How To Install A Fishfinder

Save a bundle and get the latest tech on your boat, by installing a new fishfinder.

If you’ve done your research and figured out what the best fishfinder for your needs is, all you have to do now is install it on your boat. Piece of cake, right? Actually, in the case of units with transom-mounted transducers, it is. Yet every spring I get a half-dozen phone calls from boat-owning friends and family members, where the conversation starts with the words “can you help me.” Many boaters have a severe aversion to do-it-yourselfing any project that includes drilling holes in the fiberglass, and especially the transom and helm station. Naturally, if you have a buddy who’s familiar with this kind of work – and amongst the Rudow clan that would be me – you call him in.

fishfinder install
Do you want a new fishfinder at the helm? Of course you do!

Unfortunately, the help-me fishfinder mounting jobs tend to crop up right when the bite gets hot. And invariably, the proud new owner needs to have the unit in place and ready to fish ASAP. Hey – I can relate. But what makes the situation frustrating is that installing a new fishfinder really is an incredibly easy job, one any semi-competent mariner can do without assistance. (Again, we’re talking about straight-up units with transom mount transducers, not networked systems and through-hulls). So long as you know how, that is. So take a deep breath, charge up the drill, and dive in.

Wired Up

Regardless of what type of fishfinder you’ve purchased, there will be two wiring jobs involved: The power lines, and the transducer cable. The power wire’s the easier of the two to handle: just tap into the fuse box or breaker bar behind the dash, give yourself a positive lead (red) and a negative lead (black,) and crimp-connect them to the power line that came with the unit. Hold on a sec – this is a boat, remember? You can’t get away with simple crimp connections, which will corrode and/or vibrate apart. Finish this part of the job properly by putting a heat-shrink tube around the connection, and making the seal watertight. Ready to go the extra mile? Now give the connections a thorough gooping of liquid electric tape, which will give you an added water-barrier and peace of mind.

fishfinder install
Getting power and transducer connections behind the helm is obviously an important part of the job.

The transducer line is usually more of a fight because you’ll have to fish the wire and connector from the transom of the boat to the helm station. Virtually every boat has a rigging tube that holds wires and cables running to the motor and controls, and nine times out of 10 it’ll be running under the starboard-side gunwale or under the deck. You also need to identify the transom access point, before beginning to fish the wire. (Remember, you’ll have to start at the transom and fish the plug-end forward.) On most boats, this is a black rubber or plastic boot, where you see the motor cables running out to the transom. Remove the screws securing this boot, and start the job by pushing the plug end of the transducer wire through. Next comes the tough part: fishing it to the dash. This will be a bit easier if there’s already a string left in the rigging tube by a kind builder or from a previous job; in that case, you can use it to pull the transducer cable through. Otherwise, you’ll need to start by fishing a piece of cloths line or heavy string through first, to use as a pull-chord. Warning: Don’t use braid fishing line for this job. It’ll cut into the wire’s protective casing when you apply pressure and can even cut the plug end off (yup, been there/done that).

fishfinder install
If this is what your rigging tube looks like, you’re in for a rather tough job. Lube the cable, so it moves through the tube easier.

If your rigging tube is on the cramped side, this job can be quite tough. The plug and/or the wire may grab, catch, and generally not cooperate. Don’t force it or you could damage the wire, take your time, and lubricate the plug and wire with dish soap, which can make a huge difference. When forward progress stops, pull the wire back out a few inches and see if you can jiggle it past the obstruction. Whatever you do, DO NOT cut the plug off the end of the wire and re-attach it later. Transducer connections are finicky, and you’ll probably end up having to buy a new transducer.

Mounting The Head Unit

Okay, you’ve got the wire behind the dash? What comes next depends on if you’re binnacle (top) mounting the unit, or flush-mounting it in the dash. Flush mounting, of course, means you’ll be cutting a big, gnarly hole in the dash. Do not (do not!! DO NOT!!) do this without stenciling your lines onto the dash with a template, and checking behind the dash between three and 5,000 times to make sure you won’t cut something critical like gauge wires or a hydraulic line when the saw blade starts flying around back there (Yup, been there/done that, too). Then drill a pilot hole in one corner and you’ll have a place to insert your jigsaw blade.

When the hole’s cut and mounting bolt holes are drilled out (again, via the stencil), check to make sure everything fits by dropping the unit in place. Then take it back out, seal the deal with a bead of silicon, bolt the unit in place, and run another bead of silicone around the edges.

fishfinder install
Check, check, and check again to make sure you won’t inadvertently saw through anything back there!

