FINAL CONCLUSION: The BISMARCK's internal vitals could not be directly reached through the side belt armor under any normal circumstances due to the sloped "turtle-back" armored deck design, making its design the best of all given in this article for this purpose. However, there are several costs for this:
(1) Due to the main armored deck's low position in the ship, extensive flooding of the ship above the sloped/flat armored deck is likely if the side armor is holed, which could cause serious stability problems and which reduced protected reserve bouyancy by one complete deck
(2) The upper hull area can be destroyed at much longer ranges than any other design due to the weak side belt armor. Furthermore, some important equipment, cables, etc. were in this region, compromising the effectiveness of the protection to some (possibly critical) extent
(3) The weak lower main deck armor design -- especially the close-range zone of vulnerability after the projectile penetrated the 1.97" weather deck and was deflected downward through the thin 3.15" main armor deck over the amidships region -- allowed the possibility of reaching the vitals by hits that were deflected off of other structures, such as barbettes, or which hit "shot traps" where ricochet was inhibited (such as where a solid object was bolted to the armor deck and the projectile hit the joint, requiring the projectile to lift the solid object up or to punch through it in order to ricochet)
(4) The requirement for a rather heavy upper side hull armor belt to protect the thin main armor deck from side hits above the main armor belt, which costs considerable weight that could be used to beef up the deck armor or belt armor or both
(5) Unlike the USS SOUTH DAKOTA (and USS IOWA) or the VITTORIO VENETO, the BISMARCK's side armor does not ensure that a completely penetrating projectile is virtually always shattered and rendered "ineffective" by being decapped prior to hitting the face hardened belt armor, which reduces the damage that the projectile will usually case even if it does not penetrate through the belt
(6) The armored transverse bulkheads at each end of the Citadel were weakly protected and had no sloped deck behind them, making the BISMARCK very vulnerable to raking fire from either end, especially as the main magazines were located directly behind these bulkheads
(7) The shallow extension of the belt allowed hits below it to frequently occur, as was demonstrated during the fight with the HMS Prince of Wales, bypassing the main armor belt and aggravating any flooding effects that projectiles punching through the belt above the low main armored deck might cause
The USS SOUTH DAKOTA (or, better yet, the USS IOWA) armor scheme shows that for most naval battles, an improved "conventional" side armor design (thin armored weather deck, high mounting of the heavy main armor deck at the top edge of the main armor belt, thin upper belt armor, inclined main armor belt, thin fragment screen plating spaced behind the belt armor, decapping plate in front of the main belt, and tapered lower belt armor to protect against diving projectiles) gives protection to the vitals that is just as good, if not better, than the BISMARCK's side armor protection with equal weight of armor and without most of the bad points that the BISMARCK's low and, in the flat regions, thin main armor deck gave. If the enemy can get close enough to frequently punch through an Iowa-type belt, the battle is probably already lost, anyway, as the last battle of the BISMARCK demonstrates.
