All vessel engines must be equipped with an underwater exhaust system or effective muffling device. Vessel operators may not hear sound signals or voices if the engine is not adequately muffled.
- Motorboats and PWC may not emit noise in excess of 86 decibels measured at a distance of 50 feet. At “idle speed,” exhaust noise may not exceed 90 decibels when measured by a sound meter held one meter (about three feet) from the muffler.
- More restrictive noise standards are in effect for Flathead Lake (Flathead and Lake Counties), Echo Lake (Flathead County), and Swan Lake (Lake County) in northwestern Montana because of population density and heavy recreational use. On these waterways, noise near the shoreline may not exceed 75 decibels.
- Certain exceptions are made for state-sanctioned regattas or boat races and by special permit.