Japanese Monograph No. 160

OUTLINE OF NAVAL ARMAMENT AND PREPARATIONS FOR WAR

Part III

PREPARED BY MILITARY HISTORY SECTION HEADQUARTERS, ARMY FORCES FAR EAST

DISTRIBUTED BY OFFICE OF THE CHIEF OF MILITARY HISTORY DEPARTMENT OF THE ARMY

This is the third of a series of six monographs in regard to Japan's naval armament in preparation for and during the Pacific War.

Additional monographs covering Japan's naval armament plans

are:

Mono No

Outline of Naval Armament and Preparations for War, Part I

145

Outline of Naval Armament and Preparations for War, Part II

149

Outline of Naval Armament and Preparations for War, Part IV

169

Outline of Naval Armament and Preparations for War, Part V

172

Outline of Naval Armament and Preparations for War, Part VI

174

--iii--

Preface

Through Instructions No. 126 to the Japanese Government, 12 October 1945, subject: Institution for War Records Investigation, steps were initiated to exploit military historical records and official reports of the Japanese War Ministry and Japanese General Staff. Upon dissolution of the War Ministry and the Japanese General Staff, and the transfer of their former functions to the Demobilization Bureau, research and compilation continued and developed into a series of historical monographs.

The paucity of original orders, plans and unit journals, which are normally essential in the preparations of this type of record, most of which were lost or destroyed during field operations or bombing raids rendered the task of compilation most difficult; particularly distressing has been the complete lack of official strength reports, normal in AG or G3 records. However, while many of the important orders, plans and estimates have been reconstructed from memory and therefore are not textually identical with the originals, they are believed to be generally accurate and reliable.

Under the supervision of the Demobilization Bureau, the basic material contained in this monograph was compiled and written in Japanese by former officers, on duty in command and staff units within major units during the period of operations. Translation was effected through the facilities of Allied Translators and Interpreters Service, G2, General Headquarters, Far East Command.

This Japanese Operational Monograph was rewritten in English by the Japanese Research Division, Military History Section, General Headquarters, Far East Command and is based on the translation of the Japanese original. Editorial corrections were limited to those necessary for coherence and accuracy.

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TABLE OF CONTENTS

Chapter

Page

I

GENERAL SITUATION

1

Preparation of Armament Against the United States

3

First Requisition, Conversion and Equipment of Vessels

6

Extraordinary Supplementary Armament Plan

7

Urgent Armament Plan

15

Second Requisition and Equipment of Vessels

29

Acceleration of other War Preparations

30

War Preparations Upon Decision to Go to War

31

Organization of Special Naval Units

32

Progress of Armament and Other War Preparations at the Time of the Outbreak of War

34

II

OUTLINE OF THE FIFTH AND SIXTH NAVAL ARMAMENT REPLENISHMENT PLANS

41

Charts

No. 1

Extraordinary Supplementary Armament Plan Other Supplementary Armament Plans between

the Fourth Armament Replenishment Plan

and the Urgent Armament Plan

9

2

Progress of Shipbuilding

13

3

Urgent Armament Plan

17

4

Urgent Armament Plan Approved Budget

21

5

Urgent Armament Plan Progress of Shipbuilding

23

6

Air Strength at time of Outbreak of War

37

--vii--

7

Roster of Naval Officers at time of Outbreak of War

39

8

Fifth Armament Replenishment Plan

42

--viii--

Chapter I General Situation

With the outbreak of World War II on 1 September 1939, the United States policy appeared to be to assist Great Britain and France in every way possible. The arms embargo was lifted at the beginning of November 1939, and, at the same time, the construction of an additional 40 US destroyers was approved. To offset the preponderance of German submarines in the spring of 1940, the United States leased 50 escort destroyers to Great Britain and extended positive aid in regard to the strengthening of the escort provided Europe-bound merchant ships.

In November 1939, having been notified of the commencement of the Japanese Fourth Armament Replenishment Plan, the United States Navy submitted to Congress the Third Vinson Plan, and, on 4 June 1940, it was signed by the President. The plan called for the construction of 3 aircraft carriers, 8 cruisers, 52 destroyers and 32 submarines, having a total tonnage of 400,000 tons. Also included in the plan was the construction of auxiliary vessels amounting to 125,000 tons, making a total tonnage of 525,000 tons. In addition, provision was made for 6,000 Navy planes and 36 flying boats. The appropriations for this plan amounted to $1,300,000,000, and the year of completion was set at 1944. It was estimated that the United States would have a main strength totaling more than 2,100, 000 tons, with a nucleus of 23 battleships, when this plan was completely realized.

--1--

In the spring of 1940, at the same time as the Third Vinson Plan began to get under way, the United States Navy decided to station permanently in the Hawaiian area the main force of the fleet then bound for that area to take part in large-scale maneuvers.

