Airmar TM256 Owner'S Manual & Installation Instructions Download Page 3

 

Fiberglass hull

 

—To minimize surface cracking, chamfer the 

gelcoat. If a chamfer bit or countersink bit is not available, start 
drilling with a 6mm or 1/4" bit to a depth of 1mm (1/16"). 

 

Note: To prevent drilling too deeply, wrap masking tape around 
the bit the desired distance from the point.

 

4. Apply marine sealant to the four #10 x 1-1/2" self-tapping 

screws to prevent water seepage into the transom. Fasten the 
mounting plate and any shim(s) to the hull with the screws 
positioned at the bottom of each slot.

5. Attach the transducer assembly to the mounting plate with the 

hinge pin and safety ring (see Figure 4).

6. Check again for clearances with the transducer in the “up, 

“down”, and “released” positions.

7. Using a straight edge, sight the underside of the transducer 

relative to the underside of the hull (see Figure 7). 

 

Being sure

 

 

the transducer face is nearly parallel to the water surface, adjust 
the angle of the mounting shoe to make the aft end of the 
transducer 3mm (1/8") below the rounded forward end. Tighten 
the center screw.

8. The large diameter line is the hoist line. Tie it to the upper latch 

pin 

 

inside

 

 the support tube with a boline knot. The small 

diameter line is the latch line. Tie it to the latch lever with a 
boline knot (see Figure 4).

You may want to hoist the transducer into the “up” position at 
planing speeds [especially above 30kn (35MPH)]. To operate 
the latch release line and the hoist line, it may be necessary to 
devise a system of sheaves and eyes to a remote location.

 

Testing on the Water

 

1. Test the transducer at 200kHz with the engine off.

2. Become familiar with your echosounder’s performance at a 

speed of 4kn (5 MPH).

3. Gradually increase the boat speed and observe the gradual 

decline of performance due to turbulent water flowing over the 
transducer’s face. 

 

Note: As the speed increases the performance at 50kHz will 
deteriorate more rapidly because more acoustic noise is 
generated at low frequencies.

 

4. If the decline of 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. 

5. If the performance improves when turning, the transducer’s 

position probably needs adjustment. The transducer is probably 
in turbulent or aerated water. Move the transducer farther down 
into the water in increments of 12mm (1/2"). If the performance 
does not improve satisfactorily, move the transducer closer to 
the centerline (keel). Fill unused screw holes with marine 
sealant.

 

Bolting the Assembly

 

When you are satisfied that the mounting location provides good 
performance, reinforce the mounting bracket with bolts, washers, 
and nuts (not supplied). Since transom thicknesses vary and most 
installations require a spacer, the installer must determine the 
appropriate length bolt.

1. Separate the bracket from the mounting plate by removing one 

safety ring and withdrawing the hinge pin (see Figure 4).

Figure 7. Transducer angle (TM256 shown)

 

Warning

 

: Always wear safety goggles and a dust mask.

 

2. Drill the two reinforcement bolt holes through the hull and 

spacer (see Figure 6).

3. Apply sealant to the bolt threads to prevent water seeping into 

the transom. Tap the bolts into place. Apply the washers and 
nuts inside the transom.

4. Reattach the bracket to the mounting plate with the hinge pin 

and safety ring.

 

Cable Routing

 

Route the cable over the transom for a detachable installation. For 
permanent mounting, route the cable through a drain hole or 
through a new hole drilled in the transom 

 

above the

 

 

 

waterline

 

.

 

Caution

 

: Never cut the cable or remove the connector; this will 

void the warranty.

 

Warning

 

: Always wear safety goggles and a dust mask.

 

1. If a hole must be drilled 

through the transom,

 

 

 

choose a location 

 

well 

above the waterline

 

 (see 

Figure 8). Check for 
obstructions such as trim 
tabs, pumps, or wiring 
inside the hull. Mark the 
location with a pencil. Drill 
the 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 one cable 
clamp 15cm (6") above the 
bracket. Mark the mounting 
hole with a pencil.

Figure 8. Cable routing

 

cable
clamps

cable
cover

15cm (6")

4" of
slack

10cm

 

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