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The Schwerer Gustav is one of the largest guns ever used - by many measurements, it is the largest. But it lacks range. It was, however, designed to be an extreme bunker buster, a job which it could do very well.

On the other end of the spectrum, the Paris Gun had an incredible range - But, being designed as a psychological weapon, it lacked power and accuracy.

What they both lacked was mobility, which severely limited their usage.

Here's where we get theoretical: Presume we have a gun that combines the power and accuracy of the Schwerer Gustav with the range of a Paris Gun. Of course, it's not as powerful as the Gustav, and it doesn't have the entire range of the Paris Gun, but it's both long range and heavy hitting.

Now, we're on a world with WWII-level warship and metalworking technology. The order goes out: We want one of these guns put on a ship, to be able to sail around the coasts of our enemies and be capable of bombarding inland targets. The requirements:

  1. The ship only requires one of these Bombardment Cannons.
  2. The Bombardment Cannon must be able to traverse independent of the ship, as much as possible without compromising the ship or weapon.
  3. The Bombardment Cannon must be able to fire while the ship is under way. While obviously not preferred, we can't always have things our way.
  4. The Mounting Ship must be able to have secondary weapons for self defense. While aircraft are not currently a large threat, they are beginning to show up, so some anti-air capability is required.

So, now that we have the background and required information: What would such an "Extreme Bombardment" ship even look like, and how large would it be? I'm presuming that a turret mount is out of the question due to the sheer size of the gun, so what sort of traverse would we be able to get?

Some Notes: Aircraft are not developed at this time, nor are aircraft carriers. While aircraft will be superior, that fact is irrelevant. This also is not necessarily something that would be practical - More of an influential politician's "good idea" that's pushed through regardless of some better ideas.

Some Further Notes There have been many answers and comments that this is probably the least practical way to bombard someone. That's entirely the point. In-universe, it's the idea of a particularly influential politician, influential enough to get it designed and built. It doesn't have to be particularly effective, it just has to exist.

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  • $\begingroup$ In WW2 they had bomber aircraft. They are much better at bombarding inline targets than a super-cannon. (Think: the super-cannon can fire maybe one shell per minute at a target no further than 60 km away, and has a resource of maybe 100 shots before it needs to be refurbished. A bomber wing can deliver hundreds of tons of bombs in a few minutes on a target 1000 km away, and can come day after day after day.) And bomber aircraft can be carried on ships appropriately called aircraft carriers... $\endgroup$ – AlexP Mar 22 '17 at 12:04
  • $\begingroup$ I mentioned in the question that aorcraft don't exist as a threat yet. They'll get there, but not for a number of years $\endgroup$ – Andon Mar 22 '17 at 12:50
  • $\begingroup$ What does it mean to "traverse independent of the ship"? Does traverse mean aim - you do not have to move the ship to aim the gun? $\endgroup$ – Willk Mar 22 '17 at 13:06
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    $\begingroup$ Reading on the Schwerer Gustav I think the key is that they were coming at the project in the 1930s from a WW1 mindset. Plane tech makes cannons less useful as noted elsewhere here. But a boat mounted supercannon would be great for the Crimean war, and all the guys would have those sweet moustaches. $\endgroup$ – Willk Mar 22 '17 at 13:09
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    $\begingroup$ Some things to keep in mind for a fictional setting with big shore-bombardment monitors: Aircraft have to get through the enemy fighters and Flak. There may be no realistic appreciation how easy that is until the shooting starts. Getting ballistic missiles right is literally rocket science, while guns are well known. So big guns are not all that insane. The US reactivated the Iowa class several times for shore bombardment because big guns can be useful in that role. A dedicated shore bombardment platform would be smaller, slower, less armored, more bang for the buck. $\endgroup$ – o.m. Mar 22 '17 at 17:11

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Those guns are useless

The 16" gun emplacement on a Iowa-class Battleship (which is three guns, incidentally) weighs 1700 tons. Schwerer Gustav weighed 1350 tons, but was not armored. The 3 16" guns themselves only weighed 121 tons each. So you could mount about 10 16" guns in the same weight as one Schwerer.

The effective firing range of Schwerer Gustav is given as 39 km on the Wikipedia page you linked. The range of the 16"/50 is 39km. A 1987 test of the re-activated ships were able to put 14 of 15 shells within 230m of the target at 31.9 km. The official Navy range tables at the time put the weapon's range at 35 degrees elevation to be 36,788 yards, or 33.6 km.

