Pop quiz: which is better, a through-hull, transom-mount, or shoot-through transducer? Answer: it's hard to know which is the best bet. Oh, sure, lots of people will give their opinion. But ask for the cold, hard facts, and most experts draw a blank.
To work out a tested answer, I rigged a boat with all three. My platform: An Albemarle 268 with a 1,000-watt Simrad CA44 fishfinder/chartplotter. The boat was built with a Simrad through-hull, and I added an Airmar P-66 transom-mount transducer to the stern. My shoot-through was also an Airmar, an oil-filled P-79. Airmar produces more than three-quarters of the transducers on the market, after all, and provides the transducers sold with the vast majority of the electronics units available today.
LENNY RUDOWThe author mounted all three transducers – a through hull, a transom mount, and a shoot-through – on his test boat, an Albemarle 268. The wires from each terminated in a quick-release plug, so that I could switch views quickly. Once the Albemarle was rigged, I pointed the bow toward an artificial reef 12 miles off the coast where the bottom is littered with decommissioned M-60 battle tanks and trucks.
HOW IT WORKS
Transducers are the hardware that allows fish finders and depth sounders to "see" below the water, using sound to do the job. The easiest way to think of a transducer in operation is to imagine it as both a speaker, and a microphone. The speaker shouts, then the microphone listens. To shout, the transducer produces an acoustic wave, the famous "ping" you've heard in all those submarine movies. Lean down next to a transducer while it's active some time, and you'll hear the pings, too, but they really sound more like clicks above the waves.
These pings travel through the water at about 4,800 feet per second. They have specific wave forms. Long, penetrating waves with lower detail are commonly sent out at 50-kHz, also called low frequency. Short, shallower-penetration waves with higher detail are commonly sent out at 200-kHz, and are called high frequency. This is why you'll get better detail on your fishfinder with high frequency, but you'll also lose the bottom if it's more then a few hundred feet deep. Low frequency will take you far deeper, even thousands of feet in some cases, but with less detail.
When the wave form bounces off anything with a different density than water, the transducer "feels" the difference in the wave bounced back through the water, and shoots off a signal to the fishfinder's processor. The processor then translates the anomaly into a picture.
AIRMARThrough-hull transducers do, unfortunately, require you to cut a hole in your boat. So, how exactly does the transducer "feel" the difference? Inside its housing is a crystal, made of either lead zarconate or barium titanate. The transducer's frequency is dictated by the crystal's shape. The crystal is soldered to wires, which lead to the transducer's cable. Much like a telephone wire, the waves the crystal feels are transmitted to the processor.
THROUGH-HULL
On location, I found a nice return on the bottom with the through hull transducer. The through hull is hard-mounted in the bottom of the boat, and is the heaviest, largest transducer. They have a reputation for performing the best, but they also have some significant disadvantages.
For example, you have to drill a big hole into the bottom of your boat. Also, through hulls are sometimes large and heavy enough to create lots of drag and affect a boat's running attitude. They can knock an MPH or two off top-end, and create turbulent water that can cause ventilation if it hits the propeller.
With the through hull plugged into the Simrad, I made slow passes over the battle tanks and trucks laying 65 feet below the boat. I can see the body of a tank displayed in bright red on the fishfinder screen, and amazingly, even get a glimpse of something long and thin sticking off of it – the cannon turret, maybe? Fish are clearly visible, clustered just above and below it.
