[bluGill]

I would expect counter rotating props to be more reliable. Instead of one big engine/prop you have two smaller ones. While it is more likely that something will break, it isn't a catastrophic failure: you continue to your destination using just the other engine/prop.

[krschultz]

Note that they're not talking about two separate props on separate shafts, they're talking about 2 props rotating in opposite directions on the same shaft. Many ships don't have a 1:1 ratio between propellors and engines anyway, if 2 separate shafts & props were faster they could do that easily. But counterrotating on the same shaft adds more critical moving parts in an already sensitive area.

[bluGill]

I stand corrected.

[brandmeyer]

Just because you replaced 1 thing with 2 things doesn't mean you've increased the system's reliability. In this case, you've also added a lot of complexity in the shafts, seals, and gearboxes. That means more things that can go wrong. Most of those things can affect both screws when they fail, so there isn't any gain in reliability due to redundancy at all.

[Grazester]

Counter rotating props run off the same engine(usually how its done) would offer nothing in terms of redundancy if anything fails.

[ceejayoz]

More reliable than one propeller, sure, but many (most?) large ships already have multiple, don't they?