Definition
Vertical clearance is printed in brackets against a bridge, overhead cable or pipeline, in metres above MHWS, so an entry reading 18m gives the least safe air draught at spring high water. Overhead power cables carry a separate safe clearance allowing for electrical arcing, usually less than the physical clearance. Horizontal clearance between bridge piers is given in metres between the vertical faces. Always add a personal margin for masthead instruments and swell.
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