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DeliciousFart

Torpedo protection system of BB57 (South Dakota) and BB61 (Iowa) classes

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INRO Vol 57, No. 3 provided some interesting information on the caisson tests of the torpedo protection system found on the BB57 and BB61 classes, and I'll try to summarize it to the best of my abilities.

As with the BB55 system, the BB57 system was designed to protect against a 700 lb TNT torpedo warhead; where it primarily differed was its use of an internal belt, which tapered and extended down to the triple bottom as the No. 3 bulkhead of the torpedo defense system. Inboard of this was a 25# STS holding bulkhead, and outboard there were two 25# HTS bulkheads. The liquid loading of the compartment was, from outboard to inboard, liquid-liquid-void-void, or as the publication abbreviates it, water-water-air-air (WWAA). The live fire test of the system was Caisson H, which was fired on 28 September 1939. Caisson H was a half scale unit of the midships section of BB57, and tested with a 128 lb charge in accordance with the 2.45 exponent rule that was empirically determined to represent a 700 lb charge.

The results of this test was significant deflection of the inner holding bulkhead, 11 inches in the test unit, equivalent to 22 inches in full scale. This was within the 24 inches of clearance for vital engineering equipment, although the margin is rather thin. Notably, the system was "entirely effective in preventing flooding of the machinery spaces... During the two hours which elapsed between the explosion and the docking of the caisson, there was no appreciable leakage into the machinery spaces." (verbatim from the test report).

While the system was successful in the Caisson H test, the margins were thin and inspection highlighted some of the weaknesses of the design. Regarding the small margin, although the system succeeded in meeting its goal, the deflection was substantially more than that of the BB55 design, which was tested at half scale in Caisson F. That unit had a deflection of just 3 inches, or 6 inches in full scale. Of note is the substantial deflection of the armor bulkhead; in particular, the buttstrap at the slightly "knuckle" joint between it and the triple bottom failed, although it was stated that the welded seam it self was intact. Nonetheless, the deflection of this armor bulkhead greatly contributed to the deflection of the holding bulkhead especially compared to the BB55 design that was tested in Caisson F. Despite the prevention of flooding, horizontal plating was severely wrinkled, and some brackets were found to not only be ineffective, but actually detrimental, particularly the top and bottom 45 degree brackets on the longitudinal bulkhead inboard of the armor belt beteeen second and third deck, and the lower bracket on the holding bulkhead. LCDR Henry Schade made several recommendations for the torpedo defense system as a result of the Caisson H test, and significant late design changes in BB61 are consistent with those recommendations, such as reinforcement of third deck inboard of the armor belt and elimination of the aforementioned brackets. One problem with the design of this torpedo defense system that can't be remedied is that it's very expensive compared to the system on BB55.

After Caisson H, numerous 1/16 scale caisson tests were conduct to test arrangements of stiffeners, brackets, and liquid loading arrangements. The results were found to largely corroborate that from Caisson H. It was also found that WWAA and AWWA were largely identical in terms of effectiveness. Furthermore, tests showed that generally, there should not be liquid against the armor bulkhead, and that for the purposes of counterflooding, starting with WWAA, then WWAW if necessary, and then WWWW if absolutely necessary. The tests also showed that the change from HTS to STS "did not materially affect the results." A final set of 1/16 scale caisson tests were conducted to evaluate the effects of underbottom explosions, and these showed considerably more damage than expected. It was noted that these tests may not be entirely accurate since the shortness of the models didn't allow for the full representation of ship girders. In any case, the underbottom tests were too late to affect the design of BB61.

Much has been said about the caisson tests that painted the BB57 torpedo defense system as inferior compared to the BB55 design. I believe that this may primarily be due to the greater deflection of the holding bulkhead of Caisson H compared to Caisson F. Additionally, it appears that initial caisson tests involving internal belts showed greater problems than had been anticipated; these caisson tests may in fact be of Caisson 74 and 75, which represented the initial protection scheme where the upper belt sloped inwards from the shell plating to the main deck, somewhat like a German "turtleback". Of course, this was abandoned in favor of a single sloped armor belt as represented by Caisson H, which met the design goal.

