I don`t know the Trumpeter kit, but in real life, most of the details of a submarine have a purpose. Both MAN and Germaniawerft dieselengines used in the VIICs have a dry oilsump, hence they need a sumptank. The tanks are situated between the two engines and the port and stb. luboilpumps are independently connected to each relevant sumptank. As you probably know the frames of the VIICs are numbered from aft to fwd., in the engine room this means that frame 18 is just passing the centerline of the main clutch, and the location of the tanks between the engines are: between frame 18 and 19 dirty luboil tank storage for discarded oil, frame 19-21, port sumptank, frame 21- 25, stb. sumptank and finally a fuel oil draintank collecting the fuel leakages from the engines. These tanks have all a solid bottom and top and are separated by liquid tight bulkheads. underneath the engines are the lubeoil storage tanks 1 and 2. they contains the fresh luboil for topping up if needed. The frames goes right through these tanks and the reason for that is to reduce the free surface effect which might upset the trim, hence the frames act like an important baffle in the tank system. You have cuttings in the baffles allowing free flow to the pump suction pipes. If you wander why these intricate collecting tanks, they are not an early environment action, but prevent a pollution of the bilge and thus the risk of a treacherous trail of oil on the surface.
Tore