It was felt that the tightening of the political and economic encirclement of Japan by the United States, Great Britain, the Netherlands, Australia and China, coupled with the protraction of the China Incident, was making Japan's position more and more untenable. In order to surmount this difficulty, Japan took steps to strengthen her tripartite ties and to advance to the south. This caused the Anglo-American bloc to stiffen their attitude toward Japan. To cope with this critical situation, the Japanese Navy carried out various emergency measures. In 1941, to counteract the Third Vinson Plan, the Fifth Armament Replenishment Plan was adopted. (See Chapter II) However, on 19 June 1941, the astronomical United States' Stark Plan was approved. The Stark Plan was a two-ocean fleet plan with a 70 percent increase in the total tonnage of naval vessels and an increase in Navy planes to 15,000. The appropriation for this 6-year plan was $10,000,000,000.

Although it was realized that the fulfillment of the Fifth Armament Replenishment Plan would be most difficult from the standpoint of finance, materials and equipment, Japan considered it necessary to continue in the naval armament race, which resulted in the drafting of the Sixth Armament Replenishment Plan. (See Chapter II)

--2--

The strained relations between Japan and the United States led the navies of the two nations to compete in an unlimited and endless armament race.

Preparation of Armament Against the United States

The armament policy of the Japanese Navy had hitherto been based on the requirements of the national defense policy which aimed to maintain a strength in the ratio of 7:10 with the United States. The object of the national defense policy was to guarantee the security of the nation by securing the supremacy of sea and air in the Western Pacific. The 7:10 ratio was based on the tactical principle accepted at that time by the navies of all countries, which concluded that the strength of the offensive force must be at least 50 percent superior to that of the defensive force. While a navy of 7:10 ratio would be insufficient for offensive warfare, it would be enough for defensive warfare.

Because of the completion of the naval armament replenishment plans in the United States, the Japanese Navy's policy for maintaining the 7:10 ratio with the United States became futile, and it was estimated that in a short space of time the ratio of Japan's strength would decrease to less than 6:10 with the United States. Consequently, strenuous efforts were made to offset the quantitative deficiency by such means as effective armament preparations, skilled strategy and an effort to meet quantity with quality. Toward the end of 1940 Japan was forced to revise the armament policy generally as follows:

--3--

As it was predicted that in the near future the ratio of battleships would become less than 5:10 with the United States, the idea of meeting quantity with quantity was abandoned. It was decided that by building such huge battleships as the Musashi and Yamato quantitative deficiency would be balanced by the superior power of individual vessels. In accordance with the sudden change in the situation at the end of 1941, however, the construction of battleships, other than the almost completed Musashi and Yamato, was suspended. After the defeat in the Midway Battle in June 1942, it was decided to complete the reconstruction of the battleship Shinano into an aircraft carrier by the end of 1944. Aircraft Carriers

Although at the end of 1940 there was no real difference in strength between the United States and Japan in regard to this type of vessel, there was every possibility that the United States would build more aircraft carriers in the near future. Therefore, without adhering to the tonnage ratio, Japan adopted a policy of creating parity on the number of vessels built by the United States. Finally, having exhausted her capacity for shipbuilding, Japan decided to convert first-class merchant ships into aircraft carriers. This plan was studied for some years before being put into operation. Cruisers

It was felt that there would be no difficulty in maintaining the 7:10 ratio with the United States in regard to cruisers, as

--4--

Japan was in quite a favorable position in regard to this type of vessel, especially in regard to heavy cruisers. It was decided, therefore, that the Japanese Navy policy of giving priority to superiority of fire power, torpedo power and speed should be continued. Destroyers

Although the ratio with the United States in regard to destroyers was approximately 7:10, it was felt that within a short period of time the difference in strength would be great, because of the United States' gigantic armament plans, and her capacity to reduce the time required in shipbuilding. Another factor was that the Japanese Navy required their destroyers to carry heavy torpedo equipment to be used for night combat. This meant that the destroyers had to be larger, and, in consequence, not able to build as many Submarines

From the start, the quality and number of this type of vessel was not based on the ratio with the United States. As the Japanese Navy employed these vessels chiefly in interception actions and fleet warfare, they were all large, but they were few in number. In order to remedy this defect, midget submarines were built. Aircraft

Until the Third Armament Replenishment Plan, which counteracted the Second Vinson Plan, Japan aimed generally at an equal number of planes with the United States, but because no measure could be taken to cope with the Third Vinson Plan, which called for 6,000 Navy planes lost in the China Incident, it became more and more difficult

--5--

to maintain the ratio. By estimating the potential air strength of the United States which could be engaged in an offensive in the Far East, the minimum strength required to intercept this force was to be maintained. There was a tendency, however, to underestimate the power of the United States, and the Japanese Navy hoped that the lack of planes could be compensated by the mobility of the forces stationed at dispersed air bases.

First Requisition, Conversion and Equipment of Vessels

At the outbreak of the China Incident, vessels were requisitioned and equipped as part of the preparations for war. The Japanese Navy being unable to take any measures to counteract the Third Vinson Plan, decided on 15 November 1940 to requisition a total of 550,000 ship tons. These ships would be converted into aircraft carriers, transports, cruisers, gunboats and other auxiliary vessels as well as used as general naval transports. The work was undertaken gradually and, for the most part, was completed within six months.

