As far as backstage doubletalk goes, the nacelles are basically the propellers of the ship, pushing her forward with power that is separately generated by a big boiler elsewhere in the ship.
Expanding the analogy is easy enough. Small ships may have one propeller, because that's cheap. Big ones would benefit from having more, though:
1) They can translate power to motion better, beyond the limited capability of any single given propeller to do so.
2) Redundancy is nice.
3) Two or more propellers are needed if you also want steering authority (unless you go Azipod or the like).
Even today, one prop is for cheap warships; two is very common in warships; three or four is for large warships (but also used to be for tiny boats that could not mount large props and had lots of power to translate to lots of motion - today, single or twin waterjets are the preferred solution); and stealth or economy generally favor a single large propeller, to be found in subs and big civilian ships.
Perhaps a Constellation has only a tad more power than a Constitution, but prefers to distribute the translating-to-motion job to more pairs for reduced wear and tear. Or then it does pack twice the power. And perhaps needs it due to having twice the mass or whatever (it also has two sets of impulse systems).
Timo Saloniemi
