The Derrick is a single rod, set in the middle of the ship's centerline hatch, hanging rod head does not set a stable cable, and in the head on both sides of the 1000-pound cable, it is called double-weight cable.
The movable end of the left and right sides of the heavy cable are connected with two cargo freighters by their own guide pulley. When the 2000 kg cable winch with the same rotational speed of the screw into the rope, the boom will rise, if the same speed of synchronization to loosen the rope, the boom will drop. When the winch is operated on one side of the hoist and the other is loosened at the same speed, the derrick can be controlled to turn out the side of the winch. In order to ensure safe operation, the two winch can work synchronously, the two winches are equipped with a dual controller, up and down with a controller, rotate left and right with a controller. The crane is suspended from the cargo pulley and the guide pulley to the third hoist to control the vertical fluctuation of the cargo.