In contrast to the nine aircraft carriers built under the Vinson Plans and supplementary armament plans, Japan's Third and Fourth Replenishment Plans made provisions for only three carriers. In consequence, in order to maintain parity, Japan had to make further plans. As a result, it was decided to convert first-class merchant ships. The Kasuga Maru was requisitioned and the Isumo Maru and Kashiwara Maru were purchased. The work of converting them into aircraft carriers was started in early 1941. The Taiyo (Kasuga Maru)

--6--

15,000 tons was completed 25 August 1941; the Junyo (Kashiwara Maru) 19,000 tons was completed 3 May 1942, and the Hiyo (Izumo Maru) 19,000 tons was completed 31 July 1942. It was planned that after careful study and preparation other merchant ships and some warships would be converted into aircraft carriers in order to increase the aircraft carrier strength.

In accordance with the Fourth Armament Replenishment Plan, the keels of the battleships Shinano and Kii were laid at Yokosuka and Kure Naval Yards respectively, and their construction up to double-bottoms was completed. The work was suspended temporarily on account of the delay in the construction of armour, and later, because of the major changes in armament and war preparation plans affected by the outbreak of war, it was decided to discontinue construction. Because of the sudden decrease in carrier strength caused by losses in the Battle of Midway, however, it became necessary to convert the Shinano into an aircraft carrier.

Extraordinary Supplementary Armament Plan

The Navy Ministry and the Naval General Staff agreed on the details of the Fifth Armament Replenishment Plan in order to counteract the Third Vinson Plan, but it looked as though there might be some difficulty in obtaining the necessary finances, materials and plants from the Finance Ministry, Planning Board and War Ministry respectively.

It was thought wise for the Naval General Staff to consult the Army General Staff in regard to the Fifth Armament Replenish-

--7--

ment Plan. The Army General Staff agreed to the plan under the following conditions: that it should be discussed again if it showed signs of hampering the established armament plan of the Army; that the Army would deliberate the matter of allocation of materials included in the material mobilization plan; and that it would decide the allocation of necessary manpower. Since the outbreak of the China Incident, the Japanese Navy had recognized the necessity for giving priority to the Army in regard to armament replenishments, in consideration of the nature of the operations. Unless priority was granted to the Fifth Replenishment Plan, however, it would be impossible to realize the plan. Therefore, the conditions imposed by the Array made it clear that they were rejecting the plan. From this time forward there was a constant struggle between the two services in regard to munition production.

The Japanese Navy felt keenly the necessity for taking extreme measures in order to accelerate their war preparations, and in May 1941 it established, and immediately put into execution, the Extraordinary Supplementary Armament Plan, known in Japanese as the "Maru Rin." (For details see Chart 1.)

Additional Supplementary Plans were placed into effect between the Fourth Armament Replenishment Plan and the Urgent Armament Plan and their progress, on the whole, was satisfactory. (See Charts 1 and 2)

There was no new plan for air armament. The air armament plan of the Fourth Armament Replenishment Plan had progressed compara-

--8--

Chart 1 - Extraordinary Supplementary Armament Plan

Standard Displacement per Vessel

Number of Vessels

Total Tonnage

Speed

Main Armament

Remarks

Submarines (Medium)

970

9

8,730

19.5 (Surface)

80mm gun

1

Approved budget the fiscal year 1941

General account: 196,424,750.00

25mm machine guns

2

Torpedo tubes

4

Submarines (Small)

525

9

4,725

14.2 (Surface)

25mm machine guns

2

Torpedo tubes

4

Gasoline tankers (Medium)

7,950

2

15,900

16.0

127mm gun

4

Gasoline tankers (Small)

4,460

4

17,840

l6.0

120mm gun

2

Subchasers

440

16

7,040

16.0

80mm dual purpose gun

1

Auxiliary minesweepers

215

6

1,290

9.5

80mm gun

1

Approved budget the fiscal rear 1941

Extraordinary military expenditure: 21,030,000.00

Auxiliary minelayers

280

4

1,120

9.5

80mm gun

1

Refrigerator ships

920

3

2,760

14.0

80mm gun

1

Motor torpedo boats

20

4

120

35.0

25mm machine guns

2

Total

59

59,525

220,934,750.00

--9--

[--10-- blank]

Chart 1 - Other Supplementary Armament Plans between the fourth Armament Replenishment Plan and the Urgent Armament Plan

Standard Displacement per Vessel

Number of Vessels

Total

Speed

Main Armament

Remarks

Cable laying boats

1,564

4

6,256

14.0

80mm dual purpose gun

1

Approved budget the fiscal year 1939

Extraordinary military expenditure: 13,667,000.00 (of which 3,480,000.00 allotted to not or torpedo boats)

Refrigerator ship

920

1

920

14.0

80mm dual purpose gun

1

Refrigerator ship

640

1

640

13.0

80mm dual purpose gun

1

Suchasers

440

7

3,080

16.0

80am dual purpose gun

1

Approved budget the fiscal year 1940

Extraordinary military expenditure: 8,820,000.00

Training ship

5,800

1

5,800

18.0

140mm guns

4

Approved budget the fiscal year 1941

General account 7,660,000.00

127mm dual purpose guns

2

Torpedo tabes

4

--11--

[--12-- blank]

Chart 2 Progress of Shipbuilding

Extraordinary Supplementary Armament Plan:

Type of Vessel

Class

Name of Vessel

Number of Vessels

Period of Construction

Remarks

Submarines (Medium)

Ro 35 Type

Ro 35 & others

9

Oct 41 to

Dec 43

Submarines (Small)

Ro 100 Type

Ro 100 & others

9

Jun 41 to

Apr 43

Gasoline tankers (med)

Ashizuri class

Ashizuri & others

2

Jul 41 to

Nov 43

Gasoline tankers (small)

Sunozaki class

Takasaki

1

Jul 42 to

Sep 43

Gasoline tankers (small)

Sunozaki class

3

Construction suspended

Subchasers

No 13 Type

No 24 & others

16

Hay 41 to

Feb 43

Auxiliary Minesweepers

No 1 Type

No 1 & others

ft

Jun 41 to

Oct 42

Auxiliary Minelayers

No 1 Type

No 1 & others

4

Jun 41 to

Aug 42

Refrigerator ships

Kinozuki class

Hayasaki & others

3

Dec 41 to

May 43

Motor Torpedo boat

No 1 Type

No 1 & others

6

Nov 41 to

Dec 41

Total

59

56

3

May 41 to

Dec 43

Completed

Other Supplementary Plans between the Fourth Armament Replenishment Plan and Urgent Armament Plan:

Type of Vessel

Class

Name of Vessel

Number of Vessels

Period of Construction

Remarks

Cablelayers

Kashima class

Kashima & others

4

Oct 39 to Aug 41

Refrigerator ships

Kinozaki class

Kinozaki & others

2

Oct 39 to Mar 41

Subchasers

No 13 Type

No 17 & others

7

Oct 40 to Nov 41

Training ship

Katori class

Yashiwara

1

Aug 41 to Oct 41

Construction suspended

Total

14

13

1

Oct 39 to

Nov 41

Completed suspended

--13--

[--14-- blank]

tively well due to the stalemate in the China Incident with resulting small losses in aircraft. In addition, 100,000,000 was allotted for the immediate preparation and activation of 17 of the 93 units of the Training Air Group included in the Fifth Armament Replenishment Plan. (See Chart 8)

Urgent Armament Plan

At the liaison conference between Imperial General Headquarters and the Government, held on 2 July 1941, the "Outline of the National Policy of the Japanese Empire in connection with the Change of Circumstances," which emphasized the drive southward and the inevitability of war with the United States and Great Britain to achieve Japan's objective, was agreed upon. In order to comply with this policy, the Japanese Navy, being well aware of its great deficiency in armament as compared with the United States, realized that it was necessary to speed up war production. In the latter part of August 1941 it decided on the over-all execution of the Expeditionary Preparation Plan. As part of it, the Wartime Armament Preparation Plan was immediately enforced. This plan became known as the Urgent Armament Plan. (See Charts 3, 4 and 5)

In order to equip oil tankers to carry airplanes, an additional 10,264,000 was approved for 1944 (2,566,000 for each vessel). Because it was decided to construct an aircraft carrier of 12,500 tons instead of a cruiser, a further 10,610,000 was approved for 1945.

--15--

Chart 3 - Urgent Armament Plan

Type of Vessel

Number of Vessels

Total Tonnage (Standard Displacement)

Speed

Main Equipment

Remark

Aircraft carrier

1

17,150

34

127mm dual purpose guns

12

UNRYU

25mm machine guns

31

Aircraft

49

Cruisers

2

24,400

35

200mm guns

10

After launching one cruiser converted into aircraft carrier (IBUKI Class), but conversion work suspended. Construction of 1 cruiser suspended.

127mm dual purpose guns

8

Torpedo tubes

14

Aircraft

3

Destroyers (Class A)

16

41,104

35

127mm guns

6

Construction of 8 destroyers suspended.

Torpedo tubes

8

Destroyers (Class B)

10

27,000

33

100mm dual purpose guns

8

Construction of 1 destroyer suspended.

Construction of 3 destroyers cancelled.

Torpedo tubes

4

Submarines (Class B)

6

13,110

Surface 23.6

140mm gun

1

Torpedo taboo

4

Aircraft

1

Submarines

(Class C)

6

13,060

Surface 23.6

140mm gun

1

Construction of 3 submarines cancelled.

Torpedo tubes

8

Submarines

(Medium)

12

11,412

Surface 19.7

80mm guns

1

Construction of 4 submarines cancelled.

Torpedo tubes

4

Submarines

(Small)

9

4,662

Surface 14.2

25mm machine guns

2

Torpedo tubes

4

Seaplane tender

1

4,620

19

127mm dual purpose guns

4

Construction suspended.

(Installation for carrying one large flying boat)

--17--

[--18-- blank]

Urgent Armament Plan (Cont'd)

Type of Vessel

Number of Vessels

Total Tonnage (Standard Displacement)

Speed

Main Equipment

Remark

Escort ships (Class B)

16

15,040

19.7

127mm dual purpose gun

1

120mm dual purpose gun

1

Depth charge throwers

2

Escort ships (Class A)

14

12,040

19.7

120mm guns

3

Depth charge thrower

1

Minesweepers

28

18,200

20

120mm guns

3

Construction of 17 minesweepers cancelled.

