Garmin R109LM 2kW CHIRP Thru-Hull Transducer, Bare Wires

The R109LM is the thru-hull variant of Garmin's CHIRP-ready Low/Medium transducer family, sold under the Garmin part number 010-12809-20. The thru-hull approach is what most serious offshore captains specify when they're picking between transducer mounting styles. The transducer stem protrudes through the hull and sits in direct contact with the water, with the high-performance fairing on the outside cleaning up the water flow at speed. The acoustic performance is the best of any mounting style because there's no hull material between the transducer face and the water. The trade-off is that a thru-hull punches a hole through the hull, which is a haul-out install and a permanent commitment to that transducer position.

The L/M dual-band designation tells you the frequency ranges the transducer covers. The low element CHIRPS from 38 to 75 kHz with a 19-degree-to-10-degree port-starboard beamwidth (10-degree-to-5-degree fore-aft). The low band is where deep-water tracking lives. Long wavelengths penetrate the thermocline and reach the bottom in 5,000-foot canyon edges where higher frequencies lose the picture. The medium element CHIRPS from 80 to 130 kHz with a 13-degree-to-8-degree beamwidth. Medium frequencies give you better target separation and resolution in moderate depths, and the 80-130 kHz band covers the historically popular 88 kHz and 107 kHz frequencies that older sounders default to. Total usable bandwidth across both elements is 87 kHz, which gives a modern CHIRP-ready sounder enough range to dial the optimal frequency for whatever depth and target you're working.

This transducer specifically targets the offshore deep-water fishing profile. Commercial and recreational captains chasing canyon-edge tuna, swordfish at depth, sea-mount tracking, and other applications where the bottom is deeper than what a single-element 1kW transducer can reach. Bottom detection drops to 3,000 meters (about 10,000 feet) on the low band under ideal conditions, with practical performance depending on bottom hardness, thermocline activity, and how much aeration sits around the transducer at speed. The CHIRP-ready dual-band design also pays off when you're working at different depths through the day. You can set the sounder to sweep the low band on one channel and the medium band on a second channel, displaying both at once. The low channel gives you the deep-water picture. The medium channel gives you the higher-resolution shallow picture. Most modern 2kW sounder modules support this dual-display configuration natively.

The housing is urethane potted with a high-performance fairing that mounts to the hull at the transducer's exit point. The fairing cleans up turbulent water flow over the transducer face at speed, which keeps the sonar picture clean at cruise speeds rather than washing out from cavitation around the stem. The 15.1 kg (33.3 lb) weight is substantial because the transducer is sized for serious offshore work and the housing is built to handle long-term immersion in salt water with no degradation. Hull deadrise compatibility runs from 0 to 25 degrees, which covers most modern sportfish and trawler hulls.

The R109LM is built for boats 40 feet (12 meters) and above. That's the size class where the cost of the transducer plus the 2-to-3 kW sounder module needed to drive it makes sense, and where the deep-water fishing profile actually matches the transducer's capability. Hull material can be fiberglass, wood, or metal, which is unusual for thru-hull transducers (most are fiberglass-only). The bare-wire pigtail on this SKU means you wire directly to the sounder module's input terminals, which is the right approach when you're hardwiring to a Garmin sounder black box. For chartplotters with specific connector requirements, the connector-equipped variants of the same element are alternative SKUs. One important note from the manufacturer: this product cannot be returned to the original point of purchase, and the California Prop 65 warning is on the documentation regarding chemicals known to cause cancer or reproductive harm. Warranty service goes through Garmin directly.