The QM2 does indeed have 8 propeller blades on her foredeck. If you look closely then you'll notice that there is a small hole in each spare propeller blade; this means that divers can use specialist cranes to replace damaged blades underwater, rather than having to take the ship into dry dock. Sometimes spare blades do have to be shipped in from elsewhere which does of course take time.
However, it is fair to say that the spare blades on QM2's foredeck may just be for any of the thrusters...I'm not sure. Would have to ask my company later in the course.
If a spare blade is damaged it can usually be removed and replaced without being in dry dock. However problems start occurring if the azipod unit itself was to be damaged (e.g. by a submerged object), which could potentially require the ship to enter a dry dock.
Remember the forward pair of azipods on QM2 are fixed (or we sometimes call them "static"), whilst the aft pair can be rotated 360 degrees ("steering"). Virtually all ships which are steered by azipods carry spare propeller blades; however the place in which they're stored varies depending upon the ship.
In 2006, one of QM2's propeller blades had to be replaced by divers after the ship hit an underwater port wall - however I can't remember exactly what happened after the event, and whether there was an issue with dredging, or a navigational error by one of the Officers on the bridge. I've uploaded a photo of this to my website to show one of the damaged propeller blades from QM2 (although unfortunately I can't remember exactly where this image was from). Notice the damage which was sustained to the blade - apparently there was a shudder throughout the ship, and this was clearly the result:
http://www.jamesf.co.uk/images/QM2azipodblade.jpg
James
