Question 1

During your oral exam you are asked: what is "squat" and why is it particularly relevant when navigating in shallow water or narrow channels?

Accepted Answer

Squat is the increase in a vessel's effective draft (and change in trim) caused by the interaction between the hull and the seabed/restricted water at speed, due to changes in water flow around the hull reducing pressure beneath it; squat increases with speed and decreases with under-keel clearance, meaning a vessel that has adequate clearance at rest may experience reduced clearance — potentially leading to grounding — if speed is not reduced appropriately in shallow water Squat results from the Venturi-like effect of water being forced to flow faster beneath and around the hull in restricted depth, reducing pressure and causing the vessel to sit lower in the water (and often trim by the stern or bow depending on hull form) than its static draft would suggest. This effect increases disproportionately with speed, meaning a relatively modest increase in speed in shallow water can produce a significant reduction in actual under-keel clearance — numerous groundings in approach channels and rivers have been attributed to vessels maintaining a speed appropriate for the static draft calculation without accounting for squat, particularly relevant for any vessel transiting shallow approaches at or near maximum permitted draft.

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.