Edited by DeliciousFart
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Given they were up against Long Lance (1080 lb) warheads, that's worrisome.

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2 hours ago, DeliciousFart said:

INRO Vol 57, No. 3 provided some interesting information on the caisson tests of the torpedo protection system found on the BB57 and BB61 classes, and I'll try to summarize it to the best of my abilities.

As with the BB55 system, the BB57 system was designed to protect against a 700 lb TNT torpedo warhead; where it primarily differed was its use of an internal belt, which tapered and extended down to the triple bottom as the No. 3 bulkhead of the torpedo defense system. Inboard of this was a #25 STS holding bulkhead, and outboard there were two 25# HTS bulkheads. The liquid loading of the compartment was, from outboard to inboard, liquid-liquid-void-void, or as the publication abbreviates it, water-water-air-air (WWAA). The live fire test of the system was Caisson H, which was fired on 28 September 1939. Caisson H was a half scale unit of the midships section of BB57, and tested with a 128 lb charge in accordance with the 2.45 exponent rule that was empirically determined to represent a 700 lb charge.

The results of this test was significant deflection of the inner holding bulkhead, 11 inches in the test unit, equivalent to 22 inches in full scale. This was within the 24 inches of clearance for vital engineering equipment, although the margin is rather thin. Notably, the system was "entirely effective in preventing flooding of the machinery spaces... During the two hours which elapsed between the explosion and the docking of the caisson, there was no appreciable leakage into the machinery spaces." (verbatim from the test report).

While the system was successful in the Caisson H test, the margins were thin and inspection highlighted some of the weaknesses of the design. Of note is the substantial deflection of the armor bulkhead; in particular, the buttstrap at the slightly "knuckle" joint between it and the triple bottom failed, although it was stated that the welded seam it self was intact. Despite the prevention of flooding, horizontal plating was severely wrinkled, and some stiffeners and brackets were found to not only be ineffective, but actually detrimental. LCDR Henry Schade made several recommendations for the torpedo defense system as a result of the Caisson H test, and significant late design changes in BB61 are consistent with those recommendations. One problem with the design of this torpedo defense system that can't be remedied is that it's very expensive compared to the system on BB55.

After Caisson H, numerous 1/16 scale caisson tests were conduct to test arrangements of stiffeners, brackets, and liquid loading arrangements. The results were found to largely corroborate that from Caisson H. It was also found that WWAA and AWWA were largely identical in terms of effectiveness. Furthermore, tests showed that generally, there should not be liquid against the armor bulkhead, and that for the purposes of counterflooding, starting with WWAA, then WWAW if necessary, and then WWWW if absolutely necessary. The tests also showed that the change from HTS to STS "did not materially affect the results." A final set of 1/16 scale caisson tests were conducted to evaluate the effects of underbottom explosions, and these showed considerably more damage than expected. It was noted that these tests may not be entirely accurate since the shortness of the models didn't allow for the full representation of ship girders. In any case, the underbottom tests were too late to affect the design of BB61.

Much has been said about the caisson tests that painted the BB57 torpedo defense system as inferior compared to the BB55 design. Reading this publication, it appears that these caisson tests may in fact be of Caisson 74 and 75, which represented the initial protection scheme where the upper belt sloped inwards from the shell plating to the main deck, somewhat like a German "turtleback". Of course, this was abandoned in favor of a single sloped armor belt as represented by Caisson H, which met the design goal.

they dont care...they do what they want...this game is not realistic ..get use to it

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1 hour ago, Sventex said:

Given they were up against Long Lance (1080 lb) warheads, that's worrisome.

Yes, especially since the details of the Long Lance were a well kept secret that the USN didn't know of until funny stuff started happening around Guadalcanal and the Solomon Islands in 1942.  The German and Italian torpedoes used in WWII didn't exceed the 700 lb explosive tests.

