A vessel nearing a bend in a channel or fairway where other vessels may be obscured by an intervening obstruction may sound one prolonged blast. If an approaching vessel that may be behind the obstruction hears the signal, it should be answered by one prolonged blast in response.

Note: These are not the only rules concerning sound signals in restricted visibility; there are additional rules concerning sound signals that may be found in the Rules of the Road or in Chapman's Piloting.