Why no turtleback?

[marcelo_malara]

I had a 170.7 metres citadel of 241.5 metres at waterline or 70.7% but it is less if you use the overall length from https://www.kbismarck.com/proteccioni.html.

Ok, let me use LOA for consistency:

Bismarck

LOA 252 m
Citadel 170.7 m
Machinery length (including intermediary compartments, excluding auxiliary boiler room) 69 m
Distance from after end of Dora barbette to after end of Caesar barbette 18 m

Citadel/LOA = 0.677
Citadel less Dora turret/LOA = (170.7 - 18) / 252 = 0.605

South Dakota

LOA 210 m
Citadel 115 m (scaled from plans)
Machinery length 50.4 m

Citadel/LOA = 0.54

If to Bismarck you subtract one turret the relationship would be 0.605 vs 0.54. And, we discussed it recently, Bismarck´s machinery was not compact, despite the higher steam pressure adopted.

So, I would conclude, that the shorter citadel of South Dakota class is due to one less turret and more compact machinery, and not due to a higher protected deck.

[Thorsten Wahl]

Double

[Thorsten Wahl]

Bismarcks size of citadel was dictated by lenght of machinery and ammunition space/turrets..

KAmt compared original Richelieu scheme and a adopted german scheme based on the same armour weight per meter.
I suspect the modified german scheme offers significantly improved protection.

6067558_4a317d203bef69b121bb389328869e49_p.jpg (40.9 KiB) Viewed 1416 times

[Pukovnik7]

The Germans really liked their turtleback armor, which they used in all their big ships. This can also be seen in the never built H-class battleships, which would have had a slightly thicker turtleback and slightly thinner main belt.

The heavily angled 110-120mm scarp behind the main belt certainly added a lot to the total, effective side armor of Bismarck, which was thicker even than Yamato.

But what was the reason other nations, like Great Britain, dropped it in their new battleships?

Weight? Space?

I believe there were several reasons, most of which had to do with evolving nature of threats.

1) Appearance of air power meant that deck armor suddenly became far more important. This also meant that you could not risk leaving something important above the deck as bombs would obliterate anything not protected.

2) Between the potential splash damage from bombs and appearance of torpedo aircraft, chances of ship flooding from hits became that much higher - and you could not simply extend armored belt lower and lower to protect against it. This in turn meant that:
2a) Armored deck had to be positioned much higher in the ship, as flooding above the deck risked creating a free surface effect, which would have negative effect on ship's stability.
2b) Armored deck had to be positioned high enough that it would remain above water even with ship sitting down quite low as a result of combat damage.
High armored deck and deep belt of King George V class were driven precisely by these considerations.

3) Increasing engagement range meant that benefits which turtleback offered at close range were being significantly negated.

[chuckfan3@gmail.com]

A ship’s armored citadel has to be protected against several threats:

1. Low trajectory shells impacting against the ship’s sides
2. Plunging trajectory shells impacting against the ship’s deck
3. Near vertical strikes by bombs against the ship’s deck

Between the wars ship’s citadel nominally emphasizes 1 and 2 with the concept of immunity zone, inside which shells from the guns of a specified hypothetical enemy battleship can penetrate neither the sides nor the deck. Fulfillimg 3 is perhaps considered a nature outgrowth of the effort to fulfill 2.

I think Bismarck’s design focused on seeing whether a protection scheme that affords greatly superior protection against 1 compared to contemporary foreign ships without sacrificing 2 can be achieved on reasonable displacements. The Germans succeeded by incorporating the ship’s side defenses into its scheme to defeat plunging shells. Bismarck’s upper belt is designed so any shells coming in from abeam must pass through it first before impacting on armored deck. This gave Bismarck what its designers sought, which is protection against plunging shells equal to foreign contemporaries, and protection against low trajectory shells superior to contemporaries. However, adding upper belt to horizontal defence means armored deck needs to be thinner to keep total weight reasonable.

So unlike with foreign contemporaries, on the Bismarck fulfilling 3 became somewhat decoupled from fulfilling 2. Bismarck had horizontal protection against plunging shells as good as any of her contemporaries except the Yamato. But her protection against vertical bomb impacts are rather less.

So other countries chose to strengthen protection against vertical bomb impacts at the expense of protection against close range low trajectory side impacts by APC shells. The Germans chose the reverse.

[chuckfan3@gmail.com]

A ship’s armored citadel has to be protected against several threats:

1. Low trajectory shells impacting against the ship’s sides
2. Plunging trajectory shells impacting against the ship’s deck
3. Near vertical strikes by bombs against the ship’s deck

Between the wars ship’s citadel nominally emphasizes 1 and 2 with the concept of immunity zone, inside which shells from the guns of a specified hypothetical enemy battleship can penetrate neither the sides nor the deck. Fulfilling 3 is typically closely coupled with fulfilling 2 because an armored deck thickened to defeat plunging shells enjoy proportionally increased defence against bombs.

But on the Bismarck 2 and 3 are somewhat decoupled. Bismarck used a layered defence in depth approach to protection against plunging shells. Any shell coming from abeam in expected battle ranges will first strike the upper side belt before it can strike the armored deck. This allowed the armored deck itself to be thinner to save weight, without compromising defence against plunging shells. This approach allowed bismarck to adopt a lower main armored deck position that greatly strengthened the ship;s defenders against 1.

So amongst the three things armor must protect against, Bismarck adopted a scheme that made defence against 1 exceptionally strong, defence against 2 equal to her contemporary, at the expense of making defence against 3 somewhat weaker than her contemporaries

It’s clear when designing the KGV and Yamato, the British and German took defence against bombs very seriously, and perhaps accorded that a higher priority than the Germans did..