 

However, even within the USN, their own torpedoes would become more powerful and pack more explosives passing that 700 lb test of 1939.  Either that, or start using material that was more powerful than TNT.

 

It's just the Japanese really went hardcore for torpedoes as their "Silver Bullet" in ways no other nation did.  IJN submarine I-19 fired a deadly salvo with high results, sinking CV Wasp with stray torps hitting a DD and Battleship North Carolina way behind Wasp.  NC ate a Type 95 torpedo.  Lifted from wikipedia:

"The hit on North Carolina struck the ship 20 ft (6.1 m) below the waterline on her port side and tore a 32-by-18-foot (9.8 by 5.5 m) hole in the plating. Five men were killed in the attack, but the torpedo inflicted little serious damage, apart from the shock of the blast that disabled the forward turret. Flooding occurred and North Carolina took on a list of 5.5 degrees to port, but this was quickly corrected with counter-flooding and she was able to remain on station with Saratoga, cruising at a speed of 25 knots (46 km/h; 29 mph)."

The Type 95 was the submarine variant of the surface launched Type 93 oxygen "Long Lance" torpedo.  Type 95 Model 1 had an explosive charge of "893 lbs. (405 kg) Type 97" or "1,213 lbs. (550 kg) Type 97" for Model 2.

 

So I guess NC's scheme held up, preventing worse damage, so that it can be corrected.  This is the only case I know of in WWII where a USN Battleship ate a torpedo while underway in combat.  I don't know if such cases happened outside NC, i.e. if her sister Washington ever got a torpedo hit, as well as the South Dakota and Iowa-class BBs that succeded NC-class.

 

The fun part is WoWS rates North Carolina's TDS is one of the worst for a Battleship in the game, vastly inferior to the South Dakota-class TDS.

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1 hour ago, HazeGrayUnderway said:

This is the only case I know of in WWII where a USN Battleship ate a torpedo while underway in combat.  I don't know if such cases happened outside NC, i.e. if her sister Washington ever got a torpedo hit, as well as the South Dakota and Iowa-class BBs that succeded NC-class.

 

The fun part is WoWS rates North Carolina's TDS is one of the worst for a Battleship in the game, vastly inferior to the South Dakota-class TDS.

Reading the damage reports on USN Battleships, apparently Pennsylvania was the last major US ship damaged in the war.  She was torpedoed at night at Okinawa by a torpedo bomber.  As far as I know, Japanese air torpedoes only had warheads of 713 lb, corresponding to the test, although Pennsylvania was an old Super-Dreadnought.

"Pennsylvania was severely damaged by a torpedo on 12 August 1945, two days before the cessation of hostilities" -https://en.wikipedia.org/wiki/Pennsylvania-class_battleship

Edited by Sventex

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1 hour ago, Sventex said:

Reading the damage reports on USN Battleships, apparently Pennsylvania was the last major US ship damaged in the war.  She was torpedoed at night at Okinawa by a torpedo bomber.  As far as I know, Japanese air torpedoes only had warheads of 713 lb, corresponding to the test, although Pennsylvania was an old Super-Dreadnought.

"Pennsylvania was severely damaged by a torpedo on 12 August 1945, two days before the cessation of hostilities" -https://en.wikipedia.org/wiki/Pennsylvania-class_battleship

The torpedo hit Pennsylvania aft of the TDS, near the shafts, which is why it caused so much damage. Similarly, North Carolina was hit forward of it's TDS, and held up well anyway.

The closest a South Dakota got to testing it's TDS was when USS Washington had a run in with USS Indiana, which was more a test of compartmentation.

The only US Navy battleships that I know of that took hits to the TDS were at Pearl Harbor, and that was more a case of how many torps they were hit with than the strength of the TDS or warhead size.

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Also, the IJN used a wide variety of air dropped torpedoes.  Most below the 700 lb explosive threshold of the 1939 USN test.  But in 1944 they had more powerful ones starting to show up.

