Do-It-Yourself
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How To Install A Fishfinder

By Lenny Rudow
Published: June/July 2012

Take on this simple electronics installation, and you’ll see what lies beneath your boat.

Whether you're a diehard angler, a casual fisherman, or a weekend cruiser, you probably have or would like to have a fishfinder onboard, so installing or replacing a fishfinder is a job you’ll probably tackle sooner or later. Lucky for us, when it comes to binnacle-mounted units that use transom-mounted transducers — the most common fishfinder installations on the water — this is an easy DIY job you can handle. All you need are a few tools and the know-how, which you'll find right here on these pages.

Illustration of Fishfinder installation

Step I: Installing The Binnacle Mount

Before you begin this mounting job, you'll need to find the best location. Try to keep the fishfinder as close as possible to center on the helm, which provides the best viewing angle. The unit should be between waist- and shoulder-high; stay away from overhead mounting locations such as in an electronics box or hanging down from a hard-top, which will force you to crane your head back when using the unit and may lead to neck strain. Also, make sure the location allows you to tilt and/or turn the unit without hitting the windshield, throttle, compass, or other affixed items.

Inspect the helm station from the back or underside, to ensure the area underneath is clear of obstructions, and has sufficient room for protruding bolts and wires. This is also a good time to locate the power bus and/or fuse block, and make sure you have terminals available for use; ABYC specs allow for up to four terminals per stud. Once you've chosen the location, either use the binnacle template included with the fishfinder, or use the binnacle itself as a template, to mark the location of the mounting holes with a pencil. Drill out the mounting holes with your power drill, running it at full speed at all times (a slowly turning drill bit is more likely to catch the gelcoat's edges, causing it to splinter and chip).

Now you need an entry/exit point for the unit's wires. Before you drill one out, inspect the console to make sure there isn't already a hole from a prior gear installation, which you can re-use — the fewer holes in your helm, the better. Assuming you don't get off so easy, you will need to drill a hole directly behind the binnacle mount. Make sure it's large enough to accept all of the unit's wires, and before you drill, put the binnacle and fishfinder temporarily in place to ensure there's enough clearance between the mount and the hole for the wires to pass through, without making any sharp bends. Otherwise, the wires might rub against the edge of the hole and chafing could become a problem down the road.

With all holes drilled, run a bead of silicon sealant around the base of the binnacle mount and around each hole. Put the binnacle in place, and before running the bolts through, give each a dab of silicon on their ends. Now secure the bolts to the helm with Nylock aircraft-style locking nuts.

Photo of positioning a fishfinder
First things first: Find the best location for your fishfinder.

Step II: Running The Power Leads

Depending on the length of the supplied wires, you may or may not need to lengthen or shorten the power leads. If you need to lengthen them, be sure to stick with the proper color-coding and tinned-copper wire (boat cable) of the manufacturer's recommended gauge. Wire-to-wire connections should be made with crimped barrel connectors — never solder — and should be protected by heat-shrink tubing. And whatever you do, don't cut out the manufacturer's included in-line fuse — get rid of it and you might fry the unit, as well as invalidating the warranty.

Push the power leads down through the exit hole you drilled, until the plug has enough freedom to reach the unit without extra wiring exposed outside of the helm. Then run the wire to your helm's fuse block. Secure the wires every 18 inches with tie-wraps or cushioned clamps, while making sure to minimize droops and slack. If your fuse block has male spade terminals, crimp female spade connectors to the ends of the power leads and protect the connections with heat-shrink tubing. If it has screw-type terminals, use ring connectors of the appropriate size. Leave the terminal ends disconnected, while you complete the installation.

Step III: Mounting The Transducer

Finding the perfect location is a must before you begin the actual installation. Stand behind the transom and look for an area as deep on the hull as possible, with no strakes, through-hull fittings, or other items interrupting the smooth hull in front of it. Anything that disturbs water flow will create turbulence, which degrades the fishfinder's performance.

