The essential in 30 seconds
- Through-hull = maximum accuracy. Depth sounder recessed into the hull, in direct contact with water. Accuracy: depth ±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. Mounted via clamp on the transom, partially submerged. Installation in 1-2 hours without water ingress. Accuracy loss: 10-15% on speed depending on point of sail (cavitation at high speed), depth OK up to 50 m.
- Offshore racing = through-hull is standard. All performance sailboats (IMOCA, Class40, Mini) use through-hull depth sounders — accuracy and stability essential for VMG calculation and polars.
- Coastal cruising = transom-mount is sufficient. For 80% of 30-40 ft cruising sailboats, transom-mount is more than adequate. Installation cost savings: 200-400 €, easier maintenance (removable at the dock).
- Brands distributed: Airmar (global OEM standard), Raymarine (native Axiom integration), B&G (Triton²/H5000 integration). 2026 list prices HT: 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 the GPS-depth sounder combo that uses these measurements, see our GPS-depth sounder 9 vs 12 inch comparison.
How a marine depth sounder works
A depth sounder combines three separate 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 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 — useful for fishing or navigating uncharted waters.
Through-hull depth sounder — technical details
The through-hull depth sounder is recessed into the hull, in direct contact with water. It replaces a section of 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 test. 4-6 hours labor.
- Optimal position: forward of the boat, below the keel, away from bow waves and propeller bubbles. On a 40 ft sailboat, typically one-third 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 attached via a clamp to the transom, the vertical element at the stern exterior. It submerges 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.
- Limitation: above 8-10 knots, cavitation creates bubbles around the paddlewheel, distorting the measurement. On a fast sailboat (Pogo, JPK, racer), this is a dealbreaker.
Airmar — the global 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 (you 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 chart
2026 list prices HT for authorized distributor, standard 50/200 kHz + speed + temperature depth sounder, excluding wiring and workshop installation.
| Criteria | Through-hull | Transom-mount |
|---|---|---|
| Depth sounder price HT | 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 replacement (5-7 years) | Removable from dock |
| Housing lifespan | 15-20 years | 10-15 years (UV + impacts) |
| Racing compatible | Yes, standard | No (cavitation) |
Workshop verdict by program
Coastal cruising 30-40 ft — transom-mount
Skysat solution: Airmar P79 or Raymarine CPT-S transom-mount depth sounder (~130 € HT) + 1-2 h installation. Total ~250 € HT. 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 (~380 € HT) + 4-6 h dry dock installation. Total ~700 € HT. Accuracy 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 € HT each). Total ~1,500 € HT installed. Without redundancy, depth sounder failure = no polars = no race strategy.
5 common installation errors
Depth sounder errors — observed at Skysat workshop
- Transom-mount on the propeller side. Propeller cavitation destroys speed measurement above 4-5 knots. Always mount on the opposite side of the propeller.
- Through-hull positioned too far aft. Too close to the keel = keel bubbles in waves. Optimal position = one-third 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 building silicone.
- Cable routed near engine or winch cables. The depth sounder cable carries a weak signal. 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, the speed-through-water vs speed-over-ground discrepancy is unusable for current calculation.
FAQ — Practical depth sounder use on sailboats
Can you convert a transom-mount depth sounder to through-hull?
Not directly. Conversion = new through-hull depth sounder + dry dock + hull drilling. Plan this during the next haul-out: ~700 € HT total (depth sounder + installation).
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 € HT, replacement 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 30-40° beam (bottom coverage). 200 kHz = high resolution (detailed relief), narrow 10-15° beam (vertical accuracy). 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 emitting a single frequency. Advantage: better resolution, separation of close targets near the bottom (fish + structure). Useful for fishing, not for pure cruising. Upsell ~200 € HT vs standard depth sounder.
What is the maximum depth for a recreational depth sounder?
200 kHz only: 100-150 m useful. 50 kHz: 600-1,000 m depending on power. Beyond that, commercial professional depth sounder (Furuno FCV, Simrad ES) at 3,000-8,000 €. For recreational cruising, 50/200 kHz standard covers 99% of situations.
3 real-world workshop cases — experience 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 depth sounder (~480 € HT). Dry dock installation 5 hours. 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 the 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 depth sounder (~165 € HT) + Axiom+ 9 integration. Installation 1.5 hours from dock, no water ingress. Depth up to 50 m more than adequate for coastal navigation. Accepted limitation: reliable speed up to 6.5 knots (inaccurate beyond). Total installation cost: 280 € HT. Client feedback: "Exactly what I needed for my use."
Case 3 — Class40 IRC racing (offshore competition)
Full refit 2024. Installation: 2 × Airmar P319 bronze through-hull depth sounders (bow + stern) for racing redundancy (~380 € HT each). Integration with NKE Multigraphic and Gyropilot 3 calculator. Depth accuracy ±0.1 m. Speed-through-water calibrated against GPS over 8 workshop sessions before Route du Rhum departure. Material cost: 760 € HT + 12 hours labor.
Alternatives ruled out — why some technologies are not recommended
- Interlift depth sounders (retractable): allow retracting 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-minute installation). Reality: accuracy -30% to -40% vs traditional through-hull, and modern fiberglass hulls (balsa sandwich, foam core) 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, upsell ~200-400 € HT not justified. Relevant for fishing boats, not for cruising 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 is erratic. Error observed 2-3 times per year in DIY installation rework.
- Depth sounder coaxial cable in the same conduit as fluxgate compass cable. The compass measures very weak magnetic fields. The depth sounder cable carries a CHIRP signal with several kW peak power (in short pulses). Inductive coupling → compass drift after a few months. Always maintain at least 30 cm separation between the two.
- Speed-through-water calibration never redone after antifouling. New antifouling slightly alters the flow around the paddlewheel. Initial calibration right after launch = -5 to -10% on reading after a few months. Recalibrate mid-season.
Skysat distributes Airmar, Raymarine, B&G, Garmin, and Navico brands. This article reflects our depth sounder installation experience on over 100 sailboats between 2020 and 2026. 2026 list prices HT are indicative for authorized distributors, excluding workshop labor.
