In the end as pointed out above, Bismarck was excellent protected against belt penetrations (but the SD´s 16"/45 is a worrisome deck penetrator) while South Dakota is excellent protected against deck penetration (but the Bismarck´s 15"ers are flat trajectory, belt penetrator guns)
Armor penetration is one thing, another is fuse quality.
A good number of hits at POW (?6 out of 8 ) went into the ship and did not blew up. The POW was safed by this.
A 8" AP shell from Prinzu Eugen penetrated the belt and moved into the starboard ammo handling room and did not blew up. It is considered a dud. Another 15" AP shell penetrated the bridge, killing all but the captain and one more. This shell also did not blew up.
I am still reading the details but I have found some interesting stuff. The german fuzes seems to have failed a number of times, exclusively if facing the british cementated armor.
The US AP shells have the best quality and showing a little number of duds, mostly against japanese face hardened armor.
I believe, but am not sure in all, that this not exclusively reflect the fuze quality but to a high degree reflects the ability of the armor to resist the projectile, break it´s body and fuze mechanism. Britisch cementated was by far the best face hardened armor it showed extensively bending abilities, japanese was the worst of all and simply cracked or shattered away. I compared test results of 15"/52 against KC armor plates and found out that there were a number of duds (around 12%), which was usual for the time. Post war tests with britisch cementated showed that US AP failed a lot of time to blew up after passing the plates (~20% duds), something what rarely happened against japanese armor.
This has some significance here. At first, it wasn´t the german worse fuze, which safed Prince of Wales in the Denmark Street but the superior quality of british cementated armor.
Second, the reliability of german AP fuzes would increase against US class A armor (which is of worser quality than British cementated) while in the same reverse situation the reliability degree of US AP fuzes would decrease while facing KC new armor (which is of better quality than japanese ones).