450mm Type 91 Mod 7 (1944):  926 lbs Type 97 explosive charge

450mm Type 4 Mark 4 (1944):  919 lbs Type 97 explosive charge

 

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20 hours ago, dadeoo said:

they dont care...they do what they want...this game is not realistic ..get use to it

This isn't about gameplay...

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I'll just vaguely say that modern US carriers went back to a variant of the earlier multiple bulkhead system used on US BB's prior to the S. Dakota and Iowa classes.  That speaks volumes about what works best.

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On 6/21/2021 at 11:57 AM, Sventex said:

Reading the damage reports on USN Battleships, apparently Pennsylvania was the last major US ship damaged in the war.  She was torpedoed at night at Okinawa by a torpedo bomber.  As far as I know, Japanese air torpedoes only had warheads of 713 lb, corresponding to the test, although Pennsylvania was an old Super-Dreadnought.

"Pennsylvania was severely damaged by a torpedo on 12 August 1945, two days before the cessation of hostilities" -https://en.wikipedia.org/wiki/Pennsylvania-class_battleship

That hit was near the stern and outside the torpedo defense system, so it's not a great example of torpedo defense system effectiveness.

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:cap_book::cap_popcorn:

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On 6/21/2021 at 2:49 PM, HazeGrayUnderway said:

Also, the IJN used a wide variety of air dropped torpedoes.  Most below the 700 lb explosive threshold of the 1939 USN test.  But in 1944 they had more powerful ones starting to show up.

450mm Type 91 Mod 7 (1944):  926 lbs Type 97 explosive charge

450mm Type 4 Mark 4 (1944):  919 lbs Type 97 explosive charge

 

The problem is that by 1944 trying to torpedo a warship using an aerial torpedo was near suicide for the aircraft.

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19 hours ago, Murotsu said:

I'll just vaguely say that modern US carriers went back to a variant of the earlier multiple bulkhead system used on US BB's prior to the S. Dakota and Iowa classes.  That speaks volumes about what works best.

be curious to know if the tests on the USS America before it was scuttled led to any changes in the Ford-class.

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1 hour ago, Murotsu said:

The problem is that by 1944 trying to torpedo a warship using an aerial torpedo was near suicide for the aircraft.

Depends on the navy in question that's being attacked :Smile_trollface:

Try to attack an American task force in 1944?  A USN capital ship with its screen of ships and possibly even fighters?  Good luck.

 

An Axis navy force?

hqdefault.jpg&f=1&nofb=1

Edited by HazeGrayUnderway

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On 6/21/2021 at 7:51 PM, HazeGrayUnderway said:

The fun part is WoWS rates North Carolina's TDS is one of the worst for a Battleship in the game, vastly inferior to the South Dakota-class TDS.

 

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2 hours ago, Lord_Slayer said:

be curious to know if the tests on the USS America before it was scuttled led to any changes in the Ford-class.

I don't know, but I'd doubt there were any really large changes to underwater protection systems between the two.  I can't get into details in any case because those are still classified.

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4 minutes ago, Murotsu said:

I don't know, but I'd doubt there were any really large changes to underwater protection systems between the two.  I can't get into details in any case because those are still classified.

Just saying, you don't sacrifice a ship that size without learning something from it.

The only other ship lost that size/tonnage was the Yamato class, and no one is going to stop a battle and let everyone see how the ships defenses are performing.

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1 hour ago, Lert said:

 

That doesn't work considering the historically better North Carolina-class TDS is rated at 19% torpedo damage reduction in the game while the inferior type as found on South Dakota-class is at 49%.

 

That's a damn big difference.

Edited by HazeGrayUnderway

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23 hours ago, Murotsu said:

I'll just vaguely say that modern US carriers went back to a variant of the earlier multiple bulkhead system used on US BB's prior to the S. Dakota and Iowa classes.  That speaks volumes about what works best.

