How to Install Depth Finders

by Will Charpentier
A depth finder can keep you off a shallow bottom.

A depth finder can keep you off a shallow bottom.

Fishing the flats is fun, but unless your boat's equipped with wheels on its keel, you can find yourself aground in water that's too shallow. As with so many other things, a bit of thought before you head out saves you from a world of hurt. With a properly mounted sound head, or transponder, proper wiring installation and a good receiver installation, you'll know where the bottom lies. You might even find a few fish.

What Goes Where and How

1. Select the locations for the receiver, the display screen that shows the bottom, primarily, and fish between you and the bottom incidentally. The receiver must be visible and, ideally, mounted near your boat's controls. The receiver mounting bracket will, to a certain extent, dictate that location: you can't mount a flat bracket on a round surface. The receiver's power wire has an inline fuse and must reach your boat's breaker panel, either directly, or through an existing circuit. You can, for example, wire the depth finder into the breaker panel by way of a connection to the ignition switch. A connection through the accessory pole on the ignition switch has an advantage: when you turn the ignition switch off, the depth finder will shut off as well.

2. Choose the location for the transducer, the business end of the depth finder that generates and receives sound reflected from the bottom. Turbulence in the water around the transducer -- a hull or propeller wake, for example -- will give false bottom readings or turn the reflection of a minnow into that of a 1,500-pound Goliath grouper. Select a location for the transducer that's on the lower outside of your boat's transom and away from through-hull fittings, drains or odd shapes, such as the fitting for a zinc.

3. Make a plan for all the wires to and from your depth finder. Don't let the depth finder's wiring hang on the inside of your boat like vines in a jungle. At a minimum, cover the wire with a cable cover strip. The depth finder wiring isn't shielded cable. This means that, if you run it alongside or near any of your boat's native wiring, such as the wires for your power trim system, you're likely to experience electrical interference that might interfere with the depth finder's operation. If you run the depth finder's wiring near your VHF radio's wiring, you might not only experience problems with the depth finder, you might have trouble with the radio.

Installating the Transducer

1. Shift your boat until it's level from side-to-side. Use a pencil and a straight edge to lightly draw a line from the top of the transom to the bottom of the boat, where you plan to install the transducer. Inspect the area one more time, to ensure there's nothing that will create a wake around the transducer.

2. Hold the transducer to the stern of the boat centered on the vertical pencil line. Slide it down the pencil line until its end extends about 1/8 inch below the bottom of the transom. Mark the location of the top of the transponder mounting bracket. Draw a pencil line perpendicular to the vertical line, using a combination square. The sound head extends about 1/8 inch below the bottom of the transom. While you're holding the transducer in place, use its mounting base as the template to mark the mounting holes with a china marker. Grab your drill and drill the holes for the transducer bracket's mounting screws, but don't mount the transducer just yet.

3. Pull the wire from the transducer through the appropriate hole in the transom and pull the wire forward, to the receiver. Unless you're replacing an existing depth finder -- if you are, you can use the old transducer wire to fish the new wire through tight spots -- you'll need a wire fish tape to pull the transponder wire into place. If you have excess wire -- you probably will -- when you reach the receiver, coil it, secure it with a wire tie and tape the coiled wire to a location away from the wires to your electronics, radio and radio antenna. Connect the transducer wire to the receiver.

4. Slather copious amounts of marine adhesive sealant in the holes you drilled and apply a thick coat to the back of the transducer bracket. Coat the threads of the mounting screws with the sealant as well. Set the bracket in place and install the screws, tightening them with a screwdriver or, if the depth finder's manufacturer specifies a torque, to the torque specified for the particular transducer, using a torque wrench. Marine adhesive sealant may be "tack-free" in as little as 48 hours; however, allow the sealant to set for at least four days before you splash the boat in the water again.

Items you will need

  • Cable cover strip
  • Pencil
  • Straight edge
  • Combination square
  • China marker (grease pencil)
  • Drill and bits
  • Wire fish tape
  • Marine adhesive sealant
  • Screwdriver
  • Torque wrench
  • 5/16-inch box-end wrench
  • Propeller wrench

Warnings

  • Because the transducer location is on the stern of the boat near the propeller, disconnect the negative cable of the boat’s battery, using a 5/16-inch box end wrench. Lift the cable from the battery, move it outside of the battery box and close the lid of the battery box. After the work is complete, reconnect the negative battery cable.
  • Whenever you work on or near your boat’s propeller, remove the propeller nut with a propeller wrench. Slide the thrust hub, propeller and washers from the propeller shaft. Failure to remove a propeller before working on an engine that’s out of the water is an invitation to a propeller-strike injury that can maim or kill you.

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

Photo Credits

  • Jasper Juinen/Getty Images News/Getty Images