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Installing a fish finder on a kayak

Craig Vertigan
If you want to take fishing from your kayak to a new level you should consider installing a fish finder. All those boat owners out there using them can attest to the benefits of a fish finder. They help you locate structure, discover what depth you are fishing, the nature of the bottom structure, the temperature of the water and if you're lucky a few fish may show up too, giving you an obvious indication that you are in the right spot.

Generally though a fish finder isn't going to "find the fish for you". Instead you use it in your arsenal to work out where the fish are most likely to be. Use it along with reading the weather, using your polaroid sunglasses, watching for surface activity from birds and fish and looking for any structure above water which extends sub surface. In clear and shallow water you can see all you need to ascertain the bottom structure and depth. But those kind of conditions aren't always the case. It's then that a fish finder becomes an incredibly useful tool. You can use it to find any drop offs where the fish are likely to be holding, ready for an ambush attack. Sometimes the fish will be where the weed beds are. At other times the fish may be holding at a certain depth. In that situation it is a good idea to cover the water at different depths until you find the fish. If you start catching them consistently at a certain depth then that's a good indication that you've discovered "the zone'. An example of this technique is fishing soft plastics or hard body lures to trout in one of our many fine lakes. Use the wind and a sea anchor to set up a drift along the shore. If you don't hook up, then paddle back to where you started the first drift and paddle out a bit further and deeper. Continue this until you get some action. Then concentrate on that depth for a bit to see if it becomes consistent action.
Essentially a fish finder is made up of the fish finder unit with screen and menu buttons, a transducer which emits the sonar signal and receives the echo of the signal and a battery. Generally fish finders are sold without the battery unit so you'll need to buy the bits to make this yourself.
Most kayaks weren't built with fish finders in mind so you'll need to use a bit of ingenuity when installing one. The Ocean Kayak Prowler series are one of the few exceptions, since the new models all come with a specially designed forward scupper hole to install the transducer flush with the hull. Unfortunately for me I bought my Prowler before they started putting these fish finder-ready scupper holes on them. So I had to come up with another method for mounting the transducer.
The following is a run down of the method I used when installing my fish finder.

Choosing where to place the screen, transducer and battery pack
You need the display close enough to actually see the detail on the screen. But you'll also want it to be in a place that's out of the way enough to not impede your paddling or fishing, nor get in the way when landing fish. For me that ended up being the forward cup holder between my feet. I can lean forward to use the controls, see the details on the screen clearly and it's low enough not to get in the way of rods when fishing or when landing fish.
The transducer needs to go somewhere out of the way, where it can shoot a signal straight down, either through the hull or directly through the water via a special fish finder-scupper hole if you have one, or an outrigger setup that you'll need to construct. Also some people have mounted their transducer on the rudder successfully. The main thing with your transducer is that it is again out of the way of paddling and fishing and that it gets a direct signal straight down to the bottom without any air bubbles or cavities. The transducer must be horizontal with the water.
The battery pack can be placed anywhere out of the way. The fish finder will come with a plenty long enough cable to place the battery a fair way away from the unit. So the best spot for it is out of harms way inside a hatch in the hull of your kayak, where it will be nice and dry.

Mounting the transducer
Mounting the transducer is arguably the trickiest bit of the fish finder install. It is critical that you get the transducer as close to square as possible, so that it is pointing directly down to the bottom rather than off at an angle. And it is equally important that you install it in such a way that it is sitting in the water or shooting the signal through your hull without any air bubbles/cavities. Air is the enemy when installing the transducer. The sonar signal needs to be shooting through water or something as close to the density of water as possible. If it encounters air bubbles or cavities of air then you'll get a bad signal. Transducers are designed to work in water. The signal from the transducer is reflected right back on itself by air bubbles and cavities. So if you have air directly below the transducer, where the signal is at its strongest, then the reflected signal will be very strong and will therefore interfere with the weaker bottom, structure and fish return signals, making them difficult or impossible to see. The plastic of a kayak is a very good density for shooting a sonar signal through, with minimal loss of signal.
On a power boat there can be problems with air bubbles created by turbulence at high speeds at certain parts of the hull. This generally isn't going to be much of a problem with a kayak, unless you've got a motor attached to your kayak. But it may be best not to put it directly behind the mirage drive on a Hobie since this area is likely to produce some turbulence bubbles at times.

