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

By Tom Burden, Last Updated 8/19/2019

semi-flush mushroom thru-hull transducer

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.

Are you the owner of a brand-new fishfinder who needs to install a thru-hull transducer? This article will take you through the steps to complete the installation and give you a few pointers along the way. Since the installaton will require that you cut a hole in the bottom of your boat, it is a good idea to schedule this project for when the boat is hauled out for winter layup, application of bottom paint or other maintenance.

Before the scheduled haulout, make sure you have the correct transducer. The transducer you select must be compatible with your fishfinder and include the specific capabilities you desire. It must also be compatible with your boat's hull material. Bronze transducers are recommended for fiberglass or wood hulls. Plastic transducers are recommended for fiberglass or metal hulls. Stainless steel transducers are recommended for steel or aluminum hulls. When installing a stainless steel transducer into an aluminum or steel hull, care must be taken to isolate the transducer from the hull to prevent galvanic corrosion. Plastic transducers are not recommended for wood hulls, since the expansion of wood can damage a plastic transducer. Your fishfinder owner's manual will help you in this regard. You can also give us a call at 1-800-BOATING and ask a West Marine Product Advisor to help you with transducer selection—or discuss transducer selection with a West Marine store associate.

Mounting Location Guidelines

thru-hull transducer with standard fairing block

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.

Mounting your transducer in an incorrect location can cause a problem due to turbulence, air bubbles and obstructions such as the boat's keel. So do not mount the transducer in line with or near the engine water intake, discharge openings or behind strakes, struts, fittings, or hull irregularities that will disturb the water flow. And do not mount the transducer where the boat may be supported during trailering, launching, hauling, or storage to avoid damaging the transducer’s face. Some additional guidelines:

  • 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 machinery 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 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.
  • For displacement hull powerboats, locate the transducer 1/3 of the way along the LWL and 6" to 12" off the centerline.
  • For planing hull powerboats, 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.
  • For outboards and I/Os, mount it just forward and to the side of the engine(s).
  • For inboards, mount it well ahead of the propeller(s) and shaft(s).
  • For stepped hulls, mount it just ahead of the first step.
  • For boats capable of speeds above 25 knots, review transducer location and operating results of similar boats before proceeding.
  • For fin keel sailboats, mount it to the side of the centerline and forward of the fin keel 1' to 2'.
  • For full keel sailboats, locate the transducer amidships and away from the keel at the point of minimum deadrise angle.

Boat Types

Best transducer location for different hull types

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"); For Airmar B45, SS505 transducers: 22mm or 7/8"; For Airmar B258, B271W, B285HW, B285M, SS258 transducers: 30mm or 1-3/16"; For Airmar B260, B265LH, B265LM, B275LHW, SS260, SS270W transducers: 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

Drill bits, hole saws and materials for installation of common Airmar transducers in a cored fiberglass hull:

  • For Airmar B45, SS505 transducers: 35mm or 1 3/8"; For Airmar B258, B271W, SS258, B285HW, B285M transducers: 40mm, 41mm, or 1 5/8"; For Airmar B260, B265LH, B265LM, B275LHW, SS260, SS270W transducers: 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. 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.
Diagram for how to cut a standard fairing

Cutting the Standard Fairing Block

To safely cut the fairing, you’ll need to determine the hull deadrise angle using your bevel gauge. 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, can be dangerous. If you are new to the use of power tools such as these, make sure you understand how to use them safely and that you wear goggles and other protective apparel.

Diagram showing what deadrise angle is

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 at right).
  2. Tilt the bandsaw table to the measured angle and secure the cutting fence (see Figure 2).
  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 2). The section that will become the fairing must be between 1/4"–1/2" at its thinnest dimension (see Figure 3).
  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 3).
  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. For a stainless steel transducer installed into a metal hull, slide the isolation sleeve over the bedded transducer stem as far down as possible (see Figure 3). 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 fairing that will contact the hull, the backing block that will contact the hull interior and to the hull nut that will contact the hull backing block. (see Figure 4)
  5. For a standard faiting, 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.
diagram showing how to bed and install a transducer in a metal hull

diagram showing how to bed and install a transducer with a standard fairing

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 3). 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 3). 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.
Preparing a cored fiberglass hull to install a transducer

Installation 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.

Cable Routing and Connecting

If the transducer includes 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.

Check 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.