Thinking that we need a thread for all old (pre-1880s?) projects.
Came across HMS Duke of Kent design for a four-deck ship of the line on Reddit. c1809. Never built. Original source http://collections.rmg.co.uk/
Mark K. Ragan in his excellent book “Union and Confederate Submarine Warfare in the Civil War” (pp. 108-109) mentioned such a boat. Below is the part of the text, that is the only source of information about this novel craft:
While Marshall and the other founders of the Triton Company tried to...