Quote of the Day
The success of the British aerial torpedo attack [at Taranto, Italy] ... suggests precautionary measures be taken immediately to protect Pearl Harbor ... the greatest danger will come from the aerial torpedo.
— Frank Knox, US Secretary of the Navy (1940), issuing a warning before the Pearl Harbor attack on Pearl's vulnerability to torpedoes. The warning fell on deaf ears. The main resistance he faced was the general belief that Pearl Harbor was so shallow that dropped aerial torpedoes would impale themselves on the muddy bottom. Since American aerial torpedoes could not be dropped into Pearl Harbor, it was assumed that no one else's could either – a combination of hubris and lack of imagination. People are at their most dangerous when their certainty exceeds their actual knowledge.
I just watched a wonderful BookTV presentation by three authors on the WW2 attack on Pearl Harbor, which occurred 75 years ago today. While I am generally familiar with what happened during that attack, I had not looked at the details of the attack. In particular, this show motivated me to look at the Imperial Japanese Navy (IJN) attack force composition and what happened to these ships over the course of the war. The fate of these ships reflects what happened to the rest of the IJN during the Pacific War.
For those who are interested in reviewing my work, here is my spreadsheet.
The following background material describes how the US became so vulnerable to an aerial torpedo attack. To some extent, it was a lack of imagination.
The Importance of the Torpedo in WW2
People often think of WW2 naval battles in terms of surface actions involving battleships and heavy cruisers. However, relatively few ships in WW2 were sunk by gunfire – when you needed to sink a ship, you used a torpedo. A case in point is the British destruction of the battleship Bismarck. While the ship was struck with hundreds of shells, it was actually rendered unmaneuverable and eventually sunk by torpedoes. It took the US Navy a long time to appreciate the importance and danger presented by Japanese torpedoes, which may have been the world's best.
The following quote from Capt. Wayne Hughes' book Fleet Tactics explained that the US Navy performed so poorly in sea battles during the early part of World War II because:
(1) The United States failed to grasp that the killing weapon was the torpedo; (2) The United States had no tactics suitable for night battle at close quarters; (3) The United States was slow to learn. Because of the rapid turnover of tactical leaders, the pace of the battles overwhelmed the Americans; (4) Above all, the United States did not exploit its potentially decisive radar advantage.
Warnings About Pearl Harbor's Vulnerabilities Went Unheeded
The vulnerability of ships in harbor to aerial attack had been foretold a number of time prior to Pearl Harbor:
- Bradley Fiske (1911), who patented the concept of the aerial torpedo, developed an attack method for use in shallow water. The first actual aerial torpedo attack occurred in 1915.
- Billy Mitchell (1924) warned that war with Japan would begin with a surprise attack on Pearl Harbor.
- In the Battle of Taranto (1940), the British used obsolete Swordfish torpedo planes to deliver a devastating attack on the Italian Navy while in port – the Japanese studied this battle closely. Following the same shallow water attack method recommended by Fiske, the British used torpedo bombers (Figure 2) flying at low speed dropping torpedoes modified with special fins that reduced how deeply the torpedo would dive after launch. This prevented the torpedo from burying itself in the mud at the bottom of the harbor.
- Frank Knox (1940), US Secretary of the Navy, warned that precautions needed to be taken to protect ships in Pearl Harbor from torpedo attack. Clearly, no effective actions were taken in response to his warning.
IJN Type 91 Torpedo
The IJN had just one aerial torpedo – the Type 91. It was produced in five different models and was considered a workhorse weapon. Figure 3 shows an old Type 91 on display in a park (Source). It strikes me odd how old torpedoes end up in parks – there is a Torpedo Mark 14 on display at a zoo near my home.
Figure 4 shows one of the Type 91 after having been pulled from the bottom of Pearl Harbor.
Torpedo Shallow Water Modifications
An aerial torpedo will follows an in-water trajectory that may take it as deep as 200 feet (source: personal experience) before it pulls up to its running depth of ~ 15 feet. Because Pearl Harbor has a depth of ~45 feet, the Japanese modified the Type 91 torpedo with tail fins that would enable the torpedo to quickly pull out of its post-launch dive (Figure 5). This modification was similar to that done by the British on their torpedoes for the Battle of Taranto. The details of this modification are well described on this web page, and I refer you there for more details.
I should mention that using wooden attachments on torpedoes was common during WW2. Appendix A shows how the US used wooden attachments with aerial torpedoes.
Fate of The Pearl Harbor Attackers
Most discussions of the Pearl Harbor strike force focus on the surface ships, and I will focus on those units here. However, there was a significant submarine component that I will address in another post.
Pearl Harbor Strike Force's Eventual Destruction
Historians debate the exact number of ships involved in the Pearl Harbor task force. My work here is based on the Wikipedia's list. The numbers debate usually centers on the facts that (1) two destroyers were split off to attack Midway, and (2) difficulty in counting the large submarine force in the region – some with a direct role and others in a support role.
Figure 6 is my version of the Wikipedia's list of surface combatants, augmented with their date of destruction and the means of their demise. The Wikipedia lists 30 surface warships (including Midway attackers) and 32 submarines. All but one of the surface ships assigned to the IJN's Pearl Harbor strike force were sunk during the war. One ship, the destroyer Ushio, survived the war and was scrapped a few years after the war's end.
Figure 7 shows a timeline that indicates the rate of destruction of these ships. Clearly, 1944 was a pivotal year – the rate of destruction peaked at 14 ships during that year, which means that nearly half of the strike force was destroyed in that one year.
Means of Destruction
Figure 8 shows that most of the ships were destroyed by aircraft or submarines. Only one ship, the battleship Kirishima, was destroyed by gunfire. It was sunk by the USS Washington in the Second Naval Battle of Guadalcanal – a classic nighttime action between battleships. These surface actions were rare. Most battleships spent the war doing shore bombardment or convoy protection.
While the Pearl Harbor attack was a spectacular tactical success, the strategic situation unraveled at the Battle of Midway – a mere 6 months later.
Appendix A: Wooden Attachments on US Torpedoes.
Figure 9 shows a Torpedo Mark 13 with a wooden nose cape and rectangular tail assembly.
Appendix B: First Aerial Torpedo Attack.
Figure 10 shows a report of the first aerial torpedo attack conducted by the British against the Turks in WW1.