Hi Hans,
"Free Surface" is ofc a commonly understood concept by anyone with a maritime background. On the other hand it does not mean anything to a landlubber. Thus I would translate the passage as:
'sloshing above the floor plates in the after part of the bilges'.
All bilges have the same...
Remember, boat's total bilge area was also subdivided as per compartments above, i.e. they were watertight from one another. Only connected through bilge pump suction manifold, thus there was not neccessarily the same level of bilge water in all of them. Though suction and discharge manifolds it was possible to pump bw from one bilge to another to help trim the boat, if it was too deep to pump it outboards. Rgds, RB