On the armor of the cruisers Izumo and Bayan
In my previous article, I examined the armor of the cruiser Bayan in detail. Now let's look at its Japanese opponents.
Izumo Reservation Map
You can find various diagrams on the Internet, but the most illustrative one, in my opinion, is presented by the respected A.S. Alexandrov and S.A. Balakina in their work "Asama and Others."
Unfortunately, this diagram contradicts in some places the description of the armor protection provided by A.S. Alexandrov and S.A. Balakin. For example, according to the diagram, a 178 mm thick section of the main armor belt extends behind the feed pipe of the rear turret, further aft, while according to the description, it should have begun and ended under the main caliber turrets.
There are other inaccuracies in it, but more on that below.
Main armor belt (MAB)
The Izumo's main armor belt was 178 mm thick at its thickest point. It stood 2,13 meters high and 84,13 meters long. This length allowed the Izumo's main armor belt to cover the engine and boiler rooms, as well as the turret compartments, with 178 mm of armor. However, whether this belt covered the ammunition magazines for the 8-inch guns is questionable, as the diagram of the Asama shows these magazines located forward of the turrets, not under them. The same was true of the stern, albeit to a lesser extent.
The designs of the Asama and Izumo were quite similar, with minimal differences in their internal layout. Therefore, I cannot vouch for the Izumo's magazines being protected by a 178mm armor belt; they were likely located entirely (fore) and partially (aft) outside the protection of its seven-inch armor plates.
At the same time, the Izumo's ends were significantly lighter protected—the main battleship's bow and stern were now covered with 89mm armor plates. At the bow, these plates extended to the stem, but at the stern, they fell just short of the sternpost, leaving approximately five meters of unprotected side. Therefore, the main battleship's bow had no beams, while the stern was made of plates the same thickness as the ends—89mm.
At normal displacement, this armored belt rose above the water by 2 feet, or 61 cm. In addition, the citadel was protected by an armored deck, 63 mm thick throughout. The armored deck was sloped within the 178 mm belt, and was carapace-shaped at the ends, but there are issues here too, and we'll get to them later. The horizontal portion of the armored deck was located at the level of the upper edge of the main armor belt, and the slopes connected to its lower edge.
If we examine the main armor belts of the Bayan and Izumo, we can conclude that the differences are not that significant, but the Bayan is still inferior to the Japanese cruiser in literally every way, albeit slightly. The Bayan's armor plates are 33 cm thinner, and the length of the "thick" section of 200/100 mm plates is shorter (approximately fourteen meters). Even taking into account the 150/90 mm plates, it still falls short of the 178 mm length of the Izumo section, albeit only slightly. Furthermore, the "thick" section of the Bayan has an average thickness of 150 mm and 167 mm at the waterline, compared to 178 mm on the Izumo. Only at the ends does the cruisers' main armor resistance become comparable, or perhaps even better, if the 120/80 and 100/70 armor plates were cemented. However, the Bayan's unarmored stern was approximately four times larger than the Izumo's (20,7 meters versus approximately 5 meters).
In general, it's certainly not true to say that Bayan's main armor belt was completely useless compared to Izumo's, but it was still noticeably weaker. Both Izumo and Bayan had coal pits behind the main armor belt, but Bayan's were comparatively narrow—according to naval engineer N.N. Kuteinikov, at approximately three meters (10 feet) from the waterline, the width of the coal pits was 7 feet, or 2,13 meters. On the other hand, higher up, opposite the armor belt, the pit widened—possibly reaching 2,5 meters, and the width of Izumo's coal pits is unknown to me. It's likely that the cruisers were roughly equal in terms of coal pits, which was a significant component of the citadel's defense.
If the comparison were limited to the GBP and coal pits, it could be said that Izumo had a definite, but by no means decisive, advantage. However, Izumo's citadel was also protected by sloped armor decks, while Bayan's was lacking these. Given this, it should be noted that Izumo's vertical citadel protection significantly exceeded that of Bayan. Calculations show that in the most heavily protected part of the citadel, Izumo (178 mm GBP + coal + 63 mm slope) outperformed Bayan's (167 mm + coal) armor-piercing shells by approximately 40%.
