NEVER 100% ACCURATE 30
ALL AT SEA JUNE 2020
In this second extract from Understanding A Nautical Chart, Paul Boissier explains why the accuracy of some charted depths On 7 August 1992, the QE2 grounded on a shoal off the coast of Martha’s Vineyard, on the eastern seaboard of the United States. Her draught at the time was 32ft 4in, the height of tide was about 1ft 6in, and the charted depth of water at the point where she grounded was 39ft. By straightforward calculations she would have had about 8ft of water under her keel. At the time she grounded, she was travelling at a speed of 25knots. It transpired that the most recent survey of the area had been conducted in 1939 using an echo sounder. A post-event side scan survey, however, showed that the shallowest depth of water in the area was 31ft and that the depth of water at the precise point where she grounded was 33ft below Chart Datum, 6ft less than the charted depth. It is likely that this patch of shallow water had quite simply fallen into one of the gaps left by the earlier echo sounder
IN my opinion Admiralty charts are still the best, most accurate and most reliable charts available, but even they are less than 100 per cent accurate. As a mariner you need to understand how and when to make allowances for these. Many charts contain areas of old surveys because more modern data is just not available. But there are other reasons to question the accuracy of
information that you are getting from your chart: the seabed may have shifted since the most recent survey; the chart may not be up-to-date. This means you may, from time to time, experience tidal surges caused by strong winds and atmospheric pressure variations which have the power to change the depth of water dramatically. There are also places where the seabed is
just too mobile – with strong tides and a soft seabed – and the hydrographer simply does not bother charting the area at all; the area of the chart is left white. The chart compiler generally tends to err on the side of caution to give that extra margin of safety to keep us from hitting the bottom, but even so, it pays to treat the information very carefully.
IN LOCKDOWN...
Dhara Thompson, Yachtmaster instructor, shares his experience of sailing in the Algarve on board Jalapeno as lockdown hit. in the marina opposite where we were anchored. One of my crew only had two weeks supply of his insulin, so for him it was back to Faro for a flight out the next day. Meanwhile my remaining crew and I motored further east in calm, dolphinfilled waters to the Guadiana River. This forms the southern border between Portugal and Spain. Anchored inside the river entrance, listening to Spanish radio, it appeared that the border might be shutting at midnight. We made the decision to fill up with fuel on the Portuguese side, Villa Real. If the border was to shut I needed to be upriver, away from this wide stretch of border river open to weather from the Atlantic. Quarantine and isolation did not seem a very good prospect here.
SAILING to the Algarve from the UK for a six month break, I did not expect the start of my return trip home to be interrupted by an emerging pandemic. I had taken one of our sail training vessels south for the winter after more than 10 years involvement in Sail Boat Project, a community sailing school based in Chichester Harbour. Now it was time to return; I collected my crew from Faro airport, watching their flight arrive from the anchorage directly under the flightpath. With thoughts turning to home, it occurred to me how much Faro lagoon reminds me of Chichester Harbour, but with storks and flamingos!
REALITY HITS Being on board, at sea and at anchor, is a fairly isolated existence in normal times, but even we could not escape from the news and updates pouring out of our phones. It was only on going ashore at Portimão that reality clicked though: police roadblocks stopping cars, businesses already shut in the boatyard and people scrambling to get back afloat after rumours of cases of the virus in town. We did a calm sweep of Lidl, as we had already planned to stock up for the journey, but with an added sense of urgency - we had already heard of quarantine being mentioned to boats
TRAPPED AT SEA The final barrier was the reasonably low road bridge. From previous crossings under it I worked out I would have 1m clearance above the top of the mast from 21:00 Portuguese time (22:00 Spanish time), two hours before the border was closing. I was cutting it fine. Crossing under this bridge, with its spooky vibrations in the wind, challenges your faith in your tidal calculations at the best of times. I turned the corner to see clusters of flashing blue lights at either end of the bridge. Motoring slowly under, I slipped into the countryside darkness thankful for the newly-installed navigation lights
survey. A ship moving fast in any shallow water is certain to squat a foot or so and suddenly, in the blink of an eye, you have reduced a confidently predicted clearance of 8ft under the keel into an agonising incident with an awful lot of paperwork to fill in. As a mariner, you need to know how to look at a chart in order to assess the risks and inaccuracies that it contains, so that you use it intelligently, recognising its limitations and identifying where you have to navigate with more caution. The information is often right there – it is just a matter of knowing where to look, how to interpret the information and how much of a margin of error to apply. ■ Next month: What the Chart Maker Does for Us
About the author: Paul Boissier was formerly a senior Admiral in the Royal Navy, and has spent much of his career at sea in a wide variety of vessels, commanding two submarines and a warship. He is also a very experienced yachtsman and has cruised extensively. In 2019, he retired after 10 years as Chief Executive of the RNLI, the charity that saves lives at sea, and operates over 340 lifeboats around the UK and the Republic of Ireland. www.fernhurstbooks.com
that everyone complained ruined the charm of the place. The next day I let the tide take me the final miles upriver just ahead of Spanish and Portuguese police RIBs and patrol boats spending a few days making themselves visible on the river. Occasional helicopter patrols still appeared to make sure we all ‘stayed at home’. We had some strong winds and thunderstorms to contend with, but this would have been a lot worse on the coast where only after a month of lockdown were boaters able to make sense of the mixed messages from authorities. A Facebook group ‘Trapped at Sea’ sought to clarify these messages, provide solidarity to boats really caught out by quarantine (for example medical reasons) and lobby consuls as well as local authorities. IN A ROUTINE At first my time was spent warning friends in the UK to take it seriously and to prepare themselves for their own lockdown. Structuring my day really helps and I have my own personal version of a watch system, so if I drift I can look at my list for some ideas of things to do to reset my day. I schedule in calls with friends to have things to look forward to, especially in the evenings. During the day, weather permitting, there are plenty of boat jobs to engage me. I am growing herbs and small veg plants on board. There is also
the rest of the team at Sail Boat Project to keep in touch with as we navigate our way through the impact on our business and how we make our living. I have started doing short video diaries and, of course, there are our online navigational theory course students to continue to support. I have heard more from people in the last month of lockdown than in the previous six months of being away from home! As sailors we are well placed to cope with periods of isolation, but instead of constantly shifting seascapes surrounding us, it is changes of a larger scale affecting everyone. On the other side of this, though, the sea will still be there, and passages will be made again. Watch at: youtube.com/sailboatproject Find out more: sailboatproject.org
DHARA’S UP-DATE: We are now allowed to move our boats within Portuguese waters and after two months at anchor in the same spot, apart from removing the odd tree from my chain, I went a few miles upriver for a change of scene. No one here in the Guadiana is too keen to leave the river for the coast yet in case things change and we end up quarantined somewhere more open to weather or less accommodating. I do not have any immediate plans to return to the UK as yet...