Why do documentaries on Denmark Strait have this obsession of showing the deck hit as some kind of deeply plunging shell? The putative deck hit would have passed through weaker part of the belt and then hit a steeply downward sloping part of the deck. It's best generalized as penetration of weak spot in the side armour.
This was because Holland's tactic actually worked. He successfully closed the range to the point Bismarck would have been ineffective against the conventional horizontal part of Hood's deck. Only to be ruined by ridiculous chance hit...
What critical parts specifically were unprotected? At least of the kind that could be feasibly protected and were so on other ships of this kind? Mind you, when the Prince of Wales was hit in stern by torpedo, she did not get a jammed rudder but rather suffered a massive failure of watertight integrity and sank.
The armour scheme was a fine modern version of the old incremental protection, was weight efficient for side protection because it also used the main deck for that purpose (important in that WW2 BB duels happened at relatively short ranges) and showed apparently advanced understanding of spaced armour.
Bismarck did have issues. One key one that became critical was the absence of pumping gear and valves between the fuel tanks, which meant that the hit she suffered at the hands of
Prince of Wales had a detrimental effect upon seakeeping and slowed her down more than would have been the case. This was corrected in
Tirpitz. AA protection was not ideal, maybe a result of the KM not really having to face air attack as the RN had in Italy and Norway. The 3.7cm guns had too low a rate of fire and there were not enough 2cm AA. Also, there seems to have been no AA training against low flying aircraft, which is odd as you would expect that from a Swordfish or Albacore.
Not the fault of
Bismarck, but Lutjens did have a habit of making long radio transmissions to Berlin, making him vulnerable to Huff Duff detection. Also, the lucky hit at 9.02 that knocked out two turrets at once is not really her fault, no more than the freak hit on
Hood. But this was from
Rodney, and the examination of the wreck has shown that
Rodney's high velocity 16" shells were able to penetrate the turret facings and CT and the main belt of
Bismarck, although the low velocity 14" shells of KGV did not: but she was generally further away in the engagement. One question is the ineffectiveness of
Bismarck's gunnery against Tovey's force, when she was on the button at Denmark Strait. I know an 8" shell took away her forward director above the CT.
What finally did for her, the results of the torpedo hit aft, was a flaw carried over from the
Bayerns: a three screw arrangement made her more vulnerable, whilst the access and emergency equipment was poor verging on non-existent. But looking at film from the wreck, I am not sure anything would have helped as it is so stuffed into the stern.
(With
Prince of Wales, by the way, it is worth mentioning the flooding hit came after a near miss had thrown generators off their mountings. The result being that the interior was plunged into darkness that hampered the work of damage control teams and stopped the pumps working. She was by no means as vulnerable as your comment suggests. The risk of machinery coming adrift from mountings was corrected in the other KGVs).
There were also issues such as habitability, general wetness and poor seakeeping. These are not, by any means, going to make Bismarck a bad design (the
Nelsons and
Hood all had 'submarine propensities') but the designs showed the lack of study and experience the Germans suffered from in the interwar period. It is not like they laid down the
Gniesenaus and shook them down, learned lessons and then moved forward. As soon as
Gniesenau and
Scharnhorst were out of the slips,
Bismarck and
Tirpitz were on them. No time taken to learn.
K