Boating in Fog: When to Stay In, Sound Signals, Safe Speed and Using Your Electronics

Check the marine forecast for fog, dense fog advisories or "visibility less than 1 nautical mile." The National Weather Service says fog is usually considered dense for mariners once visibility drops below 1 mile, and its marine forecasts describe visibility as very poor (under 0.5 nautical mile), poor (0.5 to under 2), moderate (2 to 5) or good (over 5). On land, a Dense Fog Advisory usually means visibility of a quarter mile or less. NOAA Weather Radio on your VHF's weather channels also broadcasts updates on fog forming, moving or lifting. Look out at the water too: if you can't see the far channel markers from the dock, stay put or wait for it to burn off. Morning fog often lifts by late morning; Mercury notes it commonly sets in early and clears as the day goes on.

If you go out on a day when fog is possible, plan a route that keeps you in water you know, away from shipping lanes, with a sheltered spot where you could anchor.

Do this at the dock, not when the fog is already around you:

• Horn: test it. A canister air horn runs out fast when you're giving a long blast every two minutes; BoatUS recommends buying canisters several at a time. Keep a loud whistle as a backup.
• Lights: check that navigation lights work. The rules require them in restricted visibility, day or night.
• Electronics: chartplotter and depth sounder on, radar warmed up if you have one, VHF on channel 16.
• Compass: make sure you can read it at the helm and know your heading home.
• Life jackets on. In fog, a person who falls in can vanish from sight in seconds.

The rules say every vessel must go at a safe speed for the conditions, and a power boat must have its engine ready to maneuver right away. There's no fixed number. Mercury's rule of thumb: go about half as fast as you think you could, so you have time to see and avoid another boat that may be coming at you just as fast. If you can only see 100 feet, you need to be able to stop well inside 100 feet.

Going slower also keeps the engine quieter so you can hear other boats (step 5).

Under Rule 35 (33 CFR 83.35), in or near restricted visibility, day or night, at intervals of not more than 2 minutes:

• Power boat moving through the water: one prolonged blast (4 to 6 seconds).
• Power boat stopped and drifting, engine running but no way on: two prolonged blasts about 2 seconds apart.
• Sailboat, fishing vessel, boat towing, or boat that can't maneuver: one prolonged blast followed by two short blasts.
• At anchor: ring a bell rapidly for about 5 seconds at least every minute. You may also give one short, one prolonged and one short blast to warn an approaching boat.

Small boats: a boat under 12 meters (about 39 feet) isn't required to give these exact signals, but if it doesn't, the rule says it must make "some other efficient sound signal" at intervals of not more than 2 minutes. In practice, most small-boat owners simply use the signals above with a horn, which other boaters will recognize. Boats 12 to 20 meters don't have to carry out the bell signals but must then use another efficient sound signal.

Set a phone timer so nobody forgets. See boat horn and sound signals for the full list.

Rule 5 requires a proper lookout by sight and hearing at all times. In fog:

• Put one person on lookout who does nothing else, ideally at the bow if it's safe.
• Turn off the stereo and keep conversation down. BoatUS notes fog tends to hold sound close to the water, so you may hear engines and buoys sooner than on a clear day, but the direction can be deceiving.
• Every so often, stop and shut the engine off for a minute to listen. The BoatUS Foundation recommends this for getting your bearings near big ships.

Under Rule 19, if you hear another boat's fog signal apparently ahead of your beam, slow to the minimum speed that keeps you on course, stop completely if needed, and go with extreme caution until the danger is past.

With nothing to look at, it's easy to steer in slow circles. Mercury suggests watching the line where the water meets the fog rather than staring into the gray, and asking a crew member to watch your wake astern and tell you if you're curving. Don't keep turning around yourself; you'll lose your reference each time. Follow a compass or chartplotter heading, and keep checking it.

Your plotter shows where you are, but it doesn't show other boats (unless they send AIS) or every rock. BoatUS describes a cruiser that went hard aground in fog at about 5 knots while the skipper navigated by radar; radar doesn't see underwater ledges. Watch the depth sounder constantly and know what depth means you're getting too close to shore. A bottom-view fish finder makes shoaling easy to spot.

Lay a track or route from the harbor mouth to your dock on a clear day. A BoatUS writer whose radar failed in thick fog inside a crowded harbor later set an autopilot track down the middle of the channel for exactly this situation.

Two traps owners have written about:

• A frozen plotter looks normal. A Practical Sailor contributor sailing in zero visibility found his chartplotter had stopped updating his position, while it still let him zoom and scroll the chart. Every few minutes, check that your position, speed and the depth sounder agree with each other, and keep a second GPS source (a phone app or handheld) ready to compare.
• GPS heading wanders at very low speed. A Boating Magazine reader creeping through dense fog found the boat was drifting so much that the GPS "heading" kept changing, and he had no compass aboard. At a crawl, steer by the compass and use the plotter for position.

• Radar shows boats, buoys and shorelines, but only if you know how to read it. BoatUS stresses that a single look doesn't tell you where a target is heading; you need several observations over time (plotting) to see whether it's a collision course. Rain and waves create clutter, and small boats can be hard to see on it.
• AIS shows ships and many larger boats with name, course and speed, but only those that transmit. Most small recreational boats don't, so an empty AIS screen doesn't mean empty water. See AIS setup.
• Be seen: a properly mounted radar reflector helps ships pick you up, but BoatUS says not to rely on it alone.

If you don't already know your radar well, treat it as a bonus, not your main tool. See radar care.

Mercury warns that boats navigating by electronics in fog tend to steer straight for the channel markers on their plotters, which funnels traffic there. Don't stop next to a marker, in a channel or in a shipping lane. Big ships can't stop or turn quickly, and the BoatUS Foundation says to avoid crossing a ship channel in fog if you can. If you must cross, cross at a right angle, quickly and at a safe speed, after listening carefully.

Also stay away from the shoreline unless you know it's clean. BoatUS describes a boater who hugged the beach to avoid getting lost and ended up on a reef; open water free of hazards was the safer choice.

If you're not comfortable continuing, the best move is often to stop. Get well clear of channels, traffic, surf and strong current, set the anchor with plenty of scope (BoatUS suggests 5:1 is better than 3:1), and wait. Keep your anchor light on, keep sounding your anchored signal, keep a lookout and keep listening. See anchor rode and scope.

If you're drifting with no anchor that will reach bottom, sound the "stopped" signal (two prolonged blasts) and keep watch.

• Lost and drifting toward hazards, or a near-collision you can't sort out: call the Coast Guard on VHF 16 and give your best position (your plotter's latitude and longitude).
• Engine trouble or low fuel in fog: anchor first, then call your tow service.
• Overdue: if you planned to be back and won't be, call or text the person holding your float plan so they don't panic.