GARMIN GT12M-THF Transducer Instruction Manual

: June 12, 2024
: Garmin

Table of Contents

Important Safety Information
Important Hull Material Considerations
Software Update
Tools Needed
Mounting Location Considerations
Deadrise Angle
Preparing the Hull
Installing the Transducer
- Installing the Transducer in a Fiberglass Hull
- Installing the Transducer in a Metal Hull
Routing and Connecting the Transducer Cable
Maintenance
- Cleaning the Transducer
Specifications
Limited Warranty
References
Read User Manual Online (PDF format)
Download This Manual (PDF format)

GARMIN GT12M-THF Transducer

GARMIN GT12M-THF Transducer

Important Safety Information

WARNING

See the Important Safety and Product Information guide in the chartplotter, fishfinder, or sounder product box for product warnings and other important information.
You are responsible for the safe and prudent operation of your vessel. Sonar is a tool that enhances your awareness of the water beneath your boat. It does not relieve you of the responsibility of observing the water around your boat as you navigate.

CAUTION

For the best possible performance and to avoid potential injury, damage to the device, or damage to your vessel, installation by a qualified marine installer is recommended.
Failure to install and maintain this equipment in accordance with these instructions could result in damage or injury.
To avoid possible personal injury, always wear safety goggles, ear protection, and a dust mask when drilling, cutting, or sanding.

NOTICE

When drilling or cutting, always check what is on the opposite side of the surface to avoid damaging the vessel.
Read all installation instructions before proceeding with the installation. If you experience difficulty during the installation, contact Garmin® Product Support.
To prevent interference and avoid possible overheating damage, do not install the transducer near the engine or in the engine room.
To prevent damage to the cable and the transducer, do not use the cable to pick up or pull the transducer.
Always operate the transducer when it is submerged. Operating the transducer when it is exposed to the air may result in damage due to overheating.
To prevent permanent damage to the surface of the transducer, do not use solvents such as mineral spirits, acetone, Methyl Ethyl Ketone (MEK), or similar products when cleaning. Do not use a power sander or pressure washer to clean the transducer.

Important Hull Material Considerations

Not all transducer models can be installed in all hull types. When installing this transducer, it is critical that you install the type of transducer that is designed for use with the material of the boat hull.
The GT15M-THF transducer is made from stainless steel, and can be installed in a fiberglass, wood, composite, aluminum, or steel-hulled vessel. When installing this model in an aluminum or steel hull, you must install the included galvanic isolation components as instructed.

NOTICE
Installing the GT15M-THF transducer in an aluminum or steel hull without the galvanic isolation components will create galvanic corrosion that may result in water entry around the transducer.
The GT12M-THF transducer is made from brass, and can be installed in a fiberglass, wood, or composite-hulled vessel. You must not install this model in an aluminum or steel hull.

NOTICE
Installing the GT12M-THF transducer in an aluminum or steel hull will create galvanic corrosion that may result in water entry around the transducer.

Software Update

You must update the Garmin chart plotter software when you install this device. For instructions on updating the software, see your chartplotter owner’s manual at support.garmin.com.

Tools Needed

Drill
3 mm (1 /8 in.) drill bit
Adjustable wrench suitable for nuts up to 75 mm
Marine sealant (flexible, fast-cure type, for below the waterline)
Mild household detergent or rubbing alcohol
Sandpaper
Masking tape
Waterproof electrical tape
Grommets (optional)
Water-based anti-fouling paint (optional)
These additional items are needed depending on the installation. For mounting in a fiberglass hull: • 73 mm (2 7
/8 in.) spade bit or hole saw
Marine grade epoxy resin for fiberglass (cored fiberglass hulls)
For mounting in a metal hull:
File
75 mm (3 in.) hole saw

Mounting Location Considerations

On outboard and sterndrive vessels 1 , the transducer should be mounted in front of and close to the engine or engines.
On inboard vessels 2 , the transducer should be mounted in front of and far away from the engine propeller and shaft.
On step-hull vessels 3 , the transducer should be mounted in front of the first step.
On full-keel vessels 4 , the transducer should be mounted at a slight angle that aims at the bow, parallel to the centerline.
On fin-keel vessels 5 , the transducer should be mounted from 25 cm to 75 cm (from 10 to 30 in.) in front of the keel and a maximum of 10 cm (4 in.) to the side of the centerline.
On vessels with displacement hulls 6 , the transducer should be mounted approximately 1 /3 aft of the waterline length of the vessel from the bow, and from 150 to 300 mm (from 6 to 12 in.) to the side of the centerline.
The transducer should be mounted parallel to the bow-stern axis of your vessel.
The transducer should not be mounted behind strakes, struts, fittings, water intake or discharge ports, or anything that creates air bubbles or causes the water to become turbulent.
The transducer must be in clean (non-turbulent) water for optimal performance.
The transducer should not be mounted in a location where it might be jarred when launching, hauling, or storing.
On single-drive boats, the transducer must not be mounted in the path of the propeller.
The transducer can cause cavitation that can degrade the performance of the boat and damage the propeller.
On twin-drive boats, the transducer should be mounted between the drives, if possible.

