Compared to 55 American submarines in the Pacific at the out-break of hostilities, the Japanese had 60; 47 of the fleet type called I- boats which designated them as front line boats. There were also 13 of the much smaller RO class which were ineffective as offensive submarines. They were inferior to the American S-boats. Nevertheless, at around 2500 tons submerged the usual I-boat had a fast maximum surface speed of 24 knots making it much faster than any merchantmen it would encounter. The cruising range was normally 10,000 miles to a massive 17,500 miles. They were armed with the deadly Type 95 torpedo and various sizes and types of deck guns and anti aircraft weapons. The I-boat did possess the potential to do great damage to Allied shipping. On the other hand I-boats were more susceptible to both air and depth-charging than their American counterparts. They were slow and shallow divers, possessed of poor subsurface manoeuvrability, and they vibrated excessively creating noise which often resulted in the destruction of an I-boat.²⁰

The Imperial Japanese Navy entered the war without radar. Surface warships received their first radar (a crude radar) only after the battle of Midway in 1942. It was nearly two years later when submarines would receive radar, despite much pleading. The submarine crews in the forward areas, recalled Mochitsura Hashimoto, were longing for radar as farmers look for rain in a long drought."In our despair we were almost driven to the view that a single radar set would be of more value than a hundred submarines."²¹ They were fighting blind against a foe who could usually see them. Admiral Paul Wenneker, Germany's naval attache to Tokyo stated, "The Japanese had poor types of submarines in this war". Admiral Wenneker also argued that the IJN was grossly over confident.²²

Beside this Japan built a needless diversity of designs. Instead of concentrating on the mass production of proven designs, the Japanese constantly created new modifications thus decreasing the efficiency of their already overburdened shipyards, while the Americans were producing large numbers of Gato and Balao class submarines.

The gigantic 6560 ton ( submerged ) I-400 class submarine could carry three or four aircraft and had a massive range of 37500 nautical miles. They were built for the bombing of the Panama Canal. However, like most I-boats they were used instead as underwater supply vessels. The reason why these boats were never put to use for the purpose they were originally intended remains a mystery to the USN.²³ If they had been used in their proper capacity they could have dealt a severe blow to allied shipping and supply in the Pacific.

On the other end of the scale, the suicide submarine (Kaiten) was more or less a man-guided torpedo (Japanese midget sumarines were as a whole more or less suicidal owing to the fact they lacked generators). The Kaiten was responsible for sinking three relatively minor ships. The cost in Japanese lives was 900 dead and the loss of eight I-boats used to ferry the Kaiten.²⁴ Apart from damaging HMS Ramillies and sinking a tanker the midget had little success.

By 1944 Japanese submarine strength had been cut in half. In Japan because of the battleship mentality submarine replacement held a relatively low priority. Scores of midget submarines and Kaiten were instead built in anticipation of an Allied invasion. These vessels never had an effect on the outcome of the war even though there must have been some grimaces on the faces of Allied commanders who knew thousands existed.²⁵

Two areas where the Japanese had an advantage was in the quality of their optical equipment and particularly in the Type 95 torpedo, which was developed from the famed Type 93 "Long Lance". This was the best torpedo of the war. It packed nearly twice the punch of its American counterpart, was faster, and left no tell-tale wake.

20. Wheeler, p. 99.

21. Ibid.

22. Robert Walters, "The Undersea War", in History of the Second World War, edited by Basil Liddell-Hart (London, Purnell, 1966) pp. 2544-2548.

23. Blair, p. 872.

24. Yokota, p. 272.

25. I.C.B. Dear, The Oxford Companion to the Second World War (Oxford, 1993). p. 748

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