Tom Cunliffe offers advice on bridge clearances and passing through unscathed.
Bridge clearances on Admiralty charts are given at ‘HAT’ (Highest Astronomical Tide) – you’re unlikely to find any less air draught. All very well until your calculations suggest there’s only 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 little boy of no consequence’ at the masthead. 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. You’ll have total control over your speed and cam ram the throttle hard down to escape unscathed if you scrape the ironwork with your spar.