Fog and restricted visibility

Fog & night · Power & sail

In fog the Navigation Rules change how you must behave: go at a safe speed with engines ready, keep a lookout by sight and hearing, sound the fog signals and use radar properly. AIS helps, but many boats don't transmit it.

What the Rules require

• Rule 19 applies when vessels cannot see each other in or near restricted visibility. Every vessel must go at a safe speed for the conditions, and a power-driven vessel must have its engines ready for immediate maneuver.
• Rule 6 lists what sets a safe speed: visibility, traffic, your stopping distance and turning ability, background lights at night, wind, sea and current, depth, and the limits of your radar. There is no fixed number.
• Rule 5 requires a proper lookout at all times by sight, hearing and every available means. In fog, post a lookout forward, away from engine noise, to listen for signals, bells and surf.
• Rule 19(e): if you hear another vessel's fog signal apparently forward of the beam, or can't avoid a close-quarters situation with a vessel forward of the beam, slow to the minimum speed that keeps steerage, stop if needed, and go with extreme caution until the danger is past.

Sound signals (Rule 35)

• Power-driven vessel making way: one prolonged blast (4 to 6 seconds) at intervals of no more than 2 minutes. Underway but stopped: two prolonged blasts about 2 seconds apart, at the same interval.
• Sailing vessels, fishing vessels, and vessels towing, not under command or restricted in their ability to maneuver: one prolonged and two short blasts at intervals of no more than 2 minutes.
• At anchor: ring the bell rapidly for about 5 seconds at least every minute, and you may add one short, one prolonged and one short blast to warn a vessel approaching. Boats under 20 m need not use a bell but must make some other efficient sound every 2 minutes.
• A 42 ft (12.8 m) boat is over 12 m, so Rule 33 requires it to carry a whistle, and the underway signals apply in full. Boats under 12 m may use any efficient sound signal instead.

Radar, AIS and navigation

• Rule 7 requires proper use of radar if fitted and working, including long-range scanning and plotting or equivalent systematic observation. Don't draw conclusions from scanty radar information.
• Rule 19(d): if you detect a vessel by radar alone and a close-quarters situation is developing, act early. As far as possible, avoid turning to port for a vessel forward of the beam (unless you are overtaking it) and avoid turning toward a vessel abeam or abaft the beam.
• If a target's compass bearing isn't changing appreciably as it closes, risk of collision exists (Rule 7). Take repeated bearings on radar targets to check.
• AIS is required only on certain commercial vessels, such as self-propelled commercial vessels 65 ft or longer. Many recreational boats, fishing boats and paddlecraft don't transmit it, so a clear AIS screen doesn't mean clear water. AIS never replaces lights and sound signals.
• Turn on your navigation lights in fog, day or night (Rule 20). If you can't fix your position, the NWS suggests staying put in a safe spot until the fog lifts. If you have an outside helm, use it; if not, open a window so you can hear.

Fog in the Northwest

• Advection fog forms when warm, moist air moves over colder water. It can persist day or night, and a slight increase in wind can even thicken it. The USCG handbook notes fog is frequent along the Pacific coast from northern Washington to Santa Barbara.
• The NWS issues a Dense Fog Advisory when visibility drops to a locally set limit of 1 nm or less. Listen for it on NOAA Weather Radio before you leave and while underway.

General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.