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Oral History -The Sinking of USS Indianapolis

Interrogation of Commander Mochitsura Hashimoto, former commanding officer of the Japanese submarine I-58 regarding the sinking of USS Indianapolis (CA-35) as well as Japanese Navy tactics and technology

Painting of USS Indianapolis (CA-35)

Adapted from: Commander Mochitsura Hashimoto interrogation in box 13 of Submarine/Undersea Warfare Division, Series III, Archives Branch, Naval History and Heritage Command.

Interrogator: J.H. Alberti
Interpreter: R.J. Fox

Hashimoto stated that submarines were always considered as secondary targets, to be attacked only if nothing more important were available. He had heard of other U-boat commanders attacking submarines, although he never practiced such tactics. Hashimoto cited two cases in which U.S. submarines were sunk by Jap [Japanese] submarines. One was sunk in early 1942 between Hawaii and San Francisco [pencilled note: "didn't have any there"] [no allied submarine confirmed sunk in this area.] by the I-26 or I-27 and the other was sunk in 1943 in the Solomons area by the I-176. [Confirmed as USS Corvina sunk on 16 November 1943 south of Truk by I-176.] In addition, a British or Dutch submarine was sunk by a Jap sub near Singapore in early 1942 by the I-165 or I-166. [No allied submarine confirmed as sunk in early 1942 in this area.]

The general policy of the Imperial Japanese Navy regarding submarines was to make use of these craft as offensive weapons. However, this was impossible due to unwise orders which exposed subs to excessive risks, thereby resulting in heavy losses. Hashimoto criticized severely the submarine policy upheld by the Japanese Navy.

Questions Submitted by Operational Research Group:

I. Organization and Tactical Employment of Jap Sub Force.
[Question] What were the main objectives of the Jap Sub Force? Were they used exclusively on scouting and supply missions or were they employed primarily to attack U.S. Task Forces? Were any patrols made with the object of sinking U.S. Subs?
[Answer] At one time half of the Jap fleet was used for supply purposes, the balance being used for offensive action. The policy of the Japanese Navy was to attack escorts (Task Forces) primarily, merchant vessels being considered very secondary. In one instance two submarines (Jap) were sent out specifically to sink subs, but the efforts were fruitless and the practice discontinued.

II. [Question] What working arrangements were there between Jap Subs and anti-submarine forces? What recognition methods were used to prevent Jap subs from being sunk by own forces? Were any arrangements specifically made for Jap subs to collaborate with their own A/S forces (e.g. planes) against U.S. subs?
[Answer] Working arrangements were not very thoroughly worked out between Jap Subs and anti-submarine forces, as proven by the case of a Jap sub sunk by a Jap plane. The only protection against their own forces was notification of time, course and area. In spite of usually faulty working arrangements, the sinking of HMS Prince of Wales [British battleship] was the result of a combined air/sub action. The sub spotted the target and called in the planes.

III. Tactics used on Patrol.
[Question] What areas were patrolled? Was the sub surfaced or submerged? What sort of doctrine did they follow on this? Did Jap subs make much use of radar? For surface search? Air warning? How about radar search receivers?
[Answer] Normally, Jap submarines submerged by day and surfaced at night, although wide latitude was given to the discretion of the Commanding Officers. In the early part of the war the area of patrol averaged 20 to 30 miles from a given point, but heavy losses resulted, so the areas were enlarged to an average area of 200 to 300 miles. Excessive risks were involved due to rigidity of patrol areas and consequently the Commanding Officers simply ignored strict adherence to instructions. No set pattern (like the German) was used and usually Jap subs followed a zig-zag course along a definite line.
Although there was much opposition to the use of radar and many Naval leaders did not believe in it, Hashimoto made full use of it against surface targets. He used radar search receivers against planes.

IV. Special Equipment.
[Question] Had he heard about the echo nullifier for use against sonar? How does it work?
[Answer] Hashimoto had rubber-coating on his sub against sonar and radar, considering it effective. The conning-tower and most of the hull to well below the water line were covered and experts considered the rubber-coating 30% effective. However, as much as one-third would come off on patrol. The rubber contained powdered iron. (Similar to the "Wesch" coating on the German Schnorkel).

