Service motor boat
A launch carried by a lighthouse tender or buoy tender.
These boats are strongly built and are often certified as approved lifeboats, with a capacity to suit the crew of the tender. Their service duty is the transportation of personnel, stores or working equipment to and from the tender.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM