Coastal Skipper and Yachtmaster Quiz · Tidal calculations
The chart gives the clearance under this bridge as 20 m, measured above HAT, and HAT here is 6.0 m above chart datum. With the figures shown, how much room is there above the masthead?
A cross-section of a yacht passing under a bridge. From the top: the underside of the bridge, a dashed line labelled HAT 6.0 m, the sea surface, and a dotted line labelled chart datum. Labels: charted clearance 20 m, measured from the HAT line up to the bridge. Height of tide now 3.0 m, from chart datum up to the sea surface. Mast top 21 m above the water. A gold arrow with a question mark spans the gap between the masthead and the bridge.
Show the answer
2.0 m to spare. On modern UK charts the clearance under bridges and cables is given above highest astronomical tide (HAT), so the charted figure is the room left at the highest predicted tide. The tide is 3.0 m below HAT (6.0 minus 3.0), so the bridge is 23 m above the water now, which leaves 2.0 m over a 21 m mast. Weather can lift the sea above the predicted level, so keep a margin, and check the chart title notes: some older and foreign charts use a different level.
