Maybe, start with something that resembles the Conestoga Class.
http://team.enterprise-tcw.com/Flanker/Extra/Scifi/STARFLEETMANUAL/ConestogaClass_page021.jpg
Give the "head" part a more submarine look, engines that look like this:
A quick MSPaint concept.
http://team.enterprise-tcw.com/Flanker/Extra/Scifi/STARFLEETMANUAL/ConestogaClass_page021.jpg
Give the "head" part a more submarine look, engines that look like this:

A quick MSPaint concept.
