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Boat Tech Guide: Fishfinders
Fish Finders - current as of March 2009
by Chuck Husick

There is no doubt that you can catch fish with nothing more than a length of line, a hook and some bait (optional for certain types of fish). However, in the face of this well known fact, both commercial and recreational fishermen willingly invest hundreds to many thousand dollars in equipment intended to improve the odds of their catching the fish in which they are most interested, not just any fish that happens to swim by. Sonar technology, which has contributed so much to undersea warfare and to peaceful uses such as underwater surveying and locating sunken ships has also made it possible for today's fishermen to visualize what is beneath their boats. The depth of the water, the nature of the sea bed and the presence of fish are easily determined using modern sonar equipment. Equipped with specialized signal processing circuits and a time line display the sounder can become a fish finder. Carried to its limit, the fish finder can become an undersea radar, searching through all or any part of a 360 degree azimuth and sweeping from straight down to horizontal in its search for underwater objects, including fish.


While the basic technology in a fish finder is identical to that in a depth sounder there are significant differences in the signal processing, display and control elements of the system. The transducers used may be identical to those used in a sounder or they may provide enhanced capabilities such as multi-frequency and multi-beam operation. A number of fish finders can operate with more than one transducer. In some units it is possible to tune the operating frequency away from the commonly used 50 and 200 KHz frequencies to eliminate interference from the signals being used by other nearby fish finders.

Fish finders detect the presence of fish primarily by detecting the air in their swim bladders. All depth sounders and fish finders rely upon the reflection of a portion of the transmitted acoustic energy back towards the transducer. Since the density of the flesh of a fish is quite close to that of water there is little reflection from the fish's body proper. The air contained in the swim bladder presents a major change in the acoustic path, giving rise to useable amounts of reflected energy. Signal processing in a fish finder is optimized for the detection of these small sonar energy reflections. Some fish finders allow the operator to assign a different on screen icons to each type of sonic reflection, creating a pseudo image of the actual fish life being detected beneath the boat.

The display screen may be the most critical factor in determining your satisfaction with the fish finder. Evaluation factors include overall size, screen resolution (the ability of the screen to show detail) and visibility in all lighting conditions. Regardless of all other measures, the screen must be clearly visible in the most difficult lighting conditions you are likely to encounter, usually direct sunlight. A monochrome screen will usually be easier to read under this condition than low cost color screen units. Unless you are planning to do all your fishing on dark days don't make your purchase decision on the basis of how the screen looks in the store, get outside where the light is close to what you will have to deal with on your boat. Depending on the location of the fish finder on the boat a monochrome LCD display may be the better choice. Many of these screens are trans-reflective and provide vivid, high contrast images in even the most intense, direct sunlight. Color LCD screens must be chosen with care, many are only marginally readable in direct sunlight. Brightness, although important in display screens is not in itself sufficient. Image contrast must be maintained if the displayed information is to be readily seen. Some of the best sunlight readable color LCDs use trans-reflective bi-refrigent technology, assuring adequate contrast in full sunlight conditions. Overall, sunlight readable color LCD screens tend to be costly. Some, which use high brightness back-lighting to achieve sunlight readability consume significant amounts of electrical power and may tend to overheat when operated for long periods of time in direct tropical sun conditions.

The ability of a display screen to show detail can be important when the acoustic image of a fish or other small object is close to the resolution limit of the system. The highest resolution is obtained on the CRT screens. While once very common, this type of display has largely been replaced in the small and medium size fish finders by LCD technology. The move to LCDs was prompted by a number of factors. While CRTs require high voltage power supplies LCDs operate quite happily from low voltage. It is relatively easy to splash proof or waterproof an LCD equipped unit. Doing so with a CRT based system can be more difficult. The CRTs used in fish finders were also used in small TV sets and other types of instruments. When the use of CRTs in those devices decreased the cost of CRTs for fish finders and related products increased significantly. At the same time, the quality, durability and overall value of the LCD technology improved to the point where it became really useful for even the larger fish finders. However, it is interesting to note that the professional level sonar systems used on commercial fishing vessels still use large color CRT displays.

When selecting a screen size it is important to consider both the overall size of the screen and the screen resolution, usually expressed in pixels. There may not be much gained by buying a unit with larger size screen if the pixel count is no greater than that of the smaller screen. Pixel counts of 640 x 480 are increasingly common on the high price units. Display screens are available in varying sizes, from about 2 1/2" x 3" up to about 7 inch diagonal. The size you buy must take into account the space available at your helm station and the distance from which you plan to look at its data display.

