The essentials in 30 seconds
- Through-hull = maximum precision. Hull-mounted transducer in direct contact with water. Depth accuracy ±0.1 m, speed ±0.1 knot, temperature ±0.5°C. Installation dry-dock only, hull drilling, 4-6 h labor.
- Transom-mount = simplicity. Clamp-on bracket on transom, partially submerged. Installation in 1-2 h without water exit. Speed accuracy loss: 10-15% depending on point of sail (cavitation at high speed), depth OK up to 50 m.
- Offshore racing = through-hull standard. All performance sailboats (IMOCA, Class40, Mini) use through-hull transducers — precision and stability essential for VMG calculation and polars.
- Coastal cruising = transom-mount sufficient. For 80% of 30-40 ft cruising sailboats, transom-mount is adequate. Ease of maintenance (removable at dock).
- Brands stocked: Airmar (global OEM standard), Raymarine (native Axiom integration), B&G (native Triton²/H5000 integration).
The transducer is the #1 sensor for coastal navigation. It measures depth, surface speed, and water temperature — three critical data points for safety (grounding), performance (VMG, polars), and strategy (thermal fronts, currents). The choice between through-hull and transom-mount affects 10-15 years of sailing.
This article compares the two families technically, lists the brands distributed by Skysat, and provides a workshop verdict by program. For the GPS-fishfinder combination that uses the measurements, see our GPS-fishfinder 9 vs 12 inch comparison for sailboats 2026.
How a marine depth sounder works
A depth sounder combines three distinct sensors in a single housing:
- Ultrasonic transducer for depth. Emits a sound pulse (typically 50 or 200 kHz) and measures the round-trip time to the bottom.
- Paddlewheel or electromagnetic sensor for speed through water. The wheel turns with the water flow; electronics count the revolutions.
- Thermistor for water temperature. Accuracy ±0.5°C.
Modern 2026 depth sounders often add a fourth sensor: downvision/sidevision (advanced sonar imaging for fishing or obstacle detection). On a cruising sailboat, this is unnecessary — it is useful for fishing or navigating in unmapped areas.
Through-hull depth sounder — technical details
The through-hull depth sounder (through-hull in English) is embedded in the hull, in direct contact with the water. It replaces a section of the hull with a bronze cylinder (older models) or high-performance plastic (modern generation).
- Installation: exclusively in a dry dock. Hull drilling, epoxy or Sika sealing, validation by controlled water test. 4–6 hours of workshop labor.
- Optimal position: forward on the boat, under the keel, away from the bow’s bubbles and the propeller. On a 40-foot sailboat, this is typically one-third of the length from the bow.
- Accuracy: ±0.1 m on depth up to 200 m, ±0.1 knot on speed at all points of sail (no cavitation), ±0.5°C on temperature.
- Maintenance: annual cleaning of barnacles, paddlewheel replacement every 5–7 years. The housing remains in place for 15–20 years.
Transom-mount depth sounder — technical details
The transom-mount depth sounder is clamped to the transom, the vertical element at the outside of the stern. It dips into the water when the boat is at rest or moving slowly.
- Installation: 1–2 h, no water exit. Light drilling of the transom for 4 stainless steel screws, silicone seal, cable pass-through via watertight gland.
- Optimal position: opposite side from the propeller (to avoid propeller cavitation), 10–15 cm below the waterline, aligned with the transom.
- Accuracy: ±0.2 m on depth up to 50 m, ±0.3–0.5 knots on speed depending on point of sail (cavitation beyond 8 knots), ±0.5°C on temperature.
- Maintenance: removable from the dock (4 screws), quick cleaning. Impeller replaceable in place without water exit.
- Limit: beyond 8–10 knots, cavitation creates bubbles around the impeller that distort the reading. On a fast sailboat (Pogo, JPK, racer), this is a deal-breaker.
Airmar — the global OEM standard
Airmar (New Hampshire, USA) supplies transducers for Raymarine, B&G, Garmin, Lowrance, Furuno — covering 90% of the recreational marine market. When you buy a "Raymarine transducer", you are actually getting an Airmar transducer with a Raymarine connector.
Skysat distributes the Airmar range as a white-label product for multi-brand refits (you keep your Axiom chartplotter but change the transducer), as well as co-branded Raymarine/B&G/Garmin transducers for native installations.
Technical comparison table
| Criteria | Through-hull | Transom-mount |
|---|---|---|
| Installation (workshop) | 4–6 h (dry dock) | 1–2 h (from dock) |
| Depth accuracy | ±0.1 m up to 200 m | ±0.2 m up to 50 m |
| Speed accuracy | ±0.1 knots all points of sail | ±0.3–0.5 knots, cavitation > 8 knots |
| Temperature accuracy | ±0.5°C | ±0.5°C |
| Maintenance | Water exit for impeller (5–7 years) | Removable from dock |
| Housing lifespan | 15–20 years | 10–15 years (UV + impacts) |
| Racing compatibility | Always | No (cavitation) |
Workshop verdict by programme
Coastal cruising 30-40 ft — stern-mounted
Skysat solution: Airmar P79 or Raymarine CPT-S stern-mounted + 1-2 h installation. Accuracy more than adequate for summer cruising, maintenance simplicity gain.
Offshore cruising 40-50 ft — through-hull
Solution: Airmar P319 bronze through-hull or Raymarine CPT-S BR through-hull + 4-6 h yard installation. Accuracy essential for estuaries and true-wind-compass navigation.