Upper armor belt
This element of Izumo's armor protection was quite interesting. The upper armor belt covered the side of the hull only in the center—a length of 53,34 meters (on Izumo's sister ship, Iwate, it was even shorter, at 51,21 meters). Here, it was positioned so that its lower edge abutted the upper edge of the main armor belt.
Beyond this protection, the upper armor belt "folded" into the ship's hull, transforming into armored beams of the same thickness, extending all the way to the feeder pipes of the bow and stern main battery turrets. And here, the esteemed A.S. Alexandrov and S.A. Balakin presented us with two very interesting riddles.
First, they pointed out that the height of the upper armor belt amidships was only 1,53 m. And this is extremely strange, because in the armor diagram given above, we see that the belt converged with the lower edge of the casemates of the 6-inch guns.
But then it turns out that the height of the Izumo's interdeck space was only 1,53 meters! This, pardon me, is a bit excessive even for the short Japanese, so I can't rule out an error here. On the other hand, perhaps the diagram itself is incorrect, and the upper armor belt didn't connect to the casemate floor, leaving an unprotected gap? Theoretically possible, but somehow doubtful.
The second mystery. In the diagram, we see that the horizontal portion of the armor deck runs along the upper edge of the main armor belt within the citadel, that is, where the Izumo's sides are protected by 178 mm of armor, and only further forward, toward the bow and stern, does it become carapace, gradually tapering toward the stem and sternpost.
At the same time, A. S. Alexandrov and S. A. Balakin write that the armored traverses, which continued the upper armor belt, rested with their lower edge on the armored deck and gradually increased in height, verbatim:
As the protective deck dropped significantly toward the ends, the height of the athwartship bulkheads gradually increased. And where they surrounded the bases of the main battery turrets, the 127mm plates reached a height of 10 feet (3,05 m).
If this is so, then it must be stated that the diagram is incorrect, and that the armored deck turned into a carapace deck already within the citadel, despite the fact that the carapace at the ends was located much lower than in the diagram.
As for the Bayan, things were much simpler. The upper armor belt, 60 mm thick, made of non-cemented steel, was likely thicker than the Japanese one (where it covered the side), as it occupied only the space between decks and extended along the side from the stem to the edge of the aft casemate.
The Japanese logic is generally understandable: we followed a similar approach when designing the armor for the Peresvet-class battleships. The Izumo's citadel is quite powerful, but it rises only 61 cm above the waterline at normal displacement, and in a real combat situation, with coal reserves close to full, it's significantly lower. Consequently, even a slight list is sufficient for the upper edge of the main armor belt to be submerged. Moreover, the closer to the ends, the greater the list required for the main armor belt to be submerged, since closer to the stem and sternpost, the distance from the centerline to the main armor belt becomes increasingly smaller until it disappears. Therefore, a "short" upper armor belt in the middle of the hull is needed to provide armor protection to the side even with a slight list.
Nevertheless, I am inclined to view the 127mm upper armor belt and the main gun turret athwartships as one of the most unfortunate armor protection solutions for Japanese armored cruisers in general, and the Izumo in particular. The athwartships were long, tall, and very heavy. However, they did not ensure watertightness, leaving the ends of the Japanese armored cruisers exposed.
Many naval lovers stories It is believed that since the Izumo and other Asamoids had a continuous main armor belt, five meters short of five meters thick, covering the ends, and with carapace protection behind it, the issue of unsinkability of the ends was satisfactorily resolved. But this, of course, was completely untrue, as we can see from the damage to the armored cruiser Asama in the Battle of Tsushima.
Two ten-inch shells struck the armored cruiser's unarmored starboard stern very close together, suggesting they were fired from a salvo from a 10-inch turret firing two barrels. Moreover, they struck quite high above the waterline, just below the middle deck. The shells' caliber was determined by their entrance holes—one was 11 inches wide and could not have been fired from a 12-inch shell.
Both shells easily penetrated the plating, apparently hitting the bulkhead at frame 201 and detonating under the middle deck. Given the range, it's unlikely these shells were armor-piercing, and domestic 10-inch high-explosive shells were loaded with pyroxylin. The amount of pyroxylin in two such shells wasn't at all astounding, amounting to approximately 13,42 kg (16,39 Russian pounds in each shell). As a result, the middle deck was heavily damaged, which, however, turned out to be of little importance. But besides these shells, the
Bulkheads:
Lower deck:
The left side of the Asama was heavily damaged by shrapnel from the shells that hit the starboard side:
Interestingly, the cruiser's steering gear was damaged, though unlike the Rurik, it was quickly restored to service: the Japanese estimated it would take six minutes. I'm unclear what happened; the hull deck wasn't breached and should have protected the steering gear.
