GT15M-THF 20° Through-Hull CHIRP Mid-Frequency Fishfinder

The Garmin GT15M-THF transducer is made of stainless steel, making it compatible with a wide range of hull materials. It can be installed on hulls made of:

FiberglassWoodCompositeAluminumSteelImportant precaution: If you install this transducer on an aluminum or steel hull, you must absolutely install the galvanic isolation components provided. Failure to do so will result in galvanic corrosion, which could cause water ingress around the transducer.

The Garmin GT15M-THF transducer is preconfigured for a 20° deadrise angle. For optimal performance, it is designed for hulls with a deadrise angle between 17° and 24° at the installation location. Deadrise angle is the measurement of the angle between a horizontal line and the outer hull at a given point.

To measure the deadrise angle of your hull, you can use:

A smartphone app
A digital angle gauge
A protractor
A spirit level
You can also contact your boat manufacturer to obtain this information. It is critical to verify this angle precisely before drilling holes.

For optimal performance, the transducer must be installed in an area of clean water, free from turbulence generated by propellers, keels, or other hull appendages. Here are general recommendations:

Outboard and sterndrive boats: At the bow and near the engine(s).Inboard boats: At the bow and away from the propeller and propeller shaft.Catamarans: At the bow, ahead of the first chine.Long-keel boats: Slightly angled toward the bow, parallel to the centerline.Fin-keel boats: 25 to 75 cm (10 to 30 in) ahead of the keel and no more than 10 cm (4 in) from the centerline.Displacement hulls: About one-third of the waterline length from the bow, and 150 to 300 mm (6 to 12 in) from the centerline.Avoid mounting it behind planking, frames, joints, water intakes, or outlets, or any feature that could create air bubbles or turbulence. The transducer must be parallel to the bow-stern axis of your vessel.

Yes, it is mandatory to update the software of your Garmin chartplotter when installing this transducer. For detailed instructions on updating the software, please refer to your chartplotter’s user manual. These manuals are usually available on support.garmin.com.

Preparing a fiberglass hull with a core is a critical step to prevent water infiltration and damage. Here are the steps to follow:

From outside the hull, drill a 3 mm (1/8 in) pilot hole at the chosen location for the transducer. Apply masking tape around the pilot hole and the surrounding area on the outside of the hull to reduce the risk of gelcoat cracking. Use a 73 mm (2 7/8 in) hole saw to cut the transducer hole completely through the outer hull. From inside the hull, use a slightly larger hole saw to cut only the inner fiberglass skin and the core. Be careful not to cut the outer skin with this larger saw. Seal the core inside the hull using either fiberglass with resin or casting epoxy. If using epoxy, create a 73 mm (2 7/8 in) diameter cylinder as a spacer while the epoxy cures. Proper sealing of the core is essential to prevent water damage.

To ensure the longevity and proper functioning of your Garmin GT15M-THF transducer, please follow these precautions:

Cable handling: Never use the cable to lift or pull the transducer, as this could damage it. Immersion: Always operate the transducer when it is submerged. Using it when exposed to air can cause heat damage. Cleaning: Do not use solvents such as mineral spirits, acetone, methyl ethyl ketone (MEK), or similar products for cleaning, as they could permanently damage the transducer’s surface. Also avoid electric sanders or pressure washers. Location: To prevent interference and potential heat damage, do not install the transducer near the engine or in the engine room.

It is strongly advised not to cut or splice the transducer cable. This should only be considered if you are connecting the transducer to a "black box" sonar module that requires bare-wire connections.

Warning: Cutting the transducer cable outside of this specific case may void your warranty.

The Garmin GT15M-THF transducer is made of stainless steel and is compatible with a wide range of hull materials. It can be installed on fiberglass, wood, composite, aluminum, and steel hulls. For metal hulls (aluminum or steel), it is mandatory to install the supplied galvanic isolation components to prevent corrosion.

The main difference lies in the manufacturing material and compatibility with hull types:

The GT15M-THF is made of stainless steel and is compatible with fiberglass, wood, composite, aluminum, and steel hulls. It requires the installation of galvanic isolation components for metal hulls. The GT12M-THF is made of brass and is limited to fiberglass, wood, or composite hulls. It must not be installed on aluminum or steel hulls to prevent galvanic corrosion.

