That's an RCABS system. You do need a large tank to get scale waterline on a U47, generally 750-1000ml depending on what you have modelled above the waterline. So it will need a large bag, which isn't always easy to find in the right shape (one of the chief disadvantages of using a bag).
A system you may like to consider is a Kavan geared water pump, pumping in and out of a vented fixed tank. The vent should go into the conning tower, and the boat trimmed so the top of the conning tower is afloat when the tank is fully filled. This will enable the tank to be emptied. This system doesn't give full static dive, but it's inexpensive (about £30-35) and easy to build and will work very well with the Robbe boat, and you don't require any valves. The tank can be constructed from ordinary plastic pipe (PVC, ABS, acrylic etc..) with solvent welded endcaps.