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When it comes to naval guns, bigger isn't always better. The 46cm/18.1" guns on the Yamato-class battleships didn't have much in the way of advantages over the Iowa-class's 40.6/16" guns. Yet the Japanese were planning on an even larger 51cm/20.1" gun for their Design A-150 battleships.

In designing a world that has WWII-level (Maybe a little further, too) tech, delayed development of aircraft, and no equivalent to the Washington Naval Treaty, I'm trying to design mostly realistic battleships.

Washington Naval Treaty's effects

In the chart above, you can see the definite upward trend of battleship size that is very suddenly halted by the Treaty. Interestingly, the Yamato fits very nicely where the line would be projected to sit at about 1936 when it was being built. Continuing this line would result in 90-100+ thousand ton battleships at around 1945-1950, the tech area I am aiming for. Such large vessels will have a significant amount of armor, and significant guns to defeat it. I'm using the Yamato as a baseline for ships that would exist in this time, but I'm wondering what sort of guns I should arm them with.

With that in mind:

  • What advantages do extremely large guns give?
  • What disadvantages do they have?
  • Why would a nation use them despite the disadvantages?
  • And, for bonus points: What would the largest practical battleship gun be?

I am defining Extremely Large Guns as guns in the range of 20-25 inches/50-65cm.

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    $\begingroup$ There are other constraints to battleship size you may want to consider, namely the ability to fit through the panama and suez canal locks. The Iowa BBs are -just- small enough to fit in the panama canal locks. Obviously this is more of a consideration for a two ocean nation like America than a purely pacific ocean nation like Japan, but you can't just increase ship size with no thought to how you are limiting it's movements due to shallow ports, bridges, and canals. $\endgroup$ – Jason K Mar 30 '17 at 13:04
  • $\begingroup$ Of course, but that depends largely on geography and where a country sits. Honestly, I haven't figured that out yet for my world. $\endgroup$ – Andon Mar 30 '17 at 15:39
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    $\begingroup$ I like how Britain built one more design right after the treaty line, so rebelious. $\endgroup$ – DasBeasto Mar 30 '17 at 18:03
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    $\begingroup$ @Andon true, but you can have the largest gun in the world, but it is useless if it can't get to where it is needed. Details like this add a lot of authenticity to worldbuilding. Maybe you have a larger gun (technically a naval rifle) but it sits in a solo turret, instead of a triple one, because the beam of the ship can't be widened anymore. You also have to consider the fire control system. No point in having a gun that out shoots your ability to aim it. $\endgroup$ – Jason K Mar 30 '17 at 19:38
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    $\begingroup$ @DasBeasto the one construction by Britain after the treaty date was specifically mentioned (and allowed) in the treaty. $\endgroup$ – Paŭlo Ebermann Mar 30 '17 at 20:04
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First off, this is the best place on the internet for information about the Pacific Theater of WWII; and specifically this page is for you. Also try here; I'm going to pull a lot of statistics from there without siting them.

What advantages do extremely large guns give?

The range advantage of larger guns is not that significant. First off, range is determined by a whole slew of factors, and smaller guns can have longer ranges than larger ones. For example, the French 380mm/45 gun had longer range than both the Iowa's 16"/50 and the Yamato's 46cm/45. The German Scharnhorst and Gneisenau had a 28cm gun (equivalent to 11") that had better range than the American 16"/45 gun. Scharnhorst, in fact, with its 11" guns scored the longest range naval gun hit of all time, at 26465 yards, on Glorious, a British aircraft carrier.

So, having demonstrated that range isn't improved that much by larger size, the only advantage remaining is penetration power. This is somewhat significant, because light cruiser's 6" guns were generally unable to damage the belt or tower armor of a 40,000 ton+ battleship. The larger calibers could always pack a larger shell. A larger shell can penetrate a given armor thickness at more angles. Armor that shrugs off a glancing blow from a 12" shell might buckle from the same hit from a 14" shell.

