Worked example: replacing depth instruments on a 10 m sailing yacht

The boat: a hypothetical 10 m fibreglass cruising yacht with 1.8 m draft, an unreliable depth display and an owner who wants a clear helm readout. The owner is not asking for fishing imagery or a complete electronics replacement. All dimensions and observations in this example are invented to explain a decision process.

The first decision is whether the existing sensor and wiring are serviceable. A new display is not the first diagnostic tool. An installer should identify the old equipment and investigate power, connections and sensor performance before the owner commits to a replacement package.

#Step 1: build a small evidence sheet

Photograph the labels and cable terminations. Record whether the old instrument drives the sensor directly or receives network data. This makes later supplier advice specific to the boat rather than a guess based on its age or brand.

#Step 2: compare two conditional plans

If inspection confirms a supported, healthy SeaTalk sensor arrangement, quote a compatible i40 replacement path. If little of the old installation can usefully be retained, quote a complete dedicated depth system such as NASA Clipper, subject to hull and sensor suitability. If the owner now wants shared network data, create a third brief instead of assuming either simple option fulfils it.

Product reference points are Raymarine i40 and NASA Clipper Depth. The one-on-one comparison separates package contents and connectivity. These references do not confirm compatibility with the unidentified sensor in this fictional boat.

#Step 3: make the depth number unambiguous

Suppose a separate check establishes 4.0 m of water, the sensor face is 0.4 m below the waterline, and draft is 1.8 m. The geometry is:

These are three different descriptions of the same simplified situation. They are not alarm recommendations, safe-clearance limits or instructions for a particular instrument's offset menu. Waves, loading, measurement uncertainty and movement can change practical clearance. Follow the specific instrument's reference and sign convention, then independently verify the display.

#Step 4: agree an acceptance record

The installer and owner should record the selected reference, how it was checked, the alarm setup and how lost data appears. Observe performance under appropriate representative conditions, including normal operating speeds and sailing angles where relevant. Do not repeatedly alter offsets to conceal an unexplained sensor fault.

The final quote should identify display, sensor, any interface, panel work, cable work, hull access and commissioning. No actual purchase or price is represented here. The example ends with a documented working measurement, not simply a new screen. Use the commissioning guide to integrate that record with the boat's other equipment.