Mont. Code Ann. § 23-2-511

23-2-511 . Operation of unnumbered motorboats or sailboats prohibited -- display of registration decal. (1) A motorboat on the waters of this state that is propelled by a motor or an engine of any description or a sailboat on the waters of this state must be properly numbered and display a valid registration decal. A person may not operate or give permission for the operation of any motorboat or sailboat on the waters of this state unless the motorboat or sailboat is numbered and displays a valid registration decal in accordance with this part and applicable federal law or with a federally approved numbering system of another state and unless:

(a) the certificate of number assigned to the motorboat or sailboat is in effect;

(b) the identifying number set forth in the certificate of number and the valid registration decal are displayed on the motorboat or sailboat; and

(c) a temporary permit has been obtained from the county in which a motorboat is being operated if that county requires a temporary permit for out-of-state motorboats, as provided in 7-16-2121 .

(2) Upon transfer of ownership of a motorboat or sailboat, the transferred motorboat or sailboat may be operated on the waters of this state for 40 consecutive calendar days immediately following the transfer of ownership without displaying the registration decal required by subsection (1) if, when the motorboat or sailboat is operated during those 40 consecutive calendar days, a temporary registration permit has been issued under 61-3-224 and is shown to a warden or other officer upon request.