Rules of the Road
Sound and light signals
Rule 34
Maneuvering and Warning Signals
International (Signals of Action)
(a.) When vessels are in sight of one another, a power driven vessel underway, when maneuvering as authorized or required by these rules, shall indicate that maneuver by the following signals on her whistle.
-One short blast to mean "I AM altering my course to starboard";
-Two short blasts to mean "I AM altering my course to port";
-Three short blasts to mean "I AM operating astern propulsion".
Inland (Signals of intent)
(a) When a power driven vessels are in sight of one another and meeting or crossing at a distance within half a mile of each other, each vessel underway, when maneuvering as authorized or required by these rules:
(i) Shall indicate that maneuver by the following signals on her whistle: one short blast to mean "I intend to leave you on my port side" (turning to starboard.); two short blast to mean "I intend to leave you on my starboard" (turning to port); and three short blasts to mean "I am operating astern propulsion"(This is the ONLY action in Inland)
Rules of the Road
Sound and light signals
Rule 34
Maneuvering and Warning Signals
International (Signals of Action)
(a.) When vessels are in sight of one another, a power driven vessel underway, when maneuvering as authorized or required by these rules, shall indicate that maneuver by the following signals on her whistle.
-One short blast to mean "I AM altering my course to starboard";
-Two short blasts to mean "I AM altering my course to port";
-Three short blasts to mean "I AM operating astern propulsion".
Inland (Signals of intent)
(a) When a power driven vessels are in sight of one another and meeting or crossing at a distance within half a mile of each other, each vessel underway, when maneuvering as authorized or required by these rules:
(i) Shall indicate that maneuver by the following signals on her whistle: one short blast to mean "I intend to leave you on my port side" (turning to starboard.); two short blast to mean "I intend to leave you on my starboard" (turning to port); and three short blasts to mean "I amoperating astern propulsion"(This is the ONLY action in Inland)
Read more: What_do_two_short_horn_blasts_from_another_vessel_mean_in_a_meeting_situation
Rule 34
International:" I AM altering my course to port."
Inland: " I INTEND to leave you on my starboard side or I AGREE to a starboard to starboard passing or I INTEND to overtake you on your portside."
They intend to pass starboard to starboard side.
In the Navy, it means there is an eminent collision.
With all due respect, this is an incorrect answer.
From the 1972 Convention of International Regulations for Preventing Collisions at Sea.
International
Sounds and Light signals
Rule 34
(D) When vessels are in sight of one another and from any cause either vessels fails to understand the intentions or actions of the other, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt SHALL immediately indicate the doubt by giving at least 5 short and rapid blasts of the whistle.
there is a dangerous situation
They intend to pass starboard side to starboard side.
Rule 34International:" I AM altering my course to port."Inland: " I INTEND to leave you on my starboard side or I AGREE to a starboard to starboard passing or I INTEND to overtake you on your portside."They intend to pass starboard to starboard side.
The Vessel is turning to Starboard .
You are operating a motorboat. You hear one prolonged blast from the horn of another vessel. What should you do?
B
One prolonged blast from the horn of another vessel indicates reduced visibility (and the presence of that vehicle). The reduced visibility may be due to geography, such as a river with significant bends, or weather features, such as fog.
In reduced visibility, it means the presence of another watercraft.
Rules of the RoadSound and light signalsRule 34Maneuvering and Warning SignalsInternational (Signals of Action)(a.) When vessels are in sight of one another, a power driven vessel underway, when maneuvering as authorized or required by these rules, shall indicate that maneuver by the following signals on her whistle.-One short blast to mean "I AM altering my course to starboard";-Two short blasts to mean "I AM altering my course to port";-Three short blasts to mean "I AM operating astern propulsion".Inland (Signals of intent)(a) When a power driven vessels are in sight of one another and meeting or crossing at a distance within half a mile of each other, each vessel underway, when maneuvering as authorized or required by these rules:(i) Shall indicate that maneuver by the following signals on her whistle: one short blast to mean "I intend to leave you on my port side" (turning to starboard.); two short blast to mean "I intend to leave you on my starboard" (turning to port); and three short blasts to mean "I am operating astern propulsion"(This is the ONLY action in Inland)
Two short horn blast from a vessel means that she is turning port and one short horn blast that she is turning starboard
Rules of the Road.Rule 35InternationalSound Signals in Restricted visibility.(c) 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, and a vessel engaged in towing or pushing another vessel shall, instead of the signals prescribed in paragraphs (a) or (b) of this rule, sound at intervals of not more than two minutes three blast in succession, namely one prolonged followed by two short blast.
Assuming you are referring to a fog horn or a warning signal, generally find where the other vessel is and change course accordingly