111's armor thickness

[CmdrKeen]

According to a few references I found, the fourth Yamato type battleship would have been Number 111. I wish I could find where I saw it, but someone reported that whereas Yamato's belt armor was 410 mm thick, 111's would have been 400. And where Yamato's deck armor was 200 to 230mm thick, 111's would have been 190. The main turrets were treated similarly. They would have then added a little more armor to the lookout area and some other parts of 111's superstructure than Yamato. Why would they do this? I'm guessing that if they had built 111 at all, there was not that much of a shortage of steel that they would have had to shave off 10mm here and there. Was it to make the ship weigh less and give an extra knot of speed?

[Dave Saxton]

It would have a negligable impact on the immunity zone against a given gun in most cases. Once armour gets so thick, additional thickness does not increase ballistic resistance proportionally. For face hardened armour about 320mm is the point of additional thickness not being worth the extra weight. For homogenous armour thinner plates are always of better quality than thicker plates. 150mm is considered the point were it's becoming difficult to produce quality homogenous plates. They may of also considered the type of threats they were more likely to face and decided the weight would be better put to use elsewhere, such as you say speed.

The German H-class would have got improved belt protection while actually decreasing the thickness of the main belt from 320mm to 300mm as another example. This is because in that system the thickness of the scarp is more important than the thickness of the belt. By increasing the thickness of the scarp they could decrease the thickness of the belt and still significantly increase belt protection.

[Dave Saxton]

And where Yamato's deck armor was 200 to 230mm thick, 111's would have been 190.

To follow up a bit more: another factor here is that deck penetration does not increase at a linear rate per range. It increases gradually up to long ranges and then at extreme ranges it sky rockets. For example Yamato's 18" gun could penetrate about; 50mm at 15km, 80mm at 20km, 115mm at 25km, 160mm at 30km, 180mm deck armour at 32km, battle range, but by 35km it would be more than 255mm single plate. Reducing the thickness from 200-230mm to 190, results in very little change in the outer IZ limit against 18" fire. When you factor in that the 190mm plate is probably of higher quality than the 230mm plate, there is probably no practical difference in IZ.

The Japanese engineers were getting smarter and more efficient with their designs.

[CmdrKeen]

You've certainly given me a most knowledgeable answer which I very much appreciate. Thank you very much.
In a book on the Russo-Japanese war of 1904-5, I seem to recall reading that armor penetration decreased with range.
But perhaps that had to do with vertical side plate, whereas you seem to be speaking of long-distance deck penetration, where perhaps gravity may play a factor?

[Dave Saxton]

It's because of the trajectory. Against vertical armour the striking angle is becoming less favorable with increasing range, while at the same time the velocity is also decreasing. Hence the penetration of vertical armour decreases with range. However against decks the striking angle is become much more favorable at long rangs, so although the velocity is decreasing the penetration actually increases at a increasing rate.

A warship can not be made proof against all shells at all ranges. Therefore, battleships were designed to have vital compartment protection at the most likely battle ranges. This is known as the immunity zone concept (The Germans used the terms the zones of penetration and the zone of protection) where at a certain range a given shell can no longer fully penetrate the vertical protection but the shell can not yet fully penetrate the horizontal protection. For example, the USS North Carolina had a designed immunity zone extending from 18,000 yards to 28,000 yards vs 14" shell fire, depending on target angle.

For almost all battleship caliber shells the effective deck penetration is no more than about 5-inches until the angle of fall reaches or exceeds about 30*. This, depending on the ballistics of the gun, will occur between 25km and 35km battle range. Therefore, about 5" effective deck protection is sufficient for a nominal IZ extending to around 30k. More is needed if the IZ is to be extended to even greater ranges. However, because of the expontential increases of deck penetration that occur at these extreme ranges it rapidly becomes impossible to provide enough deck armour. Even a doubling may only provide a few km more IZ. A compromize based on what is to be considered realistic battle ranges must be made. Before and through the time of WWI battle ranges beyond about 20km were not practical. During the interwar period advances in fire control made such battle ranges more practical.

During the WWI era vertical protection equal to the caliber of the oppossing shell was considered adequate. Following WWI advances in armour piercing shell design made this no longer the case. For example, Bismarck's 15" gun could penetrate more than 11" of vertical armour all the way out to 30km despite the increasingly acute striking angle.