I was planning to add another post as I forgot to mention a few details in my reply, but it is covered in your question about Bismarck vs KGV so I will answer it in this post.
Pips wrote:Thanks Terry, your explanation clear's a lot of things in my mind. Goes a long way to explaining why the upper decks of the Bismarck became a slaughterhouse with resulting loss of control and command, yet the hull remained (comparatively) intact.
The superstructure, everything above where the side of the ship ends, is almost entirely only plated to keep out lighter shells at long and medium ranges, not to stop heavy shells as that would mean too much weight high up in the ship. At close ranges most shells from anything larger than a destroyer, have a good chance of penetrating anything but the heaviest armour, so adding more and more weight is pointless. The thin armour ships did carry above the deck area would typically not be thick enough to detonate a heavy shell, or slow it down enough for it to explode inside the ship, the theory being it would pass out the other side and explode there. With Bismarck the armour on the side was good against short to medium ranged shells, but allowed anything passing over the belt to explode inside the ship, above the armoured citadel, but amongst the crew areas and crew passages throughout the ship, which in turn would make evacuating harder as areas became impassable, and see any crew in this area very vulnerable anyhow.
Pips wrote:That link for the Bismarck v KGV is interesting. If I'm reading it right, it shows that the KGV was better armoured (as in thicker) over it's key area's than the Bismarck. Is that right?
This is the part I sort of forgot a detail about previously. Shells have a fuse that is set to explode the burster inside the ship fractions of a second after the armour sets it off. This is why the US tried to have the 'decapping' armour before the main armour, the intention being to either distort the nose cone of the shell and render the shell dud, or to set the fuse timer running so the shell would explode harmlessly on the main armour as it would not have time to penetrate now. We are talking hundreths of a second, but in that time the shell can travel dozens of feet, or pass through several inches of armour if it is still intact. With the KGV the armour is designed to all be in the area the shell strikes first, putting all the defence on the front line effectively, so shells without enough mass and velocity would explode on the very thick armour this approach allowed, but also ensuring the armour slowed any shell passing through it enough that it would explode just inside the armour where the standard 10mm plating or so of a deck or two would see the fragments kept out of the vitals even if they did then wreck the crew areas above them. With Bismarck the ship is relying on several layers of less thick armour to slow or detonate the shell, but as the thinner plates offer less effective resistance than one single thick plate, there is a lot of weight wasted. The main risk is that a shell penetrating the main armour would not be slowed enough, or rendered dud after doing so, and still have enough energy and mass to allow it to penetrate the plate behind the main armour, then explode. For KGV you would have three decks below the main armoured deck before you reached the vitals, meaning a shell was unlikely to get beyond the armoured deck and first internal deck before exploding, and the fragments would then penetrate only one or two more decks at a much reduced velocity. On Bismarck this would mean any shell penetrating the main armour and the medium plate behind it would be exploding directly in the vitals themselves or only one deck above them. If the main armour fails, serious damage is far more likely.
Pips wrote:How does the KGV, and Rodney, compare in the upper deck protection to the Bismarck? Same? Better?
KGV is far superior in armour layout, that is where the weight saved by using 14" guns went. The main deck is comparable with any other ship bar Yamato, give or take fractions of inches. Rodney is a curious case as she was a great design cut down all round to meet a strange treaty imposed limit, and then accidently coming out 1,500 tons below the limit meaning more could have been done without cutting so much weight. The belt armour is internal and inclined, like on some US ships, which is great at resisting damage, but a real pain to repair as you need to remove the side of the ship to access the belt armour! On Rodney is also is very shallow, meaning a lucky shell could strike underneath the belt entirely and enter the vitals, effectively the 'freak' hit scored in Japanese tests that led to them trying to build a shell that would dive through the water and strike under the belt if it fell just short of the target. Rodney is also unusual as all the best armour is clustered around the main armament, allowing the thickest armour to be over a very short length of the hull. The machinery is protected well, by a secondary thickness in all cases, meaning that the ship is easier to disable regarding maneuverability than it is to disable the main armament of detonate a magazine. Given the limits of the treaty it was a very good design, the best of the pre-1935 ships, and other than the Nagato whos speed was unknown, the fastest battleship in the world when entering service. The 'she was very slow' is totally hindsight from a WWII perspective, though if the desired design had been built it would have ben capable of 31-32kts. Bismarck would have done well to avoid Rodney if given the chance, her armour really cannot cope with the 16" shells. Against KGV it is far closer, but mostly in that whilst Bismarck is unlikely to hurt KGV unless she closes to close range, by entering a range where her own guns can penetrate with certainty, she has also entered the range where KGV's shells are also capable of penetrating her own armour. After that it is down to pure luck really.
All of this makes it look like Bismarck was a really bad design (she sort of was) but that is not entirely the case. She was a good ship, could outrun any ship she couldnt stand a good chance of beating (only the three British battlecruisers had much chance of catching her when she was designed), had a good main battery, good secondary and tertiary batteries too, enough armour to keep out shells from almost any ship than came close to her speed, and keep out shells from every ship that could catch her. She is a very large ship, hard to sink, a good gun platform, and equipped with very good fire control for the time. Where she fails, and fails really badly, is in that she is well over 42,000 tons, and on the full load displacement over 50,000 tons (full load is useful as 'standard tonnage' differed from nation to nation as they all sought ways to snip a few hundred or thousand tons off the 'standard' figure, but full load is often closer to the operational weight to be expected), and only really comparable to ships 10,000 tons lighter than her. There is so much wasted weight in the design because of inexperience in the design field post-WWI, so an interesting comparison is with the British G3 or even H3a/b/c designs from 1921, which are of similar weight.
PS. You may also like to see my recent posts in the pinned 'Z Plan' thread as they deal with a Tirpitz vs Massachussetts scenario, but with a bit more discussion on the armament area.