Questions Submitted by Naval Ordnance Laboratory:

1. [Question] Were the exploders (devices used to initiate the explosion of the [torpedo] warhead) used in the torpedoes fired at the U.S.S. Indianapolis a German or a Japanese development?
[Answer] The exploders were Japanese-built, but Hashimoto did not know whether or not of German or Japanese design. (Description indicates this to be a copy of the German Pi 3).

2. [Question] If German, what was the German name for this exploder?
[Answer] No answer.

3. [Question] Was it necessary for these torpedoes to strike the ship in order for the exploder to function?
[Answer] Impact is not necessary for the exploder to function.
4. [Question] If not, how far below the keel of the ship could the torpedo pass and still fire?
[Answer] The torpedo could pass below the ship a maximum of 4 meters and still fire.

5. [Question] Were your instructions for depth setting on torpedoes carrying a magnetic exploder the same as those for torpedoes carrying a contact exploder only?
[Answer] No, the instructions were different. Magnetics were fired on deeper settings.

6. [Question] Is it true that the torpedoes fired at the U.S.S. Indianapolis carried exploders that were actuated by the magnetic field of the ship?
[Answer] Yes.

7. [Question] When you fired these torpedoes did you choose a depth setting which would permit the exploders to operate magnetically?
[Answer] According to Hashimoto there was no time to make a more suitable setting. Consequently it was left as it was, at 4 meters.

8. [Question] Did the torpedoes carry a mechanical exploder which would operate on contact with the ship as well as the magnetic exploder?
[Answer] The torpedoes carried combination magnetic and impact exploders.

9. [Question] Was there any other reason for choosing the depth setting used than that of exploder performance? If so, what was the reason or reasons?
[Answer] See #7.

10. [Question] Do you believe that the torpedoes which sank the U.S.S. Indianapolis did so by striking the ship before exploding?
[Answer] Yes.

11. [Question] If not, how far below the ship do you believe the explosions occurred?
[Answer] No answer.

12. [Question] Could you see well enough through the periscope to tell whether the torpedoes exploded on the near or the far side of the ship? If so, describe where you believe they exploded.
[Answer] Yes, the torpedoes hit the near side of the ship.

13. [Question] What did you estimate the draft of the U.S.S. Indianapolis to be?
[Answer] 7 meters. Hashimoto believed his target to be an old-style BB [US battleship].

14. [Question] If the exploders used against the Indianapolis were not magnetically operated, what was their operating principal? (Acoustic, electromagnetic, electric.)
[Answer] They were combination magnetic and impact.

15. [Question] How many pockets were provided in the torpedo to house various parts of the exploder? Where were they located?
[Answer] There were two pockets, the magnetic one in the nose of the torpedo and the impact on the top. The impact exploder was believed by Hashimoto to be an inertia type (pendulum).

16. [Question] Did the exploders used against the Indianapolis require any maintenance aboard the submarine? Explain.
[Answer] There was no maintenance aboard.

17. [Question] Did the exploders require any servicing upon return from a war patrol? Explain.
[Answer] Probably.

18. [Question] Did you have torpedoes aboard equipped with exploders of any other type? Why didn't you use them?
[Answer] There were no torpedoes of any other exploder-type aboard. All were combination magnetic and impact, except the KAITENS which were impact only.

19. [Question] Did you ever have torpedoes equipped with an external towed devised for controlling or exploding the torpedo?
[Answer] Hashimoto never used such devices, but was familiar with them.

20. [Question] Do you consider the towed exploding device satisfactory? If not, why not?
[Answer] Hashimoto considers the device no good.

21. [Question] Did you ever have trouble with torpedoes exploding before they reached the target? What type of exploders did this?
[Answer] Both magnetic and impact exploders are subject to premature explosions. The reason for this was never cleared up properly, but magnetics were almost sure to premature in a rough sea.

22. [Question] Did you have any instructions regarding the minimum safe depth for firing torpedoes with special exploders? What were they? What was the minimum safe depth?
[Answer] The minimum safe depth was 1½ meters.

23. [Question] Did you have trouble with torpedoes coming out of water in a rough sea and firing before they should have?
[Answer] Hashimoto stated they had little trouble with torpedoes coming out of the water (but see #21 above).

24. [Question] What were the special instructions for firing the torpedo due to the use of the magnetic exploder? Was it necessary to know the size of the [target] ship, material of ship construction, ship's heading, latitude, etc.?
[Answer] There were no special instructions as regards ship's heading or latitude, but depth settings for magnetics were deeper.

