Fog can reduce visibility from miles to meters in a matter of minutes. Unlike a storm, which announces itself with wind and waves, fog arrives quietly and eliminates the single most important sense a mariner relies on: sight.
Collision risk increases sharply, familiar landmarks vanish, and even experienced mariners can become disoriented. Knowing what to do before, during, and after a fog encounter is a core seamanship skill for every vessel operator, from a recreational boater to a licensed deck officer on a commercial vessel.
Why Fog Is Dangerous at Sea
Fog does more than block the view ahead. Sound behaves unpredictably, bouncing off the water and refracting through moist air. A foghorn may sound like it is coming from a completely different direction. Depth perception disappears, and without a visible horizon, a helmsman can drift off course unknowingly.
On commercial vessels, radar and AIS provide electronic situational awareness, but small vessels, kayaks, and floating debris may not produce a usable radar return. Dense fog is defined as visibility below half a nautical mile. Very poor visibility, below a quarter mile, creates conditions where visual detection may occur too late for effective avoidance.
What to Do When Fog Closes In
When visibility starts to deteriorate, a series of immediate actions reduces risk. Treating fog as an emergency from the first moment it appears prevents the small delays that lead to dangerous situations.
Reduce Speed Immediately
COLREGs Rule 6 requires every vessel to proceed at a safe speed at all times, accounting for visibility, traffic density, and the vessel's stopping distance. In fog, safe speed means slow enough to stop the vessel within half the distance of visibility. A vessel that cannot stop before reaching an object it can see is moving too fast. On commercial vessels, reducing speed to maneuvering speed or dead slow is standard practice. On recreational boats, bringing the throttle back to idle or bare steerage is appropriate.
Post a Dedicated Lookout
COLREGs Rule 5 requires a proper lookout by sight, hearing, and all available means at all times. In fog, this obligation intensifies. On a commercial vessel, the master should be called to the bridge immediately, and an additional lookout should be posted forward where engine noise is reduced. On a smaller vessel, assign a passenger or crew member to the bow to listen and watch. Reducing engine noise periodically, even briefly, allows the lookout to hear fog signals, breaking waves, or the sound of another vessel's engine.
Switch On Navigation Lights
Navigation lights must be displayed during any period of restricted visibility, regardless of the time of day. Sidelights (red to port, green to starboard), a stern light, and a masthead light make the vessel visible to other vessels that may detect it visually at close range. On smaller boats, turning off all unnecessary white lights (cabin lights, spotlights) preserves the crew's night vision and reduces glare that can make fog appear thicker.
Sound the Correct Fog Signals
COLREGs Part D requires specific sound signals in restricted visibility. A power-driven vessel making way through the water sounds one prolonged blast (four to six seconds) every two minutes. A power-driven vessel underway but stopped sounds two prolonged blasts every two minutes. A vessel at anchor rapidly rings a bell for five seconds at intervals not exceeding one minute. Vessels over 100 meters at anchor also sound a gong aft immediately after the bell forward. A sailing vessel underway sounds one prolonged blast followed by two short blasts every two minutes.
Fog signals are not optional. Failure to sound the correct signal in restricted visibility is a COLREGs violation and may result in legal liability if a collision occurs.
Fix Your Position and Monitor Continuously
Before visibility drops further, fix the vessel's position on the chart or electronic plotter. Mark the position, note the time, and begin dead reckoning from that point. Monitor the GPS or chartplotter continuously to track the vessel's progress relative to charted hazards, channel boundaries, and shallow water.
Cross-check the electronic position against the depth sounder. Comparing actual water depth to charted depths provides an independent confirmation that the vessel is where the GPS says it is. On vessels equipped with ECDIS, anti-grounding alarms and CPA/TCPA alerts provide automated safety warnings that are especially valuable when visual detection is impossible.
Use Radar and AIS Effectively
Radar is the primary collision avoidance tool in restricted visibility. Set the radar to an appropriate range scale, typically three to six miles, and watch for developing targets. On commercial vessels, engage the Automatic Radar Plotting Aid (ARPA) to track target movement, calculate the closest point of approach (CPA), and determine the time to CPA (TCPA).
AIS supplements radar by providing identity, course, speed, and position data from other AIS-equipped vessels. However, not all vessels carry AIS, and small craft may be invisible to both radar and AIS. Relying on radar and AIS alone without maintaining a listening watch is not compliant with COLREGs.
Proper navigation equipment, regularly maintained and operated by trained crew, is the foundation of safe fog navigation aboard any vessel.
COLREGs Rules for Restricted Visibility
Rule 19 governs the conduct of vessels not in sight of one another. When a vessel detects another by radar alone, Rule 19 requires the vessel to determine if a close-quarters situation is developing. If the risk of collision exists, avoiding action must be taken in ample time.
Rule 19(d) contains a critical restriction: a vessel shall avoid altering course to port for a vessel forward of the beam, except when overtaking. Altering to starboard is generally the safest response to a radar contact detected ahead or nearly ahead. Altering to port risks turning into the path of the approaching vessel.
For vessels detected abeam or abaft the beam, the rules require caution but do not restrict the direction of course alteration as strictly. The key principle throughout Rule 19 is early, substantial action. Small adjustments are dangerous because they may not change the radar picture quickly enough to resolve the situation. Operating with proper ECDIS alarm settings for CPA and TCPA ensures the bridge team receives timely warning of developing risks.
When to Anchor or Seek Shelter
Anchoring is a valid option in fog, provided the vessel is away from shipping channels, fairways, and traffic separation schemes. Stopping in a busy channel is far more dangerous than proceeding at a safe speed with proper precautions. Once at anchor, switch to the at-anchor signal (bell every minute), display an anchor light, and maintain a radar watch for approaching traffic.
Identifying safe anchoring options along the intended route as part of pre-departure passage planning is a simple precaution. Vessels carrying nautical charts showing designated anchorage areas can identify shelter options before conditions deteriorate.
Fog Lifts, Lessons Stay
Every fog encounter reinforces the same lesson: preparation and procedure matter more than experience alone. A vessel with a trained crew, working radar, a current chart, and a captain who follows COLREGs will come through safely.
A vessel without those fundamentals is at risk regardless of how many voyages the operator has completed. For navigation charts, bridge instruments, and maritime publications, contact American Nautical Services at +1 (954) 522-3321 or sales@amnautical.com.
Frequently Asked Questions
Q. What sound signal should a power vessel make in fog?
A power-driven vessel making way sounds one prolonged blast (four to six seconds) every two minutes. A power-driven vessel underway but stopped sounds two prolonged blasts every two minutes.
Q. What is the correct fog signal for a vessel at anchor?
A vessel at anchor rapidly rings a bell for five seconds at intervals not exceeding one minute. Vessels over 100 meters also sound a gong aft immediately after the bell forward.
Q. What COLREGs rule applies to fog navigation?
Rule 19 governs the conduct of vessels in restricted visibility. Rule 19 requires early, substantial avoiding action when risk of collision exists and prohibits altering course to port for a vessel forward of the beam, except when overtaking.
Q. Should navigation lights be on during fog in the daytime?
Yes. COLREGs require navigation lights to be displayed during any period of restricted visibility, regardless of whether fog occurs during daylight or darkness.
Q. Is anchoring in fog a good idea?
Anchoring can be a safe option if the vessel is away from shipping channels and traffic. Once anchored, the vessel must sound the correct at-anchor fog signal (bell every minute) and display an anchor light.
Q. What radar range should be used in fog?
A range of three to six miles is typical for fog navigation, allowing enough time to detect, track, and avoid developing targets. Shorter ranges may be appropriate in confined waters or heavy traffic.