The existing 16" weapon had the range of the Schwerer, and could mount 10 guns to the Schwerer's one by mass. Where the Schwerer could fire a 7 ton projectile, the 16" turret could put out 10x 1.2 ton projectiles, more iron downrange. Rate of fire is also relevant, though I can't find numbers, I suspect the 16" fired faster.

If you want longer range, what is wrong with an aircraft? The Avenger had a range around 800 km, which is way more than any gun. Why maneuver your gun-ship within 100 km of the shore when you can pummel it with air raids from over the horizon; something that was repeatedly done to Japanese strongholds during WWII.

Think about it this way, WWII was a total war. Anything that could be useful for the war effort was invented and/or built (jet fighters, ballistic missiles, nuclear bombs, etc). If putting a 1350 ton, 80m gun on a ship was actually useful, someone would have done it.

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    $\begingroup$ I did mention in the comments that the gun for this ship has a significantly better range than the Gustav. I'm also thinking the ship is a design similar to other historic superweapons - Practicality and realism took a second seat to "I have a really big gun on my really big ship and it can get you!" shenanigans $\endgroup$ – Andon Mar 22 '17 at 12:55
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    $\begingroup$ @Andon From kingledion's answer you can see that it'd probably be theoretically possible to put 1 Gustav in an Iowa sized hull, but it'd be far less effective than the battery the Iowas were fitted with. The main problem with the Gustav (quite apart from its barrel only lasting 100 shots or so) was the terrible rate of fire (a few shots per day, reloading took literally hours). OTOH the battery on an Iowa can be reloaded in minutes. $\endgroup$ – jwenting Mar 22 '17 at 13:58
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    $\begingroup$ This is an excellent answer to the question "Is the proposed weapon viable and practical?" But that's not the question asked. $\endgroup$ – Guran Mar 22 '17 at 14:44
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    $\begingroup$ @kingledion Sure. But this was neither "won't work" or "Not possible" but "there are better solutions". After all, it's not like real military history is short on stories about working but horribly uneffective super weapons... $\endgroup$ – Guran Mar 22 '17 at 15:03
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    $\begingroup$ And to put things in a bit more perspective, working but horribly ineffective is exactly what I want $\endgroup$ – Andon Mar 22 '17 at 16:09
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Well, there were historical ships like the British Erebus-class monitors with 15" guns. One could imagine a similar design with 16" or 18" guns. On a slightly larger scale, there were coast defense ships or coast defense battleships, but those are probably too large for your purposes.

  • The Erebus had twin 15" guns. One could envision a design with a single larger gun instead.
  • A turret mount is not out of the question, but there could be limits to the safe arc of fire.
  • For very long range fire, it is important to get a stable and well-surveyed firing platform, like the reinforced rail lines of the Schwerer Gustav.
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  • $\begingroup$ I think you are on to something here. For OP's purposes you might want to up-scale a coast defence ship to be able carry a 25"+ gun. $\endgroup$ – Flummox Mar 22 '17 at 9:21
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I tried to describe this idea a couple times, then gave up and slapped it together in paint...

Supercannon Catamaran

Join two large battleships together at the bow and the port to form a giant catamaran. Then hang the supergun in between on a single axis gear which allows it to fire straight ahead at a variety of ballistic angles, plus fire straight behind, also at a range of angles.

integrate bullpup concussion absorption into the barrel and gear assembly so that the gun doesn't break the spines of both boats during its first use. You might also be able to spread the kickback out in the water beneath the boat since the shell chamber sits under the surface at most firing angles.

Since the supergun hangs between the two hulls, the decks are available for defensive and anti-aircraft weapons. The turreted guns should also be bullpup and perhaps scaled down a bit to keep their recoil from breaking the structural elements which unite the ships. Either that or put hinges on the cross-members so that they can bend whenever the hulls buck during turret fire.

It might be a good idea to keep each battleship whole and operational, with separate bridge, radio and engineering sections. This redundancy would help the ship absorb battle damage while remaining combat capable. It would also allow the healthier hull to blast itself free (from the gun and the crippled hull) to serve as a lifeboat/escape craft for both crews in a worst case scenario.