25. [Question] What material is the [torpedo] warhead made of?
[Answer] Probably steel.

26. [Question] How far must the torpedo run before the exploder can operate? Is this distance the same for all types of exploders? Explain.
[Answer] The torpedo must run from 300 to 500 meters. Hashimoto did not remember exact information on this subject.

27. [Question] Was it necessary to stop the submarine screws [propeller] before or immediately after firing torpedoes with any type of exploder you have used?
[Answer] It was not necessary to stop the screws.

28. [Question] How long has the magnetic exploder used on the Indianapolis been in use?
[Answer] It has been in use since the middle of 1944.

29. [Question] Where was it manufactured?
[Answer] It was manufactured in Kure.

30. [Question] Did ships' noise influence its operation?
[Answer] The ships' noise did not influence its operation.

31. [Question] Under what conditions would magnetic or other non-contact exploders not have been used?
[Answer] Magnetics would not be used in a rough sea.

32. [Question] Was there more than one type of non-contact exploder in use? Describe the significant features of each type.
[Answer] There was no more than one type, i.e. the magnetic.

33. [Question] Under what conditions would you use a non-contact exploder? Under what conditions would you use a contact exploder?
[Answer] Contact exploders are generally preferred, especially in rough seas, because of the possibility of premature explosions in the magnetics. The submarines are usually loaded about half and half of each kind of exploder torpedoes. Also many sub commanders lacked confidence in the magnetic exploder and used the impact exploders at all times.

34. [Question] What is the recommended time interval between successive shots in a spread?
[Answer] 3 seconds.

35. [Question] What type of exploder do you believe was most effective in sinking ships? Why?
[Answer] Relatively few magnetics were used. Hashimoto personally was very favorable to magnetics except in rough seas.

36. [Question] What weight of explosive material was used in torpedo warheads fired at the Indianapolis?
[Answer] Although he was not certain, Hashimoto thought the weight was 550 kilograms [approximately 1,210 lbs].

37. [Question] What is the composition of the explosive material?
[Answer] It is Shimose powder.

38. [Question] Have you fired torpedoes with aluminum included in the composition of the explosive?
[Answer] Hashimoto is not familiar with the composition of explosives.

39. [Question] Have you noticed any significant difference in damage resulting from the use of different weights or compositions of explosive material? Explain.
[Answer] Hashimoto has not noticed any notable difference.

40. [Question] Could any submarine fire the non-contact type torpedoes. If not, what types could?
[Answer] All types could fire the non-contact torpedoes.

41. [Question] Were non-contact exploders used in aircraft torpedoes?
[Answer] No answer. (Lack of information].

42. [Question] How many tail fins did the torpedoes fired at the Indianapolis have?
[Answer] Four.

43. [Question] Fill out the following table concerning torpedoes carried on your last cruise:
Torpedo Type: Oxygen
Torpedo Designation: 95
Warhead Designation: 95 Magnetic
Exploder Designation: Magnetic No. 5, Impact No. 2

44. [Question] Did Japanese submarines use torpedoes of German design?
[Answer] No information.

45. [Question] Were German ideas used in the design of Japanese torpedoes?
[Answer] No information.

46. [Question] Did any electrically-driven torpedoes reach service use? What was its range and speed?
[Answer] Very few electrically-driven torpedoes reached service use. Range and speed are 7000 meters and 28 knots.

47. [Question] When did Japanese submarines first employ torpedoes powered with fuel and oxygen?
[Answer] In 1940.

48. [Question] How many different types of fuel-oxygen torpedoes were used?
[Answer] Three (93, 95, 97).

49. [Question] What was the speed and range of each type?

93 - 45 knots - 9000 meters
95 - 48 knots - 5000 meters
97 - no information.

50. [Question] How much was known about the German Ingolin torpedo? Were any of them used against the U.S. Navy?
[Answer] Nothing.

51. [Question] Were you troubled with accidental oxygen explosions? What precautions were necessary?
[Answer] In the early days of the war the [Japanese] were troubled with accidental oxygen explosions. However, fuel and oil were kept away from the oxygen until the last minute. Every effort was made to avoid heat.

52. [Question] What types of fuel were used with the oxygen torpedo? What form was the oxygen supply? Liquid or gas?
[Answer] Coal-oil was used with the oxygen torpedo. The oxygen supply was in gas form.

53. [Question] How much was known about American torpedoes? Name various types known, together with their significant properties.
[Answer] Nothing was known regarding American torpedoes in this case.

