Definition
Available clearance equals charted vertical clearance plus HAT minus the current height of tide on Admiralty charts, because charted clearance is referenced to HAT. A bridge charted at 14 m with HAT of 5.6 m and a present tide of 2.1 m offers about 17.5 m. Add a margin for VHF aerials, wind indicators, heel and swell, and remember overhead power cables require extra safe distance for electrical arcing.
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