Transducers: 600 W or 1 kW, frequencies, angles and mounting
The screen gets the glory, but what sees the fish is the transducer. It turns electricity into sound and echo into image, and it decides how deep you reach, how much water you cover and with what detail. This guide gathers data from the Garmin and Airmar catalogue, Lowrance and ONNautic to choose wisely.
Fixed frequencies and CHIRP bands
A fixed transducer transmits on one or two frequencies (50/200, 83/200 or 77/200 kHz). A CHIRP one sweeps a whole band on each pulse and separates targets better: two tuna together, or a fish tight to the bottom. The bands, with small differences between brands:
| Band | Lowrance | Garmin | Airmar | What for |
|---|---|---|---|---|
| Low | 28–51 kHz | — | 28–60 or 42–65 kHz | Deep (200–1,000 m), wide or narrow cone by model |
| Medium | 85–155 kHz | 80–160 kHz | 80–135 kHz | The all-rounder for mid-water and depth: 50–400 m with good detail |
| High | 140–250 kHz | 150–240 kHz | 130–210 or 150–250 kHz | Down to 150 m, maximum detail, separates bait from fish |
| Scanning | 455 / 800 kHz | 455 / 800 / 1,000 kHz | — | DownScan, SideScan, ClearVü, SideVü: structure and bait, 60–150 m |
Lowrance's rule: inshore and mid-water, wide-beam high CHIRP with 300–600 W; deep, narrow-beam low CHIRP and 1 kW. And a Garmin caveat: some fish show better on a fixed frequency than on CHIRP, so it pays to have both.
The angle: how much water you see
Over 50 m of water a 12° cone covers a 10 m circle; 25°, 22 m; 52°, 49 m. A fish passing 15 m off the boat does not exist for the narrow cone. Narrow = detail and target separation (ideal for vertical); wide = coverage and better-defined arches (ideal for searching). A split screen with both is the skippers' setup.
Power: how deep you reach
Watts do not give fish: they give usable echo deeper and with the boat under way. Catalogue maximum reach (Garmin/Airmar 2025), in salt water and good conditions; in practice count on half to see fish clearly:
| Model | Power | Frequency | Angle | Reach | Price (Spain, 25-09-2026) |
|---|---|---|---|---|---|
| TM150M (popa) | 300 W | 95–155 kHz | 26°/17° | ~230 m | ≈ 460 € |
| B619 (pasacascos) | 500 W | 77/200 kHz | 45°/15° | ~275 m | ≈ 240 € |
| B60 (pasacascos) | 600 W | 50/200 kHz | 45°/12° | ~250–365 m | 320–460 € |
| GT15M / GT51M (popa) | 600 W | 85–165 kHz | 24°–13° | ~550 m | 290 € / 580–630 € |
| B75M (pasacascos) | 600 W | 80–130 kHz | 24°/16° | ~335 m | ≈ 1.150 € |
| B75H (pasacascos) | 600 W | 130–210 kHz | 15°/9° | ~275 m | ≈ 945 € |
| TM185M / B175M | 1 kW | 85–135 kHz | 16°–11° | ~460–520 m | 1.410–1.490 € / 1.550–1.700 € |
| B175H | 1 kW | 130–210 kHz | 6°–10° | ~365 m | 1.565–1.610 € |
| TM185HW / B175HW | 1 kW | 150–250 kHz | 25° | ~150–180 m | 1.705–1.805 € / — |
| B175L | 1 kW | 40–60 kHz | 16°–25° | ~610 m | 1.700–1.945 € |
| B265LH / TM265LH | 1 kW | 42–65 + 130–210 kHz | 16°–25° / 6°–10° | ~915 m | 2.470–2.580 € / ≈ 1.935 € |
| R111LH (interior) | 2 kW | 38–75 + 130–210 kHz | 10°×19° / 4°–8° | ~2.400 m | ≈ 4.500 $ (EE. UU.; sin precio en España) |
| CM599LHW | 1–3 kW | 28–60 + 150–250 kHz | 5°×9°–11°×23° / 24°–26° | ~915 m | — |
Prices from Spanish shops (Iterin, GPS Náutico, Equiposnauticos, ONNautic), VAT included, for the Garmin or Lowrance/Simrad connector version; the same model changes price with connector and tilt. No visible Spanish distribution: R111LH (US price) and CM599LHW.
Quick read: 600 W covers trolling from 20 to 80 m and bottom to 200 m; 1 kW is the step for jigging and bigeye at 150–400 m, and for holding bottom at 20 knots; 2–3 kW is swordfish boats and bottoms from 800 m on. The maker says it plainly: a badly aimed 1 kW through-hull sees less than a well-placed 600 W.
Mounting
- Transom (skimmer): the most common. Half the body in the water, away from strakes and prop wash (starboard is better), on a plastic plate so you do not drill the transom. Loses reading with bubbles at speed.
- Through-hull: the best reading under way and the least noise. Measure the hull deadrise (a phone level will do) and choose the 0° (hull 0–7°), 12° (8–15°) or 20° (16–24°) version, or fit a fairing block. Bronze for fibreglass and wood; stainless for aluminium.
- In-hull: glued inside a solid fibreglass hull (never cored or wood). No hole, but loses power and gives no temperature or speed.
- Two transducers: 5–10 cm minimum apart so they do not interfere.
Three typical setups
- Inshore trolling (RIB or small boat, 20–80 m): 600 W transom with medium and high CHIRP (GT51M, GT15M or B75M type). Wide cone to search, high to separate bait.
- Trolling and jigging (7–10 m boat, down to 250 m): 1 kW medium through-hull (TM185M/B175M) plus DownScan/ClearVü for bait. The setup most trolling and jigging skippers run.
- Deep (bigeye, swordfish, 300 m and more): 1 kW low + high (B265LH/TM265LH) or 2 kW in-hull (R111LH), with a display that supports that power.
And live sonar (ActiveTarget, LiveScope, the 1 kW PS70 down to 300 m): good for seeing bait and the reaction to a jig vertically, not for trolling.
How to read what these units draw: fish on the fish finder and how to see tuna on the fish finder.
Sources
- Garmin, EMEA Transducer Selection Guide 2025 (power, bands, angles and reach of every Garmin and Airmar model).
- Lowrance, Sonar and Transducer Basics (CHIRP bands, power rule by zone, mounts).
- Garmin manual: sonar frequencies.
- ONNautic, frequencies and angles, transducer installation and through-hull deadrise (Spanish).
