[TEC] Silent Hunter IV damage control/repair system

Hello everyone,
I am new to this forum and from Germany so there may be a few spelling/grammar mistakes ^^

First things first:
I play Silent Hunter IV 1.5 UBM vanilla Steam version.

I wanted to test, rather get used to the damage system. So I started a mission with the Balo class and made myself to an easy target. The depth charges has been dropped quickly and they have hit me mostly at the stern side.
My rear torpedo room was flooded completly, other things broke and I got 32 of hull damage.

Hours later I managed to get away, pumped out the water and repaired everything which has not been destroyed completely but I could not repair the stern torpedo tubes. Three out of four were damaged. (Not destroyed; around 0.45)

I am not that new to the game so I considered about everything that might stop the crew from repairing the tubes.
Anyway, I accelerated time and I saw the tube at 0.44 damage for a short time but it switched back to 0.45.
Could it be due to hull damage? Like the crew is repairing all the time but the hull damage "adds" damage to the tubes either?