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The Fish Finder-equipped Canoe

By Joe Hyde, Lawrence, KS

Sooner or later, every canoeist and kayaker discovers what happens when a solid object or raised thick substance becomes attached to their boat hull's smooth surface.

I was paddling the Kansas River. It was the fall season. I steered my solo canoe into an eddy whose surface was covered with floating leaves and twigs. There was no sensation of impact like happens when you collide with a submerged branch, but upon exiting the eddy the canoe moved noticeably slower despite my paddle strokes maintaining the same effort and tempo.

Viewed from my paddling station amidships the canoe looked normal, but something was wrong. My ears began detecting a faint swirling noise coming from somewhere forward, a fluid sound. Curious about what was causing it, I employed braking strokes to slow my canoe and then back-paddled a few feet. What floated into view ahead was a clump of leaves and some twigs, material that had become wrapped around my canoe's bow stem during its pass through the eddy. Wasn't much of anything, really, but this small cluster of vegetation had significantly slowed my canoe's forward speed.

On another trip a similar thing happened. After getting into my canoe following a rest stop, once underway again my canoe kept turning left between paddle strokes, turning of its own accord? I stopped again, got out of my boat and dragged it ashore, tipped it over to inspect it and found, below the waterline on the left side, a small raised patch of hard muck clinging tenaciously to the hull. After I scraped it off and rinsed the spot clean, no more left turn problem.

Point being: canoes and kayaks are very sensitive to surface drag. Attach a raised object— anything, even something as seemingly insignificant as a leaf— and the hull's hydrodynamic capabilities are adversely affected. Once I understood this reality it took a few years before I let myself even consider using an electronic fish finder on my canoe. At issue were two technical problems I felt had no solutions.

First, a portable fish finder (with self-contained batteries) seemed the common-sense choice given my circumstances. But every portable model I looked at employed a suction cup mount to hold the ultrasonic transducer to the outside surface of a boat's hull.

An external suction cup mount works fine on a square-stern canoe. But my Wenonah Rendezvous solo canoe's hull is tapered at both ends, which means a suction cup mount attached anywhere on the hull constitutes a large raised object. When I'm underway at any speed the suction cup mount creates drag that disrupts my boat's normal handling characteristics. I didn't want that happening.

Second, there seemed no way for even a portable fish finder to be used as a hand-held device; at least, not for a solo canoeist like me. I'm the only person propelling the boat, so I need both hands on the paddle. And when I'm fishing, both hands are busy holding my fly rod and floating line.

How exactly, then, am I supposed to use a fish finder, portable or otherwise?

During my shopping research, I learned that a transducer (the device that generates the ultrasonic search beam) can be mounted permanently on any boat with a fiberglass hull. With epoxy resin you glue the transducer to the inside— repeat, inside— of the boat's hull. So long as the epoxy is free of air bubbles and the boat's fiberglass and gel coat are free of air pockets the transducer will shoot its ultrasonic beam directly through the boat's hull. It's as if the epoxy resin and the hull material are fluid.

THAT was interesting, to say the least. A "shoot through the hull" fish finder might work for me— my canoe has a fiberglass hull! I suddenly became very interested in buying a fish finder.

However, I was stymied by the idea of a permanently-mounted transducer held in place by epoxy resin. Suppose I want to sell the canoe but keep the transducer? Do I go at my canoe's thin-walled hull with a hammer and chisel, chipping through the epoxy to liberate the transducer? Sorry, I don't think so.

Conversely, suppose I keep the boat forever? An epoxy-mounted transducer creates a new set of problems due to the long signal wire that reaches from the transducer to the fish finder's display screen. This wire emerges from the transducer whole; the screw-together connector is located at the other end of the wire, on the back side of the display unit. On trips when I'm not using the fish finder, what do I do with that long wire?

I am ignorant in all matters involving electrical wiring and electronics. So unless I hired someone to install a disconnect device very near to the transducer, this long signal wire would also be a permanent fixture inside my canoe. Its presence would cause entanglement problems every time the canoe got used for non-fishing purposes.

