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

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 that could cause water ingress around the transducer.

The Garmin GT15M-THF transducer is preconfigured for a 20-degree angle. For optimal performance, it is designed for hulls with a deadrise angle between 17 and 24 degrees at the installation location. The 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 appA protractorAn angle gaugeA digital levelYou can also contact your boat manufacturer to obtain this information. It is crucial to verify this angle precisely before drilling holes.

For optimal performance, the depth sounder must be mounted in an area of clean water, free from turbulence generated by propellers, keels, or other hull appendages. 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.Step-hull boats: At the bow of the first step.Long-keel hulls: Slightly angled toward the bow, parallel to the centerline.Fine-keel hulls: 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 strakes, frames, fittings, water discharge outlets, or anything that could create air bubbles or turbulence. The depth sounder must be parallel to the bow-to-stern axis of your vessel.

Yes, it is mandatory to update the software on your Garmin chartplotter when you install this depth sounder. 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 cored hull is a critical step to prevent water intrusion and damage. Follow these steps:

From the outside of the hull, drill a 3 mm (1/8 in) pilot hole at the chosen depth sounder location.Place masking tape over 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 depth sounder hole completely through the outer hull.From the inside of 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.Correct sealing of the core is essential to prevent water damage.

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

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

Cutting or splicing the transducer cable is strongly discouraged. This should only be done if 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 provided 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 avoid galvanic corrosion.

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

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

To install the Garmin GT15M-THF transducer, you will need the following tools:

A drill and a 3 mm (1/8 in) drill bitAn adjustable wrench (up to 75 mm / 3 in)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 must 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 depth sounder must be mounted in an area of clean water, free of turbulence. Avoid zones behind frames, struts, fittings, water intakes or outlets, or anything that could create air bubbles. Here are some general guidelines:

On outboard or inboard/outboard motorboats, mount the depth sounder ahead and near the motor(s).On inboard motorboats, mount the depth sounder ahead and away from the propeller and the motor shaft.On stepped hulls, mount the depth sounder ahead of the first step.On full-keel hulls, mount the depth sounder slightly angled toward the bow, parallel to the centerline.On fin-keel hulls, mount the depth sounder 25 to 75 cm (10 to 30 in) ahead of the keel and no more than 10 cm (4 in) to the side of the centerline.On displacement hulls, mount the depth sounder 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 depth sounder 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 either.

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

Yes, when installing the depth sounder 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 depth sounder. You can hold the depth sounder's shaft steady while tightening.

Preparing a fiberglass hull with a core is critical to prevent water infiltration that could seriously damage the boat. Follow these 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 for the epoxy to cure. Once cured, 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 so that the internal angle of the transducer aligns correctly with your boat's deadrise angle, ensuring optimal performance.

In general, it is not recommended to cut or splice the transducer cable, as this could void your warranty. The only exception is if you are 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 depth sounder 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.