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 | Bright, sharp, high-pitched metallic strike | Deep, low, resonant — must be distinct from the bell |
| Position on board | Forepart | After part |
| When used | All anchor and aground bell signals (Rule 35) | 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) | — | — |
| Fog underway — making way / stopped / NUC / RAM / sailing / fishing / towing (Rule 35a-d, f) | yes | — | — |
| Vessel at anchor in fog (Rule 35g) | optional warning (1 short + 1 prolonged + 1 short) | yes — rapid ~5 s every ≤ 1 min, forward | yes — rapid ~5 s after the bell, aft, only ≥ 100 m |
| Vessel aground (Rule 35h) | optional warning | yes — 3 distinct strokes + rapid + 3 distinct strokes | yes — same as anchor, only ≥ 100 m |
| Distress (Rule 37) | continuous sounding; SOS in Morse | — | — |
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 in doubt about another vessel's intentions, use the prescribed warning signal early rather than waiting for the situation to deteriorate.
Exam Focus
Three short blasts are astern propulsion, not a fog signal.
In restricted visibility, think 'every two minutes' for underway signals and 'every one minute' for anchor bell signals.
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
Bell goes in the forepart, gong in the after part — sounded one after the other at anchor in fog
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 (since the 2009 Annex III amendments)
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
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