All bridges on Admiralty charts now have their clearances given at ‘HAT’, or Highest Astronomical Tide. This means that whatever it gives as the clearance, you’re unlikely to find less. All very well, until your calculations suggest there’s only going to be a metre or two above the windex. Then, as you approach the span, it all starts to look frightfully tight. Obviously, one answer is to proceed as slowly as possible. A better plan is to station ‘a boy of little consequence’ at the masthead. He’ll be able to see clearly if all is going to be well. When there’s a strong fair tide and no volunteers for the climb, try stemming the stream and letting it carry you through astern over the ground. That way, you’ve total control over your speed and can ram the throttle hard down to escape unscathed if you scrape the ironwork with your spar.