hi there
I was thinking a Harbour Defence Launch, but, when you look at the size of the cabin door to the wheelhouse; take it that is say between 5-6 foot in height; that would give an approximate length of say between 42-46 foot - way short of a Harbour Defence Launch - which are 72 foot in length.
Tend to agree with what Christian says, i.e. a gentleman's luxury yacht
However, I would have thought the 1930's-1940's era. If it is a Military style vessel - then - how's about an Admiral's Launch? mmmm possibility