[hilifta]

With the spreader closed we can carry 1x20ft container, extended we can carry 1x40ft, or by dropping centre pins we can carry 2x20ft boxes.
If a truck trailer is configured correctly we can lift 2x20ft boxes off a truck at once. Often though trailers aren't suitable for this.

If you look at a container you see in the corners, top and bottom there are whats called "corner castings". On a truck the bottom ones are locked on to the truck by his twistlocks. A straddle and the container cranes use exactly the same method in the top castings.
Likewise on the ships, a thing called a super twistlock is used to lock the boxes together.
These, together with lashing bars, are all that hold the boxes secure to the ship.
To unlock them so they can be discharged a toggle is pulled by a lasher on the ship, and this unlocks the bottom box. The crane then lifts it off and a lasher on the berth then removes it from the box before a straddle then takes it to a stack.

Hope this helps.

Rgds/Mike (hilifta)