[CaptBones]

Changing battery cells, that is...

The battery cells for USN “Fleet” boats, and the Guppy conversions, measured approximately 15”x21”x54”. Given the technology of the day, I’d suspect the battery cells for U-boats were about the same size. So, would they fit through a 21” torpedo loading hatch? Probably, given that the torpedo is 21” in diameter and the loading hatch would be somewhat larger…about 26” on our boats, IIRC.

But, that isn’t necessary, or even a good idea. First, battery cells are rather fragile (that’s why they are installed in “jars” in USN subs). You do not want to be handling them in any orientation other than vertically and with special lifting/rigging equipment attached, which makes them a bit bigger all around.

So, even as K-61 alluded to, the engineers who designed the boats (ours and theirs) incorporated battery loading hatches exactly where they were needed. According to David Westwood’s excellent book “The Type VII U-Boats” (Anatomy of the Ship series), there was a battery loading hatch in the Officers Quarters/Wardroom, directly above the forward battery compartment and another one in the Engineers’ and Senior Rates’ Mess, directly above the after battery compartment. In fact, on pg 69, the “Battery service and lifting tray” for the aft compartment is shown, along with the clearances for hoisting the tray up to and through the hull opening. Judging from how the forward battery loading hatch is depicted on pg 79, the loading hatches were fitted into a coaming and bolted down; to be unbolted and removed only when changing cells.

As for the KDB…cutting and welding on the pressure hull of a WWII era sub would not have presented any particularly difficult challenges. The hull plating in the forward sections of a Type VII U-Boat pressure hull was from 12 to 18mm in thickness. The welding and weld QA testing processes of the day would have been up to the task of cutting a hole and welding a packing tube fitting for the KDB. But, as also mentioned, the external KDB apparatus was very vulnerable to damage.

Frankly, it would be tactically more sensible to just maneuver the boat to establish a satisfactory “baseline” for passive triangulation of a hydrophone contact. That’s exactly what we…er…”they” do today. The passive SONARs consist of hydrophone arrays installed all along the length of the sides of the hull, with further “extensions” through the use of towed arrays.

"Press on" mates...good hunting.