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  • $\begingroup$ A couple of thoughts: Recoil compensators (muzzle brake, Cutts compensator) would have the side effect of washing hot gas over the decks. Taking the strain of supporting the gun and firing it would require a complete rebuild of the ships, so you'd be better off buidling a new one. How about a SWATH catamaran? There would have to be a working area for loading, so the gun can't be rotated to fire backwards and it can't elevate too much. As painted, the breech of the gun would be submerged, which increases the draft of the ship. But it might help with cooling. $\endgroup$ – o.m. Mar 22 '17 at 7:06
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    $\begingroup$ This design will have basically zero stability in the ocean. Its beam is so wide that it will rolls significantly with normal sea waves. Normally, the beam of a ship is narrow, and sea wave lengths are long, so as long as the ship is moving forward (~5 knots minimum) the hydrodynamic effects along the sides of the hull keep the ship from rolling. With this design, the beam will be large enough to be affected by sea waves even while moving. The ship will unacceptably roll unless headed directing into the wave direction. There is a reason no catamarans are this big. $\endgroup$ – kingledion Mar 22 '17 at 12:23
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By WW2 technology had progressed to where very large guns were no longer considered the way to go, so by your own statement that that's the level of technology your ship would be obsolete by the time it was launched.

For shore bombardment, a larger number of smaller guns, possibly in combination with rockets and/or aircraft is far more effective. Larger spread of fire per salvo if you so wish, shorter reload times (thus higher fire volume on target), more stable (and thus more accurate), easier to transport (smaller ships, or same size ships with more guns), and losing one ship is less likely to cost you the bulk or all of your firepower.

As said in the other answers already, the stability of a ship as a firing platform for something this massive is highly questionable to put it mildly, another reason shipboard cannon peaked at the 18" mounted on the Japanese Yamato class battleships, and at 15-16" for other navies.

Another major problem for a weapon that big is that the barrel of the gun only lasts a very limited number of shots, and replacing the barrel while underway is next to impossible. After a few days of firing (at a rate of 1-2 shots per hour) your ship is useless unless it were as a command ship or for its secondary batteries and sensors, just a big inviting target for enemy forces. Not only do the barrels of smaller guns last longer, they're also easier to replace (whether that was ever attempted at sea with main battery guns I don't know, most likely they were replaced regularly during port visits).

Another point not mentioned yet is that the weapon, in order to have both the shell weight of the Gustav AND the range of the Paris gun, would have to be humongous. Just adding a longer barrel to the Gustav (which was a mortar btw) wouldn't do it. You need a lot more steel, making the barrel extremely thick (and thus heavy) in order to contain the propellant gasses from the massive charge (several times larger than the charge used in the Gustav OR the Paris gun). And to elevate that massive weight you need far larger mechanisms as well. End result is a weapon of utterly impractical dimensions, a weapon that's probably too large to fit on any ship envisioned in WW2 or since.

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Now, we're on a world with WWII-level warship and metalworking technology. The order goes out: We want one of these guns put on a ship

Then the engineer comes in and says: Herr Oberleutnant, have you considered, you know, rockets? It's 1942, we have build them and called them V1 and have plans for V2.

The forfeit in usage of such guns was that

  1. they have very strict usage. You can cover the belt 150 km from the coastline but only in reduced line. So if your ship have a radius of fire 150 km but only 45 degrees how fast he can move 300 km further the coast. It can be used on finished amount of target. After they are conquered the ship and it's gun is useless.
  2. The problem with artillery (which is mentioned in linked wiki page) is that the barrel get worn-out. So in you usage you are not only limited by the projectile per time but also the time when the whole ship is out of service because the barrel need to be refitted
  3. It's mobility. you have one ship with one gun that can go 25 knots (38 km/h). Versus, let say 20 Katyusha with speed 60km/h. So katyusha can fastly go to the coast and flood you with rockets. Of course you can have your ship far away from their range but it will also decrease your range.
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    $\begingroup$ The Oberleutnant would answer, "I know, but the Dear Leader demands at least 50% bigger guns than our rivals. With 50% he means calibre and not throw weight. So the Großadmiral said 'yes, sir!' and you will build it." Many German WWII designs were mostly done to make sure that the designers would find themselves in a cozy office and not with a rifle at the eastern front. $\endgroup$ – o.m. Mar 22 '17 at 17:02
  • $\begingroup$ I honestly forgot about rockets, and will certainly remember them if I need a practical bombardment ship $\endgroup$ – Andon Mar 22 '17 at 21:26
  • $\begingroup$ @o.m. Germans dodged western front by inventing flying wing, Russians after fighting on said front invented Kalashnikov :D $\endgroup$ – SZCZERZO KŁY Mar 23 '17 at 8:40
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I think to get an idea of what your asking you need to look at the wooden sail ships and how they handled cannons.

There were (and still is) always three main problems. Weight, recoil, and maintainability.

