Furuno TM904 Installation Guide

OWNER’ S GUIDE & INSTALLATION INSTRUCTIONS

Transom Mount
Side Scan Transducer

Model: TM904

Follow the precautions below for optimal
product performance and to reduce the risk of
property damage, personal injury, and/or death.

WARNING: Always wear safety glasses, a dust mask, and

ear protection when installing.

WARNING: When the boat is placed in the water,

D-20990-rev.1 102642-rev.1 09/28/22

immediately check for leaks around the screws and any
other holes drilled in the hull.

CAUTION: Never pull, carry, or hold the sensor by the
cable as this may sever internal connections.

CAUTION: Never strike the transducer with anything
except the palm of the hand. Never strike the paddlewheel.

CAUTION: Never use solvents. Cleaner, fuel, sealant, paint
and other products may contain solvents that can damage
plastic parts, especially the transducer’s face.

IMPORTANT: Please read the instructions completely
before proceeding with the installation. These instructions
supersede any other instructions in your instrument manual
if they differ.

Record the information found on the cable tag for future reference.

Part No:____________________Serial No:________________________

Date_______________Frequency____________________________kHz

Mounting Location

CAUTION: Do not mount the sensor in line with or near water

intake or discharge openings; or behind strakes, struts, fittings, or
hull irregularities that will disturb the water flow.

CAUTION: Do not mount the sensor in a location where the boat
may be supported during trailering, launching, hauling, or storage.

• For the best performance, the sensor must be in contact with
smooth water. To identify an area of “clean” water, observe the
water flow off the transom while the boat is underway.

• Mounting the sensor on the side of the transom where the
propeller blades are moving downward is preferred (Figure 1).

• Mount the sensor as close to the centerline (keel) of the boat as
possible to ensure the transducer’s face remains in the water
when the boat is turning.

Applications

• Not recommended for boats with inboard engine(s).

• Not recommended for a stepped hull

• Adjusts to transom angles from 0

• Vertically orients sound beam on hull with deadrise angle up to 28

• Recommended for boats capable of speeds up to 30kn
(35MPH). Optimal speed is 1.5 - 8.5 kn (2 - 10MPH).

–30

Tools & Materials

Safety glasses
Dust mask
Ear protection
Pencil
Electric drill
Drill bits and hole saws:

Bracket holes 5mm, #4, or 7/32"
Transom hole (optional) 2mm or 1/16" larger than connector Ø
Cable clamp holes 3mm or 1/8"

Masking tape
Angle finder
Grommets (some installations)
Marine sealant (suitable for below waterline)
Socket wrench
Torque wrench
Screwdrivers
Straight edge
Cable ties
Water-based anti-fouling paint (mandatory in salt water)

Boat Types

- Single drive boat—Mount a minimum of 15cm (6") beyond
the swing radius of the propeller (Figure 1).

- Twin drive boat—Mount between the drives a minimum of
15cm (6") beyond the swing radius of the propeller.

- Trim tabs—Mount inside the trim tab, space permitting.

- Stepped transom—Mount the transducer on the lowest step.

NOTE: The side
of the transom
where propeller
blades are
moving downward
is preferred.

parallel to
waterline

hull projection
0–3mm (0–1/8")

15 cm (6")
minimum beyond
swing radius

Figure 1. Mounting location on single drive boat

Copyright © 2022 Airmar Technology Corp.

washer (2)

cable

bracket

spacer

lock nut

Mounting the Bracket

CAUTION: Metal hull—The stainless steel bracket must be

isolated from a metal hull to prevent electrolytic corrosion. Place
non-metal insulating washers between the bracket and the metal
hull.

bolt

transducer

Figure 2. Attaching the bracket to the transducer

Copyright © 2022 Airmar Technology Corp.

Installation

Attaching the Bracket to the Transducer

1. Slide a washer onto the bolt (Figure 2).

2. Align the bracket to the transducer and insert the bolt through
the upper hole in the transducer. Slide the spacer onto the bolt
and push the bolt through the remaining hole in the transducer
and the bracket. Ensure the cable passes between the bracket
and the spacer.

3. Slide the remaining washer onto the bolt and hand tighten the
lock nut onto the bolt.

Hole Drilling

CAUTION: To prevent drilling too deeply, wrap masking tape
around the bit 22mm (7/8") from the point.

NOTE: Fiberglass hull—Minimize surface cracking by running
the drill in reverse until the gelcoat is penetrated.

1. At the selected location position the transducer, so it projects
3mm (1/8") below the bottom edge of the transom (Figure 1).

1. Remove the bracket from the transducer.

2. Apply marine sealant to the threads of the three hex-washer-head
screws to prevent water seepage into the transom. Screw the

bracket to the hull using a socket wrench. Do not tighten the

screws at this time.

3. Reinstall the transducer to the bracket (Figure 2).

Checking the Sensor Angle & Projection

CAUTION: Do not position the leading edge of the sensor deeper
in the water than the trailing edge because aeration will occur.

CAUTION: Do not position the sensor deeper into the water than
necessary to avoid increasing drag, spray, and water noise and
reducing boat speed.

1. Using the vertical adjustment space in the bracket slots, slide the
assembly up or down until the bottom inside corner of the
transducer projections 0– 3 mm (0 – 1/8") below the bottom of the
hull (Figure 1). When you are satisfied with the position of the
transducer, tighten the three bracket screws. For clear access to
the screws, remove the transducer assembly from the bracket

(Figure 2). When reattaching, be sure to include the spacer.

