Hanjin has max 13.000 TEU vessels. That's maybe 8000 containers. One crane can unload at around 30 containers per hour. You might be able to use 6 cranes on such a vessel. It works out to 45 hours to unload such a vessel if completely full. I might be off a bit in some of the calculations, but 12 vs 45 hours is quite a big difference.
There's various physical limitations. You cannot have too many cranes working on a vessel (need to have space between cranes). Further, most cranes aren't that quick. Depending on a region you might hit 45 containers per hour, but that's pretty high (usually only seen in Asia).
There's various physical limitations. You cannot have too many cranes working on a vessel (need to have space between cranes). Further, most cranes aren't that quick. Depending on a region you might hit 45 containers per hour, but that's pretty high (usually only seen in Asia).