Double Purpose AA on Battleships/cruisers question

[Parma]

Since there quite some Naval people on this forum I have a question regarding the double purpose, DP, Anti Aircraft large caliber guns on heavy naval units, like heavy cruisers, battleships and battlecruisers.

Since the last year of the Great war AA guns appeared on the decks of Battleships and Batle cruisers,
This were guns of 75mm or 88 mm range, acasionaly 10,5 mm, next to a number of machine guns.
This diameter range for the heavy AA was used during the interbellum unitl the early and mif thirties when so called dual purpose guns apeared on the decks of heavy naval units.
Thes new dual purpose AA guns where in the caliber range of 105mm up to 120 mm, and there use was to defend the ship agains high flying airplanes ans to fast sailing naval units like destroyers or motor torpedo boats.
these DP artilery is generaly praised as wheigt saving in stead of using both for example 120 mm guns against fast naval units and 88 mm or 105 against high flying aircraft.

Now my question is how effective where this so called Doual purpose guns excactly? Hwere they realy effective against both objects, ari and surface of where they only good for one purpose?
I wonder how fast a 105 or 120 mm gun could fire against a fast flying object high in the sky, and how accurate will this be?
And is a high flying reconaisance plane in range of a 120 mm DP AA gun?
And why not using a 75 mm or 88mm gun against a destroyer or motor torpedo boat?
On long range you might use the main guns hile on the short range the small guns might ahve enough damage power as a 105 or 120 mmgun.

Please note I am not discussing the AA machineguns from 7,62 mm up to 40 mm but only the large calibers.

 

[Kalga]

And why not using a 75 mm or 88mm gun against a destroyer or motor torpedo boat?

As the Royal Navy realized by the time of the Iron Duke Class superdreadnoughts that those smaller guns are not adequate against the growing size of destroyers and torpedo boats, as well as the ever longer ranges of torpedoes themselves. Many other navies came to the same conclusion as secondary armaments, which went really light with the first generation of dreadnoughts, gradually went back up in caliber and size.

 

[Orcbuster]

US 5 inch DP average RoF was 15 rounds per second per barrel in barrages coordinated by radar assisted fire control directors. They were very effective at both AA and anti surface and that was the case for most of its counterparts as well. Airburst shells in particular greatly benefit from increased range and caliber that such weapons provided in the AA role.

As said before, by the time the war starts it becomes more and more evident for most navies that the smaller calibers DP guns are no longer fit for purpose

 

[HexWargamer]

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.

 

[HexWargamer]

US 5 inch DP average RoF was 15 rounds per second per barrel

typo, I assume == 15 rps is 900 rpm, a respectable rate of fire for a machine gun !

 

[Orcbuster]

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.

These guns were not meant to be used againt high altitude bombers, they were meant to be used against exactly the sort of threat you claim them to be ineffective against, they were particularly effective at breaking up attacking aircraft formations before they got into weapons range with heavy flak screens whicb were in use well before the VT fuze was developed.

 

[Orcbuster]

typo, I assume == 15 rps is 900 rpm, a respectable rate of fire for a machine gun !

Yes, typo, apologies.

 

[naraic]

Before the mid 1930s dp guns were terrible.

The fire control was poor. The rate of elevation and the rate of traverse was poor.

That said their anti air performance wasn't much worse than heavy anti air craft guns. There was no real good heavy anti air craft guns.

Heavy anti air craft guns or dual purpose guns offered something that couldn't be matched by light anti air. They offered a threat at range. A low threat but still a threat. This served to have some effect breaking up formations of incoming planes at a range beyond which lighter anti air guns could serve rather than shooting things down.

Torpedo planes or bombers had a much greater success rate when they had a clean strike run.

Your second question was if lighter guns could have an anti destroyer role. The answer is no. It's all about range.

There is no 75 mm gun or 88 mm gun that can outrange a torpedo. If a gun can't out range a torpedo then it can't offer any defense to capital ships against destroyers.

OK in theory if there's a close range engagement and all guns can be opened up against destroyers then maybe but Laffey vs Hiei was a one off.

