There are two different laws in play here. First of all, switchable exhaust systems are illegal in New Hampshire, regardless of the sound level. The db level is a separate issue altogether. The allowable sound level is determined by the year of the engine manufacture. In most cases, this is the same year the boat was built, unless the boat has been repowered. Throw in the third law that requires mufflers or underwater exhaust and you can understand that noise enforcement is quite involved.
For the record, through the transom exhausts are not illegal as long as there are mufflers installed in the pipes and the resulting noise level is within the requirements for that model year boat. FYI, all boats built since December 31, 1981 are required to meet the same sound level, which is 82dba @50 feet. Recently, a static idle test has been implemented as a supplement to the 50 foot test. That level is higher due to the nature of the test.
There is an exception to the muffler rule regarding classic or antique boats. Those vessels are exempt from mufflers as long as they do not exceed the sound level for their vintage, which could be as high as 86dba measured at 50 feet.
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