However, once you were in the 14" plus range, the quality of the shell itself became a more significant concern. Shells that large can penetrate pretty much any armor and do massive damage, provided they explode. For example, at the Second Naval Battle of Guadalcanal, South Dakota took 26 large caliber hits from Kirishima, but many of those hits did not explode. Thus South Dakota floated on, and only lost 39 of her crew. Meanwhile, Washington opened fire on Kirishima (at 9000 yards, in case you were wondering, which is a typical engagement range on ships for WWII big guns) and hit her 9 times with large caliber. This was enough to capsize and sink Kirishima within about 4 hours. Crucially, several of Washington's rounds fell short, but hit Kirishima underwater and still exploded. That is a good, reliable shell! The moral of the story is that hitting someone is great, hitting with a round that actually explodes is much better.

What disadvantages do they have?

As you can see in my 'pro' answer, I am not sanguine about larger guns. You should only make a gun large enough to be able to penetrate the enemy's armor at all angles. After that you should concentrate on shell reliability, range-finding equipment (preferably radar...definitely invent that), armor, and damage control.

The disadvantages of large guns are in what they cost you in terms of ship design. Guns have to be mounted topside (obviously) to do any good. Also, you need a lot of them. There are no automatic loaders with the specified technology level, so you are limited by the speed at which you can have men reload them. And since you need (vulnerable) men to reload them, you need the guns to be heavily armored to prevent the men, and ammunition, from being subject to return fire. Turrets were easier to put out of action by killing the operators than by destroying the machinery. The ammunition required for the big guns represents a huge vulnerability that can exploited for the WWI or WWII version of a one-shot.

It is the armor protection for the turret that makes a large gun a liability. If the turret is not armored well enough, you can take out the turret too easily, or potentially doom the ship. The larger the gun, the larger (and heavier) the turret armor. The fact that this weight is topside is significant. You can't let the center of mass rise too high for a number of reasons (not least, the higher the center of mass, the more you heel during a turn, and the less likely you can successfully aim at or hit a target while turning).

If you mount large guns with heavy turrets, high above the water line, you ship must get more massive to compensate. Sure this allows you to add even more armor, but at the expense of even more maneuverability. And this maneuverability is the key weakness. By WWII, the torpedo was a very advanced and effective weapon, despite being unguided. Destroyers and light cruiser could launch spreads of 4 or 6 torpedoes at a time. The true effectiveness of torpedoes in main fleet engagements, battleship to battleship was not really tested in WWII.

The last true fleet battle (until space battles!?!?!) was at Jutland in WWI. Torpedoes were largely inconsequential in that battle, though a storm of torpedoes from destroyers and light cruisers may have allowed the German battleline to escape destruction. However, by WWII torpedoes meant business. At the Battle of Savo Island, the Japanese crushed the American forces, sinking 3 heavy cruisers, by opening with a torpedo salvo at about 12,000 yards. At Tassafaronga, the Japanese were ambushed by the Americans, who found them with radar in the dark. However, the Japanese Long Lance torpedo turned the battle; the Americans did not notice the torpedo launch and did not evade, causing the loss or damage of 4 heavy cruisers. The Americans got their revenge at Vella Gulf and Cape St. George, once they had perfected aiming and firing torpedoes at night guided by radar. Between the two battles, American destroyers sank 6 ships without taking a single hit in return.

Since there was no true battleship to battleship fleet engagement of WWII, I have to conclude that the danger of torpedoes had become very significant. The penetration power of the torpedoes (recall, Yamato was sunk by air-dropped torpedoes) was such that they could stop capital ships.

While the effective engagement range for naval guns, especially on ships with radar, was longer than for torpedoes, torpedoes were enough of a threat at short to medium range that maneuverability was a concern. The increase in turret weight and corresponding increase in ship mass would have been counteracted by the vulnerability of such ships to torpedo attacks. The optimal battleship would not have been any bigger than the Iowa class, and perhaps even smaller.

Why would a nation use them despite the disadvantages?

Simply put, pride and ego.

What would the largest practical battleship gun be?

Guns in the 14" to 16" range were large enough. In a war with no aircraft, the medium range power of the torpedo would have been amply demonstrated. The ideal gun would have the range, accuracy, and rate of fire to engage torpedo carrying ships (destroyers and light cruisers) long before they could launch torpedoes, and the penetration power to punch through a battleship of similar size.

