Start at the loaded waterline, finish at the highest point, and keep the buffer separate.
A dimension you can trace
A height copied from a sales listing can leave you guessing about its starting point. Your planning record should say where the measurement starts, which piece of equipment defines its top, and how the boat was loaded. Record a measurement date and whether the number came from a reliable vessel record, a qualified professional or a measurement you can repeat. Do not climb a mast or improvise an unsafe measurement just to fill in a form.
Make two configurations explicit
An upright antenna and a folded antenna describe different configurations. Record them separately rather than mentally subtracting an uncertain amount at the bridge. Include fixed arches, radar equipment, lights, outriggers and other items that might be the highest point. Before using a lower configuration, establish that the relevant equipment can actually be lowered and secured as intended. A box labelled “folded” is not evidence that the work has been done.
Worked example: feet and inches
For an illustrative boat height of 13 feet 6 inches, the decimal height is 13 + 6 ÷ 12 = 13.5 feet. With an independently chosen 2-foot planning buffer, the entered clearance requirement is 15.5 feet. These are example inputs, not recommended dimensions or a universally adequate buffer. Our calculator displays the conversion and requirement separately so a rounded number does not quietly hide the assumption.
Keep uncertainty visible
Separate the recorded air draft from your additional buffer. Combining them into one undocumented height makes later comparisons difficult: you may accidentally add the buffer twice or forget it altogether. Review the record when equipment, loading or the planned running configuration changes. A static height does not account for vessel motion, the full span geometry or a restriction on the actual passage.
Take the record into the next check
Use the calculator to convert a known measurement, then send those inputs to the bridge checker. Confirm the selected station belongs to the intended crossing and designated reference point. Read the observation time before reading the margin. The NOAA definition of air gap concerns the bridge-to-water distance; your boat-height record is a separate input supplied by you. If either reference is uncertain, retain that uncertainty in the passage checklist instead of filling the gap with a convenient estimate.
Open the air-draft calculator
Keep the inputs, source references and unresolved dependencies together.
Open the air-draft calculator