Fog is the condition that separates sailors who've built real navigation skills from those who navigate by sight and luck. On the Maine coast in July, the visibility can drop to zero in fifteen minutes and stay there for three days. On the California coast, the summer fog bank is a permanent fixture from June through August. Learning to navigate in fog is not optional for anyone cruising north of Cape Cod or anywhere along the Pacific Coast — it's a basic competency, like anchoring or reefing. The good news is that the skills are learnable and the tools available to a modern cruising sailor are genuinely capable.
Fog Types: What You're Actually Dealing With
Radiation fog forms overnight when the land surface cools and the air just above it reaches its dew point. It tends to be shallow — rarely more than a few hundred feet deep — patchy over the water, and thicker in low-lying areas and valleys. The characteristic behavior: it burns off once the sun heats the surface, usually by mid-morning. If you wake up to radiation fog and the barometer is steady, the forecast is clear, and the sky above the fog appears bright, you can often wait it out for a few hours. Don't make a potentially dangerous approach in radiation fog when you could simply wait 90 minutes for visibility to return.
Advection fog forms when warm, moist air moves over cold water. This is the dominant fog type along both the US coasts and in the Gulf of Maine. The cold Labrador Current keeps Maine coastal waters cold through summer; warm, humid air from the south rides over this cold water surface and condenses. The result is a dense, persistent fog that is not going to burn off — the mechanism that created it is still operating. Advection fog can persist for multiple days, filling in quickly after any brief clearances. When the fog rolls in off the Maine coast in mid-July, it is not radiation fog and it is not going away by noon. Plan the passage around it, not through it.
Sea fog is similar to advection fog in mechanism and behavior. Along the California coast, the North Pacific High drives cool, upwelling water to the surface while warm air flows over it. The fog bank extends far offshore in summer and moves in and out of the bays on the afternoon sea breeze — dense and cold in the mornings, sometimes clearing for a few hours midday, banking back in by late afternoon. San Francisco Bay fog is famous for a reason. Predicting it: if the temperatures at the surface and the dewpoint are within 5°F of each other, fog is forming or imminent. NOAA forecast discussions (available at weather.gov) often explicitly address whether marine layer fog will clear inshore — worth reading the full discussion text, not just the marine forecast headline.
Radar: The Most Important Instrument in Fog
A functioning radar is the difference between a navigational challenge and a genuine crisis in thick fog. Radar gives you two critical pieces of information: range and bearing to targets (ships, buoys, headlands, other boats), and the relative motion of those targets over time. It does not require visibility. It works through fog, rain, and darkness. It is not optional for serious coastal cruising.
The practical differences in hardware matter. A 4kW open array on a cruising boat 35 feet and longer has significantly better range, target separation, and performance in rain than the small dome units common on boats under 35 feet. The rotating open array provides better horizontal beam geometry, which means closer targets are displayed more accurately and the picture at 2–6 miles is sharper. Small dome units — 18-inch, 2kW — are adequate for picking up large ships and headlands but lose detail in clutter and small-target discrimination. In fog with moderate rain, a small dome's picture can become a white smear just when you need it most. If you're buying a radar for a coastal cruising boat, spend the money for a 4kW open array or, at minimum, a high-performance dome. The Garmin Fantom and Furuno DRS series have solid small-target performance; a used Furuno 1835 is one of the best values in offshore cruising radar.
Learn your radar before you need it in anger. Spend time interpreting targets in clear weather when you can verify what you're seeing. The fishing boat that shows as a small, faint target at 1.5 miles on your radar — practice identifying it when visibility is 3 miles, not when it's 200 yards.
Radar Navigation: Making Sense of the Picture
Parallel indexing (described in the GPS and charts article) is the primary technique for maintaining a safe track in restricted visibility using radar. The additional technique specific to fog navigation is target identification: learning to reliably distinguish between categories of radar returns.
Fishing boats are among the most dangerous targets in fog because they are numerous, slow-moving, may not be keeping a proper watch, and often cluster in areas where you want to be. They appear as small, bright targets that may seem stationary at first glance — they're moving, just slowly. A lobster boat sitting to haul a trap will show as a nearly stationary target at first, then begin moving unpredictably as the skipper repositions. Give them a wide berth and treat any small, slow target as maneuvering unpredictably.
Commercial shipping — a tanker, a container vessel — shows as a large, extended return at range, often with a smearing effect on the leading edge due to the vessel's length. These targets move fast, typically 12–18 knots. The critical difference from a lobster boat: they will not maneuver to avoid you. A ship in a shipping lane following a course in fog is tracking its own route; it may have radar watch, it may not. COLREGS give you no special protection. The practice: determine the closest point of approach and time to CPA early, when there is ample sea room to maneuver, not when the target is 0.5 miles away.
Landmasses and headlands return large, clearly defined returns with a hard edge. Buoys return small, often intermittent targets — they may be in your picture, they may not, depending on their radar reflector size and the sea state. Don't rely on picking up a buoy in moderate sea conditions.