Worse, during travel my canoe rides upside down strapped to my roof rack, which means the transducer wire might drop into the air flow and get ruined by repeated whacks against my vehicle, or against the canoe itself. And if that signal wire gets damaged the fish finder becomes worthless.

Only a hybrid approach could solve these problems. Use a portable fish finder, yes; but use it in a fashion not recommended by the manufacturer. Instead of suctioning it to the outside of my canoe, use it "shoot through the hull" style by temporarily holding the transducer in place with an inside-the-hull mount. "Temporarily hold" because holding it permanently means gluing it down with epoxy resin, which is unacceptable.

I would have to devise a transducer mount that sits inside my canoe without taking up space. It must be lightweight yet stout enough to hold the transducer firmly. It must let me put the transducer inside it prior to launching the canoe, then after I'm done fishing I pull the transducer out and re-stow it, and the signal wire, back inside the unit's plastic clamshell case.

Still, what about that troublesome air gap issue? Because whatever sort of mount I devise, it cannot have even the tiniest air gap beneath the transponder else the entire contraption is useless. At this point I recalled the information I'd read earlier, that a transducer properly installed will shoot its ultrasonic beam through not just the epoxy bonding resin but also the boat's fiberglass fabric and gel coat finish, as if all three of those components were themselves water.

As if they were water …

An idea suddenly hit me: I should construct a home-made "cup" that conforms to the shape of my canoe's interior hull curves. A flexible cup that will hold the transducer unit and can be glued anywhere in my boat's bilge. It must be a hollowed-out cup so that it holds the transducer, but it must be leak-proof because it will also be holding…actual water.

Taking a very large gamble, I went ahead and bought a portable fish finder then set about making a mount for its transducer. If I could build a cup mount that trapped even a tiny amount of water underneath the transducer, then direct water contact should let the ultrasonic beam shoot through my canoe's hull exactly like happens on large powerboats with transducers permanently mounted using epoxy glue.

Rummaging around through my boating and camping gear, I found what I was looking for— a scrap piece of Army surplus sleeping pad, closed cell foam. Just half an inch thick, a single layer of it was not "tall" enough to reach over the transducer's rounded shoulders and hold the device in place. No problem: I would make a foam cup mount using stacked pieces of foam.

It took three layers of the sleeping pad, and this actually helped me build the device. The base layer (the one glued to the canoe's interior) was more easily measured for the center cut-out that opened a hole for the ultrasonic beam to pass through. The middle and upper foam layers were, in turn, easily measured for the larger cut-outs that would eventually grasp the transducer's bullet-shaped length and width.

The top piece of foam needed a hole large enough to let the transducer be pushed down into the cup. But a death grip on the transducer was not required. Indeed, the happiest part of the project came from realizing that any sloppy cuts or nicks I carved in the center of the foam pieces would create gaps. Those gaps were critical to letting water seep past the shoulders of the transducer and pool up in the bottom of the cup, where the water would displace air and thereby enable the ultrasonic beam.

During the assembly process it was a simple matter to check the mount-to-transducer fit and make trim adjustments to the foam pieces. I then glued the individual pieces together sandwich fashion using Weldwood cement, following directions printed on the side of the can.

I gave the laminated mount an hour to cure then glued the mount to my canoe, up front just behind the bow flotation tank. Some prefer a transducer mounted aft. Not me; I wanted the fish finder to look into the water under the bow, not under the stern into water I'd already passed over.

Pictured below is the transducer mount I made from three small pieces of Army surplus foam sleeping pad. Next to the mount lies the black transducer unit prior to insertion.

Below is the laminated foam transducer mount, shown in close-up.

And again, this time with the transducer wedged inside.

With the transducer in place, next you must put some water inside the mount. I normally do this by dunking the blade of my canoe paddle into the lake, pulling it out then quickly positioning it above the foam cup. Water dripping off the paddle blade falls onto the transducer, trickles around the transducer and fills the foam mount.