54. [Question] What accuracy of steering was realized on Japanese torpedoes?
[Answer] Regarding steering, at 5500 meters, deviation is not over 200 meters.

55. [Question] Were any Japanese torpedoes equipped wit special figure run devices? Explain.
[Answer] No.

56. [Question] Are there any types of Japanese torpedoes that follow the target even though they are not aimed at the target?
[Answer] No.

57. [Question] If yes, what method was used to make them follow the target (acoustic, magnetic, electric, electromagnetic)?
[No response necessary]

58. [Question] Did these target-seeking torpedoes receive steering signals directly from the ship or from reflection of a searching signal transmitted by the torpedo itself?
[No response necessary]

59. [Question] When did target-seeking torpedoes come into general use by Japanese submarines?
[No response necessary]

60. [Question] Under what conditions were human torpedoes used? How many did each submarine carry? Was the operator expendable?
[Answer] Human torpedoes were used if target was out of range of other torpedoes and sometimes in order to avoid detection. Big subs carried 6 human torpedoes; 300-Type carried 3; I-Type carried 2. The operator was 100% expendable. KAITENS were about 1/3 successful. Hashimoto's only success with them was on the I-58 which sank a U.S. escort carrier (converted) near Guam in January 1945. He observed through the periscope that one hit out of the four launched.

61. [Question] Which type of torpedo was best liked by Japanese submarine commanders? Why? Was it responsible for most sinkings?
[Answer] Most Japanese submarine commanders preferred impact-type exploders. Human torpedoes were only used as emergency weapons.

62. [Question] How were torpedoes launched from Japanese submarines (compressed air, powder charge)?
[Answer] Torpedoes were launched by compressed air.

63. [Question] What steps were taken to prevent detection due to air or gas escaping from tubes after launching?
[Answer] Valves for return of air were used.

Questions Submitted by Commander D.R.E. Brown, USNR:

1. [Question] What was the name of their [the Japanese] research agency investigating:
(a) Light conditions.
(b) Visibility - day and night.
(c) Transmission of light through water.
[Answer] No information.

2. [Question] What reports were published on these subjects?
[Answer] No information.

3. [Question] Were these subjects used as part of submarine officers training?
[Answer] Yes, especially as to depth at which submarines can be seen from the air, including taking flights for this purpose.

4. [Question] How did submarine officers estimate visibility:
(a) Of their own ships?
(b) Of enemy ships?
[No answer given]

5. [Question] Do they [the Japanese] have tables or charts on the subject of visibility?
[No answer given]

6. [Question] Do they have pictures?
[No answer given]

7. [Question] What were their lookout procedures?
[No answer given]

8. [Question] What size and type of binoculars were used by their sky and sea lookouts?
[Answer] The size 7 to 10 power was standard equipment. In addition they had 15 to 20 x 150 and 20 x 120.

9. [Question] At what depth does he think his submarine was visible to aircraft?
(a) In moderate sea?
(b) In China Sea?
(c) In clear, calm water?
[Answer] In the China Sea, 25 meters, keel depth. Around the Marshals in calm sea, 35 meters.

10. [Question] How far could he be seen by a surface ship
(a) Clear starlight night?
(b) Clear moonlight night?
[Answer] With standard binoculars he could be seen at 10,000 meters by moonlight and at 5,000 meters by starlight.

11. [Question] How far could be seen CA [cruiser], BB [battleship], DD [destroyer] on a
(a) Clear starlight night?
(b) Clear moonlight night?
[Answer] With standard binoculars --
Moonlight Starlight
CA 12,000 6,000
BB 15,000 7,500
DD 10,000 5,000

12. [Question] What was Japanese Submarine Force opinion of Ship Camouflage? Their own and ours?
[Answer] They felt it was only moderately effective.

13. [Question] Were there any specific cases where our patterns led to course deception sufficient
(a) To prevent a submarine from contact, i.e. from gaining a firing position?
(b) To cause torpedo misses?
[Answer] There were none.

14. [Question] Did patterns retard or prevent recognition and/or identification?
[Answer] Not too much.

15. [Question] Did patterns or any other camouflage increase or decrease visibility of ships or aircraft?
[Answer] Only slightly.

For additional information please see The Sinking of USS Indianapolis.

9 February 2001

Published: Wed Jul 06 06:34:57 EDT 2016