A whistle or horn
whistle
Vessels that are required to carry a whistle or horn but not a bell typically include recreational boats, such as small sailboats and motorboats under 20 meters in length. These vessels are often subject to specific regulations based on their size and type, which may not mandate the presence of a bell. Instead, a whistle or horn is used for signaling in situations where audible communication is necessary. Always check local regulations for any variations.
A whistle or horn. (Ref: Eq-38)
a whistle or a horn
A whistle.
Toot
Train whistle
toot
A whistle or horn.
A whistle.
It means "I am altering my course to starboard"