Double Purpose AA on Battleships/cruisers question

[BlueTrousers]

The nature of a dual-purpose gun is a compromise. A smaller-calibre weapon would be a more effective anti-aircraft gun, since it can track targets more quickly and fire more quickly. On the other hand. you really want a 6-inch or equivalent gun to engage destroyers outside torpedo range. But a compromise calibre allows you to carry more guns than you could if you had two single-purpose batteries, which (in theory) offsets the disadvantages.

After all, did we not train our OWN pilots to ignore enemy AA ? For sure the 8th airforce over Germany stuck to their formations despite AA shells bursting all around them. So why assume the enemy pilots would be any less well trained ?

Yes, air crews were trained to press their attack despite AA fire. But they're also human. When trying to hit a precision target - and a ship is a precision target, no matter how big - even a fraction of a second of hesitation can be enough to throw off aim. And when the air around you is full of bursting shells, you'll flinch occasionally, whether you want to or not. You'll try to avoid the shell bursting ahead of you, delaying your attack by a few seconds. And thus a carefully coordinated attack that overwhelms close-range defences is broken up into penny packets of aircraft arriving over a longer period of time.

The effect of that training isn't that no attack ever gets broken up. That's not how humans work. Humans are damn good at self preservation. It's that the attack stays concentrated for longer, and is less dispersed when it reaches the target. Breaking a bomber box wasn't impossible, either. There's a reason why, when heavy AA fire was the primary threat, a looser formation was employed. When you employ a loose formation against ships, you either miss entirely, or attack piecemeal.

Lethal radius is another discussion - a big shell that breaks up into a rain of shrapnel just front of the target should logically have a better chance of bringing him down than a rain of much smaller shells that rely on hitting him.

Congratulations, you've just discovered the reason for using heavy AA guns. Before VT fuses came in, the shells were time fused. That's what all the shell bursts are in an AA barrage. Nobody seriously expected to be getting direct hits with anything other than automatic weapons.

I wonder if there's an argument for using VT fused 12" battleship guns as AA guns

The Royal Navy seriously envisaged the 8-inch guns on the County class cruisers as having a role in AA barrage fire, firing time-fused HE shells to break up raids at long range. The Mark I mounts had a maximum elevation of 70 degrees to facilitate this. It turned out to be totally impractical, and I'm not sure it was ever tried in combat.

Didn't Yamato do exactly that in that their main guns fired an anti air shell.

The Type 3 shell was slightly different: it was a time-fused shell with an incendiary payload, and seems to have been totally ineffective against aircraft. Conventional time-fused HE would probably have been more effective, but with the train and elevation rates on heavy guns, you'd be doing well to get more than one salvo off.

 

[YYJ]

Here is the US Navy analysis of the lethality of AA guns in World War 2. The whole article is worth a read.

Antiaircraft Action Summary

CONFIDENTIAL [DECLASSIFIED] Information Bulletin · No. 29 Antiaircraft Action Summary · World War II October 1945 Headquarters of the Commander in Chief UNITED STATES FLEET

www.history.navy.mil

 

[fester]

The whole concenpt, whilst a 'clever' way to have both AA guns and 'normal' armanent was flawed and a distraction.

Fact is, every high-level AA gun just took attention away from the real aerial threat to your ship, specifically torpedo bombers and dive bombers. High level bombers can't even hit the right city, let alone a moving target like a warship. Only when stationary (in port) are you vulnerable to high level bombers, and then it's the job of the port AA (and fighters) to protect the ship (and not the other way around). All of this was/shpuld have been obvious before wasting time/effort on DP...

The rate of fire and rate of traverse of DP guns is way too low to be effective aganst low level attackers. You need something like a 20mm auto-cannon. If you think there is a realistic threat from both loe level and more 'medium' level bombers, then you should focus on fast firing 37 or 40mm AA in multiple gun mounts.

The Brits almost got it more or less right with their pom-pom's, however these were really too slow firing (hence 8 gun mounts) and too short effective range. Ideal would have been multiple mount Bofers 40mm's.

Of course when Radar direction and Proxity fused ammunition became available, destroyers with DP guns did good defensive service against enemy arcraft overflying at highlevel on thier way to attack the rest of your fleet.

Disagree... Cruisers and battleships need some type of anti-destroyer/anti-small craft weapon. This weapon requires a decent size shell, good range (not main battery, but effective to 10,000 yards would be nice) good rate of fire as destroyers are small slippery bastards. You also need a decent number of them to account for multi-axis attacks as destroyers are not loners and battle damage as their firing positions will be no where near as heavily protected as the main battery so protection through attrition reserves are needed.

This gets a gun somewhere between 4 to 6 inches depending on the navy and depending on the times post Jutland construction and refits.