2. With the transducer in the operational position, use a straight edge
to sight the underside of the transducer relative to the underside of
the hull (Figure 3). The trailing edge of the transducer should be
1– 6mm (1/16 –1/4") below the leading edge. When you are
satisfied with the position of the transducer, use a torque wrench
to tighten the lock nut to 40ft lbs.

detail

2. Be sure the bottom of the transducer is parallel to the waterline.

3. Mark the screw holes with an “X” in the center of each slot in the

bracket.

4. Using a 5mm, #4, or 7/32" drill bit, drill three holes 22mm (7/8")
deep at the marked locations.

2

torque lock nut

to 40ft lbs

slight angle

angle
reversed

Figure 3. Sensor angle adjustment

Copyright © 2022 Airmar Technolo gy Corp.

angle
too steep

Testing on the Water

15cm (6")

cable cover

cable clamp

Figure 4. Cable routing

Copyright © 2022 Airmar Technology Corp.

1. Become familiar with your echosounder’s performance at a
speed of 4kn (5MPH).

2. Gradually increase the boat speed and observe the gradual
decline in performance due to turbulent water flowing under the
transducer’s face.

3. If the decline in performance is sudden (not gradual), identify
the boat speed at which the onset occurred. Return the boat to
this speed, then gradually increase speed while making
moderate turns in both directions.

4. If the performance improves while turning to the side on which
the sensor is installed, the transducer’s position probably needs
adjustment. The transducer is probably in turbulent or aerated
water.

To improve performance, try the following one at a time
in the order given, in small increments.

a. Increase the sensor’s angle in the water. Review “Checking

the Sensor Angle & Projection: and see Figure 3.

b. Move the sensor deeper into the water in increments of 3mm

(1/8") (Figure 4).

c. Move the sensor closer to the centerline of the boat.

Fill unused screw holes with marine sealant.

NOTE: Optimal operating speed for a side scan transducer is

1.5 - 8.5 kn (2 - 10MPH).

Cable Routing & Connecting

CAUTION: Do not remove the connector to ease cable routing. If the

cable must be cut and spliced, use Airmar’s splash-proof Junction
Box No. 33-035 and follow the instructions provided. Removing the
waterproof connector or cutting the cable, except when using a
water-tight junction box, will void the sensor warranty.

Route the sensor cable over the transom, through a drain hole, or
through a new hole drilled in the transom above the waterline.

1. If a hole must be drilled through the transom, choose a location
well above the waterline (Figure 4). Check for obstructions
such as trim tabs, pumps, or wiring inside the hull. Mark the
location with a pencil. Drill a hole using the appropriate size bit
to accommodate the connector.

2. Route the cable over or through the transom.

3. On the outside of the hull, secure the cable against the transom
using the cable clamps. Position a cable clamp 15cm (6")
above the bracket and mark the mounting hole with a pencil.

8. Position the two cable clamps and fasten them in place. If used,
push the cable cover over the cable and screw it in place.

9. Route the cable to the instrument being careful not to tear the
cable jacket when passing it through the bulkhead(s) and other
parts of the boat. Use grommets to prevent chaffing. To reduce
electrical interference, separate the sensor cable from other
electrical wiring and the engine(s). Coil any excess cable and
secure it in place with cable ties to prevent damage.

10.Refer to your echosounder owner’s manual to connect the
sensor to the instrument.

Checking for Leaks

When the boat is placed in the water, immediately check for
leaks around the screws and any other holes drilled in the hull.
Note that very small leaks may not be readily observed. Do not
leave the boat in the water unchecked for more than three hours.

Operation & Maintenance

Anti-fouling Paint

Surfaces exposed to salt water that do not interlock, must be

coated with anti-fouling paint. Use water-based anti-fouling paint
only. Never use ketone-based paint, since ketones can attack
many types of plastic possibly causing damage to the transducer.
Reapply paint every 6 months or at the beginning of each boating
season.

Cleaning

Aquatic growth can accumulate rapidly on the sensor’s surface
reducing performance within weeks. Clean the transducer’s face
with a Scotch-Brite
taking care to avoid making scratches. If the fouling is severe,
lightly wet sand with fine grade wet / dry paper.

®

scour pad and mild household detergent

4. Position the second cable clamp halfway between the first
clamp and the cable hole. Mark this mounting hole.

5. If a hole has been drilled in the transom, open the appropriate
slot in the cable cover. Position the cover over the cable where
it enters the hull. Mark the two mounting holes.

6. At each of the marked locations, use a 3mm or 1/8" bit to drill a
hole 10mm (3/8") deep.

7. Apply marine sealant to the threads of the #6 x 1/2" self-tapping
screws to prevent water from seeping into the transom. If you
have drilled a hole through the transom, apply marine sealant to
the space around the cable where it passes through the transom.

Sensor Replacement & Parts

The information needed to order a replacement sensor is printed
on the cable tag. Do not remove this tag. When ordering, specify
the part number, serial number, date, and frequency in kHz. For
convenient reference, record this information on the top of page
one.

Lost, broken or worn parts should be replaced immediately.
Obtain parts from your instrument manufacturer or marine dealer.

Gemeco

Airmar EMEA

USA
Tel: [phone]
email: [e-mail]

Europe, Middle East, Africa
Tel: [phone]
email: [e-mail]

3

35 Meadowbrook Drive, Milford, New Hampshire 03055-4613, USA

•

4

Copyright © 2022 Airmar Technology Corp. All rights reserved.

www.airmar.com