Fishfinders are basically a type of SONAR (sound navigation and ranging). Fishfinders use "active" (rather than passive which does not produce the sound) sonar to detect fish. Fishfinders also detect the bottom of the ocean or lake, and everything else that reflects the sound beam. The transducer (usually included with the fishfinder) produces the sound beam which reflects off anything under the boat. These reflections are interpreted and displayed on the graphic display (usually a LCD or CRT screen) of the fishfinder.

Fishfinders get their origins from fathometers. Modern fathometers (meaning fathom plus meter) are designed specifically to show depth and usually only have a digital display. Early fathometers were comprised of a flashing rotating light at the edge of a circle. Which was synced up with the received echo and corresponded to the depth of the water. They also gave a faint flicker of light for echo's off fish. After technology advanced enough CRTs were used with a fathometer and thus the fishfinder was born. In the early 1990's CRTs were replaced by lower power LCD displays which also brought the price down to a more affordable range.

More About How Fishfinders Work:

Sound waves are produced by the transducer of the fishfinder and projected down toward the ocean floor. The wave is narrow at the surface and fans out as it heads down. This forms a "beam" of sound. As this beam comes into contact with something (like a fish), some of it bounces back to the fishfinder. The fishfinder calculates the time it takes to bounce back and the strength of the reflected beam and displays it on the screen. The reflection of the beam is also affected by the material that it hits. For example a soft bottom like mud, grass, and/or sand tend to absorb the signal while hard bottoms like rock, corral, and so forth reflect more of the original signal back. All these slightly different sonar beam reflections are interpreted by the fishfinder and displayed on the screen. The clarity of the water can also affect the integrity of the sonar signal. Things like air bubbles from your prop or chop in the water. Also, things like algae, plankton, or other things in the water can reduce the effectiveness of the fishfinder.

The sonar beam or cone has a certain angular measurement or "cone angle" usually in degrees. For example a 26 degree beam is wider and covers more area than lets say a 14 degree beam, but it may not produce as good of results at greater depths. Different fishfinder models and manufacturers have different cone angles. Most fishfinders use only one sonar beam, while some (such as Humminbird) use multiple beams in order to cover a much wider area than the average fishfinder. Also the more expensive the unit usually the more beam options you have. This is something to look for when purchasing a fishfinder.

Transducers: There are several different types of transducers to choose from depending on your situation. You have the option to get a portable one which mounts by suction cup. There is also a what they call a "shoot through hull" type. These can be useful for fiberglass hulls as the signal shoots through the bottom of the boat. Another type, through hull transducers actually have to be mounted in a hole through the hull (not a very popular type). Recently tranducer floats have come out for hand held portable units, such as the ones offered by Humminbird. Lastly, probably the most popular tranducer type is the transom mount which also comes standard with most fishfinder units. You can also get transducers with more capability such as the ability to read surface water temperature and a speed sensor (which can be more accurate than a GPS at low trolling or idle speeds). Fishfinder transducers are available in single or multiple frequencies and beams. Generally two or more frequencies are useful for deep water fishing, and the single frequency are better for shallow water such as lakes and rivers.

LCD Display Screen: When choosing a fishfinder pay close attention to the display screen. This where reading through other peoples reviews of a fishfinder can come in handy. The display is probably the most important part of the system. If you can't interpret what the sonar beam is picking up then the fishfinder is pretty much useless. LCD display resolution is measured in pixels. Normally it will say something like 128 x 240 pixels. The more pixels you have the better the resolution. This type of measurement has become popular in talking about flat screen televisions (720p, 1080i). Color, black and white, and mulitple shades of gray are all options when choosing a fishfinder. Generally the color is the best, followed by the gray and then black and white. Humminbird uses a technology called FSTN ((Film Compensated STN) A passive matrix LCD technology that uses a thin film optical filter layer between the STN display and front polarizer to change the default colors from blue on yellow-green to black on silver-gray.) on their displays to offer some of the best visibility. Normally you have the option on all displays of what you would like to display the fish as. You can either choose an arch or a fish symbol. If you choose the fish symbol then the fishfinder will use a software algorithms to interpret the arch as to whether or not it's an actual fish. If your using a multiple beam fishfinder then you have the added ability to see the location of the fish in relation to your boat. Another thing to consider is the sonar update speed of the unit. The faster the sonar gets updated the better image you get.

GPS Fish Finder: With GPS (global positioning system) becoming main stream and affordable you can now get fish finders with built in GPS. GPS is a satellite based navigation system developed by the US department of defense. It's a network of 24 satellites. Originally intended for the military, in the 1980s it was made available for public use with no subscription or setup fees. A marine GPS is often referred to as a chartplotter or trackplotter. Your position is super imposed over a map and constantly updated. You can also save your favorite fishing locations and easily return to them.

All of these things (screen resolution, transducer features, ect..) should be taken into account when selecting the proper fishfinder for your application. This can be a little overwhelming and reading reviews from people just like you can help narrow down the field. You can start with the bestselling fishfinders and then narrow down by price and then review. After that you can compare the current prices on ebay to make sure your getting the best deal possible

Major Brands of Fishfinders:

Humminbird: Humminbird fish finders are the biggest name in the business. They've been around for over 30 years providing top of the line fish finders, depth sounders, radios, and GPS systems. Produced and manufactured in the USA, they continue to recieve top marks and reviews by fishermen everywhere. They stay on the leading edge of technology and have excellent craftsmanship.

Eagle Fish Finder: Eagle has been designing and building top quality fish finders since 1982. Eagle Fish Finders are made by Lowrance with the same quality and most of the features but a low price tag. Eagle Fish finders are the best price performance fish finders on the market.

Garmin: They are most recognized by there products in the GPS market, but also provide top quality fish finders. Garmin International Inc. designs, manufactures, and markets navigation and communications equipment for the aviation and consumer markets.

Furuno: Furuno has designed, manufactured marine electronics for the past 50 years. Recreational & Commercial Monochrome and color fish finders for any size boat ranging in power output from 300 Watts to 3,000 Watts. They sell a high grade fish finder, provide a nice range of boating accessory technology/price solutions for the fish finder shopper. Don't forget to compare the reviews of Furuno with the other major brands.

Lowrance: Lowrance is a world leader when it comes to design and manufacture of sport fishing SONAR and GPS (global positioning systems). Lowrance, a designer and manufacturer of Sonar, GPS and Aviation instruments, got its start in Joplin, Missouri in 1957. Lowrance has established a global distribution network, encompassing more than 1,500 domestic dealers, distributors, mass merchants and original equipment manufacturers in the U.S., as well as sales distribution outlets in 53 countries worldwide.

Norcross Hawkeye: They specialize in the portable fishfinder. Their F33P model has been getting good reviews. Don't forget to check them out if your looking for a portable fishfinder.

Raymarine: Raymarine is the worlds leading manufacturer of Recreational Marine Electronics including Marine Radar, Multifunction Displays, Fishfinders, Autopilots and more. Formerly existing as the Recreational Marine Division of Raytheon (NYSE: RTN), Raymarine was formed as a company in 2001 after a management buyout, which included the rebranding of all products. Look at their product for upper end fishfinders.

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