Unable to see more than a few feet and to focus on a fixed point of reference in fog causes spatial disorientation. If safe navigation of the craft is to be ensured, the application of sensible boat-handling strategies is essential.

Fog represents a greater danger than rough weather to a yacht. Visibility is reduced making the yacht vulnerable to a collision. Resist putting to sea unless the fog is land-bound and conditions at sea are clear. If sailing when fog descends take steps to increase safety, the yacht’s visibility and use the correct sound signals.

Fog Forecasting

Professional forecasters warn sailors of impending foggy conditions, but fog forecasting with a degree of success is possible if there is a humidity meter on the craft. When humidity reaches 90 to 95 percent, and sea water temperature is 5°C to 10°C, there is a high chance of fog.

An accurate way of fog forecasting is the use of a hygrometer to find the dew point temperature of the air. A hygrometer consists of two thermometers, one wet bulb, one dry bulb and the readings from these determines the relative humidity of the air. With the air's relative humidity, its dew point temperature is found by referring to tables. If dew point temperature is a few degrees below the actual air temperature, expect fog as night approaches.

If a hygrometer is not available, listen to the weather forecast transmissions, principally during settled conditions of high pressure and in spring and autumn. Weather Instruments

Initial Reaction

When fog appears when sailing, the safety of the crew and yacht should be the skipper’s your first consideration. Establish your position as exactly as possible at the onset of mist or fog. Knowing where you are, or cannot possibly be, is the essence of safe conduct with reduced visibility. Update your estimated position thereafter, at half-hourly intervals or when making major changes in direction.

Pay attention to tidal streams as they still function as whey would on a sunny day. Double-check all estimated positions and fixes, and verify them by using the echo sounder. If using GPS navigation, use known and tested waypoints. GPS Navigation Devices - Depth Sounders

Unless there is one permanently fitted, hoist a radar reflector high up the rigging as possible. All commercial vessels and many yachts use radar as a means of collision avoidance and it is for this reason a reflector is used. Switch on all navigation lights, delegate a crew member to use the fog horn and station extra lookouts equipped with a number of white flares. If motoring, place a lookout on the foredeck away from the noise of the engine. Radar and Sonar - Flares and Emergency Signals - Signal Horns

The miniaturization and availability of electronic navigation equipment has changed the approach of long-time sailors to the dangers of fog and mist. Their ability to revert to using navigation skills when necessary is an advantage over those who rely completely on electronics.

Sight a buoy or other object which will provide an estimation of your visibility to others, or drop a bundle of paper over the side and find the time it takes before disappearing from sight. Having knowledge of the yacht's speed allows the distance to be worked out. If the yacht is fitted with radar, station an experienced crew member to monitor it. Do not rely on radar alone and still have lookouts on deck.

Personal Safety

• Have all the crew put on life jackets, as there will little time if a collision occurs.
• Consider carefully before having the crew clip on safety harnesses, as they must be able to jump clear in a collision.
• If the weather is rough, the greater danger of falling overboard may require harnesses to be used.
• The life-raft should be at the ready for instant release if needed.
• Have red and white flares at hand.
• In calm weather, consider putting the dinghy over the side, towing it astern and being available in the event of a collision.

Sailing Strategy

Strategy when sailing in fog depends on whether the yacht is close to land, in a busy shipping area, or well out to sea.

Strategy Out to Sea

• Continue on course if out to sea, keeping a lookout and making sound signals.
• If possible, proceed under sail, as engine noise limits the ability to hear other boats.
• If in calm conditions and motoring is the only option, move at a practical speed to give good steerage way enabling the yacht to turn quickly if necessary.
• Turn off the engine at regular intervals and listen for other vessels or navigation aids.
• The mainsail should be hoisted, making the yacht visible to another vessel.

Strategy in Shipping Lanes

• If in or near a busy shipping lane the first priority is to move out of it as soon as possible.
• Sail into shallow water where there are no large vessels and if crossing a shipping channel, cross at right angles.

• Establish the exact position of the boat and use the radio to make contact with surrounding vessels.
• If there is suspicion that the yacht’s position is not being tracked by other vessel’s radar, send a Securite message giving the yacht’s position.

Strategy in Shallow Water

• When in shallow water, either heave-to or anchor until the fog lifts and keep a lookout for other vessels that are also seeking shallow water.
• When a harbour is nearby with a safe entrance and there is confidence in your navigation skills, make for it and do not rely solely on electronic aids to ensure your safe entry.
• Using a depth sounder permits making way close the shoreline by choosing a seabed contour to follow. Correct for the height of the tide, and then change direction with some degree of confidence. Depth Sounders
• Approaching land or a chosen harbour, estimate your time of arrival, consult the tidal stream atlas, and initially aim for a point some miles upstream of your actual destination.

Fog Horn Sound Signals

In fog, make use of sound signals to indicate your presence to other boats. Navigational aids, such as buoys and lighthouses, are fitted with sound signal apparatus so that they can be identified. The signal type emitted is marked on charts and in pilot books. Do not assume a direction for the sound as it can be distorted by fog so stop and double-check then proceed with caution.

A craft’s presence, its size, and activity is indicated with a combination of fog horn, bell, and gong signals. A boat under 12m (39ft) carries an sound signal in the form of compressed-air or aerosol fog horn. Larger boats over 12m (39ft) carry a bell, and vessels over 100m (328ft) will also use a gong. Signal Horns

The use of appropriate sound signals in foggy conditions indicate whether sailing or motoring, aground, or at anchor. Fog horn signals are either prolonged (four to six seconds) or short (one second). A bell can be sounded as a single ring or as a rapid ringing for five seconds, and a gong is rung rapidly. The common signals are listed below. Keep a reference book on board detailing the full list of signals as required by the International Regulations for Preventing Collisions at Sea.

• Under sail (and some other vessels): One long and two short fog horn blasts every two minutes
• Making way under power: One long fog horn blast every two minutes.
• Under way but not making way: Two long fog horn blasts at two-minute intervals.
• Aground - under 100m (328ft): Three bells, rapid ringing, three bells, at one-minute intervals.

• Aground - over 100m (328ft): Three bells, rapid ringing, three bells, a gong sounded aft, every minute.
• At anchor - under 100m (328ft): Rapid ringing of bell forward in boat at one-minute intervals.
• At anchor - over 100m (328ft): Rapid bell ringing forwards, gong sounded aft, at one-minute intervals.
• Pilot boat on duty: Four short blasts (after under way or making way) every two minutes.