I believe that maneuverability would have been prohibitively bad for battleships above 50,000 tons, and this lack of maneuverability would have made them too vulnerable to torpedo attacks from small escorts. By limiting ship mass to 50,000 tons, total armor is limited such that guns in the 14" to 16" range are all you need to punch a hole in them.

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    $\begingroup$ Few additions: larger range means longer shell flight time means enemy has more room to dodge (e.g.: wiki says that at 40 km shells would fly for 90 seconds). This is what manoeuvrability here is about. BBs were actually very fast, often matching CAs and not that much behind DDs. Difference is, DD and CA can turn quickly, BB can't, but at range long enough even BB can dodge effectively. Meanwhile, DDs and Torpedo boats can evade BB main gun fire and get very close to launch the torpedoes. Ergo: limit on range and necessity of escorts is about flight time vs manoeuvrability than anything else. $\endgroup$ – M i ech Mar 30 '17 at 4:39
  • $\begingroup$ To add on to the slight torpedo tangent: Torpedoes played a rather large part in the Battle Off Samar - Knocked a heavy cruiser's bow right off, and forced the Yamato to leave the area $\endgroup$ – Andon Mar 30 '17 at 4:39
  • $\begingroup$ And to go on @M i ech's comment - The Iowa-class was designed to be able to chase down anything Japan could put out in raiding parties. $\endgroup$ – Andon Mar 30 '17 at 4:41
  • $\begingroup$ @Andon I did not count any of the WWII battles after the Solomons campaign for analysis. Those battles were all bloodbaths where the Japanese were annihilated. Not too much tactical insight to be gained there. $\endgroup$ – kingledion Mar 30 '17 at 4:47
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    $\begingroup$ The Battle of Samar was also won in large part because the US had a vast airpower advantage of 400 aircraft off the escort carriers, but even so, that battle is pretty central to this question. For example, the tiny US destroyers still had radar-guided fire control computers that let them fire accurately despite smoke and rain, while the Japanese ships relied on visual targeting and manual fire-control systems, severely limiting their ability to hit the small, maneuverable American ships at range. The biggest guns in the world aren't worth their scrap value if you can't hit anything with them. $\endgroup$ – Salda007 Mar 30 '17 at 9:14
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While a bit of a tangent, I think you need to look at the British "N3" and "G3" class battleship and fast battleship proposals. These were perhaps the last design concepts under serious consideration prior to both the Washington Naval treaty and the development of robust and reliable naval air power.

enter image description here

British G3 Fast Battleship design. The N3 was similar, but with 18" guns

The full description of the N3 is here, and you can see the Royal Navy needed to make some radical changes to the traditional battleship design to incorporate the powerful 18" battery (the G3 used a similar design to incorporate a 16" battery in a much faster and more powerful "battlecruiser" or fast battleship) without making the ship excessively large or heavy. Some of the interesting changes, outside of the unusual placement of the gun turrets, was the concentration of the armour belt in an "all or nothing" arrangement. The bow and stern of the ship were unarmored, which saved a great deal of weight and allowed the armoured citadel to withstand much heavier shells than the more traditional medium thickness armour belt.

Despite the apparent advantages of the N3 and G3 designs, they were cancelled as part off the Washington Naval Treaty, although the British Government was certainly not anxious to spend the vast sums of money that the projected series of ships would have cost.

This leads to one of the conceptual issues of capital ship design and strategy. Since you have a limited amount of resources, you cannot build enough capital ships to conceivably sail every ocean or carry out every possible task that you want to accomplish with your Grand Strategy. While massive big gun battleships, or modern aircraft carriers provide the means of visibly projecting power abroad, you only have so many, and as high value targets, you will be up against literally everything the enemy can throw at you. In some circumstances, this might work in their favour (for example, the swarming speedboat attacks in the relatively narrow and shallow Persian Gulf work because the much larger conventional naval vessels don't have the stand off room to react or manoeuvre like they do in the open ocean or even the littoral waters outside of the Gulf).

