Not long after a jack-up rig called Octopus ran aground in Stronsay Firth in the Orkneys in 2006, we were feeling our way into a bay at nearby Stronsay. We crept through shoals and reefs relying on chartplotter and echo sounder, in strong winds and bad visibility, confident that our plotter would get us through: after all, we had checked it frequently on our passage up the English and Irish coasts and through the Hebrides, and had been pleasantly surprised never to find the GPS positions more than 50 metres out on the chart.
So confident had we become in our first chartplotter that a few days earlier, in fog rolling off the island of Hoy so thick that the boat’s bow was only just visible, we used it, along with the depth sounder, to find our way at dusk into the harbour of Stromness, which involves a tight turn round a reef at a point where the tide can run at up to 8 knots into Scapa Flow, right across the entrance to the inlet leading to the town.
If only we had known more about the quality of the local charts ….. much later, I came across a Marine Accident Investigations Branch report on the grounding of the jack-up rig Octopus, under tow by the tug Harald, which popped up in a web search for something else. (MAIB Report 18 2007).
I glanced at the summary and realised we had been playing Russian (or Orkney) roulette in relying so heavily on the charts, chartplotter and GPS while cruising in the Orkneys. The charts on the west side of the islands, where Stromness is located, were fine, because the area had recently been resurveyed. Much of the rest of the Orkneys, including Stronsay and its sound, was a very different matter: it had last been surveyed by leadline in the mid-19th century.
The new surveys had reached parts of the north-central Orkneys, in a programme that had been stepped up because of the risks to cruise ships, which were calling in increasing numbers; but by extraordinarily bad luck the survey work had stopped 200 metres north of the uncharted shoal in Stronsay Sound on which the Octopus grounded. It also turned out that local fishermen knew about the danger, but they had not told anyone.
Admiralty Chart 2250, covering the danger spot, looked modern, but it used leadline surveys overseen in 1843 and 1844 by Commander George Thomas on HMS Mastiff. The skipper of the tug was using Admiralty paper charts, which display a diagram of ‘source data’, showing the age of the underlying surveys; he also had a monochrome electronic vector chart plotter with Seatrack software, of a type “primarily aimed at the leisure market,” according to the report. The plotter would have used the same underlying data as the Admiralty chart.
Leisure electronic charts do not give source data, but there was information on the paper charts about the age and quality of the surveys on which they are based. How many of us actually take it seriously enough?
One of the MAIB report’s key conclusions was that shipowners should “emphasise to shipmasters and navigating officers the need to carefully consider source data”. Since reading the MAIB report, finding out about chart accuracy and survey age have shot up my own list of priorities; if a chart based on an old survey can cause such problems for an experienced local tugmaster, how many yacht skippers also fail to make enough allowances for the age and quality of their charts?
Post published 8 April 2013
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