The Emergency Japanese Battleship Owari

Historical what if discussions, hypothetical operations, battleship vs. battleship engagements, design your own warship, etc.
Mostlyharmless
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The Emergency Japanese Battleship Owari

Post by Mostlyharmless »

I feel a little guilty in posting a hypothetical ship, which I have named Owari, rather than an encounter and would have probably chosen to post this at the Warships Projects BB if that board had not evaporated. Sadly I wanted to discuss a ship that was never considered rather than a design like the Montana Class battleships that were never laid down. I hope that the audience here will be interested and even that some of you will be able and willing to correct my errors.

There is fortunately some background to suggest that my battleship might have seemed a good idea if it had been considered. As she was never considered, I have to make my very rough plans myself and trying to do that has certainly brought home to me that the battleship designers before WW2 did have one or two skills. As Owari will be laid down in January 1937 by the IJN, I started my calculations from Yamato, assuming that that design was being prepared in parallel and I was surprised to discover that it was very hard to make Owari very much smaller than Yamato.

The starting point for Owari is the Washington Naval Treaty of 1922 and its extension by the London Treaty of 1930. Washington brought to an end the post WW1 naval race between Japan and America with Japan being allowed to keep just 60% of America's and Britain's total tonnage of battleships. At the moment when the treaty was signed, Japan had just completed the battleships Nagato and Mutsu, the IJN's first ships with 41 cm (16.1 inch) guns, and had just launched the two Tosa Class ships http://en.wikipedia.org/wiki/Tosa-class_battleship. Four Amagi Class battlecruisers http://en.wikipedia.org/wiki/Amagi-class_battlecruiser had been laid down and the IJN was about to start construction of the next two battleships, which would probably have been named Kii and Owari (I used Owari for my emergency ship to avoid confusion with any other classes such as the Kii Class for which plans exist). Japan had built about forty 41 cm guns by the time that the Treaty was ratified and construction stopped. Under the terms of the Treaty, Japan was allowed to convert two ships into aircraft carriers and initially chose Akagi and Amagi. However, Amagi was seriously damaged by the 1923 Yokohama Earthquake and Kaga was substituted. After the earthquake damage to Yokosuka had been repaired, Kaga was moved there and converted from 1925.

Now we come to the doubtful part. During the 1920s, the IJN had plans to reconvert Kaga into a battleship if the Treaty were to break down. Thus at least initially Kaga's belt and barbettes must have been stored, presumably at Yokosuka, where Kaga was converted and where the third Yamato Class ship, Shinano, would be built later. I do not know when those parts were disposed of or if they still existed in 1935. By 1935, aircraft carriers were considered as valuable as old battleships and Kaga's armour, which was of similar quality to that of HMS Hood and inferior to that of the Nelsons and later battleships, was considered too thin and weak to defeat modern shells. Thus there is no mention of plans to reuse Kaga's armour during the thirties and if it existed, it would certainly have been recycled during the Late Thirties and Early Forties when Japan was very short of steel.

Japan gave two years notice that it was withdrawing from the Treaty system in December 1934. Thus the IJN was allowed to lay down new battleships from January 1937 and Yamato was laid down in April 1937. She was not commissioned until the end of 1941 and was not considered ready for combat until well into spring of 1942. It seems possible that over 1935-6 many IJN officers might have anticipated a crisis over China, leading to a risk of war with either Britain or America over 1939-41. However, OTL they would have to fight any such war with only their old battleships.

However, let us imagine that the IJN had obtained plans of the Italian Littorio Class, laid down in 1934, and possibly also of the German Scharnhorst Class, finally laid down in 1935. Both of these classes attempted to use two plates of armour, with the thinner in front to defeat modern shells. The idea was that the outer plate or deck would remove the cap and yaw the shell, which would then be stopped or broken up by either an inner face-hardened plate or an unusually hard homogeneous deck. Inspired by these ideas, which they could verify with their own tests, the IJN plans to use Kaga's turrets, barbettes and belt to construct an emergency battleship unusually quickly and at relatively low cost. The basic idea is to build a citadel with an outer shell of homogeneous armour and mount Kaga's belt internally within it with a new main deck above the belt but at least one deck below an armoured flying or weather deck.

There are two real advantages to using old armour and guns: the cost and the construction time. One RN estimate was that the main guns were 28% of a battleship's cost. We won't save quite that much as we are going to have to re-armour the turrets. The machinery and fire control is all new and is costly. However, the belt and barbettes will be Kaga's and that should produce another big saving in both time and money. If Owari can be built very quickly and foreign attachées discover her existence, she might give Japanese diplomats some extra backing over 1939-41.