Very little water is needed as the transducer occupies nearly all the mount's internal void. Depending on how steady I hold the canoe paddle, two or three dunks into the lake are usually all that's needed to accomplish this task. Once the transducer is surrounded by this small amount of cupped water the fish finder is ready for action.

On your drive to the lake, do you ever stop for coffee and by the time you reach the lake the coffee cup is empty? Well, a person could use that humble vessel to dip water from the lake, fill the mount halfway and then insert the transducer. This faster method is apparently too logical; in ten years of using my system I've done it that way maybe once.

Now you're on the lake happily using your fish finder. It doesn't hurt to periodically drip or pour a little water onto the transducer. If there's been any evaporation loss or a leak, topping off the mount keeps the shop open for business, so to speak. In my canoe I kneel at the center so it's easy to lean forward, dunk my paddle in the lake and touch up the transducer mount using paddle drip. Indeed, this is so easy to do that the way I normally fill my mount is to wait until after I've launched my canoe. Only then will I lean forward and fill the transducer mount using paddle drip. Filling the mount with water doesn't have to be done while standing on dry land, is what I'm saying.

In any event, below are more photos of my "shoot through the hull" fish finder setup. With this transducer mount my canoe suffers absolutely no surface drag; the entire fish finder unit is contained and operates inside the boat. When I'm done fishing the whole works come out, with the exception of the nearly weightless foam cup transducer mount.

A "shoot through the hull" fish finder probably won't work on any boat made of sandwich composite that incorporates a layer of honeycomb or closed-cell foam. Just my guess here, but such layers do contain trapped air pockets. This forms an "air gap" preventing the transponder's ultrasonic beam from passing through the hull.

I've never tried my foam cup mount on an aluminum canoe. Doubtful the ultrasonic beam can pass through aluminum, but I could be wrong. However, no law says a person can't drill a small hole through the aluminum hull, install some kind of homemade Lexan or Plexiglas "window" through which the ultrasonic beam can be shot, and combine that trick with the foam cup transducer mount glued above that sonar window.

Many canoe manufacturers build high-end boats using a construction method called "vacuum bagging." In the factory as the hull is being laid up in its mould, a layer of foam sheeting (intended for stiffening the hull) is positioned in the bilge area and sandwiched between sheets of resin-impregnated Kevlar fabric. Normally this foam sheet does not extend the full length of the bilge. So my transducer mount can be used on these canoes provided a mounting spot of sufficient size is available away from the beam-blocking foam core section.

Speaking of Kevlar, I glued one of my transducer mounts onto a Sawyer Autumn Mist solo canoe, a boat whose hull is made entirely of Kevlar fabric. The fish finder works perfectly. Nowadays, woven mats of carbon-graphite fiber are being used in canoe and kayak construction. I would guess this type of matting causes no beam interference or blockage, but I really don't know.

Removing this foam transducer mount is easy. Weldwood cement is not an epoxy, so just slip a flexible putty knife under the mount and pop it off your bilge, then remove the glue residue using acetone.

When launching your boat and also while underway, take care to keep your outer hull surface— at the spot opposite the transducer?— clean and free of mud, algae, wrapped vegetation, etc. If these solid substances get in the path of the transducer beam they will degrade the ultrasonic signal, possibly blocking it altogether.

~ Joe

About Joe:

From Douglas County, Kansas, Joe is a former municipal and federal police officer, now retired. In addition to fishing, he hunts upland birds and waterfowl, and for the last 15 years has pursued the sport of solo canoeing. On the nearby Kansas River he has now logged nearly 5,000 river miles while doing some 400 wilderness style canoe camping trips. A musician/singer/songwriter as well, Joe recently retired from the U.S. General Services Adminstration.

Joe at one time was a freelance photojournalist who wrote the Sunday Outdoors column for his city newspaper. Outdoor sports, writing and music have never earned him any money, but remain priceless activities essential to surviving the former 'day job.'

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