 

[Parma]

High level bombers can't even hit the right city, let alone a moving target like a warship

That is spot on, why having this DP haevy caliber guns for high flying targets while they are not a real threat in the thirties/fourties

 

[Parma]

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

my though as well

 

[Parma]

Thanks for the input

 

[Orcbuster]

That is spot on, why having this DP haevy caliber guns for high flying targets while they are not a real threat in the thirties/fourties

Again, high flying targets are not the primary target for naval heavy anti air. They are meant to destroy or dissuade attackers at any profile coming in from long range.

Japanese aircraft learned very quickly that US ships had a bubble of air defense that could reach them from very far away.

 

[HexWargamer]

These guns were not meant to be used againt high altitude bombers, they were meant to be used against exactly the sort of threat you claim them to be ineffective against, they were particularly effective at breaking up attacking aircraft formations before they got into weapons range with heavy flak screens whicb were in use well before the VT fuze was developed.

Can't be bothered to check, but I think it takes around 1500 AA rounds to hit a low level target ..

So, lets see, a dive bomber approaching at say 300mph (440 ft/s) coming in at lets say, 10,000 feet, bombs away at 2,000 feet .. so 8,000 feet = just over 18 seconds to 'break up the attack' .. let's ignore the time taken to traverse the gun onto the target (can DP's aim direecty upwards at a dive bomber anyway ?) .. at 15 rpm, 4s/round, in 18s you get off 4 shells ... Oh dear ....
But you may well have 4 dual turret DP's, so 8 guns, so 32 shells ... better, but still minimal chance of a kill ..

Or let's have a 8 barrel pom-pom (effective to 14,000 feet,) @ 115rpm/barrel = 920 round/min so 18s you get off 276 rounds ... Still not good ..
However you may well have 10x pom-poms, so 2,760 rounds and now we are talking ....

Yes, to protect their Pacific fleets, the US has many ships with many DP, but then we assume the attackers have many aircraft. The attack is only going to be 'broken up' by ineffectual AA fire if the pilots are (very) poorly trained. Indeed, fanatical pilots will continue their attack even against highly effective AA ...

In the end, each ship has to provide it's own point defence against an enemy that gets beyond the outer screens ... and I know what I would prefer if I was on one of those ships .. (especially as the pom-poms can be aimed, with some difficulty, by hand, after power is cut by damage, unlike the DP's, == see Force Z and sinking of Prince of Wales - not that either gun type did them much good in the end)

 

[Orcbuster]

Can't be bothered to check, but I think it takes around 1500 AA rounds to hit a low level target ..

So, lets see, a dive bomber approaching at say 300mph (440 ft/s) coming in at lets say, 10,000 feet, bombs away at 2,000 feet .. so 8,000 feet = just over 18 seconds to 'break up the attack' .. let's ignore the time taken to traverse the gun onto the target (can DP's aim direecty upwards at a dive bomber anyway ?) .. at 15 rpm, 4s/round, in 18s you get off 4 shells ... Oh dear ....
But you may well have 4 dual turret DP's, so 8 guns, so 32 shells ... better, but still minimal chance of a kill ..

Or let's have a 8 barrel pom-pom (effective to 14,000 feet,) @ 115rpm/barrel = 920 round/min so 18s you get off 276 rounds ... Still not good ..
However you may well have 10x pom-poms, so 2,760 rounds and now we are talking ....

Yes, to protect their Pacific fleets, the US has many ships with many DP, but then we assume the attackers have many aircraft. The attack is only going to be 'broken up' by ineffectual AA fire if the pilots are (very) poorly trained. Indeed, fanatical pilots will continue their attack even against highly effective AA ...

In the end, each ship has to provide it's own point defence against an enemy that gets beyond the outer screens ... and I know what I would prefer if I was on one of those ships .. (especially as the pom-poms can be aimed, with some difficulty, by hand, after power is cut by damage, unlike the DP's, == see Force Z and sinking of Prince of Wales - not that either gun type did them much good in the end)

Im sorry but you are blatantly ignoring how effective heavy AA guns were at breaking up and deterring attacks before they even happened. Heavy DP AA guns did not rely on direct hits but mechanical timed fuzes set to airburst. This was a vital part of the layered AA defense of every navy out there.

