Personal work, 2020. My first concept to be illustrated from the aft section. The hull shape was inspired by Japanese "Koryu" Midget Submarines (or Mini Submarine) from c.1945. I don't imagine this would be a Starfleet Vessel but definitely Terran with similar technology; i.e. warp nacelles and sub-light impulse engines. I will admit a little Star Wars inspiration made it into the finish, akin to the chrome Naboo Cruisers.