In 1490, Leonardo da Vinci put an ear to a tube and stuck the tube into the water to listen for faraway ships. Today, the science of underwater acoustics has developed into a fine art called sonar.
Active sonar - equipment that sends out sound waves and receives them back after they bounce off objects - was refined during World War II.
Since World War II anglers have seen dramatic leaps in the technology. Japanese marine electronics manufacturer Furuno marketed the first practical fish finder in 1948.
Today, fish finders come in a wide array of configurations and prices. Understanding these tools that are used by and affordable to anglers and boaters worldwide can help when selecting one.
With all of the technology packed into them, today's fish finders can hold more features than an angler may ever need or use. An experienced user can help a potential buyer sort out the options.
One of those users is Phil Cable from Holly Springs, N.C., who has been guiding on Triangle reservoirs since 1992 and has extensive experience in finding deep-water bass.
"Give me one that will show the bottom good and give me good clarity and show my baitfish and give arches for my fish, that's what I'm really after. I want something simple," Cable said. "Some of the technology is just above the average fisherman.
"I can't tell you what every spot is on that depthfinder or every little mark, but I do know when I see baitfish, I do know when I see arches (which represent fish on the screen).
"You just have to get used to it. You have to put it on the water and catch the fish after you see them (on the screen) to really start knowing what you are actually looking at."
Most units today are very sophisticated, packing a lot of technology behind the screen that enables anglers and boaters to get a lot of information on the screen.
Virtually all fish finders start up in an automatic mode that will cover most freshwater and inshore saltwater fishing situations.
Their complex processors sample, verify and filter signals, removing unwanted clutter from the screen. This "noise" can be caused by electronic interference or by suspended debris and air bubbles in the water.
The basic components of a fish finder are the display unit and the transducer.
The display unit houses the transmitter, the processor, controls and an LCD display. The transducer sends and receives sound waves for the processor to display on an LCD screen.
The transducer emits sound waves that travel through the water in a cone shape from the bottom of the boat, like the beam from a flashlight. When the sound wave hits an object or the bottom, it bounces back to the transducer.
Because the speed of sound is near constant in water, the processor can measure the time it takes for the waves to reflect back from the object and calculate the distance from the boat.
The processor generates a vertical slice of the entire water column one pixel wide on the right-hand side of the screen. This process is continuous, with each new vertical slice displayed on the screen alongside the previous slice creating a scrolling action across the screen.
Display units and transducers use resolution, frequencies, power and cone angles working together for top performance. Knowing specifications helps evaluate different systems.
Vertical resolution is the single most important specification for any fish finder. The higher the resolution in pixels, the more detailed the information displayed on the screen.
The first-time buyer should look for "a good sonar as far as high detail and a high-resolution display," said Mark Gibson, the senior brand manager for marine electronics manufacturer Humminbird.
Cable said a good display is essential.
"I look for the clarity. I don't like depthfinders (fish finders) that have a lot of clutter. I use Garmin units with a resolution of 240 (vertical) pixels," Cable said.
That would be a good starting point for resolution.
High detail is particularly important when looking for individual fish signatures represented by an arch on the screen.
"Fish are detected by their entire skeletal matter. Their swim bladder plays a part and the mass of the fish. It's a combination of all that," Gibson said.
The telltale signature of a fish arch is created when a fish first enters the sonar's beam. The fish moves in from farther away from the boat; as it swims under the boat, it is closer; and then it gets farther away. That movement displays as an arch on the screen. Adjusting the sensitivity and scroll speed will help display the arches most prominently.
Black-and-white displays render several shades of gray representing different signal strengths from objects. The more shades of gray, the better the detail, but the drawback is being able to distinguish between the subtle differences in black and white.
Many new models feature color displays.
"It used to be that color displays would not work in (direct) sunlight, but TFT (Thin Film Transistor) LCD screens are what the military uses, and on top of that there are types of films (on the screen) that are used so that now color screens are very viewable out in the sunlight. It is not an issue," Gibson said.
This is where the color display shines.
"There are things that make them superior, like showing a thermocline, showing the difference between grass on the bottom versus a hard bottom. They are much more brilliant in color," Gibson said. "A real hard bottom may be very red; a very soft bottom may be yellow. Compared with a black-and-white unit, all it's going to do is show you different shades of gray, which is very hard to determine."
Screen size also can be an important factor in considering displays.
On many units, displays can be split into various configurations like a standard image on one side and a zoomed-in image on the other.
When looking at transducers, consider frequencies and cone angles.
In most freshwater and saltwater applications, a frequency around 200 kilohertz (kHz) with a cone angle of 20 degrees gives a good amount of detail and works well in shallow water and at running speeds, all with less noise than other frequencies.
"The ones used most often by all competitors - by Humminbird, Raymarine, Lowrance and Garmin - is 200 kHz, 20 degrees. It is wide enough for a good amount of coverage but narrow enough to get great detail," Gibson said.
"For an angle of about 20 degrees, if you take your depth and divide it by 3, that's the circle you are looking at (on the bottom) _ if you are in 21-foot (depth), you're looking at a 7-foot circle," Cable said. "But you don't know when a fish pops up on the screen if that fish is on the left-hand side of that 7-foot circle or the right-hand side. That makes a difference sometimes, like when you are vertically jigging."
Some manufacturers have new units with multiple beams that can identify and display fish as on the left or right side of the boat.
"With one transducer, we shoot the 200 kHz, 20-degree beam for high detail, and right after that we shoot a 60-degree, 83 kHz beam," Gibson said. "That not only gives you the great detail but a lot of coverage.
"A 20-degree beam covers about one-third the depth. With the 60-degree beam, it's one to one, so in 10 feet of water it covers 10 feet across."
Expressed in peak-to-peak watts or RMS watts, power can affect performance.
"When you are running, there is a lot of acoustic noise, and when you go across a steep drop-off, more power will allow (the fish finder) to lock on the bottom and hold the bottom," Gibson said. "What you want to do is just make sure you are comparing apples to apples.
"Don't try to compare one fish finder with RMS and another one with peak-to-peak."
Many base models come with about 100 watts RMS, but individual anglers may want to consider 400 or 800 RMS watt units.
The one operation on a fish finder that every angler should learn to use well is sensitivity, Cable and Gibson agree.
"I don't do a lot of changing of the features on the depthfinder," Cable said. "The only thing I do change a little bit is the gain (sensitivity). If you turn up the gain too much, they really go nuts."
Gibson advised going easy on the changes, though, and paying attention to the conditions.
"I would say sensitivity is the most (important feature), but it does not have to be adjusted that much," he said. "With a soft, deep bottom, you may have to turn the sensitivity up; if you are in a real hard (bottom) shallow water, you might need to lower your sensitivity there."
"Depends on the water conditions. If you have a lot of murky water after a big heavy rain (and) there's a lot of debris in the water, you might need to lower your sensitivity, where if it is clear you might want to boost your sensitivity."
With so many features packed into today's fish finders, users can get overwhelmed.
The great thing is that a user can go fully automatic or all manual, so as on-the-water experience is gained, an angler can fine tune the settings and use more of the features.
As Cable said, "I want something simple."
Da Vinci would agree.