I also have some questions about ballast tanks in a type VIIC boat.

If a boat is well trimmed, say that the trimtanks are half full. Do they keep the trimtanks shut until they do another trim? So that even when running on the surface the trimtanks would be half full. And a dive would be done by completely filling all ballast tanks and then simply steer the boat with the hydroplanes too any depth and the boat would be in trim, more or less?

Could they control how much water they filled the ballast tanks with? Where there an instrument showing this or where they only intended to be either empty or full?

I read somewhere about a negative buoyancy tank which, I guess, where used to dive faster and which then would be blown once under the water, but where is that tank located. I can\'t find it on my drawings?

If a U-boat blows the main tank when submerged, could it stay down anyway by tilting the boat forward and use the force from the propellers?
I\'m writing a little U-boat simulator and this was possible by only tilting about 15-20 degrees while running at 4 knots. The algorithms are in a very early stage and I have done a lot of guessing as of sizes of forces but the question came up, would it be possible? If it isn\'t I obviously have given the propellers to much force and/or the main tank is too small.

And finally, if anyone know the sizes of the different tanks it would help me a lot. I have assumed that they totally have a capacity of about 100 tons.

Leif…