I've always wanted to know this - how do container ships keep the shipping containers from sliding off the deck during rough seas? I know the containers themselves are really heavy and big (they only look small compared to the rest of the ship!) but I would think one rogue wave and all those containers would be at the bottom of the ocean.
It is an old question, but since I inerned a year on containerships I should answer it.
The shor answer is twistlocks and lashings.
Twistlocks have an oblong part that fits in the blong holes in the corners of containers, and that can be turned 90 degrees to fix them into place. There are twistlocks connected to the ships deck, and the lowest container is put on these, after which they are locked.
Since the containers also have connection fo twistlocks on their roof, you can put a second container on the the bottom container, with twistlocks between them, and so secure them together.
For extra security lashings are also being used. These are metal chains that have a method to really tighten them so far that there is no stretch in them at all. These chains are used for diagonal connections between containers or between containers and the deck. Lashing are used in combination with twistlocks, not as a replacement.
Containerships that need these method are often general cargo ships that are being used as container ships, but can be used for other cargo as well. Ships that are build only for the transport of containers are ususaly cellular containerships.
Cellulars containerships have open holds, with a frame that goes from the bottom of the ship to higher then the deck, in which containers fit exactly. On a cellular ship, you do not need twistlocks for the containers that are placed in the frame (you can not even use twistlocks, because the fram is in the way) but they are often being loaded up to 6 containers over the edge of the frame (depending on ship size) and the higher containers do need twistlock.