Tom Cunliffe's June podcast: Will your mast fit under a french bridge? The devil’s in the detail, on which hangs the fate of your masthead burgee...
‘Every time I sail under a bridge, I worry that I’m about to wallop it’
I once caught an innocent bystander’s VHF antenna with my topping lift when heeled by a gust among moorings, and a pal left his Windex on Jack-in-the-Basket beacon outside Lymington, but nobody I know has ever hit a bridge. Nevertheless, every time I duck under a span I still have to fight the primeval instinct that says I’m about to wallop it. It’s partly perspective, but most of us feel the same pang of doubt when we’ve done our sums, committed ourselves, and what looks like certain doom approaches.
I’ve just had correspondence with an American yachtsman on a club cruise to Western France. His query was about the clearance under a particular bridge. He’d noted that heights on French charts – and British charts of France – were often given as ‘above mean sea level’ (MSL) rather than the usual ‘above mean high water springs’ (MHWS) and was concerned about his air draught near High Water. This raised a couple of issues. My man had missed the critical fact that ‘height’
(of lights and rocks) and ‘clearance’ (under bridges) are two separate items, measured differently. Home-waters Admiralty charts carry defining notes on heights under the title. Clearance definitions are also confirmed where appropriate. Those covering French waters seem to clarify heights, but not clearances.
The challenge, therefore, was to discover from where, exactly, the French measure clearances.
First, I checked with the SHOM (French charting authority) symbols book. The diagram of vertical data shows that, as on our own charts, vertical clearances are referred to HAT (the Highest Astronomical Tide possible without storm surge). However, there was a caveat. A note about the SHOM diagram recommends mariners to look for information on heights and clearances on each chart. So far, so good.
Without as much as a blush, it goes on to say: ‘Vertical clearance is generally referred to a typical High Water. On original charts published by SHOM, vertical clearance is referred to Mean High Water Springs (tide coefficient 95) except where indicated otherwise under the chart title.’ This was discouraging so, for clarification, I called the Hydrographic Office (UKHO). Now the fun began. They confirmed that the internationally accepted clearance symbol for bridges or overhead cables refers to HAT. Recent charts of UK origin will comply.
However, any chart originating with SHOM, which includes many Admiralty charts, might not. As for ‘typical’, the term was not intended to be specific. Why, I wondered. ‘It was just
a way of saying that it varies,’ said UKHO. Not very helpful. The official French diagram is unequivocal, but the published explanation muddy.
The bottom line appears to be that if the chart’s origins are French, which you can confirm from the title info, if it’s not HAT, it will probably, but not certainly, refer clearances to MHWS. If it doesn’t, there should be a note to that effect. I’m still hazy about ‘a typical tide’, but one thing’s sure, clearances do not refer to MSL.
The best practical advice with a chart of French origin seems to be to work the numbers, check the notes and, if still in doubt, use MHWS for clearances. Then adopt the policy I once used in the Rio Guadiana with little more than a sandwich between my antenna and the concrete. I went in bows-up to a strong flood tide.
The stream carried me stern-first at two knots while I steamed into it at one knot. If we’d touched, I was only making a knot over the ground and was pointing the right way to power out again. The perfect solution is to send aloft a boy of little consequence to eyeball the approaching apocalypse end-on. Failing volunteers, my American correspondent’s club commodore is recommended to permit his troops to lower burgees whenever a snapping of sticks seems likely.