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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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