Bridge Clearance Calculator (Air Draft)

Both choose the reference that makes the charted number SAFE: depths at the lowest tides (least water you'd find), clearances at mean high (least sky). Two opposite datums, one philosophy — pessimism printed on the chart. The operational consequence flips though: deep-draft boats want high tide; tall-rig boats want LOW tide. The sailboat waiting out the flood under a bridge is doing datum arithmetic.

Everything above the waterline at its highest point: masthead, plus the VHF whip (3–4 ft), wind instruments, TV antennas, and the flexible-but-not-that-flexible Windex. Measure from the WATER, not the deck, in normal load trim — and remember light load floats you higher, ADDING air draft. The classic confession at the boatyard starts 'the chart said 65 and my mast is 63...' and ends with the antenna.

More than your arithmetic: the tide boards on bridge fenders display CURRENT clearance at the low steel, read live as you approach — they incorporate the actual water level, surge and settlement no table predicted. Where board and calculation disagree, the board wins. (Note some boards reference the channel's lowest point of the span, others the navigable edge — the difference matters on arched spans.)

Three feet of computed margin is a comfortable floor in calm water; wakes (your bow rises onto them), surge and chop subtract directly from it, and a powerboat passing at the wrong moment has kissed more than one masthead into more than one girder. Below two feet computed: wait for more ebb, drop gear, or take the alternate route. Heeling the boat with crew weight or a halyard to a dinghy — the old trick — buys a foot or two and a story either way.