Sound Signals: The Rule and the Reality
COLREGS Rule 35 requires vessels underway in restricted visibility to sound signals: one prolonged blast at intervals not exceeding two minutes for a power vessel making way, one prolonged blast followed by two short blasts for a vessel not under command, restricted in maneuverability, or constrained by draft. A vessel at anchor sounds a rapid bell for about five seconds at intervals not exceeding one minute (vessels over 100 meters also sound a gong aft). Sailboats under sail are required to sound one prolonged blast followed by two short blasts.
The reality offshore and in coastal waters: very few vessels comply. Ships don't sound fog signals because they have radar and AIS and consider the signals irrelevant. Pleasure boats don't sound them because most sailors don't know the rules or don't have a horn that can be heard more than 200 yards. This doesn't change your obligation — it means you cannot assume that a vessel not sounding signals is not there. Sound your own signals. They may not help you avoid a ship, but they establish your legal position and may alert a nearby boat that's not watching their radar.
What you should actually do: station a bow watch whenever visibility drops, have someone specifically tasked with listening and watching for other vessels. Two sets of ears beat one. And in shipping lane approaches, shorten your own watch intervals on the radar regardless of what the rules say about sound signals.
Safe Speed: What Rule 19 Actually Means
COLREGS Rule 19 governs conduct in restricted visibility. The critical language: "Every vessel shall proceed at a safe speed adapted to the prevailing circumstances and conditions of restricted visibility." There is no specific knot limit. Safe speed is a judgment that takes into account your radar capability, how much warning time the radar gives you, the density of traffic, proximity to hazards, and your ability to stop or maneuver.
The practical calculation: if your radar sees 0.5 miles (the typical effective range of a small dome in fog with clutter), and a ship is approaching at 15 knots while you're making 6 knots, you have roughly 90 seconds from detection to collision if you're on a directly opposed track. At that closing speed, a radar with 0.5-mile effective range in traffic is barely adequate to avoid a casualty even if you react instantly and have sea room to maneuver. This is the argument for slowing down: a lower speed gives you more time to act on what the radar shows.
Near shipping lanes, slowing to 3–4 knots in thick fog is not excessive caution — it's arithmetic. In an empty anchorage or a well-marked channel with no traffic, 6 knots is entirely reasonable. The rule is a judgment call, not a number. Make the judgment explicitly rather than defaulting to normal speed because you have radar.
Traffic Separation Schemes: Stay Out of the Lanes
Traffic separation schemes (TSS) are the maritime equivalent of divided highways, established in areas of heavy commercial traffic. On the US East Coast: the separation zones off Cape Hatteras, the approaches to New York, Boston, and other major ports. On the West Coast: the Santa Barbara Channel and the approach to San Francisco. The IMO designates these areas specifically because the collision risk at the lane boundaries is the highest in coastal navigation.
The rule: sailing vessels under 20 meters in length and fishing vessels are not supposed to impede the passage of vessels using the separation scheme. In practice, this means getting out of the inbound and outbound lanes as quickly as possible and crossing them at a right angle if you must cross at all. In fog, this is not negotiable. A ship in a lane at 14 knots in poor visibility is following its programmed course; it is not hunting for your boat, but it is also not going to yield to you. Cross quickly and at 90 degrees so your transit is as brief as possible, confirm the crossing on radar, and maintain a radar watch throughout.
Anchoring in Fog
Anchoring in fog is sometimes the right call — when the target anchorage is more than a few miles off, visibility is zero, and the approach has tight margins. The choice of anchorage in fog should weigh: good holding (you're not able to watch your position as carefully as normal), good depth (no tide surprises), and most critically — no through traffic. An anchorage in the bight of a headland that commercial traffic typically rounds close aboard is dangerous in fog. A cove well off the main track, with a reliable bottom and no tidal complications, is fine.
COLREGS require an anchor light — a 360° white light visible for 2 miles at anchor — plus the sound signal (rapid bell for 5 seconds, every minute). The anchor light is non-negotiable in fog. The bell requirement is frequently ignored; in fog in a busy anchorage, following it is seamanlike even if your neighbors don't. Sound carries further in fog than most sailors expect.
The Fog Approach: Working Into an Entrance You Can't See
The single most dangerous moment in fog navigation is the final approach into a harbor or anchorage. This is where groundings happen. The technique that makes it survivable: never put your approach waypoint on the entrance — put it 200 to 300 yards off, in clear water, with known depths and no hazards. Navigate to that waypoint precisely. Confirm your position with multiple fixes if possible (depth sounder against chart, radar range to a headland). Only then transition from instrument navigation to visual piloting.
Slow down well before the waypoint. At 2 knots you have time to assess what the radar, depth sounder, and sound signal tell you. Post a bow watch specifically tasked with listening and watching for any visual indication of breaking water, buoys, or shore. Have the anchor ready to let go instantly. Know the abort plan — where you turn and motor back offshore if the approach goes wrong.
This sequence — off-waypoint approach, multiple confirmations, slow speed, bow watch, anchor ready — works. Fog approaches done at hull speed with a chartplotter waypoint on the entrance buoy, trusting that the GPS position is accurate and the chart is current, account for a disproportionate share of coastal groundings. The difference between the two approaches is about 20 minutes of extra caution. Apply it.