Weight Ships can not handle an unlimited amount of weight. The more weight you have the lower the hull sits in the water. The lower in the water you are the slower you can turn or move. The more fuel (or sails) you had to use to move you, which means more weight. With sail boats it means more sails, a stronger rigging and mast(s), etc. Today it means more fuel and larger engines. Keep in mind that weight needs to include ammo, tools, men, and food to fire the gun, along with the gun it's self. Lets not forget balance. You can't hang a large weight over the side of a ship without it tipping.

Recoil Back when wooden sail boats were used, cannons generally just "pushed back". Some on tracks or grooves, but many just "back". This was great as they allowed for easier reload, but too big a cannon or to big a shot and the cannon would "push back" right out the other side of the ship. A little forward in time and we started hard mounting guns so that couldn't happen, but that just destroyed the ships, so we went with compensators and the like to "absorb" the excess energy (this isn't really correct it's more like redirect) But gun size was and still is limited by this factor.

Maintainability Again lets look at the era of wooden sail ships. People died. as a matter of fact when fighting with things that go boom people die. You can't have your super gun workable by just one person or when he dies it's all over. Same is true for maintenance, cleaning, loading, aiming, etc. You will effectively need a team of people that can die to man this gun. The same is true today. Super weapons exist, but there not fitted for use in combat because the average sailor can't figure out how to use it. For example that the WWII era guns. With little training anyone could be thought to load, aim, and fire those guns. Maintenance was a bit more difficult, but a ship had several persons that could do maintenance. Today we have rail guns, lasers, plasma cannons, and a whole bunch of fun sounding weapons that look good in demonstrations, but are just plain to complex to use when someone is shooting at you. You need to make sure your weapon is actually usable, without any "special" skills.

Possible Solution

As were on world building and not really looking for "the best scientific" answer, then I suggest that your BFG be place inside the hull of the ship. Build the ship around the gun, think Death Star but V-Shaped. Firing the gun would cause some serious side effects, and aiming means turning the boat not the gun, but that has been done before.

The deck can be used for small arms and anti-air, but is largely empty.

When the cannon is fired you need to clear the sounding areas. Fire. Then the recoil can be absorbed by the water. The entire boat would "jolt" like a plastic toy boat in a tub, so everyone should strap in. There would be some pretty hard limits to the number of times you could fire, and the boat could be used to turn fast, but move forward slowly. Gasses and maybe even plasma from the firing could be used to some effect, but would likely need to be flushed out somehow. Maybe by pushing against the water, helping to reduce the "bounce".

After just a few firings though (say 5 for arguments sake) it's time to head back to port an refit some parts. After 100 or so firings it's time to scrap the ship (though the gun can probably be salvaged).

Seems like the makings of an interesting chapter or two to me.

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Considering how such a large mass installed above the water line would unbalance a ship, and how the recoil of the shot would need a well balanced ship to avoid flipping it over at the first shot, I think a reasonable choiche would be to install it on a submarine, using the entire submarine body as casing for the cannon.

When you need to fire, you manouver to align with the target, lift the cannon out at the needed angle, fire one or more shells and take cover again.

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    $\begingroup$ There were some big gun submarines which did not work out well, like the en.wikipedia.org/wiki/HMS_M2 $\endgroup$ – o.m. Mar 22 '17 at 7:00
  • $\begingroup$ @o.m. based on what I read, the failure is related to the hangar. In my answer I refer to cannons. $\endgroup$ – L.Dutch Mar 22 '17 at 7:38
  • $\begingroup$ @o.m. en.wikipedia.org/wiki/French_submarine_Surcouf always comes to my mind when talking about big gun submarines $\endgroup$ – jwenting Mar 22 '17 at 14:00
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It has already been mentioned several times that this would be a totally impractical project. That is a correct assessment, but it doesn't actually answer your question. Baring that in mind, let's talk battleships.

The largest ship-mounted gun during WW2 was the 18.1" Type 94, which was mounted on Yamato-Class Battleships. Notably, the Yamato-Class was itself the heaviest class of battleship ever built and for a variety of reasons was totally impractical because of it. However, I would contend that it's guns were not among those reasons. The 18.1" had a maximum range of ~45KM and a rate of fire comparable to the 16" guns used by the American Navy.

A single Yamato-class had 9 of these guns, the total weight of which would be comparable to a single Schwerer Gustav. You could therefore possibly mount a single gun of this caliber on a Yamato-Class ship, or on a fictional battleship of similar size. If the weight of this fictional gun was closer to the Paris Gun, then you could even mount 2 or 3 of them. This would leave room for secondaries and possibly for backup barrels, though at 7 tonnes per shot, I would expect the ship to run out of ammo before it ruined its barrel beyond "usefulness".

