Air draft becomes a real concern the first time a route includes a low bridge or a marina offers a covered slip. The number on a specification sheet is helpful, but it often doesn’t describe the yacht at the dock today. Antennas get replaced, domes are added, arches move, loading changes, and the bridge clearance itself depends on the water below it.
The arithmetic is the easy part. The harder job is making sure both measurements describe the same moment: the yacht’s highest point in its current configuration and the opening available at the planned time. A dependable answer starts with those two facts and keeps every allowance visible.
Measure the yacht as it sits today
Begin with the builder’s published air draft, then check what that figure includes. Our current 590 Pilothouse specifications list 21′ 4″ to the top of the hardtop, or 18′ with the optional folding radar arch. Those dimensions identify specific builder configurations. A 590 that has since received a taller antenna or another upper-deck installation needs a current onboard measurement.
Stand back far enough to see the full profile and identify the highest item. Often it’s an antenna tip rather than the hardtop or radar arch. Check communications domes, anchor lights, weather instruments, cameras, flagstaffs, and any brackets added after delivery. If an item comes off for a passage, record where it is stowed and how the yacht is measured with it removed.
Measure from the actual waterline to the highest fixed point using a method suited to the boat and location. Note the date and loading condition, since fuel, water, stores, tender gear, and other weight affect where the hull floats. The result should describe one unmistakable configuration, such as “normal cruising setup, antennas raised,” rather than a generic boat height.
Keep the specification and the onboard measurement together. The first tells you what the builder published; the second tells you what must clear the obstruction now.
Know what changes when the arch folds
A folding radar arch can reduce air draft, but only the installed equipment determines the reduced profile. Confirm that the exact arch is designed to move and follow its documented operating instructions. A hinge alone doesn’t answer either question.
Next, trace every item attached to or near the arch. Some antennas and lights move with it, while a dome or sensor on a separate support stays upright and becomes the new high point. Wiring, mounts, latches, and anything that must be disconnected or secured belong in the operating procedure for that yacht.
Record two complete measurements:
| Yacht state | What to record |
|---|---|
| Normal cruising profile | Loading condition, equipment positions, highest item, measured height, and date |
| Documented reduced profile | Arch position, anything removed or secured, highest remaining item, measured height, and date |
Don’t calculate the reduced height by subtracting a catalog dimension for the arch. Measure the yacht in both states. Repeat the measurement after changing an antenna, dome, light, camera, or upper-deck bracket, even if the new equipment looks no taller from the dock.
Match the bridge number to the water
Published bridge clearance and current available clearance are related, but they aren’t interchangeable. The published figure refers to a stated reference level. Rainfall, river flow, reservoir operations, tide, storm effects, and local conditions move the water away from that reference.
The NOAA United States Coast Pilot includes bridge and cable clearances as well as regional navigation information, with weekly updates available. NOAA Tides and Currents provides water-level observations and predictions. Use the bridge’s official identifier, the correct Coast Pilot edition and update, and the gauge or station identified by the controlling authority. A station many miles away won’t necessarily represent the water at the bridge.
Bridge boards are another useful observation when present, but read them carefully. Note which side of the channel they apply to, the time, and the boat’s position. A board reading doesn’t erase the charted reference or an official notice; it adds current information at the bridge.
This distinction comes up often when planning the Great Loop, where one route can include fixed bridges, movable spans, locks, rivers, and tidal water. Old cruising notes still offer local context, though their clearance numbers belong to the conditions and date when they were recorded.
Confirm that a movable bridge will operate
Enough room beneath a raised span doesn’t mean it will open on demand. Schedules, advance-notice rules, construction, maintenance, equipment trouble, and temporary restrictions can all change a transit.
Check the bridge owner or controlling authority for the normal operating procedure, then look for current changes. The USCG Local Notice to Mariners for the correct district carries temporary navigation information and is updated weekly. Read the issue date, effective dates, bridge name, and any contact instructions. A notice from last month might describe work that has ended or a restriction that hasn’t started yet.
USCG also publishes active public notices for proposed bridge activities. Those pages concern proposals and public comment; they aren’t a report of today’s operating status. For the actual passage, current instructions from the bridge authority and navigation notices carry the useful detail.
Save the phone number or radio procedure with the route notes. If the planned opening, water level, or equipment status remains uncertain, resolve it before arrival rather than trying to reconstruct the source while holding position near the bridge.
Find the lowest point inside a covered slip
Marina listings often give an entrance or roof height, yet the limiting point sometimes sits farther inside. Door tracks, beams, braces, lights, conduit, or roof hardware can hang below the advertised dimension. The yacht also needs enough room along the entire path into its final berth, not merely at the opening.
Ask the marina to identify the exact slip and the lowest obstruction over the intended path. The measurement should include its location, date, and the water or dock condition used. In a tidal marina, the relationship between a floating dock, fixed structure, and water level changes differently from the relationship at a fixed dock. The operator who knows the structure should explain which condition the figure represents.
Then compare that low point with the yacht’s measured normal and reduced profiles. A folded arch doesn’t help if a fixed weather sensor remains higher, and a comfortable entrance doesn’t help if a door track farther in sits lower. Photos with a marked path can make the marina’s measurement easier to understand, though they don’t replace a current dimension.
Air draft is only one part of berth selection. Our guide to moorage planning for 45- to 60-foot cruising yachts also covers dock length, beam, depth, shore power, access, and day-to-day use.
Keep one clearance sheet for the boat
A one-page record prevents a specification, field measurement, bridge notice, and marina estimate from blending together. Give each number its own source and date.
For the yacht, record the loading condition, arch state, removable equipment, highest item, and measured height. For a bridge, add the official name, published clearance and reference, current water information, gauge or board reading, operating schedule, and temporary notices. For a covered slip, add the slip number, entrance height, true low point, location of that obstruction, and the marina’s measurement date.
Leave unknowns blank and label them. A blank field is easier to fix than an assumption buried inside a final number. Refresh the sheet whenever upper-deck equipment changes and again close to the transit, when water levels and bridge operations are current.
No worksheet turns an uncertain clearance into a guarantee. It does, however, show exactly which measurement needs checking and who can answer the remaining question. For help identifying the documented configuration of a North Pacific Yacht, contact our team with the equipment list and your latest onboard height measurement.