Taking the Pulse
You can invest all you want in fishfinders and sonar, but it’s the transducer that can make all the difference.
And then only if it’s installed correctly.
The transducer is the heart of any sonar system since its pulses provide the stream of information on which the system depends. If the transducer fails, the display flatlines.
Transducer failure, though, is a rare occurence; a more common problem is poor performance, caused by the sonar pulses being blocked or weakened, or having the faint whispers of the returning echoes drowned out by other noise.
CHIRP sonar is less susceptible to these problems, and clever processing of echoes and sonar images can mask their effects, but the key to good sonar performance is to avoid problems altogether. And that starts even before you buy the transducer—at the planning stage, when you need to decide how and where the transducer will be installed.
The most serious threat to good transducer performance comes from, of all things, bubbles.
Sonar depends on sound waves—alternating zones of high and low pressure. When a sonar wave encounters a bubble, the high pressure part of the wave squashes the bubble slightly, while the following low pressure expands it. In effect, the bubble absorbs part of the sound energy, like an airbag absorbing the impact of a car crash.
The same general principles apply to side-scan and forward-scan transducers as to any others: They rely on sound waves, which are easily blocked by bubbles or drowned out by noise. But their distinctively shaped transducers have important installation considerations of their own.
Forward-scan transducers generally fit into cylindrical housings that are very much the same as those used for some conventional through-hull transducers, but they protrude farther from the hull, so they create more turbulence of their own. They obviously need to be mounted vertically, or else their depth displays will be significantly distorted—showing an upward-sloping bottom as flat if the top of the transducer is leaning back. But the most important consideration is that they need an unobstructed view forward. It is absolutely essential that they are well clear of through-hull pipework and protruding fittings.
Side-scan sonars work by producing a fan-shaped beam that shoots out on each side of the boat. Again, accurate vertical alignment is crucial, as is a clear view out to each side. If necessary, especially on a deep-vee hull, it is worth considering the possibility of fitting two transducers—one on each side—to eliminate the blind spot caused by the boat’s keel getting in the way.
Unfortunately, bubbles are everywhere. Wind and waves trap air in the upper layers of the water. The effect is always worst near the surface, but in rough weather bubbles can be found all the way down to 30 feet or more. The movement of the boat creates more air bubbles, and traps them in the boundary layer of water that is dragged along under the boat. More bubbles are created in the turbulence behind anything that protrudes from the hull, including spray rails and log impellers, rudders, and shaft struts, and from the inlet and outlet pipes for cooling systems. Bow-thruster tunnels are particularly bad—second only to the propellers.
Most of these problem areas are pretty obvious, but an unexpected source of trouble can be the sonar transducer itself. High-powered transducers can produce such extreme pressure waves that the water in the low-pressure part of the wave vaporizes, creating cavitation bubbles just like the ones that cause “cavitation burn” on propeller blades. This is part of the reason why many high-powered transducers are mounted in tanks filled with oil or antifreeze inside the hull, rather than in direct contact with the water.
“I like a transducer that sits on a fairing block that gets it down, away from the hull to give it the best shot at getting below the turbulence where what we call the ‘boundary layer’ travels down the hull of the boat,” says Steve Bradburn, commercial fisheries product manager at Furuno USA. “It’s not just the bubbles but the disturbed water.”
Eliminating bubbles altogether is virtually impossible, but in a low-speed displacement boat, it’s fairly easy to minimize the problem by mounting the transducer as deep as possible, close to the centerline and fairly well forward. A good spot is usually about a third of the way back from where the bow breaks the water.
Being deep means that the water pressure is higher than at the surface, which tends to compress any bubbles dragged down from the surface, while being well forward keeps the transducer in a relatively smooth “laminar” part of the boundary layer, well away from any protrusions or skin fittings that would create bubbles in front of it.
“If you have a boat with a keel and you’re using a transducer with a fairing block, position it away from the keel a little bit,” Bradburn says. “What happens is that water gets pinched between the keel and the transducer fairing block and that creates turbulence that affects the transducer. Since the keel is probably lower than the transducer the turbulent water flows right over the face of the transducer.”
Still the position should be as low as possible in the hull for best performance. “Get the transducers as close to the centerline as is reasonable,” says Peter Braffitt, general manager of Gemeco Marine, direct seller of Airmar transducers. “It tends to be an area with clean water flow and gives you consistent performance in both port and starboard turns. Some placement can mean that you turn in one direction and get good performance, but if you turn the other way it starts losing bottom.”