Your super battleships will be relatively isolated in fleet actions, up against torpedo boats, fast moving destroyers, cruisers and of course whatever enemy battleships happen to be handy. Even primitive airpower could work against you, the Bismarck was crippled by a torpedo launched from a Fairey Swordfish, allowing the British ships to close in and destroy her.

enter image description here

Fairly Swordfish

Interestingly enough, for a navy with a wide ranging area to cover and a large list of tasks for its Grand Strategy, a fleet built around Frigates is probably a much more useful way to expend the resources. If you do have giant capital ships, they will still need the small, fast Frigates to act as scouts, and possibly to spot the fall of shot for long range artillery engagements, but the day to day work of the fleet will be with the smaller, less expensive ships.

enter image description here

HMCS Antigonish. Thats more like it

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    $\begingroup$ Small ships are definitely the workhorses. Battleships are often symbols. Losing the Hood to the Bismark dealt a significant morale blow to the British, even though it was an aging, outclassed ship. It was just a well-known one. Fun fact: The Japanese had planned another Yamato-sized vessel, but had an insatiable need for cruisers and other small ships, so it was never built. $\endgroup$ – Andon Mar 30 '17 at 4:54
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What advantages do extremely large guns give?

Well power, I guess, but you do reach a point of diminishing returns.

What disadvantages do they have?

I am not an expert in material sciences but conceptually size becomes a problem for various reasons.

  • Storage space. Ammunition takes up space, really big ammunition takes up a lot of space, meaning you have fewer shots available in combat. If your targets don't need bigger rounds to take down then whats the point in bigger guns?

  • Accuracy/Aiming. How quickly can you aim a pistol? Ok now how quickly can you aim a rifle? What about a shoulder launched missile? Being able to adjust to enemy movement is important...if you can't hit your target, big weapons don't matter, and more importantly if an enemy hits you with a smaller gun a bunch of times...

  • Maintenance. Big things break more. Weight matters even with steel. Not to mentioned that larger shells require more power which means more powder and stress to the weapon itself. Keeping parts on hand would be a nightmare too.

  • Bigger guns require a bigger ship, for moving the weight, for stability when firing etc. This will tend to make the ship slower and more costly to operate.

Why would a nation use them despite the disadvantages?

Well..ego I guess? This is sort of story driven but we can look back a military mistakes and it doesn't take long to realize they aren't all that uncommon. Picket's Charge, The Maginot Line, Thermopylae.

Human ignorance/arrogance is the most likely culprit in this scenario. Often the loser in a war is the side that holds on to the past truths of warfare for too long and doesn't adjust to new realities in mobility, communication and weapons technology.

And for bonus points: What would the largest practical battleship gun be?

Well you mentioned the 40 cm/45 Type 94 earlier, and second to that was the BL 18-inch Mk I naval gun

Currently with modern arms it looks like the US is in first place with the Advanced Gun System (which is a terrible name). I would say this is about the largest practical size...otherwise larger munitions would exist.

Note:

Purpose and scenario matter. Its not that the guns larger than the AGS were a bad idea, they just aren't practical now. Think about a battleship sitting of the coast trying to hit land targets, then consider guidance tech 70 years ago. In that case bigger was most probably better. Build your weapons around the enemy at hand. Big ships will be a target, ever heard of that Goliath guy? He always goes down.

This link will probably also be useful.

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    $\begingroup$ The analog computers on the Iowa-class ships was extremely accurate. When they were updating them in the 80s/90s, these analog guidance computers were more capable and accurate than any digital version. The biggest issues in targeting were unknown factors. $\endgroup$ – Andon Mar 30 '17 at 4:43
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One REALLY cool side effect of your world (1950s-ish technology with battleships still being prevalent) is the plausability of superguns being used to launch satellites.

Check out the work done by Gerald Bull with 16" US navy surplus battleship guns.

With 18+ inch guns readily available in a post WWII, battleship-centric world, it is entirely possible that work on such a supergun would have taken place long before the mid 1960s when Bull worked on HARP. Theoretically, all you really need to get into orbit is a big enough gun (Jules Verne thought that was how we would eventually launch people into space). The end of the era of big naval guns after WWII really put a damper on the foundational technology for such superguns right as they were finally becoming large and powerful enough to really work.