Modern US aircraft carriers also aren't concerned with economizing the amount of deck armor to remain within WNT/LNT limits and underwater ballistic protection, which was the main driving factor in adopting that protection system. Even though the system succeeded in its caisson tests against the design charge (scaled, of course), it's much more expensive to construct and especially repair.

Iowa's system was notably reinforced over the based design in BB57, particularly in the longitudinal spacing of the traverse bulkheads and reinforcement of the third deck and triple bottom where the plating wrinkled in Caisson H.

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3 hours ago, Lord_Slayer said:

Just saying, you don't sacrifice a ship that size without learning something from it.

The only other ship lost that size/tonnage was the Yamato class, and no one is going to stop a battle and let everyone see how the ships defenses are performing.

That would have been the first live test of the system I'm aware of.  That would have been very useful data.

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2 hours ago, DeliciousFart said:

Modern US aircraft carriers also aren't concerned with economizing the amount of deck armor to remain within WNT/LNT limits and underwater ballistic protection, which was the main driving factor in adopting that protection system. Even though the system succeeded in its caisson tests against the design charge (scaled, of course), it's much more expensive to construct and especially repair.

Iowa's system was notably reinforced over the based design in BB57, particularly in the longitudinal spacing of the traverse bulkheads and reinforcement of the third deck and triple bottom where the plating wrinkled in Caisson H.

Well, I can't go into specifics again, but modern US carriers are about layering the armor and splinter protection more than about trying to keep incoming rounds totally out of the ship.  The idea is to contain them and prevent secondary damage from splinters so that flooding and fire isn't progressive.

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On 6/21/2021 at 4:10 PM, DeliciousFart said:

The liquid loading of the compartment was, from outboard to inboard, liquid-liquid-void-void, or as the publication abbreviates it, water-water-air-air (WWAA)

Against non contact detonations liquid filled spaces directly behind the shelf of the ship may  reduce the protective effect of the TDS.

Captain_Hook aka Thoddy @ navweaps

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I’ve received a few messages inquiring on how exactly the BB61 system was reinforced over the base design tested in Caisson H, and I think it will be helpful to share here as well. On BB61, the “skins” of the triple bottom structure was thickened, particularly the part right behind the holding bulkhead, as well as the third deck behind the main armor belt; again the inwards folding (“wrinkling”) of these plates was a big reason why the holding bulkhead deflected as much as it did. Additionally, removing the 45 degree brackets between the holding bulkhead also appears to be done to mitigate the wrinkling of the third deck and triple bottom. I’ve circled below in red where the plating analogous in Caisson H wrinkled badly that the plate folded back onto itself.

Here is where I’ll opine with some of my own analysis; there is more clarity on why the excessive rigidity of the main armor belt is considered detrimental, based on my interpretation. The force of the explosion ruptured the outer hull plating and the first two bulkheads, while the third bulkhead, which is the Class B lower belt, remained intact but the force of the explosion on it pushed the whole structure (from third deck to triple bottom) back, and caused the traverse plating, namely the third deck and the triple bottom plating to “wrinkle” and essentially crumpling inwards. Rather than elastically deforming and then rupturing to absorb energy, the armor bulkhead essentially transmitted it to the third deck and triple bottom and caved the whole structure back, which contributed to the significant amount of deflection of the holding bulkhead, despite remaining watertight. I believe this is the impetus for the reinforcement of BB61’s triple bottom and third deck, which offered greater strength and resistance in the lateral direction, which would mitigate this “wrinkling”. The closer spacing of the traverse bulkheads in the machinery spaces also helps in this regard, proving more support in this axis.

F4C4ED00-72AF-4F91-BF95-C5E2589DA9FF.jpeg

Edited by DeliciousFart

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Good info.

I always say, the appeal of a historical game is that it intrinsically makes the statement to its players, that it abstracts the reality of the things it represents. It may not be totally faithful to all historical details, but in essence it tries to be.

Otherwise, we could play a space fantasy game that has no such pretensions.

Edited by Dr_Seadog

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