Will My Mast Fit Under the Bridge? Air Draught and Bridge Clearance

Charted bridge clearances are measured above a high water level, usually Highest Astronomical Tide (HAT) on modern charts. Actual clearance = charted clearance + (HAT − height of tide). With a 12.0 m clearance, HAT 5.8 m and a 13.5 m air draught, you need the tide below 3.3 m: here after about 1333.

The question

A bridge has a charted clearance of 12.0 m above HAT, which is 5.8 m at this port. Your mast top is 13.5 m above the waterline and you want 1.0 m to spare. HW 1100 (5.0 m), LW 1700 (0.6 m). When can you pass under?

Working, step by step

1. Clearance needed = air draught + margin = 13.5 m + 1.0 m = 14.5 m.
2. Underside of the bridge above chart datum = HAT + charted clearance = 5.8 m + 12.0 m = 17.8 m.
3. Highest tide you can pass at = 17.8 m − 14.5 m = 3.3 m.
4. At HW (5.0 m) the clearance is only 12.8 m, so you must wait for the tide to fall 1.7 m.
5. Range = 4.4 m; one twelfth = 0.37 m; fall needed = 4.64 twelfths.
6. In the first 2 hours the tide changes 1 + 2 = 3 twelfths, which is 1.10 m.
7. That leaves 0.60 m to go. In hour 3 the tide changes 3 twelfths (1.10 m), so it takes 0.55 of that hour, about 33 minutes.
8. Earliest time = 1100 + 2 h 33 min = 1333.

The tides in this example are made up for practice. For real passages, use official predictions such as ADMIRALTY EasyTide or a nautical almanac, and the port's own tidal curve.

Try it yourself

Put your own numbers into the free tide calculator, which uses the same Rule of Twelfths and works out under-keel clearance.

Learn the method

• How to read a nautical chartGuide
• Rule of twelfthsGuide

More worked tide examples

• Height of tide at a given time
• Time the tide reaches a height
• Depth from charted depth
• Under-keel clearance
• Crossing a drying height
• Anchoring depth and scope
• Springs or neaps from the range
• Tidal stream rate from a diamond
• Secondary port: time of HW
• Secondary port: height of HW

Frequently asked questions