Will you clear the bridge? Subtract height of tide from charted clearance, then compare remaining height to your air draft plus a safety margin.
Use this when comparing charted bridge clearance, height of tide, your air draft, and a safety margin before committing to a transit.
Enter charted clearance, tide height, air draft, and margin. Pass if remaining ≥ margin.
Not a substitute for local notices to mariners, bridge tender VHF, or measuring air draft with antennas/radar arches up.
Available clearance is charted vertical clearance minus the height of tide above the chart datum. Remaining clearance is available minus your air draft. You “pass” the check when remaining ≥ safety margin.
available_ft = charted_clearance_ft − height_of_tide_ft
remaining_ft = available_ft − air_draft_ft
pass if remaining_ft ≥ safety_margin_ft
Confirm datum (MHW, MHHW, etc.) on your chart. Waves, squat, and antenna flex are not modeled.
Defaults: charted 45, tide 3, air draft 38, margin 2 → available = 42; remaining = 4 ≥ 2 → PASS.
| Charted | Tide | Air | Margin | Remain | Call |
|---|---|---|---|---|---|
| 45 | 3 | 38 | 2 | 4 | PASS |
| 40 | 4 | 36 | 2 | 0 | NO |
| 50 | 2 | 42 | 3 | 6 | PASS |
| 35 | 1 | 32 | 2 | 2 | PASS |
| 60 | 5 | 48 | 2 | 7 | PASS |
Rows: remain = charted − tide − air; pass if remain ≥ margin.
Assumptions: charted clearance datum matches the tide height you enter; air draft includes antennas you will not lower; margin is yours.
Where it fails: unpublished temporary scaffolding, river stages not in your tide number, and soft canvas biminis that still snag.
Not a substitute: planning aid only — when in doubt, wait for the tender or a lower tide.
Charted clearance is usually above a stated datum. Higher tide reduces available air gap — subtract tide height as the calculator does.
Height from waterline to your highest fixed point (mast, arch, antenna) in the configuration you will transit.
Under-keel is depth under the boat. Bridge clearance is air gap above.
Treat as no-go unless you have a verified lower configuration and local confirmation.