But the Asama had already had enough troubles. The armor on her ends, 89 mm thick, was not penetrated on either the port or starboard sides, and the carapace deck seemed undamaged. However, 16 fragments that punctured the port side allowed water to enter the ship, and damaged bulkheads caused a significant amount of water to spill over the stern. Worse still, the breached lower deck naturally lost its watertight seal, so the water not only spread along it but also flowed below, flooding spaces all the way to the carapace deck. As a result, without any armor penetration, the Asama trimmed up to 1,5 meters by the stern, temporarily lost speed, and was put out of action.
Meanwhile, the Bayan's belt armor, although half as thick, protected a much greater length of the side and bow, despite 60 mm being sufficient against high-explosive shells of the time. And against armor-piercing shells, even 127 mm of armor wouldn't have been enough, as the Russian 8-inch/45 armor penetrated it to 22 cable lengths with a 25-degree deviation from the normal, and 26 cable lengths if the shell struck strictly perpendicular to the plate.
Therefore, at the upper zone level, I consider Bayan’s defense to be both better and more rational.
Artillery, conning tower
Izumo's main battery turrets had 152mm armor thickness, offering virtually identical protection to Bayan's 150mm. The feed pipes, extending from the turret floor to the upper deck and then to the middle deck, were also protected by 152mm of armor, again matching the Bayan's protection. However, below, for the three meters separating the middle deck from the armored deck, the feed pipes were protected only by the extension of the upper deck's beams.
The three meters up to the armor deck of the Izumo and other British-built Asamoids were protected by 127mm of armor only on the sides and front, but the rear, facing the center of the ship, had no protection at all. Consequently, a shell coming from the sharp bow or stern angle could penetrate the thin side plating above the 127mm beam and penetrate the deck, leaving no protection at all on its way to the feeder pipe.
The Bayan's aft turret was obviously better armored—its feeder pipe was 150 mm thick across its entire height, all the way to the armored deck. The Bayan's forward turret was less well protected than its aft turret. Because the forward turret was mounted on the forecastle, its feeder pipe was longer. Over the two interdeck spaces (from the forecastle deck to the upper deck and from there to the gun deck), it had 150 mm of armor, but below that, it was only 60 mm. However, this section of the feeder pipe was covered by the Bayan's upper armor belt, giving it two layers of 60 mm each.
This section of the Bayan's armor was less well protected than the Japanese one—after all, 60+60 mm of homogeneous armor was significantly inferior in armor protection to a 127 mm thick cemented armor plate. Furthermore, there was a non-zero probability that a shell, flying over the upper edge of the 60 mm armor belt, would penetrate the unarmored battery deck and strike the 60 mm feed pipe directly.
This, of course, doesn't paint the Bayan's armor scheme in a good light, but in my opinion, Japanese ships that didn't have armor on their supply pipes, other than the beams from the middle deck to the armored deck, were much more vulnerable. Therefore, it's probably safe to say that the vertical armor of the Izumo's turrets and supply pipes was still inferior to that of the Bayan.
The same conclusion applies to the horizontal protection of the turrets. In the previous article, I discussed how the 30mm thick turret and conning tower roofs might have been insufficient against large-caliber shells. But on the Izumo, the turret and conning tower roofs were even thinner—only an inch, or 25,4mm, of armor steel.
When it came to casemates, the Japanese were clearly ahead. The Izumo had 10 individual casemates, with the front, enemy-facing side being 152 mm thick, and the sides and rear being 50,8 mm. The casemate roofs, however, were the same 25,4 mm thick as the conning tower and turret roofs. However, this protection was, of course, significantly stronger than the 60 mm casemates with 15 mm roofs of the Bayan.
