Question 1

An echo sounder is reporting a depth reading that occasionally jumps to roughly double (or some multiple of) the expected depth. What is this effect called, and what causes it?

Accepted Answer

This is called "multipath" error, caused by the transmitted acoustic pulse reflecting off the seabed, travelling back up to the surface (or hull), reflecting again, and returning to the transducer a second time — the receiver interprets this longer travel time as a deeper return, displaying an apparent depth that is a multiple (e.g. double) of the true depth; it is more likely to occur over a hard, flat, reflective seabed An echo sounder determines depth by measuring the time taken for a transmitted acoustic pulse to travel from the transducer to the seabed and back, using the known (or assumed) speed of sound in water to convert this travel time to a distance. Multipath error occurs when, instead of the simple direct round trip (transducer to seabed to transducer), the pulse undergoes additional reflections — for example, reflecting off the seabed, travelling up to the sea surface (or the underside of the hull) and reflecting back down to the seabed again before finally returning to the transducer. This longer acoustic path takes correspondingly longer to traverse, and the echo sounder, having no way to distinguish this multiply-reflected return from a "deeper" direct return, displays an erroneously large depth — often appearing as a reading that is approximately double (or another multiple) of the true depth. This effect is more pronounced over hard, smooth, flat seabeds (which reflect acoustic energy efficiently with little scattering/absorption) and in shallower water (where the geometry makes multiple reflections more likely to return to the transducer within the receiver's listening window). An OOW noticing sudden, large, and physically implausible jumps in echo sounder depth readings (especially readings that are roughly double a previously steady reading) should consider multipath error as a possible explanation, alongside other causes such as aeration/turbulence under the hull or interference from other acoustic sources.

Question 2

You're preparing a passage plan for a coastal leg in a yacht drawing 3.2m. At one point the chart shows a least depth of 3.8m at chart datum. What MUST you check before finalising this leg?

Accepted Answer

The tidal height at the time of transit to confirm sufficient under-keel clearance Charted depths are referenced to chart datum (often LAT), so you must add the predicted tidal height for your transit time to get the actual depth, then compare to your draft plus a safety margin to confirm UKC.

Question 3

During your watch the ECDIS gives a "safety contour crossing" alarm. What is the correct response?

Accepted Answer

Assess the situation against the planned route and charted depths, take action to maintain safe water, and inform the Master if there is any doubt A safety contour alarm indicates the vessel may be approaching water shallower than the set safety contour. It must be investigated against the route and charted information, with avoiding action taken and the Master informed as appropriate — never simply silenced.