BTW: This also applies to ships with 3 nacelles, which in essence have 1 backup nacelle which isn't used until one of the other 2 doesn't work any more.

The nacelles must be aligned and of the same type for this to work.

That's true enough, but I have two problems with it. One, it is hard to imagine a situation in which some nacelles would be damaged and some would not. For starters, the nacelle would have to be completely powered down at the time. Otherwise, the destruction of the nacelle would quite possibly take the ship along with it. (Uncontained warp plasma can be just as dangerous to the ship as antimatter.)

The other thing is, at what point does adding redundant equipment cease to be an effective use of resources? Taken far enough, you'll be able to just go ahead and build a second ship out of it.

While having spare nacelles is certainly an advantage of ships like the Constellation, that isn't a good enough reason to justify adding them to other ships. (And there are other advantages and disadvantages to multi-nacelled ships. I'm just saying this one isn't a big enough factor.)

Point 1: Look at the Odyssey (sp?) in DS9. The Jem'Hadar destroyed a nacelle, and the Odyssey had to go home on impuls. If the Odyssey was a 4-nacelled ship, they would still be able to go to warp and escape the Jem'Hadar.

Point 2: The best way to use this is with 4 nacelled ships, since they can use either all 4 nacelles or just 2 to go to warp. My addition is that they can use ANY 2 nacelles to go to warp.

As for 3 nacelled ships: This is the best theory I can think of for the fact that they have a 3rd nacelle.

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