Part of the middle artillery The Izumo's 4 x 6-inch guns were mounted in deck mounts and protected only by box-shaped shields with a 76,2-mm thick front plate and 31,75-mm thick side plates. While the shields were fairly thick, given their relatively small surface area, this protection was very weak, though better than nothing. But let's not forget that compared to the Bayan's 8 x 6-inch guns, the Izumo had 10 guns in better-protected casemates and only 4 in deck mounts. Therefore, the Izumo's 6-inch guns were clearly better protected.
The same can be said about the conning tower. Actually, the Bayan's wasn't bad, with 160mm thick walls, but on the Izumo, it was made completely indestructible, increasing the thickness to 14 dm (356 mm).
Horizontal protection
We've already discussed the roofs of the casemates, turrets, and conning towers; the only thing left to discuss is the armored decks. On the Izumo, it was 63 mm thick, and would have been considered very good protection, were it not for one issue. This deck was not armored at all, being constructed from two or possibly three layers of shipbuilding steel (on the Asama and Tokiwa, the decks were constructed from two layers, each 25,4 mm thick, for a total of 50,8 mm). On the Bayan, however, the armored deck was truly 30 mm thick armored steel on a backing of two 10 mm thick plates.
Considering that armor steel was approximately 20-30% stronger than shipbuilding steel, it turns out that 30 mm of Bayan's armor deck was roughly equivalent to 36-40 mm of ordinary steel. Therefore, Bayan's 50 mm deck, in "steel equivalent," could be estimated at approximately 56-60 mm. In other words, it was practically no different from Izumo's deck and stronger than Asama and Tokiwa's. However, soft armor steel resisted tangential shell strikes better than ordinary shipbuilding steel, more often resulting in ricochets. Therefore, I consider Izumo's comparatively thick armor deck to be the second significant design error—they should have reduced its thickness by using armor. Look, thanks to the weight savings, it would have been possible to place not 10, but all 14 6-inch guns in casemates...
On the protection of Bayan and Izumo in a spherical vacuum
As is well known, the Japanese armored cruiser was significantly larger than the Russian one—the Izumo's normal displacement was 9,503 tons, while the Bayan's was 7,805 tons, meaning the Izumo was almost 22% heavier. Its armor weighed 2,632 tons, while the Bayan's was only 1,449 tons, so in this respect, the Japanese cruiser's advantage was a colossal 82%!
But how did the Japanese utilize this undeniable superiority? As the comparison above shows, the Izumo gained a significant advantage over the Bayan in three areas, namely vertical defense:
1. Conning tower;
2. Citadels;
3. Artillery of 6-inch caliber.
How well protected the Izumo was against the armor-piercing shells of the 8-inch/45 guns of the Bayan is shown by the following table.
From which it is quite obvious that the Bayan’s artillery could penetrate the Izumo citadel only at a distance of 10–15 cables, while the Izumo’s artillery installations could be hit by the Bayan at a distance of 18–22 cables.
And what about the Bayan? Its citadel at the waterline (167 mm of armor plus coal) could be penetrated by an armor-piercing shell from a Russian 8-inch/45 gun at a range of 14-17 cable lengths. The Japanese eight-inch gun was somewhat more powerful, so if the Izumo's main battery had been armed with first-class armor-piercing shells, they would have been able to penetrate the Bayan's citadel at a range of 16-18 cable lengths, and its turrets, protected by 150 mm of armor, at 21-25 cable lengths.
As for the protection of the main battery turret magazines, it's very difficult to understand. On the Bayan, the magazine area was protected by 120/80 and 100/70 armor plates, with nothing behind them. However, the magazines themselves were located "one floor below," so shells penetrating the main armor belt would explode not in the magazines, but in the spaces above them. On the Izumo, the magazines were, as far as can be determined, completely outside the 178mm citadel for the forward turret and partially outside the stern turret. But this raises a question.
Unfortunately, I don't have a diagram of the Izumo, but the diagram of the structurally similar Asama shows the magazines located directly under the armored deck, as I've already shown above. So, if the diagram is correct, these magazines are quite vulnerable to the Bayan's 8-inch shells at virtually any reasonable range, as they penetrate the 89 mm side and 63 mm slope at 34-37 mm. But if A.S. Alexandrov and S.A. Balakin were correct and the 127 mm beams of the Japanese armored cruisers were 3 meters high near the main battery feed pipes, then it follows that the carapace deck was located significantly lower than in the diagrams. That is, even at the feed pipe, it wasn't level with the upper edge of the main armor belt, but rather at the waterline, if not lower!