Large sweep cables

6

Auxiliary

16

3,360

95

80mm gun

1

Light minelayers

14

10,080

20

80mm dual purpose gun

1

Construction of 13 minelayers cancelled.

Depth charge throwers

2

Subchasers

20

8,800

16

80mm dual purpose gun

1

Construction of 1 subchaser cancelled.

Depth charge throwers

2

Auxiliary subchasers (wooden)

100

13,000

11

13mm machine gun

1

Motor Torpedo boats

18

1,350

29.5

Torpedoes

12

Construction of 3 torpedo boats suspended.

Construction of 7 torpedo boats cancelled.

Oil tanker

1

18,850

Construction cancelled.

Oil tanker

2

36,800

16.5

120mm dual purpose gun

3

Later equipped to carry 6 airplanes.

Oil tanker

1

6,000

Construction cancelled.

Total

293

300,058

--19--

[--20-- blank]

Chart 4 Urgent Armament Plan

Approved Budget

Type of Vessels

Tonnage

Number

of Vessels

Cost of one Vessel

(Unit of 1,000)

Total Coat

(Unit of 1,000)

Cruisers

12,000

2

60.000.0

120,000.0

Aircraft Carrier

17,100

1

87.039.0

87,039.0

Destroyers (Class A)

2,570

16

17,424.6

278,793.6

Destroyers (Class B)

2,980

10

17.820.4

178,204.0

Submarines (Class B)

2,280

12

20,497.2

245,966.4

Submarines (Medium)

1,000

12

7.920.0

95,040.0

Submarines (Small)

550

9

4,713.5

42,421.5

Flying Boat Tender

11,00

1

47410.0

47,410.0

Escorts

1,200

30

5.112.0

153,360.0

Light Minelayers

720

14

4,334.4

60,681.6

Minesweepers

700

28

3.738.0

104,664.0

Auxiliary

Minesweepers

(Fishing Boat Type)

300

16

1.431.0

22,896.0

Subchasers

400

20

2,921.6

58,432.0

Auxiliary

Subchasers

(Wooden)

100

100

780.0

78,000.0

Oil Tankers

18,400

4

14,720.0

58,880.0

Motor Torpedo Boats

100

18

2.262.0

40,716.0

Total

293

1,672,504.1

--21--

[--22-- blank]

Chart 5 - Urgent Armament Plan

Progress of Shipbuilding

Type of Vessel

Class

Name of Vessel

Number of Vessels

Period of

Construction

Remarks

(Total)

293

228

Completed

7

Dec 41 to Apr 45

Work suspended

58

Construction cancelled

Cruisers

Suzuka class

Ibuki

2

1

Apr 42 to Mar 45 (Converted into aircraft carrier)

Work for conversion into aircraft carrier suspended

1

Jun 42 to Sep 42 (suspended)

Work suspended

Aircraft carrier

Remodeled Hiryu class

Unryu

1

Aug 42 to Aug 44

Seaplane tender

Akitsushima class

Chihaya

1

Jul 42 to Oct 42 (suspended)

Work suspended

Oil tanker

Kazahaya class

Kazahaya and others

4

2

Oct 42 to Apr 44

(Large)

1

Construction suspended

(Small)

1

Construction cancelled

Escort ships (class A)

Remodeled Shimushu class

Etorofu and others

14

Feb 42 to Feb 44

Escort ships (class B)

Mikura-class Ukuru-clas

Mikura & others

16

Oct 42 to Nov 44

--23--

[--24-- blank]

Chart 5 - Urgent Armament Plan (Cont'd)

Type of Vessel

Class

Name of Vessel

Number of Vessels

Period of

Construction

Remarks

Destroyers(class A)

Yugumo class

Hayanami & others

16

8

Jan 42 to May 44

Yugumo class

8

Construction cancelled

Destroyers (Class B)

Akitsuki class

Shimotsuki & others

6

Jul 42 to Apr 45

Akitsuki class

Mitsutsuki & others

1

Jan 44 to Mar 44 (suspended)

Work suspended

Akitsuki class

Kiyotsuki & others

3

Construction cancelled

Submarine (Class B)

Type I-15

I-40 and others

6

Mar 42 to Feb 44

Submarine (Class C)

Type I-16

I-46 and others

6

3

Dec 42 to Aug 44

-

Type I-16

I-49 and others

3

Construction cancelled

Submarine (Medium)

Type Ro-35

Ro 44 and others

12

8

Feb 42 to Sep 44

Type Ro-35

Ro 51 and others

4

Construction cancelled

Submarine (Small)

Type Ro-100

Ro 109 and others

9

May 42 to Jan 44

Minesweepers

Type No 19

No 25 and others

28

11

Apr 42 to Jul 44

Type No 19

No 31 and others

17

Construction cancelled

--25--

[--26-- blank]

Chart 5 Urgent Armament Plan (Cont'd)

