It means a vessel is going to back up.
three short blast
A boat is using astern propulsion and is in the act of backing up.
"I am about to turn to starboard".
Three short blasts means you are operating in reverse. One prolonged blast followed by 3 short blasts means boat under tow in redcued visibility conditions.
Two short horn blast from a vessel means that she is turning port and one short horn blast that she is turning starboard
A. Three long blast of the ships whistle or general alarm at an interval of three, means a man overboard. This is dismissed by three short whistle blast. B. Three Short blast can also be Rule 34 (Inland and International) "Maneuvering and Warning Signals." Three short blast means "I am operating astern propulsion."
One short blast of a horn mean while boating may mean a couple of things depending on where you are. Some of the meanings of the short blast include request for drawbridge to open, intention to pass of the left side and intention to change course to starboard among others.
A boat is astern propulsion and is in the act of backing up
Toot
You could 'toot' your horn...
toot
When two boats approach each other, they use horn blasts to signal the other about their intentions when it comes to moving to avoid collision. 5 short blasts indicates that the signaling boat is unsure about what the other boat is doing.