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Just as lights play a significant role in understanding what other boats are doing, so do sounds.  Understanding what you hear is another step towards being a "complete mariner".  Virtually every boat is required to have some sound producing device.  There is a great deal of latitude in what type of sound making device you choose, but loud is good!

Legal Requirements:

Equipment for Sound Signals is based on the length of your boat as follows:
  • Boats less than 39.4 feet in length - must carry an efficient sound producing device. In general, this may be
    a bell, whistle, or air horn.  Though guns--even pots and pans--can make a suitable sound signal useful in
    getting attention in an emergency, you should always carry the appropriate equipment.
  • Boats at least 39.4 feet to less than 65.6 feet in length - Must carry a whistle and a bell. The whistle must
    be audible for 1/2 nautical mile. The mouth of the bell must be at least 7.87 inches in diameter.
When and how to sound off

Sound signals are to be used only when vessels are in sight of each other and are meeting or crossing at a distance within half a mile of each other. These signals must never be used in fog or other conditions of reduced visibility, where the vessels are not visible to each other by eye. Only the fog signals listed under the Inland Rules, Rule 35 may be sounded at such time.

Signals

Sound signals are called "blasts".  There are two different blasts used for warning and steering signals.
  • Short Blast - Lasts about one second.
  • Prolonged Blast - Lasts from four to six seconds.