom structure like the seabed floor, artificial reefs, shipwrecks, rock piles, etc. are normally either a dark orange or red color on the color scopes screen. When game fish, bait and structure are all jammed into one spot on the bottom, the bait will typically look like a tight pale blue or green cloud, with yellow or orange marks to the sides and underneath, with a dark red signature showing the bottom or fish-attracting structure nearby.

Differentiate the Bottom Type
Soft bottoms like mud or grass usually send back a signal that appears like a thin red line that marks the bottom. In contrast, harder seabeds like sand, clay or rocks are usually identified by a very thick and dense red line that marks the bottom. The general rule that applies is that the harder the bottom, the thicker or denser the "tails" that show where the bottom is situated.


fish finder sounder screen

Single-Frequency Picture

Showing area your boat has just passed over, including:

A Surface clutter, created by turbulence around transducer.

B Individual fish.

C School of fish hovering above the reef.

D Reef area, showing rocks above and harder bottom below.

E Digital readout of depth, water temp and boat speed.

F Speed of picture advance (also called "scrolling" rate).

G Sounder depth scale.

H A-Scope Picture, showing immediate, real-time returns of echoes including:

I School of fish below the boat, just above the bottom.

J Softer bottom below boat.



fish finder sounder screen

Low-Frequency Picture

Showing area your boat has just passed over, including:

A Surface clutter, created by turbulence around transducer.

B Individual fish.

C Schools of fish hovering above the bottom.

D Digital readout of depth below boat, water temp and boat speed.

High-Frequency Picture

Showing area your boat has just passed over, including:

E Speed of picture advance (also called "scrolling rate").

F Surface clutter, created by turbulence around transducer.

G Individual fish.

H Schools of fish hovering above the bottom.

I Sounder depth scale.


We offer an extremely wide range of matched, high performance, high speed transducers for various depth sounders. There are transducers for virtually every type and size of boat -- sail boats, large power boats, trailerables and more. There are also separate sensors you can install that will read your boat speed and the surface water temperature. Fact is, our selection of transducers is so broad that it is impractical to list them all here.
For a complete run-down, you should see your boat dealer. He can recommend the best type of transducer for your boat; he'll make sure it matches your sounder; and he can help you install both your transducer and your sounder. For do-it-yourselfers, installation instructions are available for all transducers. To help make your transducer selection easier, we have provided a general guide to the basic types of transducers along with a brief description of where and how to install each.


transom mount transducer Transom Mount Transducers

As the name indicates, this style of transducer mounts on the transom. The best mounting location is at the bottom of the transom with the face (bottom) of the transducer on a nearly horizontal plane, as shown in these two diagrams. Installation is best accomplished while the boat is out of the water.

The objective in mounting this style of transducer is to keep the face of the transducer in the water -- whether the boat is at rest or underway -- while minimizing the amount of transducer protruding below the bottom of the boat -- as illustrated here.

transducer mounting angles

It is important to minimize the turbulence and aeration around the transducer. Consequently, you should avoid locating the transducer along strakes, behind thru-hull fittings or other hull irregularities that may disturb the water flowing across the transducer face.
Transom mount transducers are often used on trailerable boats, as the location does not usually interfere with the trailer bunks, rollers, struts and other objects on a boat trailer. This style of transducer is also used on boats that cannot easily accommodate a thru-hull fitting on the bottom of the boat.


thru-hull transducerThru-Hull Transducers

When properly installed, this type of transducer offers better performance and fish detection than any other style. As the name implies, it does require drilling or cutting a hole in bottom of your boat. For this reason, installation should be undertaken while the boat is in dry dock.
The location of the transducer depends on the type of hull, as shown in these diagrams for displacement and planing power boats as well as fin keel and full keel sail boats.

When mounting on a deadrise angle -- such as shown on a full-keel sailboat -- a special "fairing block" is required, so that the face of the transducer lays on a horizontal plane.

boat hull types

As with any other transducers, thru-hull types should be mounted where they will be continuously immersed in undisturbed water. Objects than can disturb the water flow around a thru-hull transducer include strakes, fittings, keels and propeller wash.
To eliminate potential leaks, it's essential to apply a liberal amount of a quality bedding compound -- one that can stand up to continuous immersion -- around the transducer's stem and fittings on both the inside and outside.


shoot thru transducer"Shoot Thru" Transducers

This type of transducer fastens inside the boat hull, transmitting and receiving signals through the hull material, as shown in this illustration. This works only through fiberglass and is how "shoot thru" or "in-hole" transducers get their name. This style does not require cutting a hole in the bottom of the boat. As with other transducers, location is critical to performance. Typically, a flat section in the aft bilge area is the best spot, although you may need to experiment to find the best location. The latest in hull transducers, the P-79® allows you to compensate for the boats dead-rise angle.

Not all boats are well suited to shoot-thru style transducers. On smaller boats, for example, there is often a layer of foam flotation material between the deck and the hull. Shooting through the foam seriously hampers performance. In such cases, another type of transducer -- such as a transom-mount -- is more practical.

There are a number of methods for securing a shoot-thru style transducer. Some can be bolted on. Some are designed to be fiberglassed or epoxied into place. Still others require a special interior enclosure that completely encapsulates the transducer in fluid such as castor oil.
Your dealer can help you decide on the installation method that best suits your needs.


speed temperature sensorSpeed & Temperature Sensors

All echo sounders will display boat speed and surface water temperature, when connected with the proper sensors. In fact, some transducers -- ones known as "multi-sensors" -- have built-in speed and temperature sensors.
Most transducers, however, handle only echo sounding/fish-finding. To read boat speed and water temperature, a separate sensor is required. These sensors come in two styles: transom mount and thru-hull (shown here).

Like transom mount transducers, transom mount speed/temperature sensors mount at the bottom of the transom. They should be mounted just low enough to allow water flowing off the bottom of the boat to turn a speed-sensing paddle wheel.

As with thru-hull transducers, thru-hull speed/temperature sensors require cutting a hole in the bottom of the boat, as shown. These sensors do not need to be mounted on a horizontal plane. They can be installed on a deadrise angle without a fairing block.


Close Window

Defender HomeBoating SpecialsRebate / Promo CenterGift CertificatesEmail News Sign-upSite Map

copyright defender industries inc