Mounting the belt internally means that we don't have to worry about Kaga's 1922 hydrodynamics and can use the best shape for the hull that we can design. The obvious problem is that we need more beam outside the engine rooms than we find in Yamato, which was anyway cramped http://www.spacecruiseryamato.com/ijn/a ... /heel.html. Thus, assuming that we use Yamato's machinery, which was available if a rather conservative choice, we are going to need to rearrange the machinery. As Kaga's belt is about 143 metres (measuring roughly from pictures) compared to Yamato's citadel of 131.6 metres, we can accept some extra length. Owari's turbine rooms will be two abreast rather than Yamato's four but there will be additional machinery parallel to the turbines. For a rough guestimate, Yamato had a beam of 38 metres with about 10.2 metres of torpedo defence whilst Owari will have a beam of about 33.5 metres with about 12.5 metres of torpedo defence. Thus the width of the machinery is three quarters of Yamato's. The less obvious problem of an internal belt is that you have to fit it before launching the ship. Thus Owari could reach Yamato or Shinano's weight before launch.

The greater length of the machinery quickly forces us to only fit four of Kaga's five turrets or at least four rebuilt turrets. Nagato's turrets weighed just over 1,000 tons as built. The armour thickness went up by 50% during the rebuild in the Thirties, so I am going to guess that Owari's turrets weigh about 1,400 tons each. That means that we have roughly the same weight as Yamato aft but probably need more length whilst we have significantly less weight forward. Thus Owari's citadel will end on a slimmed down Yamato hull at roughly the same point aft but extend further forward to preserve the trim. As I do not have the expertise to design a new hull form, I am simply going to assume that we keep Yamato's length and draft (Yamato's draft was similar to other battleships despite Yamato's size, being deeper than Tirpitz, which was constrained by the Kiel Canal, or the Littorios at 10.5 metres but less than the 36 feet or 11.08 metres of Vanguard or Iowa), whilst reducing the beam by a ratio of 0.88 (obviously a competent designer could do better). The slimmed down form might give us around two knots more speed, so we are hoping for 29.46 knots on trials at 153,000 shp. If Owari carries the same 6,200 tons of oil as Yamato, she will surely have a significantly longer range.
Mostlyharmless
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Re: The Emergency Japanese Battleship Owari

Post by Mostlyharmless »

Owari is going to have two continuous structurally significant armoured decks above the waterline, a main deck and a weather or flying deck. The main deck will follow Japanese design ideas and is positioned well above the full load waterline. Thus we will not have all the splinter proof bulkheads connecting the decks and limiting flooding as seen in Bismarck. The main connection between the two decks will be the outer hull but this will be very strong as it will be at least 100 mm thick homogeneous armour (Bismarck's outer hull could be just 15 mm of structural steel to which 145 mm of face-hardened armour was attached because of the 30 mm bulkheads going up to the weather deck). Below Owari's main deck, the connections to the keel are the main belt extension downwards, which will resemble South Dakota or Iowa's, and a bulkhead behind the main belt and its extension which will be the outer boundary of the machinery space. This inner bulkhead will hopefully stop fragments from either shells or the main belt itself, assuming that de-capped shells are at least heavily damaged by the main belt. It will also greatly strengthen the TDS.

One of the first things to decide is the height of the main armoured deck. Ideally I would have liked Owari to have its main deck at the same height as Yamato's. Then I could simply copy the deck positions of Yamato without worries about fitting the turbines into the space. Unfortunately, Kaga's belt is shallower than Yamato's. Kaga's belt covers approximately 5.5 metres http://xoomer.virgilio.it/bk/NWS/Imperi ... gaKii.html and the 1924 tests on Kaga's sistership Tosa http://en.wikipedia.org/wiki/Japanese_battleship_Tosa seemed to show that shell hits at a depth of 3.3 metres need to be blocked. Thus Owari's main deck is only about 2.2 metres above the full load waterline. Hopefully, it is high enough to squeeze in Yamato's turbines and a communication deck above (just be careful not to bang your head). 2.2 metres is higher than the 5 feet that was the late war position accepted for South Dakota, is approximately the same as Iowa's but is lower than Yamato's or King George V's 9 ft. 9 inches. As Owari only has one deck above the main armoured deck, like British, French and American battleships but unlike Yamato, Bismarck or Littorio, Owari won't have as much freeboard as Yamato or the RN ships but should roughly match the Iowa http://www.hnsa.org/wp-content/uploads/2014/08/bb64.pdf (note that the designed waterline may be below the waterline after adding anti-aircraft guns etc.).

