One prolonged blast from the horn of a vessel serves as a warning to other ships that an unseen vessel is in the water. Any vessels within earshot of the prolonged horn blast should reply with a prolonged blast of their own. A prolonged blast is four to six seconds long and is commonly used in situations of low visibility such as fog or blind corners on a river.
Smaller vessels entering or exiting a marina may issue a prolonged blast to warn other vessels that a boat is moving through the area. Situations that require prolonged blasts should be dealt with cautiously and at low speeds. Low visibility situations include obstructions in the way such as docks, equipment and outcroppings.
In Virginia, rules require powered vessels to make one prolonged blast every two minutes in areas of little or no visibility. A powered vessel underway, but not moving, sounds two prolonged blasts in succession during low visibility to warn other ships that a nonmoving ship is nearby. A sailing vessel gives a prolonged blast, followed by two short blasts, in low visibility every two minutes to signal warnings.
Boat signals on the water are used to communicate intentions to other vessels. Short blasts indicate passing on the port or starboard, moving towards the stern or a danger signal when another vessel's intentions are unclear.