If 1 DD just happens to go over the top of you, your screwed. The Active pinging that all DD's do at 14 Knts and under will get you. Its max depth is 300m. Being under TC you may have very well done the above and not been aware that you had done so.
The thing that gets me is that the Devs did the incorrect thing with the Type XXI. You see its Prop is far bigger than any other Uboat. Being so its there for must turn at a slower RPM. I wont go into properler theory here but to basicly say that the bigger a prop the slower it turns. This does not equate to a slower speed BTW. Its all due to pitch, how many blades an so forth. Anyway the Type XXI's prop RPM's is to fast.
So therefor the noise it generates is now to loud. Technicaly the Type XXI is cavitating at all speeds over 4-5 Knts. They may have used the correct RPM's for the Electric motor's, but they sure as hell didn't allow for the reduction gear (Gearbox). The most typical ratio on light craft is around the 1:1.5 mark, but on heavier craft its common to get ratios up to 1:2.3, sometimes higher (thats 1 revolution of the Prop to 2.3 revolutions of the motor)
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