It is called a short salute and is how ships greet each other.
Five or more short blast on a ships whistle, while under way, is the signal for risk of collision.
7 or more short blasts on the ship whistle and general alarm bell followed by one prolonged blasts
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.
Passing on your right side.Signals are (number of blasts):1. Passing you on your left (port) side2. Passing you on your right (starboard) side3. I am moving backwards (astern)4. Danger, don't do that (answer to the other three if dangerous)One short blast means 'I am changing course to starboard.'Two short blasts mean 'I am changing course to port.'Three short blasts mean 'I am operating astern.'Two prolonged blasts followed by one short blast to mean 'I intend to overtake you on your starboard side';Two prolonged blasts followed by two short blasts to mean 'I intend to over-take you on your port side.'A long blast every two minutes is used when operating in fog.
There are particular sound signals that are to be used when you first get underway. One prolonged blast signifies that you are about to get underway. If you are going to make way in reverse, you need to signal an additional 3 short blasts.
Pilot vessel engaged in pilotage duties
3 short blasts
Well, honey, three long blasts followed by two short blasts from a Great Lakes ship means they're backing up, just like your ex trying to come back into your life after they realize they messed up. So, keep your distance and let that ship do its thing before it ends up in hot water.
Sailboat
5 short blasts is a signal warning of imminent danger (or disagreement).
When visibility is very limited, such as in heavy fog, boaters must give an audible signal to others in the area. For a sailboat the signal is one prolonged blast plus two short blasts every two minutes.