The newer U.S. battleships that fought in World War II had many important gunnery advantages over earlier battleships, such as excellent fire-control systems and, by 1944, the best surface fire-control radar in the world. They also had a hidden advantage in their magazines: rather unusual shells.
When considering the characteristics of battleship heavy guns, normally what first comes to mind is caliber (bore diameter) and muzzle velocity (projectile speed leaving the barrel). You could also consider fire control. You probably wouldn’t go a step further to look at shells, except for noticing that some were a lot better than others at piercing armor (such as at Jutland in 1916).
The interwar U.S. Navy Bureau of Ordnance went the extra step. In the early 1930s it began to ask whether heavier armor-piercing (AP) shells would be worthwhile. Like most other naval developments of this period, the heavy-shell idea was tied up with naval arms limitation.
In particular, in 1922 at the Washington International Conference on Naval Limitation, the world’s major sea powers agreed not to build battleships with guns of greater than 16-inch caliber. That was the most powerful type then in service (or nearly in service) with the U.S. and Japanese navies, and it was what the British had adopted for their four “Super Hood”–class battlecruisers. However, all three navies were planning battleships armed with 18-inch guns. The Washington Naval Treaty was a way of stopping an incipient competition leading to heavier and heavier guns and therefore larger and larger (and more and more expensive) battleships.
The British badly wanted to cut the cost of future warships, and under the 1936 London Naval Treaty, the last of the series of interwar pacts, they managed to reduce the upper gun limit to 14 inches. The two-inch reduction might not seem very impressive, but shell weight rises as the cube of caliber, so a shell 14 percent larger in diameter (and other dimensions) is about 50 percent heavier. The standard U.S. 14-inch AP shell weighed 1,400 pounds—but the standard 16-inch shell available in 1936 weighed 2,100. The British used a heavier 16-inch AP shell, weighing 2,375 pounds, and the Japanese version weighed 2,250 pounds.
In March 1934 the Japanese had announced that they were dropping out of the naval treaty system, effective when the Washington and 1930 London treaties, which they had signed, expired in 1936. Because the U.S. and Royal navies assumed that their next war would be against Japan, both were much affected. What would happen once Japan was no longer bound by treaty? The U.S. Navy decided to consider a worst-case scenario, and the Preliminary Design Division of the Bureau of Construction and Repair sketched a ship with 20-inch guns.
Badly pinched economically, the British tended to assume that the Japanese would limit themselves to something they considered rational, which ultimately meant a 40,000-ton battleship armed with 16-inch guns. Both Western navies (and their governments) searched for an incentive to keep the Japanese within the treaty-limited system, for fear that once they broke out all limitation would collapse and the cost of new battleships would skyrocket.
They placed an “escalator clause” in the 1936 London treaty: If a non-signatory (Japan) refused to confirm that its future battleships would not be armed with guns over 14-inch caliber, then the signatories could adopt 16-inch guns and also a larger size limit for ships. That’s why U.S. World War II battleships had 16-inch guns and the Iowa-class ships were designed to a 45,000-ton, rather than a 35,000-ton, limit.
A lingering fear was that the Japanese would build much larger guns and ships. There’s no evidence that either of the two Western navies had the slightest idea if that was the case (it was), but by 1939 the U.S. Navy was wondering whether its next class of battleships, which turned out to be the Montanas, should have something like the 18-inch guns the service had flirted with around 1920. Battleship size was still limited by factors such as the Panama Canal (wider but not longer locks were planned), and going up a size in gun caliber would have necessitated reducing the number of guns.
The Bureau of Ordnance offered a way out. For some time it had been advocating heavier shells, gun for gun. With a given powder charge, a heavier shell would be slower out of the muzzle and lose some range. On the other hand, when it descended toward an enemy’s armored deck, the projectile would gain energy because of gravity. It turned out that a “super heavy” 2,700-pound 16-inch AP shell had the deck-piercing capacity of a normal 18-inch AP shell.
The U.S. Navy had invested heavily in the fire-control system needed to fight at maximum ranges. At such distances, most shells would hit an enemy ship’s decks rather than her sides, so the special quality of the new super heavy shell was particularly welcome. In effect, arming a battleship with the new shells gave her much of the armor-piercing capacity of a much larger ship with the same number of 18-inch guns.
All of the new U.S. battleships received “supershells.” Heavier weight meant much longer shells. Existing battleships had shell hoists designed for the earlier shells, so only minor improvement was possible; the new 16-inch shell for older battleships weighed 2,240 pounds, about what the Japanese achieved. Much the same was true of the 14-inch guns, whose new shell weighed 1,500 pounds—hardly super heavy. Many of these ships did benefit from wartime improvements in radar and fire control, which they demonstrated at the Battle of Surigao Strait in October 1944.
The Bureau of Ordnance also developed super-heavy cruiser shells. Thus the U.S. Navy replaced its 260-pound 8-inch shell with a 335-pounder (in the San Francisco and later classes), and its 105-pound 6-inch shell with a 130-pounder (Brooklyn and later classes, but not in the two Erie-class gunboats). The modest improvement in the light-cruiser gun suggests that it was not expected to hit at such ranges that deck hits would be very important; muzzle velocity, for a flat, controllable trajectory, was much more valuable. Heavy cruisers were expected to hit at very long ranges.
The Japanese, meanwhile, very secretly built two Yamato-class superbattleships, laying down two more but completing one as a carrier (the other was canceled after little work on her had been done). As late as 1945 the U.S. Navy assumed that these ships were armed with 16-inch guns and displaced about 45,000 tons—in other words, that they were slow (27-knot) equivalents of the Iowas. Only after the end of the war did it emerge that the Japanese, knowing they could not match U.S. economic strength, had decided to build individual ships of such superiority that numbers would not be so important (the Japanese also banked on their assumed much better fighting spirit and on higher aircraft performance, bought by omitting crew protection). The result is a continuing fascination with the possibility of a duel between an Iowa- and a Yamato-class battleship.
The success of battleship gunnery depended on fire control. Both ships had modern systems designed specifically so that the shooter could maneuver freely while firing. Neither system (nor any other of the era) could deal with a maneuvering target. If both ships could maneuver freely, neither might hit the other often enough to end the battle. In 1943 that was just what happened in an engagement between the U.S. heavy cruiser Salt Lake City (CA-25) and the Japanese heavy cruisers Maya and Nachi. During the three-and-a-half-hour fight, the three ships expended most of their armor-piercing shells but with remarkably few results.
Of course in the hypothetical Iowa-Yamato duel, if anything (such as a torpedo or an early shell hit) slowed down either battleship enough, her opponent’s modern fire-control system would work. Then it really would matter that the super-heavy 16-inch shells were just as good as Yamato’s 18.1-inch projectiles.
16-inch Armor-Piercing Projectiles
Mark 3
Weight: 2,100 pounds
Length: 64 inches
Bursting charge: 70 pounds
Muzzle velocity: 2,800 fps
Mark 5 'Supershell'
Weight: 2,240 pounds
Length: 64 inches
Bursting charge: 33.6 pounds
Muzzle velocity: 2,520 fps
Mark 8 'Supershell'
Weight: 2,700 pounds
Length: 72 inches
Bursting charge: 40.9 pounds
Muzzle velocity: 2,300 fps