Some of the options kayakers use for mounting the transducer include:
Fish finder-ready scupper hole
For those of you lucky enough to have one of these fish finder-ready kayaks you can either buy the transducer kit to install your transducer snugly in the hole or use bungy cord, rope and PVC tubing to build your own kit for holding the transducer in place in the hole. The transducer should be fairly flush with bottom of the kayak so that it is always in the water but avoids getting hit by sand and rocks when you launch and land.
Custom built outriggers made from PVC or aluminium to hold the transducer directly in the water at the side of the boat. The drawback with this technique is that they can get in the way of paddling and landing fish on one side of your boat. Also they could get weeds tangled up in them. The positive is that it can be removable and it is shooting directly into the water, avoiding any issues with bubble interference and also giving a quicker more accurate temperature reading.
Permanent in-hull
Glue the transducer to the inside of the hull with a two part epoxy or similar good quality glue or a silicon sealant. Facing the transducer down and ensuring no air bubbles. This method has the advantage of keeping the transducer out of the way inside the kayak.
Semi-permanent in-hull.
Glue a piece of foam or rubber "cup" with a cut-out to hold the transducer snugly to the inside of the hull. The join needs to be fully water tight. The hole in the foam is then filled with water or Vaseline to form a bubble free liquid between the transducer and the hull.

What I did
I decided to go with an in-hull install for my transducer. My only other option was an outrigger to hold it in the water. But I saw too many negatives with this approach. I like to keep the cockpit area of my kayak as clutter free as possible. I decided that the permanent install was probably the most straight-forward, fool proof design.
To work out the best place to mount my transducer I used a small spirit level and with the aid of a torch placed it at different spots inside the hull in my forward hatch. I made sure I was as far back as possible to make sure it was an area that is always under water. Initially I considered putting it in the centre of the hull, but changed my mind when I found a fairly level bit to the side. I like to store spare rods in my centre hatch, and a transducer there would have been in the way. I found a level bit next to the left scupper hole near my foot peg.
I prepared the surface with some fine grain sandpaper and washed it with a rag and some methylated spirits. I then did the gluing in two stages. Firstly I cut a rectangle of foam a few centimetres large than the foot of the transducer. I then traced the foot of the transducer on the foam and cut that out to form a hole to hold the transducer snug. My first stage of gluing was to glue the foam support in place, making sure that no glue went onto the hull in the centre hole of the foam. I used Selleys All Clear, which many other kayakers have used successfully. The idea of the foam holder is to keep the transducer in place while the glue is curing, you don't want the weight of it moving it out of alignment. After a day when this was dry I then filled up the hole in the foam with a generous blob of All Clear and pushed the transducer in place. I then gaffer taped the transducer in place to give it some pressure while it was curing. You could also place a weight on top to hold it down. When dry I just wrapped the coils of spare cable around the scupper hole and used a cable-tie to hold it all in place.

Mounting the screen Unit
For the display unit I wanted it to be removable so I built a holder from some PVC pipe and an end cap. This fits into my forward cup holder perfectly. I made sure it was as low as possible so that it was out of the way, yet still readable. For a permanent install you could bolt it to the hull of the kayak and use some sealant in the bolt holes to and a rubber grommet for the cable to stop any leaks into the hull from wave wash.

Building a battery pack
The fish finder needs a 12 volt battery. The easiest way to build a cheap rechargeable battery is to get a battery holder for 8 AAs. This will provide enough voltage to run the fish finder for a couple of days of fishing and the batteries can be pulled out and charged with a cheap charger. Then install an inline fuse on the red wire and join the wires from the battery holder to the cable provided with the fish finder, making sure to go red to red and black to black. Before soldering it to the cable I put the battery and inline fuse into a clip-lock plastic container and pushed the cable in through a small hole I had drilled. Some foam padding in the container stops the battery from moving about.

Fitting it all together and going fishing
My fish finder unit and battery are removable units. So when I want to use them the display goes in the cup holder, the battery goes in the forward hatch next to the transducer and the cables from the battery and transducer go under the rubber gasket seal on the bottom of my hatch to the back of the display unit. The first time I used it I discovered the mysteries of the bottom makeup of New Town Bay on the Derwent and managed to find a shallow rocky platform next to a deep drop off which held flathead, a school of passing salmon and some blue nosed bream.

Craig Vertigan