Typical fish finder operating features include zoom, bottom lock and white line, useful in analyzing the nature of the seabed. Dual frequency operation, usually at 50 and 200 kHz allows the user to optimize the system for the depth of the water and for the nature of the hoped-for targets. Some systems can operate in a simultaneous mode, showing the results from both transducers on a split screen. A number of units can combine their output information with data on chart plotters and with radar information, creating a type of multi-function display. Some manufacturers offer black-box versions of their sounders that have no display screens and present sonar data on the vessel's multi-function display. In some units the screen orientation can be selected, landscape (horizontal screen) or portrait (long axis vertical).

Some fish finders provide for a boat speed input signal from a hull speed sensor. This information can be used to modify the picture advance rate to compensate for changes in boat speed, ensuring that the image of a fish of a given size remains constant as the speed of the boat varies. Radar like controls are available on some fish finders. These allow selection of time varying gain (TVG), similar to a radar's STC (short time constant) control and clutter suppression, allowing better visualization of information in the presence of acoustic noise.

The more complex fish finders may include digital depth displays and "A" scope images. The "A" scope image, showing the progress of an acoustic pulse downward from the vessel into the water column and the time / distance relationship of any resulting echoes can be very useful in detecting fish just above the sea bed. ("A" scope images were the initial images provided on the earliest radar sets, prior to the development of the now familiar circular or Plan Position Image (PPI) seen on all modern marine radar sets.

In addition to the conventional downward looking echo sounders and fish finders some sonar systems can look sideways or forward of the vessel. These systems typically require transducers that are larger and protrude further into the water from the boat's hull than the conventional down-looking transducer. Such sounders can be very useful for boats that regularly navigate in challenging and uncharted water where a look ahead with a probing beam of acoustic energy is far easier on the boat and crew than using the keel as a sounding pole. The limit of forward visibility of such sonar equipment is usually limited to a few hundred feet and may be influenced by wave action and average water depth.

The high cost of the color displays required for high-end fish finders makes using them for additional purposes attractive for both the manufacturer and the buyer. In addition, the helm stations of many boats are becoming quite crowded, with numerous displays competing for scarce real estate in the prime viewing area. These facts have led to the creation of multi-function devices that at the extreme place all of a vessel's data displays on one screen. The system may begin with a color chart plotter, add a sonar or fish finder capability (usually accompanied with a water temperature read-out), navigation data from the GPS or Loran C and the capability of displaying or even overlaying radar data. Obviously, unless the screen is sizeable all of the data will tend to merge into a difficult to see mass.

These multi-function systems can be of value on both small and large boats. On the small boat the space saving can be a major factor in being able to place all of the desired information where it can be seen by the helmsman. Care will have to be taken to limit what is chosen for simultaneous display. On larger vessels it may be desirable to install dual display systems so that during normal operation both are used to inform the helmsman. In the event of failure of one display the other can still be used to selectively show whatever information is desired.

The electrical installation of a depth sounder or a fish finder is usually straight-forward. Most units operate from the normal 12 volt DC supply and as noted above except for the largest, sunlight readable screen units, consume only a small amount of power. Although modern sounders and fish finders are designed to limit radiation of pulse energy through their power supply lines there may be a certain amount of interference noted on radio receivers placed close to the sonar unit. For this reason it is best to keep power wiring separate from wiring going to radio equipment, especially entertainment radios that may be more susceptible to interference than VHF communication radios. Should interference be noted there are noise filters available that will usually deal with the problem.

The virtual vision provided by acoustic systems can now be augmented with actual images of what lies below the boat. Waterproof, highly light sensitive TV cameras are lowered from the boat. The controls and monitor are on deck. The seeing ability of these units will of course depend on water clarity, with visual ranges of 25 feet or more claimed in good conditions. In addition to their use in simply surveying the surrounding waters, these systems can be valuable for inspecting the boat's running gear. This capability can be invaluable when operating in water too cold for the casual diver. We can expect to see further development of this type of technology, including mini rovers that will follow navigation commands transmitted over their umbilical cables. The risk that a large fish may decide to sample the edibility of the camera may be compensated for by constantly recording the image. You might obtain some really interesting images of the feeding habits of some of the inhabitants of our seas.

As with most of today's marine electronics, there is seemingly no limit to the features and clever tricks the engineers can build into fish finders and sonars in general. Ultimately, you need to match your appetite to your budget. Be sure you get what you need before you buy features only needed to find the Loch Ness Monster.





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