The essential in 30 seconds
- Through-hull = maximum precision. Depth sounder recessed into the hull, in direct contact with water. Depth accuracy ±0.1 m, speed ±0.1 knot, temperature ±0.5°C. Installation only in dry dock: hull drilling, 4-6 hours labor.
- Transom-mount = simplicity. Secured with a clamp on the transom, partially immersed. Installation in 1-2 hours without water ingress. Loss of accuracy: 10-15% on speed depending on point of sail (cavitation at high speed), depth OK up to 50 m.
- Offshore racing = through-hull by default. All performance sailboats (IMOCA, Class40, Mini) use through-hull depth sounders — precision and stability essential for VMG calculation and polars.
- Coastal cruising = transom-mount is enough. For 80% of 30-40 foot cruising sailboats, transom-mount is sufficient. Installation cost savings €200-400, ease of maintenance (removable at the dock).
- Brands distributed: Airmar (worldwide OEM standard), Raymarine (native Axiom integration), B&G (Triton²/H5000 integration). 2026 dealer prices: through-hull €250-450, transom-mount €80-180.
The depth sounder is the #1 sensor for coastal navigation. It measures depth, speed-through-water, 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 impacts 10-15 years of sailing.
This article compares the two families technically, presents the brands distributed by Skysat, and provides a workshop verdict by program. For GPS-depth sounder combos that use these measurements, see our GPS-depth sounder 9 vs 12 inch comparison for sailboats 2026.
How a marine depth sounder works
A depth sounder combines three distinct sensors in one 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 paddlewheel spins 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 — useful for fishing or navigating uncharted areas.
Through-hull depth sounder — technical details
The through-hull depth sounder is recessed into the hull, in direct contact with water. It replaces a portion of the hull with a bronze cylinder (older models) or high-tech plastic (modern generation).
- Installation: exclusively in dry dock. Hull drilling, epoxy or Sika sealing, validated by controlled water testing. 4-6 hours labor.
- Optimal position: forward of the boat, under the keel, away from bow wave bubbles and propeller wash. On a 40-foot sailboat, typically 1/3 of the length from the bow.
- Accuracy: ±0.1 m depth up to 200 m, ±0.1 knot speed at all points of sail (no cavitation), ±0.5°C temperature.
- Maintenance: annual cleaning of barnacles, paddlewheel replacement every 5-7 years (~€80 each). The housing remains in place for 15-20 years.
Transom-mount depth sounder — technical details
The transom-mount depth sounder is secured with a clamp on the transom, the vertical element at the outside of the stern. It immerses in water when the boat is at rest or moving slowly.
- Installation: 1-2 hours, no water ingress. Light drilling of the transom for 4 stainless screws, silicone gasket, cable routed through a watertight gland.
- Optimal position: opposite side of the propeller (to avoid propeller cavitation), 10-15 cm below the waterline, aligned with the transom.
- Accuracy: ±0.2 m depth up to 50 m, ±0.3-0.5 knot speed depending on point of sail (cavitation above 8 knots), ±0.5°C temperature.
- Maintenance: removable from the dock (4 screws), quick cleaning. Paddlewheel replaceable in place without water ingress.
- Limit: above 8-10 knots, cavitation creates bubbles around the paddlewheel that distort measurements. On a fast sailboat (Pogo, JPK, racer), this is a deal-breaker.
Airmar — the worldwide OEM standard
Airmar (New Hampshire, USA) supplies depth sounders for Raymarine, B&G, Garmin, Lowrance, Furuno — 90% of the recreational market. When you buy a "Raymarine depth sounder," it’s actually an Airmar depth sounder with a Raymarine connector.
Skysat distributes the Airmar range as a white-label for multi-brand refits (keep your Axiom chartplotter but change the depth sounder), as well as co-branded Raymarine/B&G/Garmin depth sounders for native installations.
Technical comparison table
2026 dealer prices (excl. wiring and workshop installation) for standard 50/200 kHz + speed + temperature depth sounders.
| Criteria | Through-hull | Transom-mount |
|---|---|---|
| Depth sounder price (excl. VAT) | €250-450 | €80-180 |
| Workshop installation | 4-6 h (dry dock) | 1-2 h (from dock) |
| Installation cost | €200-400 | €50-100 |
| Depth accuracy | ±0.1 m up to 200 m | ±0.2 m up to 50 m |
| Speed accuracy | ±0.1 knot at all points of sail | ±0.3-0.5 knot, cavitation > 8 knots |
| Temperature accuracy | ±0.5°C | ±0.5°C |
| Maintenance | Water ingress required for paddlewheel (5-7 yrs) | Removable from dock |
| Housing lifespan | 15-20 years | 10-15 years (UV + impacts) |
| Racing compatible | Yes, by default | No (cavitation) |
Workshop verdict by program
Coastal cruising 30-40 foot sailboat — transom-mount
Skysat solution: Airmar P79 or Raymarine CPT-S transom-mount (~€130 ex. VAT) + 1-2 h installation. Total ~€250 ex. VAT. Accuracy more than sufficient for summer cruising, maintenance simplicity.
Offshore cruising 40-50 foot sailboat — through-hull
Solution: Airmar P319 bronze through-hull or Raymarine CPT-S BR through-hull (~€380 ex. VAT) + 4-6 h dry dock installation. Total ~€700 ex. VAT. Precision essential for estuaries and true wind compass navigation.
Offshore racing IMOCA / Class40 — dual through-hull
Solution: 2 × performance through-hull depth sounders (primary + forward redundancy) Airmar (~€450 ex. VAT each). Total ~€1,500 ex. VAT installed. Without redundancy, depth sounder failure = no polars = no race strategy.
