What adjustment must be made to fathometer readings to ensure accuracy?

To ensure accurate fathometer readings, it is necessary to account for the hull's draft, which is the vertical distance between the waterline and the bottom of the vessel's hull. This adjustment is essential because the fathometer measures the distance from the transducer to the seabed, not the total depth of water under the vessel. If the draft is not taken into consideration, the fathometer reading will indicate a depth that is shallower than the actual depth beneath the hull of the vessel. By subtracting the hull's draft from the fathometer reading, you can derive the actual water depth underneath the vessel, which is crucial for safe navigation and operation.

In contrast, other adjustments like adding the transducer height or accounting for wave action may not be necessary or could lead to inaccuracies in the context of fathometer readings. Similarly, multiplying by a factor doesn't apply in standard fathometer operations, which rely more on direct measurements adjusted by hull draft factors. Understanding this adjustment helps boat crew members navigate safely and avoid grounding or other navigational hazards.