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Why weren't there torpedo nets?
Some have questioned why there weren't nets around the battleships at Pearl Harbor to prevent torpedos from hitting. Such nets had been used by navies world-wide for many years, so why not the US Navy at Pearl? The answer is that essentially the US Naval leadership lost a bet. Any nation has limited resources, and any military only has so much that they can do to protect their forces. One could put them in a huge underground bunker, but even those can be devastated today, and in the mean time the forces are bottled up in a fortress that is difficult to get out of quickly.
Such was the dilemna of the leadership at Pearl Harbor. What was the biggest and most likely threat, and how best to respond? Today we routinely deploy our Navy around the world, but even in 1940 underway refueling was a new and dangerous undertaking. Major fleet units (I.E. battleships and aircraft carriers) did not regularly deploy on long, round-trip missions. Submarines, however, did. Clearly, some form of protection against torpedos was needed.
Anti-torpedo nets had been in use operationally since 18771, but by the First World War their use as part of shipboard defense had effectively ceased as they could not protect against the latest torpedos and had a high operational liability if damaged. Heavier, static nets remained in use through the war in diminishing cases. While offering more protection, the nets were cumbersum to use, requiring long anchor lines and tenders to both put them im place after a ship had moored and remove them before she could move.
The Chief of Naval Operations wrote two letters, one on February 15, 1941 to CINCPAC Admiral Husband Kimmel in which he stated the shallow depth of the harbor negated the need for torpedo nets and the heavy conjestion inside the harbor made it impractical (see full text here). The guidelines stated in that memo were not just meant for Pearl Harbor however; two days later he sent a similar memo out to the commandants of different naval districts around the world (read here).
The topic was revisited again at the middle of the year in a memo dated June 13, 1941 and we can see that intelligence of the Japanese effort to produce shallow dropping torpedos had not yet reached the American Naval command. In fact, it was not until September of 1941 that the Japanese were able to get a torpedo design that would reliably run in 12 meters of water2 (72 feet)
1) "The following year 1877 HMS Thunderer was the first operational ship to be fitted with experimental torpedo nets." - http://www.gwpda.org/naval/nets.htm
2) "At Dawn We Slept" -Gordon Prange, Page 321