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Heading inshore in fog without radar



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What is the issue?
If fog descends and you are in or near a busy shipping lane the first priority is to move out of it as soon as possible and to steer for shallow water where there are no large vessels. But this also brings about the danger of land.

Why address this?
Sometimes you may not have radar and good charts for this area so you may have to improvise to keep the vessel in safer waters.

How to address this?
In fog, you must use the boat's air-horn to indicate your presence to other boats. But few know that you can position the vessel safely offshore using the mandatory airhorn.

Compressed air 'Air horn'
Photo: Courtesy of Lauzas

A vessel underway must give one prolonged foghorn horn blast every two minutes. However, you can also use the blast to position yourself using the echo from its sound signal. Listen carefully and time the echo. If the echo takes 5 seconds to travel back to you, you are about half a mile from land, 10 seconds equals about a mile. Cliffs or forested shorelines work best. A flat sandy beach won't reflect the sound as well.

If motoring, you may have to turn off the engine at regular intervals to listen to hear the echo. You should also be listening for the sound signal of other vessels or navigation aids. Vessels under 12 metres (39ft) carry a sound signal in the form of an air horn. Larger boats over 12 metres (39ft) carry a bell, and vessels over 100 metres (328ft) will also use a gong. Sound 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 meaning of the signals for boating are as follows:

  • Under sail (and some other vessels): One prolonged foghorn horn blast and two short blasts every two minutes

  • Making way under power: One prolonged foghorn horn blast every two minutes.

  • Under way but not making way: Two long 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 underway or making way) every two minutes.

Navigational aids, such as buoys and lighthouses, are also fitted with maritime sound signal apparatus so that they can be identified.
Please note

Do not assume a direction for the maritime horn signal as it can be distorted by fog so stop and double-check the direction then proceed with caution.



With thanks to:
Michael Harpur, Yacht Obsession.
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