Party Out Back
Most “entry level” sonars are supplied complete with transom-mounted “skimmer” transducers. Mounting a transducer on the transom breaks almost every rule about transducer installation. So why are transom installs so common?
It all comes down to simplicity, and therefore price. Screwing a bracket to the back of the boat and leading the cable up over the transom or through a small hole above the waterline is a much cheaper and easier do-it-yourself job than drilling a substantial hole in the bottom of the hull, with much less serious consequences if the job goes wrong, and for outboard and outdrive-powered boats the results can be surprisingly good.
But there are a few ways to help transom-mounted transducers, such as the Airmar TM185HW (shown), work better. First, check the direction the propeller rotates, and look for a location on the same side as the upward-moving propeller blades. For a single engine, that usually means on the starboard side of the transom and at least 3 inches clear of the propeller circle. For twin installations with outward-turning propellers, it usually means between the two engines.
Then look over the back of the boat while it is under way, to see where the water flow leaving the transom is smoothest, and where it includes a lot of bubbles. Go for one of the smooth areas, rather than a bubbly bit. Attach the bracket to the transom so that the transducer is just below the bottom of the transom, and parallel with the waterline (side to side), with a very slight nose-up attitude (up to 5 degrees).
Installing a transducer in a planing hull involves more compromises, because the transducer has to be located farther aft to avoid the turbulence and aeration where the hull meets the water’s surface, but still needs to be close to the centerline, out of line with spray rails and skin fittings and in front of the propellers and shafts.
Stepped hulls are another story altogether, because the whole point of a step is to encourage air to flow under the hull to reduce the wetted area. Steps are great for reducing skin friction, but a real pain when it comes to installing sonar! The best solution is usually to go for a position as far aft as possible on the first (forward) step.
As a general rule, transducers should be installed so that their faces are in direct contact with the water. “It’s location, location, location, just like real estate,” says Andrew Fairchild, OEM technical manager for Garmin. “Transducer performance is completely dependent on the condition of the water around its position. When the water is disrupted around the transducer your signal gets distorted and it doesn’t look clear—it appears that the equipment isn’t working even though it is.”
Tilting the forward edge of the transducer upward helps avoid the formation of bubbles as the transducer moves through the water and might—arguably—overcome the fact that in the time it takes for each sonar pulse to travel down to the seabed and back, the boat will have moved on. Don’t overdo it, though: A couple of degrees is enough.
There is little doubt that the traditional “through-hull” installation—particularly with a tilted-element transducer to ensure that the sonar beam is directed straight downwards, or with a fairing block to ensure that the transducer is vertical, and to smooth the flow of water around it—gives the best performance.
But there is an alternative: Mount the transducer inside the hull, and have it “shoot through” the hull skin. In-hull installation is simpler and quicker, and there is, of course, less risk of flooding. One downside is that it is only suitable for GRP hulls, and the other is that sonar performance is likely to suffer—particularly as fouling develops on the hull.
At its simplest, “in-hull” installation involves removing a transom-mount transducer from its bracket, and gluing it directly to the inside of the hull with a rigid epoxy adhesive that will transmit the sonar vibrations through to the hull itself and then out to the water.
Such a simple system works best for hulls with a deadrise of 10 degrees or less, but the most crucial point to check is that the hull is solid fiberglass. Any voids or foam or balsa core will absorb the sound waves instead of transmitting them. It’s still possible to fit an in-hull transducer to a sandwich hull, but the process involves cutting away the inner layer of GRP and the core material so as to glue the transducer directly to the outer skin.
Some purpose-made in-hull transducers are designed to be glued in, but a more sophisticated variation uses a tube or tank bonded to the hull, and filled with a liquid such as propylene glycol (antifreeze) which carries the sonar energy through to the hull. The beauty of this is that the mounting can be cut to ensure that it is vertical, while the use of antifreeze virtually eliminates the risk of cavitation caused by the sonar waves themselves.
“To me, that’s a last-resort installation,” Bradburn says. “I’m really biased just because there’s so much performance loss. Think of it this way: You’ve got a good stereo system and then you put the speakers in the closet. Can you still hear them? Yeah you can hear them. But can you hear them as well? No. There’s no way for even the best transducer in the world to overcome the loss.”
With crisp, strong pulses in a clear, uncluttered water column, you won’t have to use your sonar’s processing technology to try to clean up a muddy picture. You can use the device instead the way its designers intended—to enhance a clear picture to help find the fish that will keep your pulse going. If you look at it that way, you might even try claiming transducer installation on your health insurance!
This article originally appeared in the November 2015 issue of Power & Motoryacht magazine.