Deadrise Angle

The deadrise angle 1 is the measurement of the angle between a horizontal line and the outer hull at a single point.
These transducers are available in a selection of pre-configured angles from 0 to 20 degrees. For optimal performance, it is critical that the transducer angle you purchase is as close as possible to the deadrise angle at the installation location.
Before drilling any holes in the hull, you should verify the deadrise angle at the installation location by measuring with a smartphone application, an angle finder, a protractor, or a digital level. You can also ask your boat manufacturer for the deadrise of a specific point on your boat hull. You should confirm that you have purchased the correct transducer for the mounting location by referring to this table.

Deadrise Angle Measurement	Appropriate Transducer Model
From 0 to 5 degrees	0 degree
From 6 to 16 degrees	12 degree
From 17 to 24 degrees	20 degree

NOTE : Your vessel’s hull may have several deadrise angles 2 depending on the hull shape. Be sure to measure the deadrise angle at the selected installation location.

Preparing the Hull

Preparing a Cored-Fiberglass Hull

NOTICE
If the core of a cored-fiberglass hull is not sealed properly, water may seep into the core and severely damage the boat.
Preparing the Hull

1 Fiberglass or casting epoxy (not included)

2 Cylinder spacer
3 Inner fiberglass skin
4 Core
5 Outer fiberglass skin

From outside the hull, drill a 3 mm (1 /8 in.) pilot hole at the transducer location.
Place masking tape over the pilot hole and surrounding area outside the hull to reduce cracking of the gel coat.
Use a utility knife to cut a hole in the tape over the pilot hole.
From outside the hull, use a 73 mm (2 7 /8 in.) hole saw to cut the transducer hole completely through the hull.
From inside of the hull, use a slightly larger hole saw to cut only the inner fiberglass skin and core.
NOTICE
Take care to cut only the inner fiberglass skin and core and do not cut the outer fiberglass skin.
Seal the core inside the hull using either fiberglass or casting epoxy.

Sealing the Core with Fiberglass

From inside the boat, coat a layer of fiberglass cloth with fiberglass resin, and place it inside the hole to seal the core.
Add layers of fiberglass cloth and resin until the hole is the correct diameter of 73 mm (2 7 /8 in.).
After the fiberglass has hardened, sand and clean inside and around the hole.
The cored-fiberglass hull is now prepared, and you can complete the transducer installation.

Sealing the Core with Casting Epoxy

To properly seal the core with casting epoxy, you must create a cylinder with an outer diameter of 73 mm (2 7 /8 in.) to act as a spacer as the epoxy sets. 1 Coat a 73 mm (2 7 /8 in.) cylinder with wax.
Insert the cylinder in the hole through the outer skin, and tape it in place on the outside of the boat.
Fill the space between the cylinder the core with casting epoxy.
After the epoxy has hardened, remove the cylinder, and sand and clean inside and around the hole.
The cored-fiberglass hull is now prepared, and you can complete the transducer installation.

Preparing a Non-Cored Fiberglass or a Metal Hull

From outside the hull, drill a 3 mm (1 /8 in.) pilot hole at the transducer location.
On a fiberglass hull, place masking tape over the pilot hole and surrounding area outside the hull to reduce cracking of the gel coat.
Use a utility knife to cut a hole in the tape over the pilot hole.
From outside the hull, use a 73 mm (2 7 /8 in.) hole saw to cut the transducer hole.
Sand and clean the inside of the hole and the area around the hole.

Installing the Transducer

Applying Marine Sealant to a Thru-hull Transducer
When installing the transducer, you must apply marine sealant to the water path to ensure a tight, waterproof seal between the transducer and hull.

When instructed in the installation procedure, apply a 2 mm (1 /16 in.) layer of marine sealant to the inner flange of the transducer housing where it contacts the hull.
Apply a layer of marine sealant along the threads of the transducer housing to fill the space between the threads and the hull.
NOTE : The sealant on the threads should extend about 6 mm (1 /4 in.) above the inside of the hull to seal the hull and secure the hull nut.

1 Transducer threads

2 Hull nut
3 Washer (when installing in a metal hull)
4 Hull
5 Thread isolator (when installing in a metal hull)
6 Hull isolator (when installing in a metal hull)
7 Marine sealant

Installing the Transducer in a Fiberglass Hull

NOTICE
When installing a transducer in a fiberglass hull, avoid over-tightening the nut to prevent damaging the hull.
NOTE : It is recommended that two installers complete these instructions, with one positioned outside the boat and one inside the boat.

