Home Cruise Sound Signals in Restricted Visibility and Rule 35

Sound Signals in Restricted Visibility and Rule 35

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Sound Signals in Restricted Visibility and Rule 35

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Fog, mist, falling snow, heavy rainstorms, sandstorms or any other similar causes can make conditions that hamper visibility. Sound signals for vessels in restricted visibility, whether underway, at anchor or aground, day or night, are the subject of Rule 35 (COLREGS). The bell and gong are added into the mix of whistle sounds used as options for restricted visibility signals. Rule 35 is fairly complex, and it is advised to have a highlighter in hand while reading through it.

The Rules of the Road state when underway ‘vessels shall sound at intervals of not more than two minutes’ the prescribed sound signal.

Why a two minute rule? A requirement for all vessels is to maintain a proper look-out by sight and hearing. In restricted visibility, the two minutes allows the look-out to listen for sound signals from other vessels groping their way through the fog. COLREGS lists the following sound signals for vessels underway:

  • A power-driven vessel making way shall sound one prolonged blast.
  • A power-driven vessel underway but stopped shall sound two prolonged blasts.
  • A pilot vessel, engaged in pilotage duty, may also sound four short blasts in addition to the sound signals required by a power-driven vessel.
  • Vessels that are encumbered, due to their maneuverability or their ranking in the hierarchy of responsibilities between vessels in Rule 18, shall sound one prolonged blast followed by two short blasts. These encumbered vessels are those not under command, restricted in their ability maneuver due to their work, constrained by draft, sailing, engaged in commercial fishing and towing or pushing another vessel.


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  • If a vessel being towed is manned, that tow or barge shall sound one prolonged blast followed by three short blasts. The tug itself first sounds its own signal for an encumbered vessel, and if possible, the manned tow sounds its own signal immediately after.
  • While at anchor, ringing the ship’s bell rapidly for about five seconds at intervals of no more than one minute is the primary sound signal for smaller vessels.
  • Larger vessels, those 100m (328ft) or more in length, also use a gong.

For these large yachts and ships, the bell is rapidly sounded from the foredeck, and immediately followed by the gong sounded rapidly from the afterdeck, both for about five seconds.

  • In addition, any vessel at anchor can also sound one short, one prolonged and one short blasts as a collision avoidance warning.

There are two classes of working vessels that do not use the bell and gong signals while at anchor.

Commercial fishing boats that are fishing while at anchor, and vessels restricted in their ability to maneuver that are carrying out their work while at anchor must use the encumbered vessel sound signal of one prolonged and two short blasts.

  • If aground, a vessel shall give the bell signal and if required the (at anchor) gong signal prescribed … and shall, in addition, give three separate and distinct strokes on the bell immediately before and after the rapid ringing of the bell.

To further finesse Rule 35, The International Regulations for Preventing Collisions at Sea (IRPCS), doesn’t oblige the bell signals to be used at anchor and when aground for vessels 12m (39ft) or more, but less than 20m (65.6ft) in length. Instead, she shall make some other efficient sound signal at intervals of not more than two minutes.

Vessels less than 12m LOA are NOT required to use any of the sound signals required for vessels in restricted visibility. They also can make some other efficient sound signal at intervals of not more than two minutes.


With the large number of sound signals available for use in restricted visibility, remembering all the possibilities can be difficult. Trying to quickly look up a sound signal heard in the fog by turning to Rule 35 in COLREGS can be a bit of a challenge.

A good alternative is to use one of the many laminated visual shorthand versions of COLREGS sold at ship chandleries.

For sound signals, a Morse code dot represents the short blast of about one-second duration. And a Morse code dash represents the prolonged blast of four to six seconds long.

Here’s the link to reference the Related, Rule 34 describing maneuvering and warning for powered vessels underweigh in international waters

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