Type of Vessel

Class

Name of Vessel

Number of Vessels

Period of Construction

Remarks

Subchasers

Type No 13

No 40 and others

20

19

Feb 42 to Jul 44

Type No 13

No 59

1

Construction cancelled

Light minelayers

Remodeled Hirashima class

Ajiro

14

1

Sep 43 to Jul 44

Ajiro class

Kamishima & others

13

Construction cancelled

Auxiliary minelayers

Type No 1

No 7 and others

16

Feb 42 to Oct 44

Auxiliary subchasers

Type No 1

No 1 and others

100

Dee 41 to Mar 44

Motor torpedo boats

Type A

No 10 and others

18

8

Jul 42 to Feb 45

Type No 10

No 18 and others

3

Mar 44 to Jun 44 (suspended)

Work suspended

Type No 10

No 21 and others

7

Construction cancelled

--27--

[--28-- blank]

as only three, months had elapsed since a part of the Fifth Armament Replenishment Plan (air section) had been put into effect, and that only after long deliberation and preparation, it was considered impossible to establish another supplementary plan to further augment air strength without causing confusion. As the Fourth Armament Replenishment Plan was progressing smoothly, it was decided to advance the date of completion of this plan by one year, the plan to be completed by the end of the 1942 fiscal year instead of 1943. The remarkable success of the Japanese Navy Air Force at the beginning of the Pacific War, and the idea of giving priority to air armament which resulted from the above decision, not only enabled the general completion of the Fourth Armament Replenishment Plan within the prescribed time, but also led to the realization of the Revised Fifth Armament Replenishment Plan.

Second Requisition and Equipment of Vessels

The vessels obtained through the First Requisition were almost completely equipped and were steadily being added to the fleet strength. Under the firm policy adopted in the summer of 1941, it was decided that an additional 600,000 tons of vessels would be requisitioned for use as converted vessels or for increasing the number of supply and transport vessels.

Included in the vessels requisitioned in accordance with this plan was the Nitta Maru, which was converted into the aircraft carrier Chuyo, and such ships as the Aikoku Maru and the Hokoku Maru, which later distinguished themselves as converted cruisers

29-

raiding merchant shipping. Most of these vessels were completely equipped by the end of 1941, but, because it was to be used to carry the mission dispatched for the conference between President Roosevelt and Premier Konoye at Juneau, Alaska, the equipping of the Nitta Maru was not started until June 1942. It was completed as the converted aircraft carrier Chuyo (15,000 tons) in the latter part of November 1942.

Acceleration of Other War Preparations

The activation of the wartime organization of the Imperial Japanese Navy was proclaimed on 1 September 1941, and the Fleet rose to unprecedented strength.

Following this, all vessels were subject to necessary repair before going into action. It was scheduled that, because of the state of emergency, this work would be completed by the end of November. The greatest problem in connection with the execution of the Expeditionary Preparation Plan was the lack of war materiel. The war materiel stockpile planned during peacetime was virtually nothing compared to the quantity required by the huge number of. units organized and for the vessels to be constructed under this plan. There was available only 10 to 30 percent of the required quantity of such materiel as large bombs, Type 91 torpedoes, 20mm. machine guns and ammunition, all of which were vital to air war. Moreover, as the output of these items was very slow, it was believed that, within several months after the outbreak of war,

stockpiles would be completely exhausted. Consequently, efforts

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were made by the Navy Ship and Ordnance Department to increase the production of large bombs by converting shells of 16-inch guns, aircraft torpedoes and machine guns. At the same time, in order to increase the production of the Navy's weapons, and also to produce the strength called for in the Fifth Armament Replenishment Plan, preparations were made for the establishment of a number of Navy plants.

Although it was planned that the facilities for producing Type 93 torpedoes for surface vessels would be converted into those for Hype 91 torpedoes for aircraft, such a conversion required a complete change-over of materials, parts, machinery, etc. As no plans had been made during peacetime for this change-over, it was expected that there would be a blank period of several months before the increased production of Type 91 torpedoes could be achieved. However, this precaution prevented the disruption of supplies in regard to air operations after the outbreak of war.

War Preparations Upon Decision to Go to War

At the conference in the Imperial Presence held on 5 November 1941, a decision was reached on the "Outline for the Execution of the National Policy of the Empire," the main point of which was "to continue negotiations with the United States, but, in case no accord can be reached, a resolution to declare war will be adopted and preparations for operations will be undertaken. If a diplomatic settlement cannot be reached by the beginning of December, the decision to declare war will be made immediately."

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As a result of this decision, actual responsibility concerning Navy war preparations was shifted from the naval administration authorities to the Supreme Command. In preparation for the unprecedented operations against the United States and Great Britain, the strongest and wealthiest nations in the world, in a theater covering almost the entire Pacific area, the Navy Ministry and all sections in charge of construction, repairs, replenishment and supply had to make all-out efforts under conditions of top secrecy.

With the launching of the United States' Stark Plan, Japan intended to set up the Sixth Armament Replenishment Plan. Just when the problem of how to adjust this plan with the Fifth Armament Replenishment Plan was being studied, the decision to declare war was reached. The Japanese Navy, therefore, ordered the construction of 32 submarines and one bombing target ship to be included

in the Fifth Armament Replenishment Plan. At the same time, they

ordered the increased production of both operational and training aircraft.

