When containers are on board the ship, are they secured in any way in transit, or just by their own weight?
Id imagine the outer ones have twistlocks (the removable type that you get on the cranes for stacking)
But I have seen some that are chained down.
Thank god Wiki is back online
Numerous systems are used to secure containers aboard ships, depending on factors such as the type of ship, the type of container, and the location of the container.[31][32] Stowage inside the holds of fully cellular (FC) ships is simplest, typically using simple metal forms called container guides, locating cones, and anti-rack spacers to lock the containers together.[33] Above-decks, without the extra support of the cell guides, more complicated equipment is used.[31] Three types of systems are currently in wide use: lashing systems, locking systems, and buttress systems.[31] Lashing systems secure containers to the ship using devices made from wire rope, rigid rods, or chains and devices to tension the lashings, such as turnbuckles.[31] The effectiveness of lashings is increased by securing containers to each other, either by simple metal forms (such as stacking cones) or more complicated devices such as twist-lock stackers.[31] A typical twist-lock is inserted into the casting hole of one container and rotated to hold it in place, then another container is lowered on top of it.[34] The two containers are locked together by twisting the device’s handle.[34] A typical twist-lock is constructed of forged steel and ductile iron and has a shear strength of 48 metric tons.[35]
The butress system, used on some large container ships, uses a system of large towers attached to the ship at both ends of each cargo hold.[36] As the ship is loaded, a rigid, removable stacking frame is added, structurally securing each tier of containers together.[36]
Theyre usually all held with twistlocks and the outer ones chained in a criss cross,or thats what they used to do anyway.
On the ferry ports they use a double twistlock with fastens two boxes together, well they do on P&O from Hull and Teesport
There was a television documentary series on Nat Geo a little while ago called mighty ships.
en.wikipedia.org/wiki/Mighty_Ships
Watch out for it if you’ve got Sky as in season 1 they went on board the Emma Maersk and showed you how they secure them.
If I remember right, they have cables they use to lock them all together from on board the ship.
Anyway it might still be on repeat and it was an interesting series so if it’s on again you want to look out for the one with the Emma Maersk. I haven’t half heard some myths about this subject. (like they don’t do anything to them).
when its done right they look like this
but when it goes ■■■■ up 
WHOA hell,great pic…tis obvious he took the island too quick
Tower of Pisa, Ital Florida, Costa Concordia…
that lasts pics definately a… well boss, moment 
Nah, sharp right will sort that lot
The Evergreen container hanging off the right-hand side of that ship shows how good the securing systems are when they work