Bridge clearance math is simple. The references are not.
The risky part is rarely subtraction. It is knowing exactly what the bridge number, water level and boat height each refer to.
Start with air draft, not boat length
Air draft is the vertical distance from the waterline to the highest point that must clear the bridge. A folded antenna may change the number; a fixed arch, mast or tower may not. Loading and trim can alter it too.
Separate charted clearance from measured air gap
A charted bridge clearance is a reference value tied to chart information and a vertical datum. A NOAA air-gap station is a live or near-live observation at a designated point. They are related concepts, but they are not interchangeable labels.
Keep a deliberate buffer
BoatWaypoint asks for a buffer because measurement uncertainty, vessel motion and real-world geometry deserve room. The site does not prescribe a universally safe number; the appropriate margin depends on the vessel, conditions, route and captain’s judgment.
Never hide stale data
If NOAA does not return a current observation, BoatWaypoint says so. A previous reading should not silently become a current clearance claim.
Best workflow
- Confirm your maximum air draft.
- Identify the bridge and current official navigation reference.
- Use direct NOAA air-gap data when that bridge has a supported sensor.
- Check tide/weather and bridge operating restrictions.
- Keep enough margin for conditions and uncertainty.