Looking through your requirements, I'm not sure what you mean by the cannon being able to move independently of the ship. If you mean that the weapon can detach from the ship, then I have good news and bad news. The good news is that the requirements for carrying the thing got a lot easier, as any sufficiently large barge should suffice. The bad news is that there is absolutely no way that this thing is firing while on-ship. Moving on, I will continue under the assumption that this bombardment cannon is mounted to the ship, rather than carried by it.

Firing while moving is tricky. This cannon would need to have enough muzzle power that I would be worried about the long-term integrity of the ship's hull. I suspect it's not particularly accurate in the first place due to the mass of its ammunition, but any accuracy it has would likely be lost while moving. Moreover, I'm concerned about the effect that firing this gun could have on the ship itself. Let's run some quick napkin math here.

The shell of the Schwerer Gustav weighs 7 tonnes or 7000KG. It has a Barrel which is 30M long and a Muzzle velocity of 720m/s (with AP Rounds). With these numbers, my conservative estimate of the force involved is 60480KN (The actual number is likely quite a lot larger). This equates to a change in velocity for the Yamato-class ship of 70m/s or 19.4KM/h! I'm not sure if that's enough to flip the ship, but it's almost certainly enough to do unpleasant things to the hull while it's sitting still, much less when it's moving. I'm sure there existed techniques to help absorb recoil on ship, but I'm unfamiliar with it as a topic and frankly doubt that it's up to the task of fully mitigating this behemoth.

(I am not a physicist, so please correct me if my numbers are exceptionally bad. That said, please also bear in mind that this is an estimation.)

Finally, I do have a small bright side in all this. As we only removed the primaries to mount this monstrosity, this ship will have just as much Anit-Air capabilities as any other Yamato-class. Sure, 2 of the 3 Yamato-class ships that were built were sunk by air-attacks, but hey, it's better than nothing.

Don't go claiming that anything containing this thing is "realistic", but I've definitely seen more outrageous weapons in fiction.

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While what you are asking for is somewhat impractical with WWII era technology, the modern USN is actually looking at getting "guns" to have similar performance metrics for near future warships.

Experiments are ongoing with electromagnetic rail guns, and the ultimate aim is to arm large warships (comparable to 10,000 tone cruisers) with batteries of 64Mj railguns. The projectile could have a range of 200km, actually exiting the Earth's atmosphere for part of the trajectory and descending to the target at Mach 6, causing destruction through kinetic energy alone.

While there are still lots of technical issues to be solved before railguns can become part of a ship's arsenal, engineers noted that the extreme aerodynamic shape of a railgun projectile could be adapted to current artillery pieces. The shell can be fired with a large charge and effectively double the range of the shell, or since the shell is flying far faster than a conventional shell of the same calibre, it can also be used at close range firing with a very flat trajectory at incoming missies.

To give you an idea of how this might work in practice, a 155mm artillery piece can fire at a range of @ 30 km. Rocket boosted and "base bleed" shells can extend the range somewhat, but with a general loss of payload and accuracy. Experiments using the new form factor suggest that a range of 70km can be expected with "normal" shells. Adding rocket boosters or base bleed should extend the range, although I have no knowledge of any experiments along these lines. A 155mm cannon using a boosted aerodynamic shell might be able to reach targets up to 100km away.

This actually has a historical precedent. German engineers experimenting with rockets in WWII also developed the so called "arrow shell". This was fired from a specially modified K5 28cm rail gun (in this case, a huge conventional canon mounted on an enormous railway flatcar). The range was effectively doubled to 127km, and the gun hidden in a railway tunnel and used to fire on the allied landing at Anzio, Italy. The gun was safe from allied fighters and bombers sent to silence her, because the search radius was set for @ 60km from the beachhead, which was considered the maximum possible range for a rail gun of that era. The downside was the Arrow shell had a very small payload, and using arrow shells eroded the barrel quite quickly compared to regular ammunition.

enter image description here

Krupp K5 similar to "Anzio Annie"

enter image description here

Peenemünder Pfeilgeschosse (arrow shells)

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  • $\begingroup$ I like the idea of specialty rocket shells, if only because it makes the weapon even more of a "Working but impractical superweapon" $\endgroup$ – Andon Mar 23 '17 at 4:29
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I would take a Yamato class battleship, strip it of the forecastle, all of its big guns, and place the Paris gun where the back turret used to be

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