A fishfinder is a type of fathometer, both being specialised types of echo sounding systems, a type of Active SONAR (`Sounding` is the measurement of water depth, a historical nautical term of very long usage.) The fishfinder uses active sonar to detect fish and the bottom then displays them on a graphical display device, generally a LCD or CRT screen. In contrast, the modern fathometer (from fathom plus meter, as in `to measure`) is designed specifically to show depth, so many use only a digital display (useless for fish finding) instead of a graphical display, and frequently will have some means of making a permanent recording of soundings (which are merely shown and subsequently electronically discarded in common sporting fishfinder technology) and are always principally instruments of navigation and safety. The distinction is in their main purpose and hence in the features given the system. Both work the same way, and use similar frequencies, and, display type permitting, both can show fish and the bottom. Thus today, both have merged, especially with the advent of computer interfaced multipurpose fishfinders combining GPS technology, digital chart-plotting, perhaps radar and electronic compass displays in the same affordable way.
In a generalized sense, a electrical impulse from a transmitter is converted into a sound wave by the transducer, also called a hydrophone, and sent into the water. When the wave strikes something such as a fish, it is reflected back and displays size, composition, and shape of the object. The exact extent of what can be discerned depends on the frequency and power of the pulse transmitted. The signal is quickly amplified and sent to the display. Knowing that the speed of the wave in the water is 4921 ft/s (1500 m/s) in sea water, 4800 ft/s (1463 m/s) in freshwater (typical values used by commercial fish finders), the distance to the object that reflected the wave can be determined. The process can be repeated up uto 40x per second this fathometer function eventually spawned the sporting use of fishfinding. Note: This discussion of the propagation of sound in water is simplified, speed of sound in water depends on the temperature, salinity and ambient pressure (depth).
If you have watched the old German U- Boat movies there is always a scene where the sonar room have to listen to the ping -ping of the sonar as an attack boat on the surface searches for the U-boat underneath before it launches the depth charges to try and blow it out of the water !
Early sporting Fathometer for recreational boating used a rotating light at the edge of a circle which then flashed synchronized in time with the received echo corresponding to depth there also gave a small flickering flash for echos off fish. They did nothing to display the trend of fthe bottom depth over time nor anything about bottom structure. They operated strictly in a snapshot mode, as do the cheap digital fathometer of today. They were hardly ideal in a wave tossed small craft or in bright light but they were good for holding the boat in the safe channel assuming one could actually see the light....
Commercial and Naval Fathometer of yesteryear used a strip chart recorder where an advancing roll of paper was marked by a stylus to make a permanent copy of the depth, susally with some means of also recoring time each mar or time tic is proportional to the distance traveled so that the strip charts could be readily compared to navigation charts and maneuvering logs (speed changes.) Much of the world's ocean depth have been mapped using such recording strips. Fathometer of this type usually offered multiple (chart advanced) speed settings, and sometimes, multiple frequencies as well. (Deep Ocean-- low frequency carries better shallower-- high frequency shows smaller structures (like fish), submerged reefs, wrecks, or other bottom composition features of interest.) At high frequency settings, high chart speeds, such fathometer five a picture of the bottom (and any intervening large or scrolling fish) relatable to mitigation position data. Fathometer of this constant recording type are still mandated for all large vessels (100+ tons displacement) in restricted waters (i.e. generally, within 15 miles of land)
Eventually, CRT's were married with a fathometer for commercial fishing and the fishfinder was born. With the advent of large LCD arrays, the high power requirements of a CRT gave way to the LCD in the early 1990's and fishfinding fathometer reached the sporting markets at prices nearly anyone of modest means can afford. Today, sporting fishfinders lack only the permanent record of the big ship navigational fathometer, and that is available in high end units that can use and ubiquitous computer to store that record as well.
Most units just attach the FISH symbol to every echo which is not connected to the bottom surface. Nowadays with advances in factory operations and in the technology fishfinders are becoming very affordable and as such an essential part of any fisherman's tackle.If you have a small boat many people will use them to aid in shallow water navigation.The growing popularity of kayak fishing has lead to people fitting them on kayaks and canoes.Anglers even use portable and casting units from the bank.Reliable, easy set up units with great LCD screens can now be had for under 100 GB Pounds.Most units carry a model number which will usually relate to the screen size eg,Cuda 168 versus Fishmark 320 - the latter unit has a bigger screen of 320 pixels.Remember also these products are continually updated each season and become very affordable as units are changed out.Some brands are also made at the same factory and are very similar but may be priced a little different.Lowrance own Eagle for example.Entry level units can be used to get to know how to use the products then upgrade to colour, bigger screens, GPS / Sat-Nav and chart plotting.A fishfinder will not guarantee you catch fish but it will make the day easier, safer and more interesting !