Lessons learned: When entering an unfamiliar harbour, check chart and pilot books on safe navigation

by 3, Nov, 2020Innovation

Incident

A traditional wooden sail training vessel was preparing to enter a small and unfamiliar harbour to berth overnight.

Although the skipper had studied the electronic charts, was concerned about the depth of water and sea room available at the berth.

In order to be certain that sufcient under keel clearance would be achieved, the skipper sent two crew members in a small tender into the harbour to check the berth’s suitability.

After a report back from the two crew in the harbour, the skipper was satisfed that all was well, so proceeded to take his vessel in to the harbour under power.

As the vessel passed through the narrow harbour entrance a loud cracking noise was heard from aloft.

Looking up, the skipper observed that the top of the mast had passed very close underneath power cables that were suspended above the harbour entrance.

Credit: UK MAIB

Once the vessel was alongside, the skipper discovered that all of the electronic systems that were connected through the mast had ceased to function.

Lessons learned:

  1. Although the mast had not touched the power cables, it had passed close enough for voltage to jump the gap and earth through the electrical cables, damaging the electronics. The overhead power cables were carrying 22,000 volts (22kV) which is high-voltage and highly dangerous. Advice on safe distances from high-voltage cables is available from the system providers and suggests that, for a 22kV power cable, the minimum clearance should have been 2.7m.
  2. Although the skipper had demonstrated due diligence in ensuring that the harbour was suitable for the safe navigation of his vessel in terms of depth of water, the air draught had not been considered. It is very understandable that the skipper focused on the under keel clearance, but this also served as a distraction from identifying all potential hazards to safe navigation.
  3. In this case, the height of the overhead cables was marked on the chart. When planning to enter an unfamiliar harbour, every potential source of navigational advice such as the chart and pilot books should be checked for advice on safe navigation.
  4. This incident also serves as an important reminder for vessels with wooden masts to provide an air termination rod, earthing conductor and surge protection to minimise damage in the event of a lightning strike.

November 2020
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