When you've found the best spot, hold the transducer bracket against the transom and adjust its position until it holds the face of the transducer perfectly horizontal and about 1/8-inch below the running surface of the hull. Using a pencil, mark the location of the bracket’s mounting holes. Before you actually mount the transducer, however, now's the time to run the transducer's wire to the dash. (Do so after the mounting procedure, and you might ruin the sealant's grip by accidentally jerking on the wire). If your boat had an old fishfinder with a transom mount transducer, cut the transducer off the end and use the old wire to draw the new one through the wiring chase. If not, you'll have to start from scratch by using a wire fish to pull the new transducer wire and plug through.

Once you've pulled all of the wire to the helm and routed it up through the hole you made, coil any extra line inside the helm, and secure it with tie-wraps or cushioned clamps. WARNING: Never cut the transducer wire down to size; the new connections you’ll have to make at the plug can create additional resistance in the wires, and the fishfinder's performance will suffer. SECOND WARNING: Don't bundle the transducer wire with the engine's wiring harness or VHF cable; they can cause electrical interference that will degrade the unit's performance.

Using the markings you made earlier on the transom, drill pilot holes for the bracket's mounting screws with your power drill. Then use 3M 5200 adhesive/sealant to liberally coat the mounting screws and the holes in the transom. While holding the bracket in place, screw in the mount. Use lots of that adhesive/sealant. The last thing you want are holes in your transom that let water soak through.

Photo of catching a fish
If you follow these steps, the end result should be more fish caught.

Step IV: Completing The Job

Take a moment to protect the wires inside your helm by sealing off the wiring hole you made behind the binnacle mount. Use a grommet to seal it if possible (silicon sealant works, if you can't get a grommet that fits), then cover it with a clamshell vent; simply hold the clamshell in place over the hole, mark the spots you'll need to drill, and screw it down. Now you're ready to complete your final wiring connections. After making sure the battery is switched off, connect the power leads, then plug the power and transducer wires into the back of the fishfinder.

Now, hold on a sec. Before you splash the boat for a test run, make sure the 5200 you used to seal the transducer mounting holes has had plenty of time to dry. In some conditions, this can take up to a full week, so don't rush it. You need to be 100-percent sure water intrusion won't take place.

When you first launch the boat and idle away from the dock, the fishfinder will almost certainly work fine. But problems often arise with transom-mounted transducers when you hit the throttle and get onto plane. If your unit loses the reading entirely, the transducer is probably a bit too high (in some cases, it may ride completely out of the water). If the bottom reading is intermittent, chances are it's just a hair too high, or it may be low enough that it's creating turbulence. Luckily, virtually all transducer brackets allow some room for adjustment. Try lowering or raising the transducer in 1/8-inch increments, until you find the best height. But remember that few transom mounts work well at very high speeds. If you can reach 30 mph or more, and still maintain a solid reading, you've done a great job.

Everything's working perfectly? Excellent job. Now trade in those tools for a rod and some bait, and find out how that new fishfinder will help you fill the cooler. And when you're done, get a waterproof cover to protect that new unit. Most covers provided by the manufacturer only protect the LCD screen, but a cover that slides over the entire fishfinder will do a better job.End of story marker



 


Tech Support

Degree Of Difficulty:
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Tools:
  • Phillips-head screwdriver
  • Power drill and bit(s)
  • Wire crimper/stripper
  • Wire fish
  • Fishing rods (for testing your handiwork, after the installation is done)
Materials
 (with West Marine pricing):
  • 3M 5200 Adhesive/sealant $14
  • Adhesive-lined heat-shrink tubing $5
  • Cable ties $3
  • Clamshell fitting $3
  • Mounting hardware (usually included with fishfinder)
  • Silicon sealant $10
  • Crimp connectors $3
  • Terminal connectors $4
Project Cost
 (based on West Marine pricing):
$42
Approximate Yard Time/Cost

This is a straightforward job and shouldn’t take a pro more than an hour or so. The national average cost for a marine electronics installer runs about $80 an hour but many have minimums, so you can plan on saving around $100 by doing the work yourself.