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.

These are likely sea bass, and I catch a few of them to prove that the transducer is telling the truth. I mark the spot by hitting the MOB button on the chartplotter, then drop a plastic milk jug rigged with a line and heavy sinker, so that I also have a visual target to work from as I make more passes over the tank.

TRANSOM MOUNT

Then it's time to pull the plug. I switch over to the transom mount. Transom mounts are loved for their easy installation. Anyone with a basic ability to use a drill and screwdriver can do it (use plenty of 3M 5200 sealant, since the screw holes are below the waterline) and there's no danger of sinking your boat if you botch the job. Like the through-hull, the P-66 transom mount I'm testing has a dual-frequency, 50/200-kHz crystal inside a plastic casing. It also has a hydrodynamic nose cone to reduce turbulence, and can be used with a wide variety of different units from different manufacturers.CREDIT TKA dual-frequency 1kW transducer; 19 degrees at 50kHz and 6 degrees at 200kHz.

The tank now appears to be a mere mass – a pile of something, made up of red and green blocky images with a few yellow blobs nearby. If the image with the through-hull was a perfect 10, the transducer mount scores an imperfect eight.

Time to switch wires again. I plug in the P-79 shoot-through, which is also a dual 50/200 frequency unit. Unlike the other transducers, this one is designed to mount inside the hull, and I bonded it to the Albemarle then filled its base with mineral oil before twisting the offset ring to account for the hull's 21-degrees of deadrise.

Obviously, there are some huge advantages to mounting your transducer inside the boat: hydrodynamics, ease of mounting, and safety all get a boost. But you can't use this form of transducer if your boat has a cored bottom. It must be solid glass, and glass bottoms laid up carelessly, with air pockets and gaps, won't work either. Lucky for me, the Albemarle has a solid glass bottom that's been built properly.

Just one problem: as I make the next pass the tank is smaller and flattened out into a fuzzy lump. The colors become even less distinguished, and what were separate targets indicating fish have become part of the whole mass. It's a score of five or six, at best. The difference between these pictures is as dramatic as the difference between first-generation digital cameras and today's tack-sharp, 10 mega-pixel professional models.

Okay: so now we know how performance relates to transducer type in shallow water, when looking at hard structure and fish. Plus, there are plenty of sea bass in the cooler at this point. But what will happen when I go deep? Will I experience the same results at 650 feet as I did at 65 feet? And what about when the boat is off the edge of the continental shelf, in water that's more than 1,000 feet deep? To find out, I point the bow for Baltimore Canyon, 65 miles off the coast of the Delmarva Peninsula.

DEEP WATER TEST

Two hours later, I switch the unit over to 50-kHz (remember, low wavelengths are for deep penetration), plug in the through-hull and take a few passes over the drop. In 723 feet, I see two red dots near the bottom, and send a few squid spiraling into the depths on deep-drop rigs weighted with three pound sash weights. It's not too long before a crewmate cranks up a 10-pound tilefish. Yup, those marks were real. Heading east, the unit holds the bottom with a solid reading to about 1,800 feet. After that it's a broken reading down to 2,010 feet, when the screen finally goes blank.

The transom mount does pretty well, but can't match the through-hull's performance. It holds bottom down to between 900 and 1,000 feet, but is faint and broken beyond the 800-foot mark. The slower I go, the better the performance, and the transom-mount works best when I shift into neutral and drift. This reinforces one of the things you've probably heard about transom mounts: You have to get the placement just right. Turbulence effects performance whenever the boat's moving. My placement is good, and I get a solid reading in water under 800-feet even when I'm running at cruising speed of 25 knots, but turbulence still has some effect.

Next I'll see if the shoot-through works any better in low frequency mode than it did in high frequency. I shut down the unit, switch plugs one more time, and"¦nothing. In water over 600-feet it doesn't get a reading at all. If I move shallow to 600, wait for it to get a reading, then putt east, it holds a faint reading to anywhere between 660 feet and 800 feet before going caput. It never shows a separate target, and really isn't much use at this depth.FURUNOThe view on the left shows detail levels at 200-kHz, while the one on the right shows the same exact fish at 50-kHz, on this dual-frequency, split-screen unit.

There is one good thing I noticed about the shoot-through, though. Unlike the transom mount, and to a lesser degree the through-hull, the performance of the shoot-through is completely unaffected by the boat's motion. Since it does its pinging and its listening from inside the hull, this makes perfect sense.

POWER RATINGS

You may have noticed that one very important factor in depth penetration hasn't been touched on: wattage. Wattage is actually not the most important factor, because a unit's probing and listening power is effected by the transducer every bit as much as it is by actual wattage. In fact, doubling the transducer crystal's diameter has the same effect as quadrupling the output power. This happens because the larger the transducer, the more focused its beam – think of a flashlight set to spotlight, instead of flood. So, a machine pushing 200 watts through an eight-inch transducer will see just as deep as a fishfinder pushing 800 watts through a two-inch transducer.

Yeah, yeah, I know: you don't believe it. Neither did I. That's why I brought Lowrance big-wig Ron Webber with me on this expedition. He promised to prove it. Now that the transducers have been thoroughly tested, Ron hooks up a Lowrance LC-X 104 C to his laptop computer. He has the codes necessary to manipulate the unit's power output from 50 watts to 1,000 watts, on the spot.

Next, we hang a massive, grapefruit-sized 200/50-kHz through-hull transducer, on a temporary bracket mount, over the side of the boat. In 1,000 feet the LC-X 104 C had no problem touching bottom with 1,000 watts. As he backed the power down, the bottom reading lost color definition and got a little spotty. But even at 50 watts of output, the reading held. Just to prove his point, Webber then hooked up a $200 cheapo 100-watt unit. With the standard (one-inch crystal) transom-mount transducer, of course, we saw nothing. But when he hooked it up to the huge transducer hanging over the side, it got a solid bottom reading in 1000 feet.

ASSESSMENTlENNY RUDOWYup, there are sea bass down there! The through-hull transducer tells no lies.

Through-hull transducers do, according to traditional theory, provide the best performance – and this testing proved it to be true. Again holding to traditional theory, the transom mounted transducer gave us the second-best view and the shoot through provided the poorest view.

But also remember that transducer size plays a huge role in how effective your unit will be on the water. Of course, if you buy a fishfinder package from a big-box store or a catalog, you'll be stuck with the transducer the manufacturer supplies with the unit. If you want your rig to be as capable as possible, then make your purchase from a marine electronics specialist who can source the transducer separately. You can choose the type that best fits your application, and get the size you feel you need, instead of taking what came out of a box.

Back at the dock I don't even need to look at my notepad. The test results are that obvious. Transducer choice clearly dictates how well your unit will perform. The through-hull provides the best performance, in all depths and situations. A transom mount comes in second, providing good shallow-water definition and acceptable deep-water performance. The shoot-through is in last place. These will be useful as depth gauges but not as fishfinders used by anglers who want to actually catch fish.

Yes, all of these types have advantages and drawbacks. But if you want the ultimate under-water view, the evidence says it's time to cut a hole in your boat.

Lenny Rudow was senior technical editor for Boating Magazine for more than 10 years, and is currently the electronics editor for Marlin and GoBoating magazines.