i always thought that the uboats were maintained intentionally abit heavy. you couldnt stay stationary whilst submerged and still maintain your depth. i thought it was related to avoiding breaking the surface once the weight of the torpedo left the tube, or as fuel weight decreases as its burnt off, etc.
bouyancy was maintained by continual weight management, ballast juggling and importantly for uboat designs; forward momentum over the dive planes.
.....or have i missed something?
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