5 common installation errors
Depth sounder errors — seen at Skysat workshop
- Transom-mount on the propeller side. Propeller cavitation destroys speed measurements above 4-5 knots. Always mount on the opposite side of the propeller.
- Through-hull too far aft. Too close to the keel = keel bubbles in waves. Optimal position = 1/3 forward from the bow, central axis.
- Neglected sealing on through-hull. #1 cause of water ingress in recreational sailing. Always use Sikaflex marine or 2-part epoxy, never standard silicone.
- Cable routed next to engine or winch cables. Depth sounder cables carry weak signals. Electrical noise = random measurements. Always separate signal from power cables.
- No post-installation speed calibration. Speed-through-water must be calibrated against GPS (speed-over-ground) in calm seas without current. Without this step, speed-through-water vs speed-over-ground difference is unusable for current calculation.
FAQ — Practical sailboat depth sounders
Can you convert a transom-mount depth sounder to through-hull?
Not directly — they are different housing types. Conversion = new through-hull depth sounder + dry dock + hull drilling. Plan this during your next haul-out: ~€700 ex. VAT total (depth sounder + labor).
How long does a paddlewheel last?
Typically 5-7 years in cruising (300-500 hours sailing/year). In intensive racing 1,500 hours/year, 2-3 years. Spare part: €60-90 ex. VAT, replaceable without removing the depth sounder on most modern models.
50 kHz or 200 kHz depth sounder?
50 kHz = depth (up to 1,000 m), wide beam 30-40° (seafloor coverage). 200 kHz = high resolution (detailed relief), narrow beam 10-15° (vertical precision). For cruising, dual 50/200 kHz = good versatility.
Is CHIRP really useful?
CHIRP (Compressed High Intensity Radar Pulse) = depth sounder that sweeps a range of frequencies instead of a single frequency. Advantage: better resolution, separation of targets close to the bottom (fish + structure). Useful for fishing, less so for pure cruising. Extra cost ~€200 ex. VAT vs standard depth sounder.
What is the maximum depth for a recreational depth sounder?
200 kHz alone: 100-150 m useful. 50 kHz: 600-1,000 m depending on power. Beyond that, commercial professional depth sounders (Furuno FCV, Simrad ES) at €3,000-8,000. For recreational cruising, dual 50/200 kHz covers 99% of situations.
3 real-world workshop cases — feedback
Case 1 — Sun Odyssey 410 sailboat (annual cruising)
Winter refit 2024-2025. Client wanted to replace an old Airmar 235M mechanical depth sounder with a modern CHIRP solution. Recommendation: Airmar B175H 1 kW through-hull (~€480 ex. VAT). Dry dock installation 5 h. GPS calibration post-launch. Depth accuracy ±0.1 m up to 200 m, stable reading up to 9 knots (propulsion cavitation limit). User ROI: better detection of rocky bottoms in Saint-Malo estuary, enhanced safety.
Case 2 — Beneteau Oceanis 35.1 (summer coastal)
First depth sounder installation 2025. Choice: Raymarine CPT-S BR transom-mount (~€165 ex. VAT) + Axiom+ 9 integration. Installation 1.5 h from dock, no haul-out required. Depth up to 50 m more than sufficient for coastal navigation. Accepted limit: reliable speed up to 6.5 knots (inaccurate beyond). Total install cost: €280 ex. VAT. Client feedback: "Exactly what I needed for my use."
Case 3 — Class40 IRC offshore racing (high-end competition)
Full refit 2024. Installation: 2 × Airmar P319 bronze through-hull depth sounders (bow + stern) for racing redundancy (~€380 ex. VAT each). Integration with NKE Multigraphic and Gyropilot 3 computer. Depth accuracy ±0.1 m. Speed-through-water calibrated against GPS over 8 workshop sessions before Route du Rhum departure. Material cost: €760 ex. VAT + 12 h labor.
Alternatives ruled out — why some technologies aren’t recommended
- Interlift depth sounders (retractable): allow raising the depth sounder into the hull at sea. Technically appealing but sealing joints = permanent weak point. Over 8 years, 60% of interlift depth sounders we replace in the workshop have minor leaks. We do not recommend for recreational sailing.
- "Tap-fix" silicone depth sounders: glued 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. Only relevant for certain aluminum or solid wood hulls.
- Premium SideVision / DownVision: advanced sonar imaging for fishing. For cruising sailboats, extra cost ~€200-400 ex. VAT is unjustified. Relevant for fishing boats, not sailboats.
3 advanced errors — beyond the basics
Subtle errors — observed on theoretically correct installations
- Through-hull depth sounder misaligned with water flow. The depth sounder has a flow direction (arrow molded on the housing). Installed 180° reversed, it still measures depth but speed readings are erratic. Error observed 2-3 times per year in DIY installations.
- Depth sounder coaxial cable routed in the same conduit as fluxgate compass cable. The compass measures very weak magnetic fields. The depth sounder cable carries a CHIRP signal at several kW peak power (in short pulses). Inductive coupling → compass drift after a few months. Always maintain a minimum 30 cm gap between the two.
- Speed-through-water calibration never redone after antifouling. New antifouling slightly alters water flow around the paddlewheel. Initial calibration right after launch = -5 to -10% reading after a few months. Recalibrate mid-season.
Skysat distributes Airmar, Raymarine, B&G, Garmin, and Navico. This article reflects our depth sounder installation experience on over 100 sailboats between 2020 and 2026. 2026 dealer prices are indicative and exclude workshop labor.
