I have seen 8-12 foot ships, and only one of them used a trolling motor. The trolling motor was under a 12 foot replica of a Fletcher class destroyer. It handled very nicely.
The rest are using direct drive pittman motors, and standard Gel cell batteries 12v, 7-24ahr

11 foot USS Enterprise 1/96
8.5 foot Iowa Class 1/96
11.39 foot Bismarck 1/72
8.13 foot USS Pensacola 1/72
8.23 foot Graf Spee about to engage an 8.08 foot Houstan 1/72
Lutzow, Houstan and the Louisville with Destroyers 1/72nd
8 foot Kinagusa 1/72
Motors in the Kinagusa Pen on the deck for referance. [8D]