It was decided that an additional 500,000 tons of vessels, including the Yawata Maru, would be requisitioned for the purpose of supplementing the lack of all types of ships. Work was begun on the Yawata Maru in January 1942, and toward the end of May it was completed as the converted aircraft carrier Unyo (15,000 tons).

Organization of Special Naval Units Construction Battalions

Eight construction units were organized for the purpose of

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constructing airfields in occupied territories. The plan was to dispatch five units to the Philippines, two to the Fourth Fleet area (Inner South Seas Area) and one to the Malay area. Regular

officers had no experience in this type of work. The units, therefore, were composed mostly of nonmilitary men who were compelled to carry a large amount of equipment with them. It was extremely difficult under the circumstances to conduct preparations and transportation at a top secret level. A meeting to discuss the duties of the leaders was delayed until after 10 November, and the men were dispatched toward the end of the month in order to join their assigned units in time for the operations. Special Landing Units

Two units were organized and trained in each naval district. They were assigned to the respective invasion forces and dispatched rapidly.

The airborne units which were being trained in the Tateyama Sector under the command of the Yokosuka Natal District were the first of their kind in Japan. They were well trained and on 20 November were assigned to the Combined Fleet. Fuel Exploitation Unit

As it was presumed that all oil refining facilities would be destroyed by the authorities in the Dutch East Indies, the 101st Naval Fuel Depot was established for the purpose of restoring such facilities speedily and effecting the greatest output in production. It was planned to send this unit to Dutch Borneo simultaneously

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with the occupation of that area. As a large quantity of machinery was necessary and technical aid from commercial firms was required, it was especially difficult to maintain secrecy with this project. Other Units

Preparation for the dispatch of fire-fighting units for oil fields, personnel for military post offices and weather units was undertaken simultaneously. No great difficulty was encountered with these units.

On 5 November 1941, the Chief of the Naval General Staff issued directives to all naval operational forces to commence defensive preparations in conformity with plans regarding their deployment. All surface units strove for the early completion of the planned emergency war preparations and all Naval Districts made efforts to intensify security patrols by organizing garrison units, comprised of members of the former naval barracks.

Progress of Armament and Other War Preparations at the Time of the Outbreak of War

Prior to the outbreak of war, successive supplementary armament and other war preparations had been undertaken and their completion had been accelerated to the utmost. The resultant progress in ship construction at the time of the outbreak of war on 8 December 1941 was generally as shown below. In addition, the Fourth Armament Replenishment Plan was almost completed and such preparations as the requisitioning and completion of converted vessels, strengthening of ground and port defenses, and procurement and

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transportation of equipment and the necessary war materials were in progress.

Number of Ships at the Time of the Outbreak of War

Battleships

10

301,400 tons

Aircraft Carriers

10

152,970 tons

Heavy Cruisers

18

158,800 tons

Light Cruisers

20

98,855 tons

Destroyers

112

165,868 tons

Submarines

65

97,900 tons

Others

156

490,384 tons

Total

1,466,177 tons

Ships under construction

Battleships

2

128,000 tons

Aircraft Carriers

4

77,860 tons

Light Cruisers

4

42,700 tons

Destroyers

12

27,120 tons

Submarines

29

42,554 tons

Others

37

57,225 tons

Total

88

375,459 tons

Requisitioned Vessels

By the middle of 1940, the vessels requisitioned for the purpose, of continuing the China Incident amounted to 200,000 tons. At the end of 1940 it was decided to requisition an additional 1,650,000 ship tons and to continue to convert these vessels until the outbreak of war. At the time of the outbreak of war, the state of these vessels was:

Completely equipped converted vessels

522

approximately 1,150,000 tons

Converted vessels being equipped

41

approximately 170,000 tons

Vessels to be converted or equipped

180

approximately 290,000 tons

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Requisitioned vessels which would not be converted

approximately 200,000 tons

Total

approximately 1,810,000 tons

Air Strength

Details of air strength at the time of the outbreak of war are shown on Chart 6. Naval Personnel

The number of naval personnel at the time of the outbreak of war was as shown hereunder:

Active Service

Recalled

Reserve

Total

Officers

10,000

506

1,338

11,844

Special Duty Officers

2,997

775

3,772

Warrant Officers

5,262

3,415

8,677

Petty Officers and men

238,235

60,312

298,547

Total

65,008

1,338

322,840

The number of officers, according to their rank and branch of service, is shown on Chart 7.

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Chart 6 Air Strength at time of Outbreak of War

Details

Operational Air Group

Training Air Group

Shipborne

Total

Outer Combat Force

Inner Combat Force

O*

R*

O

R

O

R

O

R

O

R

Fighters

Interceptors

180

51

36

12

50

31

149

10

415

104

Night fighters

Carrier bombers

9

5

36

12

38

19

135

3

218

39

Carrier attack planes

6

2

42

14

165

189

9

402

108

Land reconnaissance planes

18

6

18

6

Land bombers

Land attack planes

255

84

80

20

4

2

339

106

Patrol planes

Transports

38

38

Fighter

2-seater reconnaissance seaplanes

10

4

28

14

24

12

99

12

161

41

3-seater reconnaissance seaplanes

16

7

28

16

40

20

74

11

158

54

Submarine reconnaissance planes

Flying boats

36

12

13

5

49

17

Land trainers

286

286

286

286

Sea plane trainers

94

94

94

94

Utility

84

84

84

84

Total

568

171

263

785

631

646

45

2262

940

O - Operational R - Reserve

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Chart 7 Roster of Naval Officers at Time of Outbreak of War

