Rules 32–37
Sound Signals
Every whistle, bell, and gong signal in the Navigation Rules — play them, read them as dash-dot patterns, and switch between International and US Inland meanings.
Maneuvering & warning
Rule 34Whistle signals between vessels in sight of one another. International signals announce an action being taken; Inland signals propose a maneuver and require the other vessel’s agreement.
“I am altering my course to starboard.” An action signal — it announces a turn already being made. Sounded on the whistle by a power-driven vessel underway, in sight of another vessel, when maneuvering as authorized or required by the Rules.
“I intend to leave you on my port side.” An intent signal, not an action signal: it proposes a port-to-port passing when power-driven vessels meet or cross within half a mile of each other. The other vessel must answer — the same signal if in agreement, or the danger signal (5 short) if in doubt.
“I am altering my course to port.” An action signal announcing a turn already being made, sounded by a power-driven vessel underway and in sight of another vessel.
“I intend to leave you on my starboard side.” A proposal for a starboard-to-starboard passing, used when power-driven vessels meet or cross within half a mile. The other vessel answers with the same signal if in agreement, or the danger signal if in doubt.
“I am operating astern propulsion.” Note it signals what the engines are doing, not necessarily that the vessel is moving astern — a ship may still be carrying way forward.
Same words under Inland Rules, but sounded within the Inland intent-and-agreement exchange used when power-driven vessels meet or cross within half a mile of each other.
“I intend to overtake you on your starboard side.” Sounded in a narrow channel or fairway when overtaking can take place only if the vessel ahead moves over (Rule 9(e)(i)). The vessel ahead answers with the agreement signal if she consents, or the danger signal if in doubt.
“I intend to overtake you on your port side.” Sounded in a narrow channel or fairway when overtaking can take place only if the vessel ahead takes action to permit safe passing (Rule 9(e)(i)).
Agreement: the vessel about to be overtaken sounds “one prolonged, one short, one prolonged and one short blast, in that order” — the Morse letter C (“affirmative”). If in doubt she sounds the danger signal instead.
“I intend to overtake you on your starboard side.” Inland overtaking proposal between power-driven vessels in sight of one another. The vessel about to be overtaken answers with a similar signal if in agreement, or the danger signal (5 short) if in doubt.
Inland Rules only — your active setting is International.
“I intend to overtake you on your port side.” Inland overtaking proposal between power-driven vessels in sight of one another. Answered with a similar signal if in agreement, or the danger signal if in doubt.
Inland Rules only — your active setting is International.
When vessels in sight are approaching and one fails to understand the intentions or actions of the other, or doubts whether sufficient action is being taken to avoid collision, she sounds at least five short and rapid blasts on the whistle. May be supplemented by at least five short and rapid light flashes.
Also the mandatory reply to a passing proposal you disagree with: if you doubt the safety of a proposed maneuver, sound the danger signal and take precautionary action until an agreement is reached.
A vessel nearing a bend or an area of a channel or fairway where other vessels may be obscured by an intervening obstruction sounds one prolonged blast. Any approaching vessel within hearing around the bend answers with one prolonged blast of her own.
“When a power-driven vessel is leaving a dock or berth, she shall sound one prolonged blast.” It warns traffic that a vessel is emerging into the waterway. There is no equivalent signal in the International Rules.
Inland Rules only — your active setting is International.
Restricted visibility (fog)
Rule 35Whistle signals repeated at intervals of not more than 2 minutes, day or night, in or near an area of restricted visibility.
“A power-driven vessel making way through the water shall sound at intervals of not more than 2 minutes one prolonged blast.” Making way means actually moving through the water under power.
“A power-driven vessel underway but stopped and making no way through the water shall sound at intervals of not more than 2 minutes two prolonged blasts in succession with an interval of about 2 seconds between them.” Underway just means not anchored, made fast, or aground — a drifting ship is still underway.
Instead of the power-driven signals, a vessel not under command, a vessel restricted in her ability to maneuver, a vessel constrained by her draft, a sailing vessel, a vessel engaged in fishing, or a vessel towing or pushing another sounds one prolonged followed by two short blasts every 2 minutes. Under International Rules, a fishing vessel at anchor or a RAM vessel working at anchor sounds this instead of the anchor bell (Rule 35(d)).
The Inland list omits “a vessel constrained by her draft” (that category exists only in the International Rules) and states that the restricted-in-ability-to-maneuver signal applies whether underway or at anchor.
“A vessel towed or if more than one vessel is towed the last vessel of the tow, if manned, shall at intervals of not more than 2 minutes sound four blasts in succession, namely one prolonged followed by three short blasts. When practicable, this signal shall be made immediately after the signal made by the towing vessel.”
“A pilot vessel when engaged on pilotage duty may, in addition to the signals prescribed in paragraphs (a), (b) or (g) of this Rule, sound an identity signal consisting of four short blasts.” It is optional and sounded alongside — not instead of — the normal fog signal.
At anchor & aground
Rule 35(g)–(h)Bell and gong signals repeated at intervals of not more than 1 minute in restricted visibility — bell forward, gong aft on vessels of 100 m or more.
“A vessel at anchor shall at intervals of not more than 1 minute ring the bell rapidly for about 5 seconds.” Vessels of 100 m or more add a gong aft (see the 100 m+ variant). Vessels of 12–20 m may instead make some other efficient sound signal every 2 minutes; under International Rules a fishing vessel at anchor or a RAM vessel working at anchor sounds the hampered-vessel whistle signal instead.
Inland Rules exempt vessels of less than 20 m, and barges, canal boats, scows, and other nondescript craft, from anchor sound signals when anchored in a designated special anchorage area.
“In a vessel of 100 meters or more in length the bell shall be sounded in the forepart of the vessel and immediately after the ringing of the bell the gong shall be sounded rapidly for about 5 seconds in the after part of the vessel.” Hearing bell then gong tells you both the vessel's presence and roughly how she lies.
“A vessel aground shall give the bell signal and if required the gong signal prescribed in paragraph (g) of this Rule and shall, in addition, give three separate and distinct strokes on the bell immediately before and after the rapid ringing of the bell.” Vessels of 100 m or more also sound the gong aft. She may in addition sound an appropriate whistle signal.
“A vessel at anchor may in addition sound three blasts in succession, namely one short, one prolonged and one short blast, to give warning of her position and of the possibility of collision to an approaching vessel.” It supplements — never replaces — the bell signal.
Distress
Rule 37 & Annex IVSignals used only to indicate distress and need of assistance.
Annex IV lists “a continuous sounding with any fog-signalling apparatus” among the distress signals used to indicate distress and need of assistance. Unlike every other signal, it never stops — that endlessness is what marks it as distress. Use only when in distress; the audio here plays a short excerpt.
Annex IV includes “a signal made by any signalling method consisting of the group · · · — — — · · · (SOS) in the Morse Code” and “a signal sent by radiotelephony consisting of the spoken word ‘Mayday’.” SOS can be sent by whistle, light, or any other method — three shorts, three longs, three shorts.
Audio is synthesized to the Rule 32 timings — short blast about 1 second, prolonged blast 4–6 seconds. Whistle pitch varies with vessel size in reality (Annex III); fog signals here use a deeper horn.