Annex III — Sound signal appliances
Rule 33 says which vessels carry a whistle, a bell and a gong. Annex III says what those appliances must be. The whistle's pitch by vessel length, the audibility ranges and the warning that comes with them, where a whistle is fitted, and the bell and gong — written for the watchkeeper who has to hear them.
The technical details of sound signal appliances, in three sections. Section 1, whistles: frequencies and range of audibility, limits of fundamental frequencies, intensity and range of audibility, directional properties, positioning, fitting more than one whistle, and combined whistle systems. Section 2, the bell or gong: intensity and construction. Section 3, approval: by the appropriate authority of the flag state.
Rule 32(a) defines a whistle as a sound signalling appliance capable of producing the prescribed blasts that complies with Annex III. Rule 33(a) requires a whistle on a vessel of 12 metres or more, a bell as well at 20 metres or more, and a gong as well at 100 metres or more — all complying with Annex III.
Section 1(a): its fundamental frequency lies within 70–700 Hz. Section 1(b) narrows that by length, 'to ensure a wide variety of whistle characteristics': 70–200 Hz for a vessel of 200 metres or more; 130–350 Hz for 75 metres but less than 200 metres; 250–700 Hz for less than 75 metres.
In practice, the deeper the note, the bigger the ship is likely to be — but the bands overlap, so pitch is a clue, not a measurement.
Section 1(c) gives audibility ranges by length: 2 nautical miles for a vessel of 200 metres or more; 1.5 for 75 metres but less than 200 metres; 1 for 20 metres but less than 75 metres; 0.5 for less than 20 metres. Alongside them it sets the minimum sound pressure level each whistle must produce, measured one metre from it in its direction of maximum intensity.
The ranges are 'for information'. The annex describes each as approximately the range at which the whistle may be heard on its forward axis with 90 per cent probability, in still air, on a vessel with average background noise at her listening posts.
It then warns that in practice the range is 'extremely variable' and depends critically on the weather, and may be much reduced in strong wind or high background noise at the listening post.
Not necessarily. Section 1(d) allows a directional whistle to be up to 4 dB below the prescribed level on its axis in any horizontal direction within 45 degrees of the axis, and up to 10 dB below it in any other horizontal direction — set 'so that the range in any direction will be at least half the range on the forward axis'.
As we read it, abeam or astern of a vessel with a directional whistle, you may hear her at as little as half the range you would from ahead. A directional whistle used as the only whistle is fitted with its maximum intensity straight ahead (1(e)).
Section 1(e): as high as practicable, to reduce interception of the sound by obstructions and to minimise the risk of hearing damage to people on board. At the vessel's own listening posts her signal must not exceed 110 dB(A), and so far as practicable should not exceed 100 dB(A).
Whistles more than 100 metres apart are arranged so they are not sounded simultaneously (section 1(f)), and Rule 34(f) says only one of them is used for manoeuvring and warning signals.
Where obstructions would leave a zone of greatly reduced signal level, a combined whistle system is recommended (1(g)): whistles not more than 100 metres apart, sounded together, each differing in frequency from the others by at least 10 Hz. For the rules, a combined system counts as a single whistle.
Section 2(a): a bell, gong or other device with similar sound characteristics produces a sound pressure level of not less than 110 dB one metre from it.
Section 2(b): bells and gongs are made of corrosion-resistant material and designed to give a clear tone. On a vessel of 20 metres or more, the mouth of the bell is at least 300 mm across. Where practicable a power-driven striker is recommended, for a constant force, but manual operation must be possible. The striker's mass is not less than 3% of the bell's.
In or near an area of restricted visibility, at anchor or aground. Under Rule 35(g), a vessel at anchor rings the bell rapidly for about five seconds at intervals of not more than one minute; at 100 metres or more she rings it in the forepart and then sounds the gong rapidly for about five seconds in the after part. Rule 35(h) adds three separate and distinct strokes on the bell before and after the rapid ringing for a vessel aground. A vessel engaged in fishing at anchor, or restricted in her ability to manoeuvre and working at anchor, sounds the Rule 35(c) signal instead (35(d)).
Only a vessel of 100 metres or more sounds the gong (Rule 35(g)), and Rule 33(a) requires its tone and sound to be such that it 'cannot be confused with that of the bell'. So, in our view, a gong straight after the bell tells you a vessel at anchor or aground is 100 metres or more — and, with the bell forward and the gong aft, roughly where her ends are.
It may come up as a question about the equipment, or about why you cannot rely on hearing a signal. Examples of the kind of question to be ready for:
“What frequency range is the whistle on a 250-metre ship?”
“At what range should you hear the whistle of a 60-metre yacht?”
“In fog you have not heard anything. Does that mean nothing is near?”
“Which vessels carry a bell, and which carry a gong?”
“What is a combined whistle system?”
The common slips: treating the audibility ranges as a guarantee rather than a figure for still air; mixing up the 75-metre and 200-metre length bands; forgetting that a directional whistle may carry only half as far off its axis; and confusing which vessels carry a bell and which a gong. In our view, the point of the ranges for a watchkeeper is the warning attached to them: not hearing a signal is not evidence that no vessel is there.
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