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I have been scouring the net for data on the Iowa TDS and have found two possible concepts of fixing the TDS weaknesses and structural discontinuities
#1… was to provide No. 1 turret with a torpedo blister
And #2… (and this is the one Im most interested about) is fixing the Armor belt holding bulkhead connection to the inner triple bottom structure, the Navy considered fixing the last two Iowas BB65 and BB66 by modifying the structural discontinuity connection, Please Anybody … how were they planning to do this?
Here is all I could dig up so far……………………………….
North Carolina was the only modern U.S. battleship to be torpedoed. Her side protective system differed significantly from that used in the South Dakota/Iowa design in that there was no internal belt and North Carolina’s system was considered a superior design as a result of the caisson tests mentioned earlier. The ship was hit by a Japanese oxygen fueled long range torpedo containing the equivalent of more than 960 lbs of TNT, a more powerful charge than anticipated by the design parameter of 700 Ibs of TNT. The system failed at its weakest design point, flooding occurring beyond the holding bulkhead around No. 1 turret. Structural damage and flooding extended 108 feet along the side protective system. The torpedo hit on the left side just behind number 1 barbette (and abeam the forward magazine), and blew an 18x32 foot hole in the hull. This let in about 970t of seawater, and buckled the second and third decks. Within minutes the ship was able to make 24 knots.. Internal flooding extended over four decks for 68 feet being contained by the fore and aft athwartships vertical bulkheads. Failure of the torpedo protection system was not catastrophic, the ship counter-flooding and increasing speed to withdraw within minutes. However, her capabilities were significantly reduced. The Long Lance torpedo essentially defeated the USS North Carolina’s side protective system. The ship was hit by chance at its narrowest, and therefore most vulnerable part of the side protection system. An Iowa Class battleship would have taken lighter damage from the torpedo due to an improved torpedo protection system over the North Carolina Class. Tests are great but I'm more interested in the real life and all we have to go on there is the experience of North Carolina & Prince of Wales. The Prince of Wales TDS held against Japanese air delivered torps of 330lb and 450lb warheads, considerable doubt as to whether KGV's TDS deserved a rating of 1,000 lbs. I brought up North Carolina's torpedoing because she was hit by a much bigger warhead - 960 lbs.
The U.S. Navy has never released official details for the Iowa-class ships on the designed resistance of the side protection system to underwater explosions. However, based on the known similarity of design and construction details to the side protection system of the South Dakota-class ships, it will be assumed that the same design parameters and structural details were applied to the Iowa-class side protection system design as was used in the South Dakota design, there are continuous structural discontinuities, or weak points, built into the torpedo protection system at two locations:
-a discontinuity in the armor bulkhead connection to the triple bottom structure in way of the forward and after magazine groups -a similar discontinuity in the holding bulkhead connection to the inner triple bottom structure these are considered serious defects in the execution of the side protection system design concept. On the first 4 IOWAs the carrying of the armor belt down to the hull bottom added strength, but the rigidity of the armor plate could possibly have caused shear of the plate away from the hull bottom. Although, still a very good design additional tests in 1943 showed certain structural defects in the system. Changes were made to BB65 and BB66 that would have improved system performance by as much as 20 percent, but unfortunately, neither ship was ever completed. A good design, though not as wide as some, the carrying of the armor belt down to the hull bottom added strength, but the rigidity of the armor plate could possibly have caused sheer of the plate away from the hull bottom. Still, a very good design. Their limited beam in way of #1 turret made it difficult to provide adequate torpedo protection in that area. The final pair would have included several detail improvements, including a revised torpedo protection scheme.
