Do-It-Yourself: Installing a Thru-Hull Transducer

By Tom Burden, Last Updated 5/08/2018

Tilted /Element

This SS175 is a stainless steel semi-flush mushroom thru-hull. It has a tilted element, so it is compatible with hull deadrises from 8° to 15°. All images and diagrams in this article courtesy of Airmar Technologies, the leading manufacturer of recreational fishfinder transducers.

Thru-hull transducers are the style to choose for the best in fishfinder performance, but installing one is a complicated project. It’s usually tackled as part of a bigger list of jobs for your annual haulout since you can only install a thru-hull with the boat on dry land.

With the application of antifouling paint or other big jobs going on at the same time, the do-it-yourself boater is often overwhelmed and on a tight deadline. That’s why mistakes get made, the wrong product is chosen, or the transducer is installed in a less-than-optimal location.

That’s also why we’re asking you to answer a few questions and save yourself from the hassles that lurk in the shadows of this project. First, is the transducer compatible with your fishfinder? Sadly, many boat owners are tripped up by this one. They may have a basic Echo 301 fishfinder, for example, and are planning to ask it to drive a 3,000-watt CHIRP transducer, and that’s not gonna happen. Or they may have a fishfinder that’s one year too old to run the new StructureScan HD with twin stainless thru-hulls that they’ve selected.

Don’t make this mistake, but instead verify with the manufacturer (or with our Product Advisors at 1-800-BOATING) that your choice is a good one.Is the transducer compatible with your hull material? Remember that a bronze transducer is recommended for a fiberglass or wood hull only. A stainless steel transducer is compatible with all hull materials, and is recommended for aluminum hulls to prevent electrolytic corrosion, provided the stainless steel transducer is isolated from the metal hull. A plastic transducer is not compatible with traditional wood hulls, which expand and contract with temperature and moisture changes, and can crack the plastic transducer.

Mounting location guidelines

thru-hull mounting location

B260 bronze thru-hull with standard fairing block. It is cut at the angle of the hull’s deadrise, and part of the fairing is mounted inside the hull as a backing plate.

Have you selected the best mounting location? This varies depending on your type of boat and your specific vessel. Airmar’s Figure 1, below is a general guide.

Do not mount the transducer in line with or near water intake or discharge openings or behind strakes, struts, fittings, or hull irregularities that will disturb the water flow. Don’t mount the sensor where the boat may be supported during trailering, launching, hauling, or storage to avoid damaging the transducer’s face. Some more considerations:

  • The water flowing under the hull must be smooth with a minimum of bubbles and turbulence (especially at high speeds).
  • The transducer must be continuously immersed in water.
  • The transducer beam must be unobstructed by the keel or propeller shaft(s).
  • Choose a location away from interference caused by power and radiation sources such as: the propeller(s) and shaft(s), machinery, other depth sounders, and other cables. The lower the background noise level, the higher the fishfinder gain setting that you will be able to use.
  • Choose a location with a minimal deadrise angle, unless you’re installing the appropriate tilted element transducer for your hull’s shape.
  • Choose an accessible spot inside the vessel with adequate space for the height of the transducer stem and for getting a big wrench around it when tightening the hull nut.
  • CHIRP transducers need to be mounted in a cool, well-ventilated area away from the engine to avoid overheating.

Boat types

  • Displacement hull powerboat: locate the transducer 1/3 of the way along the LWL and 6" to 12" off the centerline.
  • Planing hull powerboat: mount it well aft near the centerline and well inboard of the first set of lifting strakes to ensure that it is in contact with the water at high speeds. The starboard side of the hull where the propeller blades are moving downward provides a cleaner flow of water.
  • Outboard and I/O: mount just forward and to the side of the engine(s).
  • Inboard: mount well ahead of the propeller(s) and shaft(s).
  • Stepped hull: mount just ahead of the first step.
  • Boats capable of speeds above 25 knots: review transducer location and operating results of similar boats before proceeding.
  • Fin keel sailboat: mount to the side of the centerline and forward of the fin keel 1' to 2'.
  • Full keel sailboat: locate amidships and away from the keel at the point of minimum deadrise angle.