If we have the belt extending a fairly safe 4 metres below the deep load waterline, the top of the belt is only 1.5 metres above the deep load waterline. Then we must follow Yamato in having slopped outer sections of the main deck at a cost in weight because those sections must be thicker. If we assume a 7 degree slope and about 5.5 metres of TDS inboard of the top of the belt, we gain 0.67 metres. Yamato had 230 mm also sloped at 7 degrees compared to 200 mm for the flat sections of the main deck, so it is reasonable to add 30 mm also to the thinner deck of Owari. The Navy Weapons site gives the penetration of the American 16 inch/45 super heavy as 109 mm when falling at 17.9 degrees and 146 mm when falling at 25.4 degrees, suggesting that adding 30 mm for 7 degrees seems to be roughly valid. For a 143 metre citadel, the cost in weight is about 380 tons. Alternatively, we can have the main armour deck flat, which will be structurally simpler, and have the main belt only extending to 3.3 metres at full load. We then need to have homogeneous armour covering the gap. If that were about 250 mm thick, we would need about 400 tons of armour.

Apart from using slightly less weight, the other advantage of following the Yamato scheme is that the extension down to the keel uses slightly less beam. German studies found that 100 mm of armour and a space of 0.6 metres was needed to decap a 38 cm shell. As we are worried about 16 inch shells, Owari may have a space of 0.75 metres between the outer shell and top of the belt, which is assumed to be sloped at 15 degrees as in the Kaga design. If the top of the belt is 1.5 metres above the waterline, the extension meets the keel 12.36 metres below and 3.36 metres inwards (multiplying by tan of 15 degrees), which is 4.11 metres inwards from the projection of the ships side downwards (I assume that Owari will have a mostly vertical outer shell). For comparison, with the flat deck option, the extension would meet the keel about 4.3 metres inwards.

It really is fair to assume a belt without a Yamato like bend and resulting weakness, as once we have made the machinery space thinner, there is no point in the kink. The extension of the belt downwards starts where the belt tapers as the backing plate and grows from 15 mm to 200 mm of VH at the bottom of the face-hardened belt, to which it is attached by armour bolts from the back. We might add keying bolts. These would weaken the bottom of the belt against shells but make it harder for explosions to push apart the two belts. The extension is homogeneous and tapers down to 75 mm where it meets the outer hull. Using thinner armour than near Yamato's magazines could be justified by the evidence that only Japan has developed shells optimised to travel through water without tumbling (we started by assuming that Japan had seen the Italian and German designs with shallow belts). However, even such a thin extension, equivalent to that over Yamato's machinery, is not light. Assuming a uniform taper over 6.9 metres, gives a weight for both sides of 6.9 x sec (15) x (200 + 75) x 8 x 143 = 2247 tons (the sp. gravity of armour is assumed to be 8.0). Note that Yamato escaped some of this weight by raising the magazines away from the double bottom and armouring beneath the magazines http://en.wikipedia.org/wiki/File:Yamat ... es_cut.svg. Owari will have oil tanks beneath her magazines and could have armour there if the weight can be accepted but the belt extension has to go to the keel to allow the inner bulkhead to function as part of the TDS. The longer citadel also costs extra weight.

We might consider reducing the thickness further to below 75 mm. However, we would then need to add oil tanks. Japanese test suggested that voids were the best defence against the pressure wave from an explosion whilst at least 80 cm of liquid was useful to stop fragments thrown inwards by a torpedo explosion. Thus Yamato was built with only voids as it had armour that was expected to defeat fragments whilst Taiho had two layers of liquid http://www.fischer-tropsch.org/primary_ ... S-01-9.pdf.

It is also worth pointing out that all the above depths are for full load. The IJN conducted trials and assumed that battle would probably occur at 2/3 fuel load. As it is reported that the smaller Littorio submerged one additional centimetre for every 52 tons, it seems likely that Owari's draft might change one centimetre for every 65 to 70 tons. Thus Yamato's full oil load of 6,200 tons will change the draft by just under a metre.
Mostlyharmless
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Re: The Emergency Japanese Battleship Owari

Post by Mostlyharmless »

Now for the details of armour thickness and weights. As an first try, we can try 125 mm for the vertical shell near the barbettes and magazines and 100 mm elsewhere. If that goes from 5 metres above the waterline to 3 metres below and 60% is 100 mm, the two sides will weigh roughly 143 x 2 x 8 x 8 x (0.6 x 0.1 + 0.4 x 0.125) = 2014 tons.

The weather deck might be 80 mm near the barbettes and 55 mm elsewhere (German 50 mm of Wotan Harte could reliably decap shells but I have given 55 mm in case Japanese armour is inferior). Again, that will probably weigh just over 2,000 tons.

