The Voyager of the Seas has port and starboard azipods, plus a fixed but reversible electric prop mounted in the skeg. Maybe I don't understand thrusters, but it seems to me that they wouldn't be able to overcome all the inertia if they're all at one end or the other. It seems to me that the ship would just pivot, rather than move sideways, if there weren't a thruster at the bow. Maybe there's some combination of RPM and prop pitch that compensates. Any physicists or engineers, out there?