The Garmin GT15M-THF transducer is a pre-configured model set at 20 degrees. For optimal performance, it is essential that the deadrise angle of the hull at the installation point matches this angle as closely as possible. It is optimized for deadrise angles ranging from 17 to 24 degrees.

Yes, specific precautions are required for installation on aluminium or steel hulls to prevent galvanic corrosion and avoid water ingress. You must absolutely install the included galvanic isolation components (hull isolator and thread isolator) as indicated in the installation instructions. Failure to install these components can result in galvanic corrosion.

For installing the Garmin GT15M-THF transducer, you will need the following tools:

A drill and a 3 mm (1/8 in) bitAn adjustable wrench (up to 75 mm)Marine sealant (flexible, fast-curing type, for below the waterline)Mild household detergent or rubbing alcoholSandpaper and masking tapeWaterproof electrical tapeHole saws: 73 mm (2 7/8 in) for fiberglass hulls and 75 mm (3 in) for metal hulls.Additional items such as marine-grade epoxy resin or a file may be required depending on the hull type.

Yes, you need to update the software on your Garmin chartplotter when installing this depth sounder. For software update instructions, refer to your chartplotter's owner's manual.

For optimal performance, the transducer must be mounted in an area of clean water, free of turbulence. Avoid areas behind frames, struts, fittings, water intakes or discharges, or anything that could create air bubbles. Here are some general guidelines:

On outboard or inboard/outboard motorboats, mount the transducer in front of and close to the motor(s). On inboard motorboats, mount the transducer in front of and away from the propeller and the motor shaft. On stepped hulls, mount the transducer in front of the first step. On deep-keel hulls, mount the transducer slightly angled toward the bow, parallel to the centerline. On fin-keel hulls, mount the transducer 25 to 75 cm (10 to 30 in) in front of the keel and no more than 10 cm (4 in) to the side of the centerline. On displacement hulls, mount the transducer approximately one-third of the waterline length from the bow, and 150 to 300 mm (6 to 12 in) to the side of the centerline. Ensure the transducer is parallel to the bow-stern axis of your boat and will not be struck during launching, towing, or storage.

To avoid permanently damaging the transducer’s surface, do not use solvents such as white spirit, acetone, methyl ethyl ketone (MEK), or similar products when cleaning. Do not use an electric sander or a pressure washer to clean the transducer either.

For regular cleaning, a mild household detergent or rubbing alcohol can be used.

Yes, when installing the transducer in a fiberglass hull, it is important not to overtighten the nut to avoid damaging the hull. The hull nut should be tightened using a 75 mm wrench or an adjustable wrench to secure the transducer. You can hold the transducer’s shaft steady while tightening.

Preparing a fiberglass hull with a core is crucial to prevent water ingress and serious damage to the boat. Here are the main steps:

Drill a 3 mm (1/8 in) pilot hole from the outside of the hull. Place masking tape around the pilot hole on the outside. Use a 73 mm (2 7/8 in) hole saw to cut the transducer hole through the entire hull. From inside the hull, use a slightly larger hole saw to cut only the inner fiberglass skin and core, taking care not to cut the outer skin. Seal the core inside the hull either with fiberglass and resin or with casting epoxy, creating a 73 mm (2 7/8 in) cylinder as a spacer so the epoxy cures. After curing, sand and clean the hole.

During installation, you must rotate the transducer from inside the hull until the arrow on top points toward the keel of the boat. This orientation is essential for the internal angle of the transducer to align correctly with your boat's deadrise angle, ensuring optimal performance.

In general, cutting or splicing the transducer cable is not recommended, as it may void your warranty. The only exception is when connecting the transducer to a black box sonar unit that requires bare-wire connections. In all other cases, avoid cutting the cable to prevent damage to both the cable and the transducer.

The Garmin GT15M-THF transducer is made of stainless steel. This material provides robustness and versatility, making it compatible with various hull types, including metal hulls, provided the supplied galvanic isolation components are used.