In short, an 8-inch shell definitely had the potential to hit the magazines of a Japanese cruiser; the only question was the probability of such a hit. I can only conclude that both the Bayan and the Izumo were in roughly equal conditions in terms of the protection of their 8-inch gun magazines, as 8-inch shells could penetrate their protection at virtually any reasonable range with armor-piercing shells of the time.
Some preliminary conclusions
If the Izumo's commander had had high-quality armor-piercing shells at his disposal, it would have meant that the Bayan would have had to close to within 10-15 cable lengths to penetrate any of the Izumo's armor, while the Izumo would have been able to penetrate any of the Bayan's armor at 16-18 cable lengths. In reality, it was even less, because I calculated the Bayan's waterline armor strength at normal displacement, whereas she would have gone into battle at, if not full displacement, then close to it, and her armor thickness at the waterline would have been greater.
At the same time, at 16-18 kbt, both the Izumo’s artillery and ammunition magazines would have been vulnerable to the Bayan’s 8-inch guns; only the engines and boilers were safe, as well as the supply pipes in the area of the 178-mm armor belt, and even then only on the condition that the Izumo was turned sideways to the Bayan.
Thus, having spent 82% more armor than the Bayan's designers, the British did not make the Japanese armored cruiser ultimately protected from 8-inch armor-piercing shells. Moreover, in a number of combat situations, the Izumo, due to its armor design, performed worse than the Bayan.
Let's imagine the Izumo trying to catch the Bayan. Yes, the Bayan's stern was downright indecently exposed, but its steering gears were still protected by a 30mm armor deck, resting on a 10mm+10mm steel backing, and this deck was located below sea level. An armor-piercing shell would likely ricochet off such armor upon impact. And then there's the 8-inch turret, the bottom of which is protected by a 200mm/100mm armor beam, meaning at the waterline—the same 167mm of armor, 150mm feed pipes, and turret walls.
And what about the Izumo, the pursuing ship? A very thin strip of armor at the bow, 60 cm above the waterline at normal displacement, and much lower in real combat. That's all! The rest of the ship's bow is just plating, which an armor piercer would pass over without noticing. And behind it is a 63 mm carapace, only it's not horizontal, but at a significant angle, sloping toward the stem, making the Bayan's 8-inch armor piercing chance of penetrating this carapace much greater than the Izumo's chance of penetrating the Bayan's deck at the stern. Where the Bayan's magazines are protected by approximately 150-200 mm of abeam armor (the narrow part of which is located under the armored deck) and 150 mm of feed pipe protection, the Izumo has three meters of 127 mm abeams, which are clearly less durable than the Bayan. Even if the Russian shell doesn't penetrate the Izumo's carapace and feed pipes, damage to the bow could cause it to flood, similar to the Asama, forcing the Izumo, having a bow trim of a meter or a half, to abandon the chase. But then the Bayan might even return...
Let's imagine a different scenario: the Bayan is catching up, and the Izumo is escaping. The Bayan is essentially invulnerable here—its relatively high 60mm upper belt will cause shells that hit the hull to ricochet. The only danger is if a shell passes directly over the upper edge of the armor plates, penetrates the upper deck, and hits the 60mm section of the barbette, but that would require some clever maneuvering. Meanwhile, the Izumo has the same problem as the bow: a relatively vulnerable carapace, three meters of weakened 127mm armor, and a very good chance of suffering serious flooding and loss of propulsion without any damage to the armor at all.
Thus, even considering a possible clash between the Bayan and the Izumo within the logic of free maneuvering zones, I don't see an overwhelming advantage for the Japanese cruiser. Especially considering that the Bayan, with a couple of knots faster, would choose a convenient engagement range, and the firepower of its two single-gun turrets is only slightly inferior (if at all) to the Izumo's four eight-inch guns.
But all of the above would only have worked if the Izumo's magazines had been stocked with fully-fledged and effective armor-piercing shells. But they weren't there, because, as I've written many times before, Japanese armor-piercing shells were essentially a type of high-explosive shell. Furthermore, a warship's protection isn't limited to the thickness and surface area of its armor. Let's consider what a confrontation between the Izumo and the Bayan could have resulted in under the real conditions of the Russo-Japanese War.
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