GT21-TM – TA ClearVü CHIRP 500W (260/455kHz) and traditional 600W (50/200kHz) transducer with temperature sensor (8-pin)

The Garmin GT21-TM is a transom-mount fishfinder designed for external installation at the rear of the boat. It combines traditional sonar and CHIRP ClearVü scanning sonar technologies.

The GT21-TM transducer uses two main sonar technologies:

Traditional sonar, which provides clear fish arches and excellent target separation. CHIRP ClearVü scanning sonar, which delivers near-photographic images of underwater structures and fish beneath your boat.

The GT21-TM transducer’s traditional sonar operates on 50/200 kHz frequencies with 600 W power output.

The CHIRP ClearVü sonar of the GT21-TM transducer operates on frequencies of 260/455 kHz (CHIRP ranges: 245-275 kHz and 445-465 kHz) with a power output of 500 W.

The GT21-TM transducer can reach a maximum depth of 457 meters (1,500 feet) using traditional sonar (50/200 kHz) and 305 meters (1,000 feet) with CHIRP ClearVü sonar (260/455 kHz).

Yes, the Garmin GT21-TM transducer has a built-in fast-response temperature sensor.

The GT21-TM transducer comes with a 9.1-meter (30-foot) cable and features an 8-pin connector.

The beam angles of the GT21-TM transducer are:

For traditional sonar: 40°/10°. For CHIRP ClearVü sonar: 2.0°x51° at 260 kHz and 1.4°x29° at 455 kHz.

The GT21-TM transducer is a transom-mounted transducer. It must be mounted to the rear of the boat’s transom using a bracket and screws. Installation requires drilling holes in the transom for the bracket and possibly for cable routing. It is critical to mount it so that it has direct, unobstructed contact with smooth water, away from turbulence.

To install the GT21-TM transducer, you will need the following tools:

A drill and drill bits.
Two 13 mm hex keys.
A Phillips screwdriver size #2.
Masking tape.
Marine sealant.

For optimal performance, the transducer must be mounted:

As close as possible to the center of the boat.Away from propellers, strakes, reinforcements, water intakes or discharges, and anything that could create air bubbles or turbulence.In an area where it is not at risk of damage during launching, towing, or storage.Parallel to the boat’s waterline.

To minimize interference:

Mount the transducer away from sources of electromagnetic (electronic devices, engines, radar) and acoustic (other transducers, boat machinery, wind, waves) interference.
Avoid turbulence zones caused by hull features or propeller wash.
If the transducer cable must cross a power cable, route the cables at 90 degrees to each other.
If using multiple transducers, space them at least 60 cm (24 inches) apart and avoid operating two transducers of the same frequency simultaneously.

For the 8-pin cable of the GT21-TM transducer, you will need to drill a 25 mm (1 inch) diameter through-hull hole in the transom.

Yes, it is essential to apply marine sealant around all screws and around the cable where it passes through the transom to ensure watertightness and prevent any water ingress into the boat.

No, it is strongly recommended to mount the transducer away from the propeller. The propeller’s wash creates turbulence that can interfere with the depth sounder’s signal and damage both the transducer and the propeller. Mount it at least 38 cm (15 inches) from the propeller and on the side opposite to the propeller’s rotation direction.

To test the installation:

Put the boat in the water and power on the chartplotter. Drive the boat at low speed and check the fishfinder is working properly. Gradually increase speed while monitoring the chartplotter. Note the speed at which the signal is lost or degraded. If the signal is lost, return to the speed where the loss occurred and make moderate turns in both directions. If the signal improves when turning, adjust the transducer 3 mm (1/8 in) lower. Repeat the steps until the degradation is eliminated. If the signal does not improve, consider relocating the transducer.

Yes, as a transom-mount transducer, the GT21-TM is compatible with all hull materials.

The GT21-TM transducer is not recommended for use on sailboats due to excessive heel. For vee-bottom hulls, mount the transducer at the lowest point before the first chine and away from other hull features that create turbulence.