Aircraft Carrier propellers

[Falling Star]

Most twin screw rotations that I've seen when viewed from the stern seem to be:
Port engine - anti clockwise
Stbd engine - clockwise

but the BBC 4 program, Britain's Biggest Warship was showing that these were the other way around.

So why is that? Do all ships have that configuration? What is the size of vessel that determines prop rotation?

 

[jfm]

Outward turning is good on our boats because increases prop kick effect, =easier berthing. But very bad if you want to run on one engine.

If you don’t care about berthing because you don’t do it much or have tugs, eg aircraft carrier, then inward turning is better if you want to be able to run on one engine in emergency.

If the props were variable pitch and there was no engine reversing, the blades might have been set in forward or reverse in whatever picture you saw- I don’t know.

 

[Portofino]

Wonder if the acoustic signature is beneficial too ?
Hunter killer subs attempting to listen .
Just guessing .

 

[Falling Star]

Thanks! I should have realised that. :redface-new:

 

[MapisM]

Inward spinning props are very popular also in speedboats (see below), because they are a tad more efficient, hence allowing to reach slightly higher speeds for any given power.
Now, I really can't tell if this is true also for ships with completely different displacement hulls, much larger and slower.
But if it would, that might be another reason for using inward spinning props, I guess.

 

[PCUK]

Inward turning props' suffer from greater ventilation so not ideal for anything other than surface piercing (or aircraft carriers apparently).