In the Yamato, the Japanese carried the internal armored belt all the way to the double-bottom to form an armored torpedo bulkhead. Although this armored bulkhead was substantially thinner than the belt armor it was joined to above, it was still very thick and rigid by comparison to the thin elastic bulkheads introduced by the Americans in 1915. Unfortunately, such bulkheads were too rigid and prone to displacement from their mountings, permitting flooding around them. Worse, in the Yamato’s case, a poorly designed and constructed joint between the armored belt and the torpedo bulkhead proved prone to failure and drove its supporting structure backward, puncturing the inboard holding bulkhead. The US also employed this variety of TDS in fast battleships of the South Dakota and Iowa classes and came to the conclusion that the heavy bulkhead was too rigid, resulting in a modest down-grading of the system’s explosive resistance rating. The Iowas vertical side armor consists of an upper and lower belt which is inclined to an angle of 19 degrees. The total depth of the belt is 38 feet 6 inches and extends from just before turret 1 to just aft of turret 3. The upper belt is Class A armor, 12.1 inches thick, while the lower belt is Class B armor, 12.1 inches thick at the top and tapered to 1.62 inches at the bottom. The USN became concerned about shellfire diving under the belt at long range. The USN didn’t know it, but Japan had actually designed all of its modern APC shells to do exactly that too, abet without great reliability. This meant that at least one thick layer of armor had to be in the TDS, even if it meant sacrificing torpedo resistance. Multi-layered systems are intended to absorb the energy from an underwater explosion, the TDS in the South Dakota and Iowa battleships were designed to absorb the energy from an underwater explosion equivalent to 700 pounds (317 kg) of TNT, the Navy's best guess in the 1930s about Japanese weapons. But unknown to U.S. Naval Intelligence, the Japanese 24-inch (60 cm) Type 93 "Long Lance" torpedo. carried a charge equivalent to 891 pounds (405 kg) of TNT. The Iowas all shared the relatively unsatisfactory torpedo-protection system devised for the South Dakotas. Detail improvements in the last two ships, the Illinois and Kentucky, were proposed on the basis of new caisson tests made in 1943, in connection with the design of the Midway-class aircraft carriers. These changes were expected to improve protection by about 20 percent and would also reduce flooding in the event the system was penetrated. However, neither ship was ever completed. The protection of No. 1 turret was discussed again in connection with the torpedoing of the North Carolina and the proposed improvements to the last two ships. After all, the only torpedo damage suffered by any of the new U.S. battleships had been in the same area on the North Carolina. The minor changes under consideration might have saved her from extensive flooding, but to provide No. 1 turret with protection similar to that of the rest of the ship would have required a blister and a consequent loss of 1.5 knots at full power. The General Board rejected it. Ten thousand tons had been spent to buy six knots; the General Board was not going to surrender a quarter of that gain. The torpedo-protection system was relatively unsatisfactory, a problem accentuated by the narrowness of the hull in the vicinity of number one turret and its magazine. The only solution would have been a blister, but this was estimated to slow the ship by 1.5kts, a price the General Board refused to pay. As it was, Wisconsin's torpedo protection system was only designed to resist about 680lbs of explosive, and this estimate may have been optimistic. This was probably her most serious deficiency. Fortunately, none of the Iowa class were ever torpedoed. There was thought of redesigning the BB-66 hull with a "Montana class" type protection system for added torpedo protection. This was rejected and the final two unfinished ships BB65 and BB66 were being built along the regular Iowa class 108’ beam (32.92m) hull
The Japanese and American tapering lower belts were rigid in the thickened regions and, as such, they compromised the anti-torpedo system somewhat, especially in the Japanese design, where the plating remained very thick (3" (76.2 mm) minimum) even at the ship's bottom and where its top connection to the main armor belt was grossly inadequate and tore free too easily, as actual torpedo hits showed. In the U.S. lower belt, the upper and lower belt plates were "keyed" using a strong slot-and-tongue design. The five-bulkhead, four-layer U.S. anti-torpedo/anti-diving projectile design was free of connection problems at the top. However, though the bottom end of the tapered bulkhead was anchored with much heavier bolts, it was still too rigid and should have tapered to its 1.625"/0.75" thickness much further from the bottom. All three other 0.75" HTS inner bulkheads were kept thin and ductile from top to bottom so the system's reduced performance due to the overly rigid tapered bulkhead could be largely corrected by redistributing the liquid (fuel and water) layers to the two outermost compartments and making the two innermost layers both voids - with the tapered armored third bulkhead between the voids and yet another heavy "holding" bulkhead behind it. This kept the tapered bulkhead out of the resistance to the torpedo explosion until all of the liquid layers, one of the void layers, and three of the five hull bulkheads (the outer hull of 0.75-1" HTS below the bottom edge of the main belt and two of the three spaced "torpedo" bulkheads) had already been expended in smothering the explosion. This made a total of 4" of HTS at the anti-torpedo system's bottom edge in five widely spaced bulkheads of roughly equal thickness. Caisson tests, which revealed the flaws in the design of the new underwater protection scheme, were not performed until all of the South Dakota class were actually under construction. Likewise, the Iowa class design was virtually finished, and its underwater scheme was similar to the South Dakota class. Nothing could be done for either class except to fill the outboard void spaces with fuel oil, in the hope that it would absorb some of the energy from an underwater explosion.
During Pearl Harbor, TDS and damage control counter-flooding saved West Virginia from nine torpedo hits, while Oklahoma, which didn’t have a TDS, capsized after just three torpedo hits. Design deficiencies, such as the discontinuities in the Iowa’s torpedo protection system, are often only correctable at the expense of major modification. The chance to correct this deficiency exists if ever there was a reactivation of Iowa, Missouri, or Wisconsin.
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Originally Posted by JasonH
