The display head contains the computer, input keys to give the computer instructions, and the display screen.
The transducer, mounted [in the case of our boat] on the outside of the boat on the transom [stern] uses Sonar technology [developed in World War II] to produce sound waves that it sends through the water. When these waves hit a fish, or the bottom, they bounce back to the transducer; by measuring the time elapsed between the sending and the reception, the computer can then calculate the depth at which the object lies and it can show the bottom of the water.
The nature of the signal received by the transducer can also tell the computer the size and shape of the object encountered. The computer then displays these shapes on the screen.
The sound waves are sent out in the shape of a cone, rather like the beam of a flashlight; this means that in very shallow water the area the transducer “sees” is very small…only a few feet in diameter. At greater depths the cone has time to spread and, therefore, allows the transducer to see a much wider area below the boat.
Obviously, this is all very useful information. Knowing the water depth and seeing the bottom are helpful for finding underwater structure [rocks, holes, etc.] that may be indicators of locations where fish will queue up to feed. Seeing the actual fish gives an indication of their size and number.

Unfortunately, seeing the fish and their habitat doesn’t guarantee fishing success. Sometimes you find likely habitat, and no fish are there. Sometimes you see the fish, and they won’t bite. Oddly, sometimes no fish appear on the screen and you still catch them. No guarantees at all.
The fishfinder in the photo tells us we are fishing in 27.6 feet of water [pretty deep for us] and the water temp is 60.5 degrees F. The clutter at the top of the screen is just that: Noise produced by the boat’s propeller and the interaction of the transducer and the water. The bottom of the water is the red line at the base of the screen. All those squiggly things between 18-feet and the bottom are fish. Unfortunately, the fishfinder can’t tell us the type of fish we’re seeing; but, as I recall, they turned out to be smaller striped bass…they were all stacked up like cordwood.
On the day the photo of the screen was taken, we did catch fish.