Coast Guard navigation rules require sound signals to be made during periods of reduced visibility and when meeting, crossing and overtaking other vessels. Any vessel less than 39  feet in length may carry a whistle or horn, or some other means to make a loud sound signal to alert others of your presence in reduced visibility or your navigation intentions.

Sound Signals
Every vessel is required to carry a sound producing device and are required to sound signals any time that they are in close quarters and a risk of collision exists.

The term “short blast” means a blast of about one second.

The term “prolonged blast” means a blast of from four to six seconds.

The following signals are the only ones to be used to signal a vessel’s intentions (inland

rules only).

One short blast - I intend to change course to starboard.

Two short blasts - I intend to change course to port.

Three short blasts - I am operating astern propulsion (backing up).

Five or more short and rapid blasts - Danger or doubt signal (I don’t understand your intent).

For more information about portable boat air horns and sound signaling devices go to Falcon Signal Air Horns.