The Montagu whaler was a design of small boats, designed by Admiral Montagu in the 1890s to replace a variety of smaller craft of other designs with one single, versatile craft. They were in use with the Royal Navy - and associated navies such as the Royal Australian Navy - until the 1960s. After that date, they were usually passed on to other groups, including the Sea Cadets.
References
Montagu whaler Wikipedia(Text) CC BY-SA