Simon.
You are right the fuel layer shall keep me busy today. I`ll start at the top. The tank you have drawn up under the pressurehull is the hearth or rather the stomach of the whole system. The tank is split in two halves along a mid shipline and function like a purifyer, clarifyer and a head/supplytank to the engines. It is fed by white pipeline on the picture below from the fuel pressure watercompensated bunkertank system to the aft end of the headtank via cocks.
During normal service you take fuel for both engines from one of the tanks, while the other is filled from the bunkertanks and then let the fuel settle so the water and impurities collects in the bottom of the tank, that takes a while but it is ready for use when the other tank is nearly empty. Below is a drawing where I have tried to show the details of the supplysystem. Amongst the mess of arrows I have indicated a valve and line in yellow, leading fuel to the Junker compressor which we shouldn't`t forget this time. The system sketch is showing as usual the MAN system with fuel supplypump driven off from the camshaft aft end, as mentioned before remember on the GW it is driven from the crankshafts forward end.
With so many arrows I think I better split this up and go into details of the supply system from the daytank in a separate post.
A small detail to mention, the tank is fitted with 3 brackets on each side to the upper frames of the pressurehull, the aft fitting is fixed by wire to welded on eyes to the plating as the tank end does not quite meet the aft frame. The light underneath the tank seems to be on the small side , the diameter is about a 5th or 6th of the length of the tank.
Tore