I also showed it at Stoneleigh and Princess Anne spoke to me about it.

It replicated all of the motions of a typical cargo ships gear.
The runners of the two derricks are joined to a common hook and they were worked in conjunction with each other.
The inboard derrick lifted the cargo out of the hold and then the outboard one pulled it across the ship until the load was over the side.
Weight was transfered from one derrick to the other as the load was moved across.
A pair of skilled winchmen working in harmony could load or unload at speed.
Tony