The centre of buoyancy on this boat is designed in, so you shouldn't need to add foam unless you've added additional buoyancy in the free flood area. This could be a C.G issue. The boat should balance at a point in line with the attack periscope with both the tank empty and filled. Bear in mind that small shift in the C.G will not necessarily make itself apparent on the surface but will become blindingly obvious once submerged.
Difficult with a large boat like this, but you should be able to get a decent idea by hanging the boat on sling at the boats C.G.
