46 C.F.R. § 90.10-23
This term means any vessel indicated in Column 5 of table 90.05-1(a) 65 feet in length or less which is propelled by machinery (including steam). The length shall be measured from end to end over the deck excluding sheer. This term includes a boat temporarily or permanently equipped with a detachable motor. For the purpose of this subchapter, motorboats are included under the term "vessel" unless specifically noted otherwise. The various classes of motorboats are as follows:
Class A-Any motorboat less than 16 feet in length.
Class 1-Any motorboat 16 feet or over and less than 26 feet in length.
Class 2-Any motorboat 26 feet or over and less than 40 feet in length.
Class 3-Any motorboat 40 feet or over and not more than 65 feet in length.
46 C.F.R. §90.10-23