 

[Kalga]

The difference between the various bombers (dive, torpedo, etc.) and missiles of later eras is that the former is piloted, and the humans piloting them are not as rational as the inhuman guidance systems of the latter. Thus when presented with incoming fire they will react accordingly, even if intellectually they should know better (and even that's not a given).

Hence there's a use for heavier AA guns.

 

[Orcbuster]

Yes, to protect their Pacific fleets, the US has many ships with many DP, but then we assume the attackers have many aircraft. The attack is only going to be 'broken up' by ineffectual AA fire if the pilots are (very) poorly trained. Indeed, fanatical pilots will continue their attack even against highly effective AA ...

This was not the case at all. One of the reasons why the japanese resorted to suicide tactics was the effectiveness of such guns to deter enemy attacks and have them break off or outright miss their approach.

 

[jlckansas]

The thing to remember about attacking dive bombers and torpedo bombers is they both have to fly straight for a while and cannot be jinking and varying their speed and altitude when they start their attack runs. Once they are committed their track is going to stay the same within their flight parameters and your guns then are able to track and fire at the targeted aircraft. When you look at a dive bomber for example, you have it starting at a certain altitude, on a certain bearing from the ship, and descending at a certain rate toward the ship. You basically just have to track it and fire upon it. Torpedo planes are the same way. They are coming in on a certain bearing at a certain altitude and at a certain speed, you just need to start firing down the bearing line toward the planes themselves and don't have to have pinpoint accuracy, just be accurate within a certain Circular Area of Probability.

 

[HexWargamer]

Again, high flying targets are not the primary target for naval heavy anti air. They are meant to destroy or dissuade attackers at any profile coming in from long range.

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)

IMHO, way too much 'theory' was banded about interwar (even today the concept of 'mission kill' is seen as more important than 'hard kill') .. the idea that you only needed to 'dissuade' the enemy and that this could be done by throwing a few shells in the air in his general direction was pure wishfull thinking (but one that Vickers used to defend the (in)accuracy of their AA guns to the RN). Any decently well trained and determined enemy is just going to ignore all those puffs of black smoke ..... (and, until Radar direction and VT shells became common, they were right to do so - after which weaving about a bit instead of flying nice straight predictable line might have been a good idea, since this might upset the radar aiming).

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 ?

Japanese aircraft learned very quickly that US ships had a bubble of air defense that could reach them from very far away.

Yes, this is the CAP, or Combat Air Patrol and it was indeed quite effective at disrupting formation attacks ...

 

[HexWargamer]

The thing to remember about attacking dive bombers and torpedo bombers is they both have to fly straight for a while and cannot be jinking and varying their speed and altitude when they start their attack runs. Once they are committed their track is going to stay the same within their flight parameters and your guns then are able to track and fire at the targeted aircraft. When you look at a dive bomber for example, you have it starting at a certain altitude, on a certain bearing from the ship, and descending at a certain rate toward the ship. You basically just have to track it and fire upon it. Torpedo planes are the same way. They are coming in on a certain bearing at a certain altitude and at a certain speed, you just need to start firing down the bearing line toward the planes themselves and don't have to have pinpoint accuracy, just be accurate within a certain Circular Area of Probability.

Yep - so long as your compute engine and gun traverse system is fast enough to lay the barrel on target you "can't miss" ... of course the ship is maneauvering at maximium speed and may well be rolling and heaving in the seas, so even today it seems less than a sure thing ... and the more shells you can throw at the incoming enemey, the better a chance you have of bringing him down ..

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.

I wonder if there's an argument for using VT fused battleship guns as AA guns
["new tactic lads, as soon as we spot the enemey torpedo planes above the horizon 15 miles away, full stop, drop anchors and turn our 16" guns to face. Set the VT to detonate at 300 yards from the target and fire when ready"]

 

[naraic]

I wonder if there's an argument for using VT fused battleship guns as AA guns
["new tactic lads, as soon as we spot the enemey torpedo planes above the horizon 15 miles away, full stop, drop anchors and turn our 16" guns to face. Set the VT to detonate at 300 yards from the target and fire when ready"]

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