Question 1

When setting up a radar display for collision avoidance, what is the difference between "sea-stabilised" and "ground-stabilised" relative motion, and when would each be used?

Accepted Answer

Sea-stabilised motion references own ship's movement through the water (using gyro and speed log/EM log input), suitable for open water collision avoidance against COLREGS, whereas ground-stabilised motion references movement over the ground (using GPS/SOG and COG), which is more useful close to land, in strong tidal streams, or for monitoring drift, but can give a misleading picture for COLREGS-based collision avoidance In sea-stabilised mode, own ship's vector and the relative motion of other targets are derived from the vessel's heading and speed through the water (gyro plus EM/Doppler log), which aligns with the speeds and aspects referenced by the Collision Regulations and is generally preferred for open-water collision avoidance. In ground-stabilised mode, own ship's vector uses speed and course over the ground (e.g. from GPS), which is useful for monitoring actual track, drift due to current, and close-quarters situations near land or in strong tidal areas, but can distort target aspects and CPA/TCPA assessments relative to the "through the water" basis assumed by COLREGS, so officers must understand which mode is selected and its implications when assessing risk of collision.

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.