A transfer vessel grounded after the master decided to reduce the required under keel clearance (UKC) and depart on a lower tide.
A transfer vessel, preparing to leave the harbour for work at a wind farm, called the harbourmaster’s office to book a departure time, knowing the main channel dried to 1m. The harbourmaster recommended either an immediate departure or a 7-hour wait to ensure a 1m UKC. The master, not ready to depart or wait that long, decided to reduce the UKC to 0.5m and depart on the earlier tide.
The vessel entered the lock a little later than planned, with actual tides lower than predicted, but the master decided to proceed slowly to avoid squat. However, after passing 30m clear of the lock gates, the vessel grounded.
The master attempted to power through the mud, but the vessel was aground.
Lessons learned
Plan: Tides are predictable, but sailing on a falling tide can lead to delays, and it may be better to wait if observed tides are lower than forecast. The master’s decision to depart on the early tide, coupled with the short delay, increased the risk. Following this incident, the harbourmaster imposed strict constraints on lock opening times.
Margin of safety: The UKC is the primary margin of safety in navigational plans and should consider vessel factors like draught, squat, heel, water density, sea state, and storm surges. Reducing the safety margin increases the operation’s risk, as demonstrated by the vessel’s grounding after a slight delay on a falling tide.
Action: Once the vessel grounded, the master ordered soundings and inspected the hull for water ingress. A plan was prepared with the harbourmaster to refloat the vessel when the tide rose sufficiently. The initial decision to power through the mud likely contributed to subsequent damage.
Check: After the vessel refloated, the master inspected the propulsion system and found one propeller shaft not responding as expected. Divers confirmed minor hull damage but significant damage to one propeller. Early checks allowed for prompt identification of damage and prevented further issues.