Question 1

When navigating in shallow water at speed, what is "squat" and why does it matter for under-keel clearance calculations?

Accepted Answer

Squat is the increase in a vessel's effective draft (and change in trim) caused by the flow of water around the hull being restricted in shallow water at speed — it reduces under-keel clearance beyond the static draft figure, and must be accounted for when planning passages through shallow areas Squat increases with speed and decreases with water depth — in shallow water, a vessel's effective draft can increase significantly compared to its static (at-rest) draft, which must be factored into UKC calculations alongside tide, density and other allowances.

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.