To get the look right for Warhammer, I reckon you need to be looking at ship from Henry VIII's navy, like the Mary Rose.
http://www.maryrose.org/
Theres a 3d model thing at
http://www.maryrose.org/explore/modelguide.htm
if you can be arsed to download it. (And actually you should, cause its quite instructive. Note the cooking bits right in the bottom there frex)
The MR there would be an Imperial Greatship though.
But a small one, since I reckon the Empire can manage far bigger ships.
So on
http://www.thabebanss.qld.edu.au/stu...ailgallery.htm
you want to be looking at Caravels or Carracks or Early Galleons (the Bretonnians should be driving Cogs to keep with their look, but its not all bad as some can look a lot hipper than those).
I don't think it would be entirely out of character for an enterprising PC to build the first race built galleon, which chops the fore and stern castles off, lengthens the hull, uses the wieght save from the lack of castles to pack in more guns and uses its superior sailing ability to stand off and pound the bad guys from a distance.
As used by the English against the Spanish Armada.
So look up the Revenge and use her as your hero ship.