Offshore racing IMOCA / Class40 — dual through-hull
Solution: 2 × through-hull (primary + forward redundancy) Airmar performance. Without redundancy, transducer failure = no polars = no race strategy.
5 installation errors observed
Transducer errors — seen at Skysat workshop
- Stern-mounted on the propeller side. Propeller cavitation destroys speed measurement from 4-5 knots onwards. Always on the side opposite the propeller.
- Through-hull positioned too far aft. Too close to the keel = keel bubbles in waves. Optimal position = 1/3 forward from the bow, on centerline.
- Neglected seal on through-hull. #1 cause of flooding in recreational boating. Always use Sikaflex marine or 2-part epoxy, never building-grade silicone.
- Cable run alongside motor or winch cable. Transducer cable carries a weak signal. Electrical noise = erratic readings. Always separate signal from power cables.
- No post-installation speed calibration. Surface speed must be calibrated using GPS (speed over ground) in calm water without current. Without this step, the difference between surface speed and speed over ground is unusable for current calculation.
FAQ — Depth sounders in practice
Can a transom-mount depth sounder be converted to a through-hull model?
Not directly — they are two different housing types. Conversion requires a new through-hull transducer + dry dock + hull drilling. If you plan to do this, schedule the project for the next haul-out.
How long does a paddle wheel last?
Typical lifespan in cruising (300–500 hours/year): 5–7 years. In intensive racing (1,500 hours/year): 2–3 years. Replacement without removing the transducer is possible on most modern models.
50 kHz or 200 kHz transducer?
50 kHz = deep water (up to 1,000 m), wide beam 30–40° (seafloor coverage). 200 kHz = high resolution (detailed relief), narrow beam 10–15° (vertical accuracy). For cruising, a dual-frequency 50/200 kHz transducer offers the best versatility.
CHIRP — really useful?
CHIRP (Compressed High Intensity Radar Pulse) = a transducer that sweeps a range of frequencies instead of emitting a single frequency. Benefit: better resolution, separation of targets close to the seabed (fish + structures). Useful for fishing, less so for pure cruising.
What is the maximum depth range of a recreational depth sounder?
200 kHz transducer alone: 100–150 m usable range. 50 kHz transducer: 600–1,000 m depending on power. Beyond that, commercial professional sounders (Furuno FCV, Simrad ES). For recreational cruising, a standard 50/200 kHz transducer covers 99% of situations.
3 real-world workshop cases — field experience
Case 1 — Sun Odyssey 410 sailboat (annual cruising)
Winter refit 2024-2025. The client wanted to replace an old Airmar 235M mechanical transducer with a modern CHIRP solution. Recommendation: Airmar B175H 1 kW through-hull transducer. Installation on dry land: 5 h. GPS calibration after splashdown. Depth accuracy ±0.1 m up to 200 m, stable reading up to 9 knots (cavitation limit of the propulsion). User ROI: better detection of rocky bottoms in the Saint-Malo estuary, enhanced safety.
Case 2 — Beneteau Oceanis 35.1 (summer coastal cruising)
First fishfinder installation 2025. Choice: Raymarine CPT-S BR transom transducer + Axiom+ 9 integration. Installation time: 1.5 h from pontoon, no water exit required. Depth up to 50 m more than sufficient for coastal navigation. Accepted limit: reliable speed up to 6.5 knots (inaccurate reading beyond). Client feedback: "Exactly what I needed for my use."
Case 3 — Class40 IRC race (offshore competition)
Complete refit 2024. Installation: 2 × Airmar P319 bronze through-hull transducers (bow + stern) for racing redundancy. Integration with NKE Multigraphic and Gyropilot 3 computer. Depth accuracy ±0.1 m. Speed-over-ground calibrated via GPS over 8 workshop sessions before the Route du Rhum start. 12 h labor.
Excluded alternatives — why certain technologies were not selected
- Interlift (retractable) transducers: allow the transducer to be raised inside the hull at sea. Technically appealing but sealing joints are a permanent weak point. Over 8 years, 60% of the interlift transducers we replace in the workshop have minor leaks. We do not recommend them for recreational sailing.
- "Tap-fix" silicone transducers: bonded against the hull from the inside (no drilling). Strong marketing promise (30 min installation). Reality: accuracy -30% to -40% vs traditional through-hull, and modern fiberglass hulls (balsa core, foam) are incompatible with this technology. Relevant only on certain aluminum or solid wood hulls.
3 advanced errors — beyond the basics
Subtle errors — observed in supposedly correct installations
- Transducer mounted with incorrect water-flow orientation. The transducer has a flow direction (arrow molded on the housing). Installed at 180° (inverted), it still measures depth but speed readings are erroneous. Error observed 2–3 times per year in DIY installation rework.
- Coaxial transducer cable routed in the same conduit as the fluxgate compass cable. The compass measures very weak magnetic fields. The transducer cable carries a CHIRP signal with several kW of peak power (in short pulses). Inductive coupling → compass drift after a few months. Always maintain a minimum 30 cm separation between the two.
- Speed-over-ground calibration never redone after antifouling. New antifouling slightly alters flow around the paddlewheel. Initial calibration right after launch = -5 to -10% on readings after a few months. Recalibrate mid-season.
Skysat distributes Airmar, Raymarine, B&G, Garmin and Navico. This article reflects our experience installing transducers on more than 100 sailboats between 2020 and 2026.
