On a marine vessel lights are required, and to the surprise of many, so is a horn or whistle. Boats in length less than 39ft must carry a sound production device. This could be a bell, whistle, or air horn. Vessels larger than 39ft must carry a whistle and a bell. The whistle must be audible for 1/2 nautical mile. Understanding the different types of signal "sound blasts" will increase your awareness, and enhance your safety.