Fog is the quiet emergency. There is no gust to warn you, no wave to brace against, just a slow shrinking of the horizon until the bow pulpit becomes the edge of the world. Most skippers who get caught out did not misread the forecast, they underestimated how much their decision-making relies on being able to see. Navigating in poor visibility is less about heroics and more about a small set of disciplines you apply before, during and after the visibility drops.

Anticipating fog before it forms

Fog is a temperature and humidity problem, and the forecast usually tells you it is coming if you know what to look for. Advection fog, the classic Channel and Atlantic killer, forms when warm moist air moves over colder water. Watch the sea surface temperature against the air dew point : when the two converge to within a degree or so, expect fog. Radiation fog forms overnight in sheltered water and burns off with the sun. Sea fog does not. It can sit for days over the same patch of cold current and drift with the wind.

Practical pre-departure checks:

  • Compare SST charts (many national services publish daily) with the forecast dew point along your route.
  • Look at wind direction. Southwesterlies over the western Channel in spring and early summer are a classic advection setup.
  • Check METAR reports from coastal airfields on your route. If Cherbourg or Guernsey is reporting BR or FG at dawn, take the hint.
  • Note the sea state. Very light winds and a glassy surface let the boundary layer stay cold and wet.

If your passage plan involves a busy area during a high-risk window, build in an escape option before you leave the dock. A fog contingency added at sea, in a shipping lane, with a tired crew, is not a plan. It is a reaction. This is especially true when crossing the English Channel, where the tidal gate can force you into the TSS just as visibility collapses at dawn.

The COLREGs rule 19 mindset

Rule 19 governs the conduct of vessels in restricted visibility, and it is worth re-reading every season because it changes the game. There is no stand-on vessel in fog. Every vessel is required to proceed at a safe speed adapted to the prevailing circumstances, and to take avoiding action in ample time based on radar or sound alone.

Two specific requirements catch skippers out:

  • If you detect another vessel by radar only, you must determine whether a close-quarters situation is developing and, if so, take action in ample time. Waiting to see the other boat visually is exactly what the rule tells you not to do.
  • Avoid altering course to port for a vessel forward of your beam (except when overtaking), and avoid altering towards a vessel abeam or abaft the beam. These defaults exist because the other skipper is applying the same logic.

Safe speed in fog is almost always slower than you think. On a planing motorboat, it usually means coming off the plane entirely. On a sailboat, it means being able to stop within half the visible distance ahead. If you can see 80 metres, you need to be able to stop in 40. That is a hard exercise the first time you try it. If your rule 19 memory is patchy, take ten minutes with our refresher on the COLREGs right of way rules.

Radar and AIS: what each actually tells you

In fog, radar and AIS are your eyes, and they do not see the same things. Understanding the difference keeps you alive.

Radar shows you anything with a return : ships, buoys, cliffs, breaking waves, rain, and eventually the small fibreglass yacht that has no AIS. Its weakness is interpretation. A cluttered display in a chop, with rain overhead, will hide a small target until it is uncomfortably close. Practical habits:

  • Tune gain, sea clutter and rain clutter manually. Auto modes are conservative and hide small returns.
  • Run two range scales. A longer one (6 to 12 NM) for situational awareness, a shorter one (0.75 to 1.5 NM) for imminent traffic and navigation marks.
  • Use MARPA or ARPA if you have it, but verify the vector against the raw echo. Auto-acquired targets can jump.
  • Practise in clear weather. Learn what your own boat's wake looks like, how buoys paint at different ranges, where your radar has blind arcs from the mast or arch.

AIS shows you cooperating vessels : commercial traffic, most fishing boats, and yachts with a transponder. It gives you name, CPA, TCPA and a heading vector, which is gold in fog. Its weakness is that anything without a transponder is invisible to it. A rowing skiff, an unlit mooring, a small open fisherman recovering pots : none of these will appear. AIS complements radar, it does not replace it. If you are unsure how to squeeze real value out of your MMSI display, our guide on reading and interpreting AIS to avoid collisions covers the CPA and TCPA logic in detail.

The two together give you a workable picture. Radar tells you something is there. AIS tells you who and where they intend to go. When both agree, you have a target. When only radar shows something and AIS is silent, treat it as a small vessel and give it room.

Sound signals, lights and lookout

Electronic aids do not remove the requirement for a proper lookout by sight and hearing. In fog, hearing is the one that matters.

Standard sound signals under rule 35, on a whistle or horn, at intervals of not more than two minutes:

  • Power-driven vessel making way : one prolonged blast.
  • Power-driven vessel underway but stopped : two prolonged blasts, separated by about two seconds.
  • Sailing vessel, vessel not under command, restricted in ability to manoeuvre, constrained by draught, engaged in fishing, or towing : one prolonged followed by two short blasts.
  • Vessel at anchor : rapid ringing of the bell for about five seconds, at intervals of not more than one minute. For vessels over 100 metres, add a gong aft.

Post a bow lookout, ideally with headphones off and radio traffic muted on deck. Engine noise from your own boat masks a huge amount. If you are motoring, throttle back to idle every few minutes and listen. It sounds excessive until the first time you hear a fishing boat you had no radar contact on.

Turn on your navigation lights, whatever the time of day. It costs nothing and helps the vessel that briefly gets you visually before you get them. Have the deck flare pack and the sound-signal appliance within reach of the helm, not stored in a locker under the sail bag.

Once you are in fog, the plan changes. You are now flying on instruments. Three things matter : position, traffic, and destination.

  1. Position. Log your GPS position every ten minutes on the chart, even if the plotter is showing it live. If the plotter dies, you want a paper fallback with a recent fix, a heading and a speed. Cross-check with depth. If the sounder disagrees with the chart, trust the sounder.
  2. Traffic. One crew on radar and AIS, full-time. Not sharing the job with cooking, not doing it from below over the shoulder of the helm. In busy waters, this is a dedicated station.
  3. Destination. Reconsider it. A harbour entrance you know well in clear weather can become genuinely dangerous in fog, especially one with a narrow channel, cross-tide, or ferry traffic. Anchoring in shallow water outside the shipping lane, well clear of the fairway, is often the mature call. Wait it out.

Approaching buoyage in fog is its own exercise. Buoys paint on radar at close range, but identifying them by top mark and colour requires visual contact you may not get until you are almost on top of them. Know the sequence of marks on your approach, the light characteristics, and the direction of buoyage on the chart. Our short primer on reading IALA buoyage at a glance is worth a re-read before any coastal passage where fog is forecast.

Finally, if the situation escalates, do not let pride delay the call. VHF channel 16, a securité broadcast to warn nearby traffic of your position and intentions, or a full DSC alert if you are in genuine trouble, is the right tool. If your VHF setup is anything less than instant to use, fix that before the next passage. Our guide on sending a Mayday and using DSC distress covers the procedure that saves the minutes that count.

What good preparation looks like

The skippers who handle fog well are not braver or luckier. They have rehearsed the drills, know their instruments cold, and treat the first wisp of grey on the horizon as a decision point rather than a curiosity. They slow down early, put a lookout forward, start their timing on the sound signal, and stop pretending the plotter is the boat.

An Oria Box logging your NMEA data in the background will not clear the fog, but it will give you a clean voyage replay afterwards : your track, your speed, your engine load and the moment you decided to alter course. That is how a skipper actually learns from a foggy passage, rather than remembering it as a blur. What did you do differently after your last one?