GT12M-THF 20° Through-Hull CHIRP Mid-Frequency Sonde

The Garmin GT12M-THF transducer is made of brass and is designed for installation on fiberglass, wood, or composite hulls.

This model must not be installed on aluminum or steel hulls, as this would create galvanic corrosion that could lead to water ingress around the transducer.

For installing the GT12M-THF transducer in a fiberglass hull (solid or cored), you will need to drill a 73 mm (2 7/8 inch) hole using a hole saw.

Choosing the mounting location is crucial for the depth sounder’s performance. Key considerations:

Mount the transducer in an area with clean, non-turbulent water, away from turbulence generated by propellers, keels, strakes, fittings, water intakes, or discharges. Avoid zones where the transducer could be damaged during launching, hauling, or storage. On outboard or sterndrive-powered boats, mount the transducer ahead of and close to the engine(s). On inboard-powered boats, mount the transducer ahead of and clear of the propeller and propeller shaft. The transducer’s axis must be parallel to the bow-stern axis of your vessel.

Yes, it is mandatory to update the software of your Garmin chartplotter when you install the GT12M-THF transducer. This update ensures optimal compatibility and the best performance of your sonar system.

The Garmin GT12M-THF transducer is made of brass. This material makes it ideal for installation on fiberglass, wood, or composite hulls.

It is generally not recommended to cut or splice the depth sounder cable, unless you are connecting it to a "black box" sounder unit that requires bare-wire connections.

Cutting the depth sounder cable outside of this specific case may void your warranty. It is important to route the cable while avoiding sharp bends or excessive pulling to prevent damage.

The Garmin GT12M-THF transducer is made of brass.

The GT12M-THF transducer is designed to be installed in fiberglass, wood, or composite hulls.

No, it is critical not to install the GT12M-THF transducer on an aluminum or steel hull. Installing this brass model on a metal hull would create galvanic corrosion, potentially leading to water ingress around the transducer.

The GT12M-THF transducer is a 20-degree model, ideal for hulls with a deadrise angle of 17 to 24 degrees. For optimal performance, the transducer angle must match the deadrise angle at the installation point as closely as possible.

Before drilling holes in the hull, you must check the deadrise angle at the installation location. You can measure it using a smartphone app, an angle finder, a protractor, or a digital level. You can also ask your boat manufacturer for the deadrise angle at a specific point on your hull.

Yes, to achieve the best possible performance and avoid potential damage to the device or your boat, installation by a qualified marine technician is recommended.

The required tools include:

A drillA 3 mm (1/8 in) drill bitA wrench suitable for nuts up to 75 mmMarine sealant (flexible, fast-curing type for below the waterline)Mild household detergent or rubbing alcoholSandpaperMasking tapeWaterproof electrical tapeCable glands (optional)Antifouling paint based on water (optional)For a fiberglass hull: a 73 mm (2 7/8 in) hole saw and marine-grade epoxy resin for fiberglass (for laminated fiberglass hulls).

To avoid interference and overheating risks, do not install the transducer near the engine or in the engine room. It should also not be installed behind frames, bulkheads, fittings, water intake or discharge openings, or anything that creates air bubbles or causes turbulent water. The transducer must be in clean (non-turbulent) water for optimal performance.

The choice of location is crucial for performance. Here are some guidelines:

Outboard and sterndrive boats: The transducer should be mounted forward and close to the engine(s). Inboard boats: The transducer should be mounted forward and away from the propeller and propeller shaft. Deep-V hull boats: The transducer should be mounted forward of the first chine. Full-keel boats: The transducer should be mounted slightly angled toward the bow, parallel to the centerline. Fin-keel boats: The transducer should be mounted 25 to 75 cm (10 to 30 in) forward of the keel and no more than 10 cm (4 in) to the side of the centerline. Displacement hull boats: The transducer should be mounted approximately 1/3 of the waterline length from the bow, and 150 to 300 mm (6 to 12 in) to the side of the centerline. The transducer must be mounted parallel to the bow-stern axis of your boat.

For a fiberglass hull:

Drill a 3 mm (1/8 in) pilot hole from the outside. Apply masking tape around the hole on the outside to reduce gelcoat cracking. Use a 73 mm (2 7/8 in) hole saw to cut the transducer hole through the hull from the outside. From the inside, use a slightly larger hole saw to cut only the inner fiberglass skin and core, ensuring the outer skin is not cut. Seal the exposed core inside the hull with fiberglass or casting epoxy resin to prevent water intrusion.

For a solid fiberglass hull:

Drill a 3 mm (1/8 in) pilot hole from the outside.Place masking tape around the pilot hole and the surrounding area on the outside of the hull to reduce gelcoat cracks.Use a 73 mm (2 7/8 in) hole saw to cut the transducer hole through the hull from the outside.Sand and clean the inside of the hole and the area around the hole.

During installation, you must apply marine sealant along the water path to ensure a watertight seal between the transducer and the hull. Apply a 2 mm (1/16 in) layer of marine sealant to the inner flange of the transducer housing where it contacts the hull. Also apply a layer of marine sealant along the threads of the transducer housing to fill the gap between the threads and the hull. The sealant on the threads should extend approximately 6 mm (1/4 in) above the inside of the hull to seal and secure the hull nut.

Yes, you must update the software on your Garmin chartplotter when installing this device to ensure compatibility and optimal performance.

Unless you are connecting the transducer to a black box sonar unit requiring bare-wire connections, you must not cut or splice the transducer cable. Cutting the transducer cable otherwise than as required to connect to a black box sonar unit may void your warranty.