If the unit’s binnacle-mounted, you’ll need to drill a large hole in the top of the dash to bring your wires through. Don’t try to drill into the gel coat slowly, because this just exacerbates cracking. Fire the drill up to full-tilt, hold it firmly, and apply steady, light pressure. Once you’ve drilled your hole and pulled the wires through, cap it off with a clamshell fitting. This is where most people stop, but you and I don’t want water dripping down that hole, do we? Cap the underside of the dash hole with a piece of duct tape (to prevent drips) then plug it up with a generous helping of silicone.

Before drilling the holes for the binnacle mount, be sure to assemble the mount and head unit and hold them in position. Otherwise, you may mount it too close to the windshield or a support strut, and discover that once it’s in place you don’t get the full range of tilt and turn motions you want. Also, make sure you mount the binnacle facing forward. I know this sounds silly, but some have the openings on the rear of the mount and others have it on the front. Make sure you’re clear on which way your unit fits, before going to town. You’ve got it in the perfect position? Good. Now drill your holes and through-bolt the binnacle down with nylock locking bolts. Naturally, you’ll want to give them a dab of silicone or similar caulk to seal the holes from water intrusion.

fishfinder install
Notice how the windshield and throttle could restrict the fishfinder’s adjustments in its mount if it were mounted too far forward or too far aft.

Transom Mounting The Transducer Bracket

Now it’s time to mount the transducer to your transom. You’d rather have a through-hull transducer because you heard this works the best? Yes, they do, but drilling a hole into the bottom of the boat is where I draw the line as a do-it-yourselfer. Call in a pro, for that job. What about a shoot-through transducer? Maybe you’ve heard these are easier to deal with because you simply epoxy them to the hull? Also true. But their performance is often lackluster. If you want to spot fish on your unit with a decent level of sensitivity, junk the idea.

The Golden Rule: thoroughly seal each and every hole you drill in the transom with a healthy heap of 3M 5200. You can make an argument for silicone at the dash since it’s much easier to remove down the line, but silicone doesn’t cut it beneath the waterline. And give the stuff a full week to cure before splashing the boat. It may be hard to hold back after a couple of days, but 5200 is a slow cure and won’t be at 100-percent strength once you dig beneath the surface.

One tip for techno-geeks who need to have the latest and greatest fishfinder every few years and foresee changing units in the future: cut a piece of Starboard to match the angle of the bottom of your boat, and mount it against the transom. Then, mount the transducer to the Starboard. When it’s time to swap out for a new one, you’ll be putting the additional holes in the Starboard instead of filling and re-drilling fiberglass.

Locating the best spot for a transom-mount can be tricky. You need to stay away from areas that have through-hulls, strakes or any other irregularity in the hull bottom forward of the transom, which will create turbulence and ruin the unit’s performance. You also need a spot as close as possible to the bottom of the V in the hull, so the transducer doesn’t ride up and out of the water when you’re on plane. Usually, this magic spot is about two-thirds of the way from the chine to the bottom of the V, between the V and the first or second running strake.

In most cases, you’ll want the bottom of the transducer flush with or just a hair beneath the hull bottom, though some side-finders need to go a bit lower. Follow the manufacturer’s recommendation. But recognize that it may still take some tweaking to get the best performance. So be sure you drill your holes with the mounting slots as far up as possible. Then, if you find you lose bottom when running, you can loosen the screws and drop the transducer down another fraction of an inch. In fact, you should expect to splash the boat and test it a couple of times and make a couple of adjustments, before finding the sweet spot.

fishfinder install
Use a straight edge to make sure the transducer’s level with the bottom of the boat and positioned as per the manufacturer’s recommendations.

Okay: the transducer’s in place, the wire goes to the helm, and the unit is mounted and has power. Think we’re done? Think again, there’s one more thing to do, and that’s secure all those wires so they won’t shake and jiggle as you run your boat. All fishfinders come with a couple of clamps you can add to the transom to hold down the transducer wire until it enters the boat. After mounting these, take a handful of tie-wraps and secure the wires where they enter the rigging tube, and under the helm at least every two feet. If you don’t, vibrations and gravity will conspire to pull at them until they chafe against fiberglass or stress your connections.

fishfinder install
With that new fishfinder at the helm, you’re ready to rock and roll.

There. That was fairly easy, wasn’t it? I’m glad you think so, too. Just remember that next spring, before you pick up the phone and start dialing my number.

Get more great boating information and tips from Lenny Rudow on BD.

Lenny Rudow …has been a writer and editor in the marine field for over two decades, and has authored seven books. He is currently the Angler in Chief at Rudow's FishTalk Magazine, is Electronics and Fishing Editor for BoatUS Magazine, and is a contributing editor to several other publications. His...