Tools and materials

  • Safety goggles
  • Dust mask
  • Electric drill
  • Drill bits and hole saws for some typical Airmar thru-hulls (Pilot hole: 6mm or 1/4"; B45, SS505: 22mm or 7/8"; B258, B271W, B285HW, B285M, SS258: 30mm or 1-3/16"; B260, B265LH, B265LM, B275LHW, SS260, SS270W: 33mm or 1-5/16")
  • Sandpaper
  • Mild household detergent or weak solvent (such as alcohol)
  • File (for installation in a metal hull)
  • Angle finder bevel gauge (for installation with a fairing block)
  • Bandsaw or table saw (for installation with a fairing block)
  • Rasp or power tool (for installation with a fairing block)
  • Marine sealant (suitable for below-waterline use)
  • Slip-joint pliers
  • Grommet(s) for cable chafe protection at bulkheads (some installations)
  • Cable ties
  • Water-based anti-fouling paint (mandatory in salt water)

Additional tools for installation in a cored fiberglass hull:

  • Drill bits and hole saws for hull interior: (Airmar B45, SS505: 35mm or 1-3/8"; B258, B271W, SS258, B285HW, B285M: 40mm, 41mm, or 1-5/8"; B260, B265LH, B265LM, B275LHW, SS260, SS270W: 42mm or 1-5/8")
  • Cylinder, wax, tape, and casting epoxy

Installation: no fairing block or standard fairing only

  1. Drill a 6mm or 1/4" pilot hole perpendicular to the waterline from inside the hull (see Figure 2). If there is a rib, strut, or other hull irregularity near the selected mounting location, drill from the outside.
  2. Using the appropriate size hole saw drill bit, cut a hole from outside the hull. Be sure to hold the drill so the hole will be perpendicular to the water’s surface. Start the drill in the reverse direction to score the surface. This will help avoid chipping the gelcoat. Then commence drilling in the forward direction.

  3. Sand and clean the area around the hole, inside and outside, to ensure the marine sealant will adhere properly to the hull. If there is any petroleum residue inside the hull, remove it with either a mild household detergent or a weak solvent (alcohol) before sanding. For a metal hull, remove all burrs with a file and sandpaper.
  4. The next step is to tape off the hull with blue #2090 masking tape. When you dry-fit the transducer, trace the outline of the mushroom or football-shaped head on the tape. Then use a utility knife or similar to cut out the hole in the tape. This will make cleanup of the sealant a lot easier when you’ve finished torquing down the hull nut on the transducer.

Cutting the standard fairing block

To safely cut the fairing, you’ll need to determine the hull deadrise angle using your bevel gauge. (Bill Bishop, in his blog, The Marine Installer’s Rant, has a clever technique using a simple piece of paper. Click on the link for more useful and entertaining advice.) You will need to build a jig out of plywood to safely hold the transducer at the precisely correct angle as you push it past the blade of your bandsaw or table saw. See Figure 3. CAUTION: woodworking machines, especially table saws, are very dangerous for the uneducated DIY boat owner. If you’re not experienced enough to understand what that means, please hire a professional who is.

Tilted Element

The arrow/pointed end of the fairing points forward toward the bow. Be sure to correctly orient the fairing on the bandsaw, so the angle cut matches the intended side of the hull and not the mirror image.

