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.

To install the GT12M-THF transducer in a fiberglass hull (solid or cored), you must drill a 73 mm (2 7/8 inches) hole using a hole saw.

The mounting location choice is crucial for the depth sounder’s performance. Key considerations include:

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 forward and near the engine(s).On inboard-powered boats, mount the transducer forward and away from 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 display when installing the GT12M-THF transducer. This update ensures optimal compatibility and the best performance from 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" fishfinder 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 tension to prevent damage.

The Garmin GT12M-THF depth sounder is made of brass.

The GT12M-THF transducer is designed for installation in fiberglass, wood, or composite hulls.

No, the GT12M-THF transducer must not be installed 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 depth sounder is a 20-degree model, ideal for hulls with a deadrise angle of 17 to 24 degrees. For optimal performance, it is essential that the depth sounder’s angle matches the deadrise angle at the installation point as closely as possible.

Before drilling holes in the hull, you need to check the deadrise angle at the installation point. 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 optimal performance and avoid potential damage to the device or your boat, installation by a qualified marine installer 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)Water-based antifouling paint (optional)For fiberglass hulls: a 73 mm (2 7/8 in) hole saw and marine-grade epoxy resin (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 turbulent water flow. 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 depth sounder should be mounted forward and close to the engine(s).Inboard boats: The depth sounder should be mounted forward and away from the propeller and propeller shaft.Deep-V hull boats: The depth sounder should be mounted forward of the first chine.Boats with full-length keels: The depth sounder should be mounted slightly angled toward the bow, parallel to the centerline.Boats with fin keels: The depth sounder 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 depth sounder 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 depth sounder must be mounted parallel to the boat's fore-aft axis.

For a fiberglass hull:

Drill a 3 mm (1/8 in) pilot hole from the outside. Place 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 epoxy casting 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 cracking.Use a 73 mm (2 7/8 in) hole saw to cut the depth sounder hole through the hull from the outside.Sand and clean the inside of the hole and the area around it.

During installation, 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 about 6 mm (1/4 in) above the inside of the hull to seal and secure the hull nut.

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

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