Air draft and passing under bridges

Navigation Rules, lights & bridges · Power & sail

Know your boat's air draft, its height above the waterline, before you reach a bridge. You may not ask a drawbridge to open if lowering antennas or similar fittings would let you pass. Chart clearances are usually measured from mean high water, not today's water level (33 CFR 117.11, 118.160).

Know your height

• Know your boat's height from the waterline to her highest point before you approach a bridge; boating guides stress this, especially for sailboats.
• You may not signal for an opening if you could pass under the closed span after lowering every lowerable, non-structural fitting that isn't essential to navigation (33 CFR 117.11(a)). Boating guides give VHF antennas and outriggers as examples.
• Part 117 treats a fitting as not essential to navigation if lowering it wouldn't affect the boat's piloting, propulsion, control or collision avoidance (33 CFR 117.4).

Reading the clearance

• NOAA charts usually give a bridge's vertical clearance above mean high water. When the water is higher than that, for example at spring tides, in a storm surge or in a river flood, you have less room than charted (NOAA).
• Where fitted, a clearance gauge on the right-hand channel pier, facing approaching boats, shows the distance from the water to the lowest steel of the closed span (33 CFR 118.160(b)).
• Clearance gauges are required on drawbridges over waters that flow into the Atlantic south of Delaware Bay or into the Gulf of America, except the Mississippi River system. Elsewhere, any gauge requirement is in the bridge's subpart B entry (33 CFR 117.47).

Passing under or waiting

• The Inland Rules let you lower navigation lights and shapes if you need to in order to pass under a bridge (Inland Rule 20(f)). The International Rules have no such clause.
• While you wait for an opening, keep the boat under control and clear of the span; current can carry a waiting boat into the bridge (Discover Boating).

The rules: 33 CFR 117.4 · 33 CFR 117.11 · 33 CFR 117.47 · 33 CFR 118.160 · Inland Rule 20

A plain-language summary of federal rules for recreational boats, checked against the rules’ own text and a second source, and written in our own words. It isn’t legal advice. States add rules of their own, and rules change — the links above go to the current official text.