  1. Measure the deadrise angle of the hull at the selected location (see diagram).
  2. Tilt the bandsaw table to the measured angle and secure the cutting fence (see Figure 3).
  3. Place the fairing on the table, so the cutting jig rests against the fence. The arrow/pointed end will be pointing toward you for installation on the starboard side of the boat or away from you for installation on the port side.
  4. Adjust the cutting fence, so the fairing will be cut in about two equal parts (see Figure 3). The section that will become the fairing must be between 1/4–1/2" at its thinnest dimension (see Figure 2).
  5. Recheck steps 1 through 4. Then cut the fairing.
  6. Shape the fairing to the hull as precisely as possible with a rasp or power tool.
  7. Use the remaining section of the fairing that’s attached to the cutting jig for the backing block that will go inside the hull.

Bedding the transducer

CAUTION: Be sure all surfaces to be bedded are clean and dry.

  1. Remove the hull nut (see Figure 5).
  2. Thread the transducer cable through the fairing (if used).

  3. Apply a 1/16" thick layer of marine sealant (3M 4200 or Sikaflex 291 are popular choices) to the surface of the transducer that will contact the hull/fairing and up the stem. The sealant must extend 1/4" higher than the combined thickness of the hull, fairing and backing block (if used), and the hull nut. This will ensure there is marine sealant in the threads to seal the hull and hold the hull nut securely in place. Stainless steel transducer/stem in a metal hull: slide the isolation sleeve over the bedded transducer stem as far down as possible (see Figure 2). Apply a 1/16" thick layer of the marine sealant to the outside of the sleeve.
  4. Apply a 1/16" thick layer of marine sealant to the following surfaces (see Figure 5):
    • Fairing that will contact the hull
    • Backing block that will contact the hull interior
    • Hull nut that will contact the hull/backing block
  5. Standard Fairing: Seat the transducer firmly in/against the fairing with a pushing twisting motion. Be sure the button on the fairing mates with the recess in the transducer housing.

Installing the transducer

  1. From outside the hull, thread the cable through the mounting hole. Then push the stem of the transducer through the hole using a twisting motion to squeeze out excess sealant. Take care to align the transducer with the blunt/button/arrow end facing forward toward the bow. The long side must be parallel to the centerline of the boat. If installing a stainless steel transducer in a metal hull, be sure the isolation sleeve is between the transducer stem and the hull (see Figure 2). However, the isolation sleeve must be below the hull nut to prevent the sleeve from interfering with tightening the nut.
  2. From inside the hull, slide the backing block (if installing with a fairing) and the hull nut onto the cable. Seat any backing block against the hull, being sure the arrow end faces forward toward the bow. Screw the hull nut in place and tighten it with slip-joint pliers (see Figure 5). If installing in a cored fiberglass hull, do not over-tighten, crushing the core. For installations in a wood hull, allow for the wood to swell before tightening the nut.
  3. Remove any excess marine sealant on the outside of the hull and fairing to ensure smooth water flow under the transducer.

Cable routing and connecting

Installing inside a cored hull requires removal of the core. In the diagram above, the core has been replaced with epoxy and a cylinder has been glued inside.

If the sensor came with a connector, do not remove it to ease cable routing. If the cable must be cut and spliced, use Airmar’s splash-proof Junction Box (33-035, available by Special Order) and follow the instructions supplied. Removing the waterproof connector or cutting the cable, except when using a watertight junction box, will void the sensor’s warranty.

  1. Route the cable to the fishfinder, being careful not to tear the cable jacket when passing it through the bulkheads and other parts of the boat. Use grommets to prevent chafing and support the cable properly with cable clamps or wire ties, per ABYC specs for marine wiring. To reduce electrical interference, separate the transducer cable from other electrical wiring and the engine. Coil any excess cable and secure it in place with cable ties to prevent damage.
  2. Refer to the fishfinder owner’s manual to connect the transducer to the instrument.

Checking for leaks

When the boat is placed in the water, immediately check around the transducer for leaks. Note that very small leaks may not be immediately obvious. Do not leave the boat in the water for more than three hours before checking it again. If there is a small leak, there may be considerable bilge water accumulation after 24 hours. If a leak is observed, repeat “Bedding” and “Installing” immediately.