OK. I know its not easy to work it out, a plan view might help but here goes.
The Towers are Fixed and do not move. They are supported by Cables passing from the ground to the tops of the Towers and they all appear to be cross supported.
Now to the Boxes.
They have a ladder going up to them but I don't think they were intended to have a Crane Driver. The faces of the Boxes have a swivel joint rivited to them and the Jibs are mounted on this joint allowing them to pivot through an arce of 180 degrees. Some boxes seem to have two Jibs, one at either side and others have three. By using this method all parts of the ship would be covered. It may have been nessessary to put the load down and then pick it up again using another Jib in order to get it to its intended place.
The Jibs appear to be "Fixed" height. ie the tip does not move up and down.
Now we get to the intellegent guess bit......
Its possible that the hooks were controlled by Winches on the ground. Note the White Cabs at the base of the Towers.
The rope comes up from the ground, round a pulley just under the Pivot then around another pulley at the end of the Jib. They would not have been capable of lifting very heavy objects, but two of three jibs could be used at once where a heavy lift is required.
Paul Hood. + WSS Tyneside Branch Hon Sec.