Five short blasts from a boat on the water signal that the pilot of the boat doubts the action of another nearby craft trying to avoid a collision, according to the New South Wales Roads & Maritime Services. These signals also mean the pilot of the ship has concerns about the proximity, course and speed of a nearby boat. Typically, the larger of two boats makes this warning.
A short blast of a horn or whistle lasts about 1 second. A prolonged blast, which is a warning to all vessels that a ship is rounding a blind corner, is 4 to 6 seconds. One short blast means a vessel is moving to starboard, two short blasts indicate a move to port and three short blasts denote a ship is moving backwards. Depending on a vessel's movement, these blasts may indicate a ship's intent to pass.
The Roads & Maritime Services also explains smaller vessels should make their intentions obvious to nearby vessels, leave enough room for safe passage and keep distance from larger ships in sea lanes. When five short blasts are heard, all vessels involved should come to an immediate stop to ascertain what actions are to be taken next. Proper sound signals are prescribed by the U.S. Coast Guard and other maritime agencies.