The main deck can 125 mm and 155 mm on the slopes. I am guessing that the weight of the main deck will be around to 4,700 tons.

Kaga's belt including 15 mm backing weighs 5.5 x sec (15) x 0.295 x 143 x 2 x 8 = 3843 tons. The barbettes assuming an 11 metres diameter are 4 x π x 11 x 0.3 x 8 x 3 = 995 tons between the main deck and the weather deck. There is also supporting structure of 50 mm thickness below the main deck which might get counted as armour but is not listed here (it would add perhaps 150 tons). Owari may need new armour for barbettes B and X above the weather deck. These are perhaps 2.2 metres high and could be 41 cm as in Yamato. Thus 2 x π x 11 x 0.41 x 8 x 2.2 = 500 tons. Alternatively, we could put a ring of diameter 12.5 metres around old barbettes if Kaga's barbettes are long enough (Help, how are barbettes made? If they are rings keyed together, we can certainly mix and match five old barbettes into four of the right length. Cutting could wear out too many cutters but we could if desperate cut with a torch and hide the weakened structure below the main deck at another cost in weight).

The inner bulkhead covers the 13 metres from keel to main armour deck. If it were 40 mm thick, it would weigh 143 x 2 x 13 x 0.04 x 8 = 1190 tons.

Then we have to seal the ends of the citadel, which will cost another 1,000 tons, protect the rudder control room or rooms, adding perhaps 500 to 700 tons, and we also need an armoured conning tower (unfortunately I have no idea of either dimensions or thickness).

Adding the armour weights together, gives us about 19,200 tons plus a conning tower, which compares with around 22,900 for Yamato. Fortunately, we have only two thirds of the armament weight and Owari's structure will be significantly lighter than Yamato because the ship's beam is reduced and because some of the weight given as “hull” in Yamato is classed as “armour” above (for example, the weather or flying deck and the upper vertical hull). Thus a hull form with 88% of Yamato's beam should be able to support Owari's weight.

Finally, we can explore the effectiveness of Owari armour scheme. The main belt is 280 mm exactly as the Littorio Class. However, the armour quality is much inferior. We can only hope that the greater spacing and the thicker armour in front, 125 mm instead of 70 mm, can yaw the shells enough to compensate for the lower quality. Certainly the 40 mm splinter protection behind the main belt is likely to get tested if the belt is hit by a shell.

The decks and barbettes can be compared with Tirpitz. Tirpitz had an upper belt of 145 mm face-hardened armour on 15 mm of ST52 and a weather deck of 50 mm and 80 mm of Wotan Harte. The main deck over the magazines was 100 mm and 80 mm over the machinery. The barbettes were 220 mm below the weather deck.

There are elements of the comparison in favour of Tirpitz. Tirpitz's barbettes were very hard face-hardened armour designed to shatter decapped shells. Owari's 300 mm barbettes are not so hard and the armour is probably up to 20% weaker. Finally, Owari's outer belt is only 125 mm homogeneous armour and will not slow or yaw shells so strongly. Thus Owari barbettes are likely to have comparable resistance to Tirpitz's but not better.

Owari's deck is 125 mm compared to Tirpitz's 100 mm over the magazines. However, again Tirpitz's armour is harder than most homogeneous armour and the space between the two decks is almost double in Tirpitz. German studies suggest that these features should compensate significantly for the thinner German armour. However, Owari is surely stronger over the machinery. The relative effectiveness partly depends on whether Owari's MNC armour can be hardened whilst retaining good extension like Wotan Harte.
Mostlyharmless
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Re: The Emergency Japanese Battleship Owari

Post by Mostlyharmless »

I would like to point out again Owari's advantages, building time and cost. Shattered Sword quotes the cost of Soryu as 40.2 million yen. Yamato's displacement was around four times that of Soryu. The aircraft carrier Soryu was being completed as Yamato was being laid down and had almost identical propulsive power. Thus we might naively expect Yamato to cost 160 million yen, especially as that would pay for four 150,000 shp power plants and four sets of anti-aircraft guns and directors. However, Yamato is quoted as costing 250,000,897 Yen by Wikipedia. The difference reveals the cost of the big guns and the armour.

Clearly, Owari does contain a good deal of new armour. However, the reuse of Kaga's armour might bring Owari's cost down to around the 160 million yen region, if we balance the medium thickness armour against only needing one set of machinery and rudders. If true, that would make Owari at 46 million dollars (using the 1937 3.47 ratio) the cheapest of the last generation battleships, costing even less than King George V's 50 million dollars.
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