Tore, I was thinking about the box keels, and how they would have rested on the sea floor. There seen to be no direct contact between them and the bottom of the keel to help with the load distributions.
I was wondering perhaps they were used to restrain that part of the boat while it was in dry-dock being constructed. I imagine that the two engines are very heavy and would put a large amount of stress on the framing around this area.
After the engines were installed and just before flooding the dock, they could not remove box keels, so build the keel around them and left them in place.