Rule 33 specifies the required sound signal equipment by vessel length.
Bell vs gong — how to tell them apart
| Aspect | Bell | Gong |
|---|---|---|
| Required from | ≥ 20 m | ≥ 100 m |
| Tone | Bell sound complying with Annex III | Tone and sound must be distinct from the bell; Rule 33 does not prescribe a particular pitch |
| Sounding positions in Rule 35(g), vessels ≥ 100 m | Forepart | After part |
| When used | Anchor and aground signals under Rule 35(g)/(h), subject to 35(d), (i) and (j) | Only on vessels ≥ 100 m, sounded immediately after the bell, in the after part of the vessel |
Which appliance for which signal?
| Situation | Whistle | Bell | Gong |
|---|---|---|---|
| Manoeuvring & warning in sight (Rule 34a-d) | yes | — | — |
| Bend / blind-channel warning (Rule 34e, 9f) | yes (1 prolonged) | — | — |
| Underway in restricted visibility: ordinary power-driven, special vessel or rigid composite unit (Rule 35a-c, f) | prescribed whistle sequence, subject to the small-vessel alternatives | — | — |
| Fishing at anchor, or RAM carrying out work at anchor (Rule 35d) | 1 prolonged + 2 short, at intervals ≤ 2 min | Rule 35(g) anchor signal is replaced | Rule 35(g) anchor signal is replaced |
| Ordinary anchor signal in restricted visibility (Rule 35g; exceptions below) | optional warning (1 short + 1 prolonged + 1 short) | yes — rapid ~5 s every ≤ 1 min, forward on vessels ≥ 100 m | yes — rapid ~5 s after the bell, aft, only ≥ 100 m |
| Aground signal in restricted visibility (Rule 35h; exceptions below) | optional appropriate whistle signal | 3 distinct strokes + rapid ringing for ~5 s + 3 distinct strokes, intervals ≤ 1 min | same as anchor, only ≥ 100 m |
| Distress (Rule 37) | continuous sounding; SOS in Morse | — | — |
Under Rule 35(d), a fishing vessel at anchor and a RAM vessel carrying out her work at anchor give one prolonged and two short blasts instead of the ordinary anchor signal. Under Rule 35
- 1, a vessel 12 m to less than 20 m need not give the bell signals in (g)/(h), but if she does not, she must give another efficient sound signal at intervals not exceeding two minutes. Under Rule 35(j), a vessel under 12 m may use that efficient-signal alternative instead of the signals listed in Rule 35.
A vessel stopped while still underway does not become a vessel at anchor. Use the signal for her actual status. For the ordinary anchor/aground signal on a vessel ≥ 100 m, the gong follows the bell.
Whistle
≥ 12 m
Bell
≥ 20 m
Gong
≥ 100 m
IMO COLREG 1972Official text
Full wording of the rule, reproduced from the official source below.
(a) A vessel of 12 meters or more in length shall be provided with a whistle, a vessel of 20 meters or more in length shall be provided with a bell in addition to a whistle, and a vessel of 100 meters or more in length shall, in addition, be provided with a gong, the tone and sound of which cannot be confused with that of the bell. The whistle, bell and gong shall comply with the specifications in Annex III to these Regulations. The bell or gong or both may be replaced by other equipment having the same respective sound characteristics, provided that manual sounding of the prescribed signals shall always be possible.
(b) A vessel of less than 12 meters in length shall not be obliged to carry the sound signaling appliances prescribed in paragraph (a) of this Rule but if she does not, she shall be provided with some other means of making an efficient sound signal.Signal Workflow
Decide the context first: manoeuvring in sight, restricted visibility, or distress.
The same whistle can mean very different things in a different context.
Check both the pattern and the interval.
For fog signals, the time spacing is part of the rule, not just the blast sequence.
When vessels in sight are approaching and you do not understand the other vessel’s intentions or actions, or doubt its collision-avoiding action, immediately sound at least five short, rapid blasts (Rule 34(d)).
Exam Focus
Three short blasts are astern propulsion, not a fog signal.
Rule 35(a)–(f) signals have a maximum interval of 2 minutes; the anchor and aground signals in (g)–(h) have a maximum interval of 1 minute.
Paragraphs (i)–(j) provide small-vessel alternatives.
Key Takeaways
≥ 12 m: whistle required
≥ 20 m: bell added (in addition to whistle)
≥ 100 m: gong added (in addition to whistle and bell)
Gong's tone must NOT be confusable with the bell's
For the ordinary Rule 35(g) anchor signal on a vessel ≥ 100 m, sound the bell forward then the gong aft; fishing/RAM-at-work and small-vessel alternatives are specified separately in Rule 35
Bell and gong may be replaced by equivalent equipment, but manual sounding must always remain possible
Common Mistakes
Thinking a 12 m vessel needs a bell — bell is only required from 20 m onward (under Resolution A.910(22), effective 29 November 2003)
Forgetting the gong on vessels ≥ 100 m
Believing the bell and gong sound the same — by Annex III they must be distinguishable
Sounding bell and gong simultaneously — at anchor in fog they go in succession (bell forward, then gong aft)
Small vessels (< 12 m) carrying no means of making any efficient sound signal
Test Your Knowledge
Practise this topic and review the explanation for each answer.
More rules in this Part
- Sound Signals in Restricted VisibilityIn restricted visibility, power-driven vessels making way sound one prolonged blast every 2 minutes. Vessels not making way sound two prolonged blasts.
- Signals to Attract AttentionIf necessary to attract attention, a vessel may make light or sound signals that cannot be mistaken for any prescribed signal.
- Distress SignalsWhen in distress and requiring assistance, vessels shall use the signals described in Annex IV (red flares, SOS, MAYDAY, DSC, EPIRB and others).
Last reviewed: