In another post I mentioned that I had once been forced to drive a Petersen 33 into a marina walkway at 5 odd knots
. The damage that occurred was actually minimal with a notch of about 1" vertically by 1/2" deep into the stem and no water into the boat. The boat lifted itself out of the water as the stem rode up the walkway but it never even came close to reaching the keel. And this walkway was "unmovable".
And if you sail into a container with the side of the hull, the chances would be that you would glance off with some damage but I really wonder how much damage a boat will sustain. If you were really unlucky and sailed into an exposed corner of the box it may be somewhat worse but I suspect that even then, it wouldn't be that bad. OK, maybe if it was a Catalina . . . .
I recall watching the video of Ellen McArthur's race when she came second in the Vendee Globe and she nailed a container near the Azores doing something like 15 knots. It damaged one of her dagger boards but the damage was only severe enough to piss her off because of lost boat speed and no talk of structural damage. She managed with some difficulty to pull the dagger board out of it's slot and went on her way.
I suspect that the dangers of containers at sea are somewhat over-rated. I once heard, a while ago now, a statistic that there are something like 4 million containers unaccounted for (lost at sea). Where the heck are they cause I've never seen one floating. 4 million of those boxes end to end would make a Trans-Atlantic boardwalk
I'd be interested to hear stats from other forumites.