Apply marine sealant to the transducer (Applying Marine Sealant to a Thru-hull Transducer, page 6).
From outside the hull, insert the transducer through the mounting hole, using a twisting motion to squeeze out excess sealant.
From inside the hull, place the nylon washer 1 and hull nut 2 onto the stem.
From inside the hull, rotate the transducer as needed until the arrow on the top points to the keel of the boat.
The arrow on the top of the transducer must point to the keel so the internal angle aligns with the deadrise angle of the boat.
Using a 75 mm wrench or adjustable wrench, tighten the hull nut to secure the transducer in the hull.
NOTE : You can hold the transducer stem steady while tightening the hull nut using slip-joint pliers, a 67 mm wrench, or an adjustable wrench on the flat areas without threads 3 .
Do not over-tighten the nut.
Before the sealant hardens, remove all excess sealant on the outside of hull to ensure smooth water flow over the transducer.

Installing the Transducer in a Metal Hull

NOTE : It is recommended that two installers complete these instructions, with one positioned outside the boat and one inside the boat.

Before applying marine sealant, place the hull isolator and the thread isolator on the transducer.
From outside the hull, insert the transducer through the mounting hole and hold it in place.
Using a knife or scissors, cut the thread isolator so it is flush with the surface of the inner hull.
NOTE : The thread isolator must be flush with the inner surface of the hull so it does not restrict the hull nut when tightening it.
Remove the transducer from the mounting hole, and apply marine sealant to the transducer and isolators
(Applying Marine Sealant to a Thru-hull Transducer, page 6).
Insert the transducer through the mounting hole, using a twisting motion to squeeze out excess sealant.
From inside the hull, place the nylon washer and hull nut onto the stem.
From inside the hull, rotate the transducer until the arrow on the top points to the keel of the boat.
The arrow on the top of the transducer must point to the keel so the internal angle aligns with the deadrise angle of the boat.
Using a 75 mm wrench or adjustable wrench, tighten the hull nut to secure the transducer in the hull.
NOTE : You can hold the transducer stem steady while tightening the hull nut using slip-joint pliers, a 67 mm wrench, or an adjustable wrench on the flat areas without threads .
Before the sealant hardens, remove all excess sealant on the outside of the hull to ensure smooth water flow over the transducer.

Routing and Connecting the Transducer Cable

NOTICE

To prevent damage to the cable and the transducer, do not use the cable to pick up or pull the transducer.

Unless you are connecting the transducer to a sonar black box that requires bare-wire connections, you should not cut or splice the transducer cable. Cutting the transducer cable other than what is needed to connect to a sonar black box may void your warranty.
If the transducer cable is not long enough to reach the chartplotter or sonar black box, you can purchase an extension cable from your Garmin dealer or buy.garmin.com.

Route and connect the transducer cable to the chartplotter or sonar black box while taking these precautions.
• Route the cable away from other wiring and the engine(s) to prevent possible interference with the sonar signal.
• Route the cable so it is not pinched by other equipment.
• Use grommets to protect the cable if it passes through the bulkhead or other parts of the boat.
• Use zip ties or other suitable fastening equipment to secure the cable where necessary to protect it from damage. You should avoid over-tightening zip ties and compressing the cable.
Connect the transducer cable to the appropriate port on the chartplotter or sonar black box.
Tighten the locking ring on the cable connector to secure it.

Maintenance

Testing the Installation

NOTICE
You should check your boat for leaks before you leave it in the water for an extended period of time.
Because water is necessary to carry the sonar signal, the transducer must be in the water to work properly. You cannot get a depth or distance reading when out of the water. When you place your boat in the water, check for leaks around any screw holes that were added below the water line.

Anti-Fouling Paint
To prevent corrosion on metal hulls and to slow the growth of organisms that can affect a vessel’s performance and durability on both metal and fiberglass hulls, you should apply a water-based anti-fouling paint to the hull of your vessel every six months.

NOTICE
Never apply ketone-based anti-fouling paint to your vessel, because ketones attack many types of plastic and could damage or destroy your transducer.

Cleaning the Transducer

CAUTION
To avoid possible transducer damage or personal injury, use care when cleaning the transducer, particularly when attempting to remove severe fouling.

NOTICE
To prevent permanent damage to the surface of the transducer, do not use solvents such as mineral spirits, acetone, Methyl Ethyl Ketone (MEK), or similar products when cleaning. Do not use a power sander or pressure washer to clean the transducer.

Aquatic fouling accumulates quickly and can reduce your device’s performance.

Remove the fouling with a soft cloth and mild detergent.
If the fouling is severe, use a non-metallic scouring pad or putty knife to remove growth.
Wipe the transducer dry.

Specifications

All Models

Specification	Measurement
Frequencies1	From 85 to 165 kHz
Beamwidth	From 24 to 16 degrees
Operating temperature range	From 0° to 50°C (from 32° to 122°F)
Storage temperature range	From -40° to 70°C (from -40° to 158°F)
Dimensions	Diameter (threads): 70 mm (2.76 in.)