The question is whether or not the additional weight and space needed to transform a single purpose mount into a double purpose mount that will be the outer part of a ships' air defense system is worth the trade-off against a marginal increase in the medium and short range AA fit of a ship. The US went with 5x2 5"/38s per side of their modern battleships, so if we assume that 4x2 single purpose 5 inch guns would fill the light ASuW role, what is the marginal value of an extra quad 40mm (not bought for the fleet when these ships were designed) if there is no longer range/heavy/super heavy AA capacity for the ship?

That is the trade-off as the anti-DD role is needed and the heavier single purpose AA guns (57mm to 90mm) can't fill that role so there will be some number of 4 to 6 inch guns on a warship massing 8,000 tons or more in 1939.

 

[Hammerbolt]

DP weapons had the massive advantage of saving a lot of ship mass and cost, not to mention crew. Look at problably the worst example of not having them: the Bismarck class. The german's lack of a DP gun meant the Bismarck had 14 twin secondary turrets, but only but only 8 were AAA. Meanwhile the Prince of Wales, South Dakota and Dunkerque, all (basically) of the same generation, had 8 twin DP. Imagine the savings in the design if the germans had a 100-130mm DP weapon.

 

[RamscoopRaider]

As may be. But I still contend that DP's were a waste of effort. The design effort could have gone into faster firing low level guns (in ww1 Imperial Germany had an experimental externally powered 12-barrel 7.6mm Gatling gun that could fire more than 7,200 rpm = so a 5 barrel 20mm at 5,000 rpm should have been 'doable' with 20 years of effort)

Doable but of little use for naval AA. Because 20mm is short ranged enough that it is near useless against a torpedo bomber and of marginal use against a dive bomber

A 20mm shell at a 45 degree angle has a ballistic range of 4800 yards, with a flight time of over 30 seconds and at that range is basically at terminal velocity. Most aerial torpedoes had ranges of 2000-3000 yards, plus a few hundred to a thousand more based on drop height/speed, with the US Mark 13 being the outlier with 6800 yards of range. So a US torpedo bomber never has to enter 20mm range at all and most other nations do not need to get into effective 20mm range, as 20mm was usually given an effective range of 1000 yards, but a director fired piece with a high rate of fire could maybe double that. That said doctrine was generally to drop closer, with IJN standard being 1100 yards and 220 of altitude, but the physical capability is there to make a 20mm weapon functionally useless

With a typical 70 degree 2000 foot release dive bomber drop a 20mm will only have a few seconds to engage given a 10,000 foot absolute AA ceiling at 90 degrees and the usual slant range, by which point the pilot is committed and the most probable result is a pilot unable to pull out crashing into the ship, which is a suboptimal outcome

DP guns have the advantage that with an absolute range of 20,000 yards, they can start shooting when bombers are well out of range of a 20mm or even a 40mm

 

[jlckansas]

The Hotchkiss Gun dated back to the 1880's with a 47mm/5 barrel revolving cannon on a naval mount. If it was followed on from then or even was updated in the interwar years by someone, bigger caliber, better power for higher rate of fire, better shell types easier ammo handling, would make a great medium AA weapon at least.

 

[Astrodragon]

The Hotchkiss Gun dated back to the 1880's with a 47mm/5 barrel revolving cannon on a naval mount. If it was followed on from then or even was updated in the interwar years by someone, bigger caliber, better power for higher rate of fire, better shell types easier ammo handling, would make a great medium AA weapon at least.

the magazines can only hold so much

 

[RamscoopRaider]

The Hotchkiss Gun dated back to the 1880's with a 47mm/5 barrel revolving cannon on a naval mount. If it was followed on from then or even was updated in the interwar years by someone, bigger caliber, better power for higher rate of fire, better shell types easier ammo handling, would make a great medium AA weapon at least.

Probably not, I wouldn't expect much better performance than a conventional autocannon. The 5 barrel hotchkiss fired about 90 pounds a minute in service, theoretically 180, the 40mm Bofors did 160 in service, theoretically 240, and with a muzzle velocity 30-50% higher. By the time you improve muzzle velocity, increase rate of fire and add a recoil system your gun is a lot beefier and you probably aren't weight for weight better than a single barrel autocannon

 

[EagleKeeper]

Near the end of WWII based on the experience of the kamikaze attacks the U.S. Navy started a program to replace 20mm and even 40mm mounts with 3" guns. The reasons were primarily two - They found that 20mm and 40mm hits did not STOP attackers that were on a trajectory to hit the ship. Even if they hit the size and momentum of the incoming aircraft tended to make it continue on its planned trajectory. The second reason was that 3" rounds were the smallest that could use a proximity fuse and have an effective round. the proximity fuse was the thing that made large er caliber guns effective as AA weapons. I don't have the sources handy but I remember reading it cut the number of rounds needed to engage a target by something like 75%.