Obviously, whatever you are launching in this way would have to withstand some incredible acceleration (mitigated somewhat by making the barrel longer), but it is entirely possible that nations which have not focused on aircraft development, and as a result rocketry is less advanced than it was after WWII might use the largest available naval guns to launch spy satellites against one another.

As a side note: Bull was able to basically double the effective range of a 16 in battleship gun thanks to a lot of supersonic aerodynamics optomization that came out of his research. This kind of thing is what your theoretical navies would be doing. Clearly, WWII era battleship guns were not even remotely at the edge of the performance envelope possible for such weapons.

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    $\begingroup$ While you can get into space with a gun, you won't get into a real orbit (one not intersecting the floor) without some second accelleration phase up there. $\endgroup$ – Paŭlo Ebermann Mar 30 '17 at 20:24
  • $\begingroup$ Just launch with enough velocity to get out of the atmosphere, then shoot another satellite even faster so that it smashes into the first one and knocks it into a circular orbit. Easey peasy lemon squeezy $\endgroup$ – etherealflux Mar 31 '17 at 1:41
  • $\begingroup$ @PaŭloEbermann if you read up on how Superguns work you will realize that Bull had perfected a hypersonic projectile with a solid fuel booster that would ignite after it had left the gun. It's really a matter of chemistry. Once the solid fuel ignites, you have plenty of energy to get into a stable orbit. $\endgroup$ – JBiggs Mar 31 '17 at 18:06
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The answers so far seem to concentrate on ship-vs-ship combat. I'm certainly neither an historian nor knowledgeable on military weapons, but it needs to be noted that these large guns were used both during WW2 and much more recently (I don't remember the date, but I recall them pulling a WW2 era battleship out of mothballs and using it prior to landing troops on the beach. Iraq maybe, not really sure. I remember the news blurb at the time describing the ordinance as the size and weight of a VW car.) to "reduce" land based (immobile) targets.

The obvious limitation is to hit there from here, you need to know exactly where "here" is, and at sea that's only possible to a certain (not so good) accuracy, especially in the days before radar and GPS type systems. Take a look at wikipedia (to start) on the V-3 cannon and the Paris gun. Range can be almost 100 miles. Ammunitions don't have to all be stored on the same ship. Stability of the platform (ever heard of recoil?) is definitely an issue. Plus, the barrels of these things wore down with each shot. You'll note that even towards the end of WW2, that the technology was evolving towards rockets rather than cannons (see V3).

What one PT boat can do with a couple of torpedos makes these hulks pretty vulnerable. Finally, given the need for trans and supersonic aerodynamic calculations for these high muzzle velocity rounds, it's pretty hard to buy into the absence of air power in your scenario. Keep in mind how easy it was for the Wright brothers (they did design both the wing and the engine for their first plane empirically, but that was in 1903. By the time of the 1950s its just not plausible that heavier than air aircraft weren't common. (Not to mention lighter than air craft).

This is moot, but brings up the question: what are the records for inventions that could have been invented years, decades, centuries earlier but weren't? Innovation generally takes both a number of inventions, but also requires economies of scale and materials availability. TV was first done in 1909, but didn't "catch on" until signal amplification became practical (mid-1920's).

The oldest "technology" which took a long time to catch on is probably vaccination. Although in all fairness, a 2% fatality rate from the inoculation was more than a little daunting.

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    $\begingroup$ Hey there Li Zhi! I think we've met before. You are currently unregistered, which is why you cannot accumulate reputation with your answers. The more reputation you get the more privileges you unlock. These let you participate in more ways in the community. Of course your help is appreciated in this form, too. I just wanted to let you know about the privileges. As a side note: your answer would be far easier to read if you formatted it a bit, for example by using line breaks. $\endgroup$ – Secespitus Mar 30 '17 at 18:06
  • $\begingroup$ As far as aircraft go, in this world it's not a matter of the technologies being unavailablen but both environmental factors (Higher gravity, slightly lower air densityn both requiring more lift and better engines) and social/political factors. Maybe a little bit of religion, too. This delayed them a little bit longer, but even then only a 15-25 years $\endgroup$ – Andon Mar 30 '17 at 20:10

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