According to the navigation rules, which must be aboard a 22-foot powerboat

The navigation rules for countries in the Unites States are compiled in the U.S. Coast Guard. There are actually boating courses that are recognized by the U.S Coast Guard. One of the things they discuss there is the emergency aids every boat should have depending on the size and purpose. For recreational vessels with lengths measuring less than 39.4 feet, they must carry aboard an efficient sound producing device. This will serve as your signal when emergencies happen and you have to get the attention of nearby boat vessels. Hence, for practical purposes, every boat should have a whistle or a horn.

The answer is a 0.64 volts