Air Draft & Bridge Clearance Tool

Air Draft • Bridge Clearance • Air Gap • Water Level • River Stage • Tide • Safety Air Gap • Motion Allowance • Route Bottleneck

Air Draft & Bridge Clearance Tool

Calculate vessel air draft, current or projected overhead clearance, physical separation and margin after operator-entered safety allowances. Simple mode checks one bridge quickly. Advanced derives vessel air draft from geometry if desired, handles tide/river-stage corrections, multiple bridges or overhead obstructions, local uncertainty, motion/heel/roll allowance, lowerable equipment, route bottlenecks and water-level sensitivity.

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Need route-level depth? Click Advanced above.Advanced adds reference-datum bridge clearance, current water level, direct air-gap measurements, vessel geometry, multiple overhead obstructions, local uncertainty, motion allowances and a controlling-clearance dashboard.

Fast6 primary inputs

Core answerPhysical gap + operational margin

Clear framingNo automatic “safe to pass” verdict

Fast Bridge Clearance Screen

Use the best current vertical-clearance / air-gap source available. Positive expected water-level rise reduces bridge clearance one-for-one.

Current bridge air gapwater surface to controlling underside of bridge.

Vessel air draftwaterline to highest relevant point.

Lowerable equipment reductionreduction after verified antenna/mast lowering.

Expected water-level rise before transitpositive = water rises; negative = water falls.

Motion / heel / roll allowanceoperator-entered vertical allowance.

Required safety air gapcompany / pilot / local requirement entered by user.

Vessel Air-Draft Basis

Air-draft method

Direct current air draft

Keel to highest relevant point

Current mean draft

Verified lowerable-equipment reductionAntenna/mast/radar reduction actually available for this transit.

Motion / heel / roll allowanceVertical operating allowance entered by user.

Vessel air-draft uncertainty allowanceMeasurement/configuration uncertainty.

Required safety air gapUser-entered company/pilot/local requirement.

Overhead Obstruction Route Register

Use Direct Current Air Gap when a trusted current measurement is available. Use Reference + Water Level when clearance is published relative to a defined water-level datum or gauge. Formula: current air gap = reference clearance − (current water level − reference water level).

Type	Clearance basis	Direct air gap	Reference clearance	Reference water level	Current water level	Local uncertainty

Transit Readiness

Controlling bridge point verifiedMinimum vertical clearance is known for the intended channel track. Water-level datum / gauge verifiedReference and current water levels use the same datum where required. Current air-gap source checkedSensor/chart/authority data timing and limitations are understood. Vessel air draft verifiedLoading condition and highest relevant point are current. Lowerable equipment confirmedAny claimed reduction is operationally available and authorized. Motion / heel / roll consideredWind, waves, squat/trim effects and manoeuvring are reviewed separately as applicable. Safety air gap matches procedureUser-entered margin reflects company, pilot and local requirements. Forecast water-level change reviewedTide, river stage, surge and timing are checked close to transit.

Navigation limitation: this tool is a planning/cross-check calculator, not an authorization to transit. Bridge clearances can depend on the controlling span, channel position, tide/river stage, bridge loading/temperature, sensor uncertainty, vessel loading, heel/roll, trim and equipment configuration. Use the current official chart/ENC, bridge authority, pilot/port information and real-time air-gap/water-level data where available. Never treat a positive calculated margin by itself as proof that passage is safe.

Current methodology context: the U.S. Coast Guard defines air draft as the distance from the vessel’s highest point to the waterline and vertical clearance as the excess distance that permits passage beneath an overhead obstruction. NOAA’s Air Gap systems measure the vertical distance from a defined bridge reference point to the water surface and update frequently because water level and bridge conditions change. NOAA’s Lower Mississippi bridge-clearance method demonstrates the datum principle directly: current river stage is subtracted from the charted reference clearance to obtain current minimum clearance. This tool generalizes that relationship without assuming one universal datum or safety air gap.

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