This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
Note that a number of things have changed by the time we see the
U.S.S. Enterprise zipping around with three nacelles in that episode:
* Warp 13 is a common cruising speed
* Warp is no longer damaging space-time
* New nacelle designs are being used on other ships
My own explanation: new nacelle designs allow ships like the U.S.S.
Pasteur to cruise at Warp 13 without frying space-time. Older ships,
like the Enterprise, can be refitted with a third nacelle (and other
wingdings and widgets) to clean their subspace emissions, so to speak.
The third nacelle also allows a more powerful field to be generated,
to drive the ship around at Warp 13, but this goes beyond TNG-era
knowledge of subspace mechanics.
So as far as strictly TNG-era ships are concerned, three nacelles are
still worse than useless.
 
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