As per the latest Safety Digest report by the UK MAIB, the log transducer of a ro-ro cargo ferry was damaged and paint had been scraped from the shell plating.
ro-ro cargo ferry was inbound to harbour and following its regular passage plan at a speed of 16kts; it was low water and the master had confirmed all the details with harbour control, including the 7m draught.
During the passage, the master and bridge team felt a heavy vibration and observed that the vessel’s wake had increased in size.
Suspecting that the ferry was experiencing squat in the shallow water, the master reduced speed and the unusual effects disappeared.
The master then increased speed and resumed the passage, monitoring the echo sounder throughout. Soon after the vibration, the bridge team observed that the log had stopped working.
Once alongside, a diver inspection of the hull revealed that the log transducer was damaged and that paint had been scraped from the shell plating.
An investigation concluded that the ferry had briefly grounded on a charted 7m shoal near the entrance to the navigational channel.
- Plan: Safe navigation relies on a berth-to-berth passage plan irrespective of the repetitive nature of the operations. The ferry grounded because the planned under keel clearance calculation made insufficient allowance for the effect of squat. The vessel’s wheelhouse poster showed that, for depths of less than 10m, a squat of up to 1m could be experienced. Had the passage plan allowed for this and accounted for the speed and height of tide, the grounding risk could have been appreciated and the planned track amended or intended speed reduced.
- Action: As soon as the vessel began to experience the telltale signs of squat – vibration, a speed reduction and increased wake – the master took the correct action. Slowing the vessel, and thereby reducing its draught, minimised the risk of further hull damage.
- Observe: The fact that there had been no alarms on the bridge dissuaded the crew from investigating the incident further. However, if they had taken the opportunity to review and assess the ferry’s passage it is likely that they would have seen that the vessel had grounded. The crew could then have reported the accident in a timely manner, confirmed the scope of any damage and taken action to prevent a recurrence.