LINE

ENGINEER

PAYMASTER

MEDICAL

PHARMACY

SHIPBUILDING

ENGINE MANUFACTURING

ORDNANCE MANUFACTURING

TOTAL

TOTAL

Active

Recall

Reserve

Active

Recall

Reserve

Active

Recall

Active

Recall

Active

Active

Active

Active

Active

Recall

Reserve

Admiral

11

11

11

Vice-Admiral

62

4

3

3

2

1

1

72

4

76

Rear-Admiral

151

21

16

18

5

1

8

199

21

220

Captain

492

232

185

11

84

12

73

15

13

49

911

255

1,166

Commander

695

26

316

2

112

2

90

4

22

13

35

1,287

30

1,317

Lt Commander

720

33

12

338

4

112

2

208

27

7

19

13

52

1,469

66

12

1,547

Lieutenant

887

8

275

338

1

77

263

2

382

112

14

38

36

170

2,126

125

352

2,603

Lt (Jr Grade)

455

1

461

144

115

427

468

29

55

122

288

1,988

7

576

2,571

Ensign

636

240

157

158

62

45

8

14

49

244

1,215

398

1,613

Midshipman

440

96

47

6

30

101

720

720

Total

4,549

325

988

1,574

18

350

1,126

18

1,287

145

62

176

278

948

10,000

506

1,338

11,844

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Chapter II

Outline of the fifth and Sixth Naval Armament Replenishment Plans

The Fifth Naval Armament Replenishment Plan was deliberated with the object of counteracting the Third Vinson Plan, but, in consideration of the huge scale of the Third Vinson Plan, and the fact that the United States had been adopting supplementary plans each year, calling for considerable strength, it was impossible for Japan, from the standpoint of her economic and industrial power, to establish a plan to counteract these plans. The Japanese Navy studied the situation carefully during 1940, and finally an understanding was reached between the Navy Ministry and the Navy General Staff around May 1941, to execute a plan starting from simple and prepared armament. At the same time, it was decided that, in regard to the budget, formal steps would be taken later. Details of the strength of vessels and air armament included in armament preparations requested in a preliminary discussion between the Navy Ministry and the Navy General Staff are shown in Chart 8.

The Navy Ministry studied these requests and decided that, in view of the budget, the materials on hand and the production facilities available, the plan for ship construction should be a 9-year plan starting in 1942 and ending in 1950. The necessary expenditure was estimated at 4,400,000,000 (4-$1). Although it was difficult to estimate the completion date of the air armament program, it was believed that it would require a shorter period

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Chart 8 Fifth Armament Replenishment Plan

Vessel strength requested

Battleships (Yamato-Class)

3

Super cruisers (new Type)

2

Aircraft carriers (Taiho Class)

3

Seaplane tenders

2

Flying boat tender (Large)

1

Flying boat tenders (Small)

6

Cruisers (Medium)

5

Cruisers (Light)

4

Submarine tenders

2

Minelayers

2

Netlayer

1

Destroyers (Class A)

16

Destroyers (Class B)

16

Submarines (Class A)

2

Submarines (Class B)

12

Submarines (Medium)

9

Submarines (Small)

9

Submarines (Navy Type large)

10

Submarines (Supply)

3

Gunboat (Large)

1

Gunboats (Small)

2

Escort ships (Class A)

4

Minesweepers

10

Subchasers

18

Light minelayers

8

Naval auxiliaries (Transport)

3

Naval auxiliary (Survey)

1

Naval auxiliary (Provision)

1

Naval auxiliary (Icebreaker)

1

Naval auxiliaries (Repair)

2

Total

159

650,000 tons

--42--

Chart 8 - Air strength requested (cont'd) Operational Air Group

Type of aircraft

Number of Units

Number of aircraft

Land fighters

14

336

Fighter-bombers

10

240

Land reconnaissance planes

2

48

Fighter seaplanes

12

288

Bomber seaplanes

2.5

40

Flying boats (Medium)

10

120

Flying boats (Large)

8

96

Patrol planes

6.5

104

Transports

2

48

Total

67 units

1,320

The grand total at the time of completion was 132 units.

Fifth Armament Replenishment Plan (Cont'd) Training Air Group

Type of aircraft

Number of Units

Number of aircraft

Land trainers

10

240

Seaplane trainers

9.5

228

Utility trainers

21.5

516

Land combat planes (Small)

22.5

608

Combat seaplanes (Small)

11

198

Land combat planes (Medium)

10.5

252

Combat seaplanes (Medium)

8

96

Total

93

2,138

The grand total at the time of completion was 156 units.

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of time than the ship program. The estimated amount required for the air armament program was 2,000,000,000.

As a tremendous amount of expansion would be required in both land and sea installations to cope with this program, 1,900, 000,000 was set aside for installations for increasing production capacity and 500,000,000 for training installations.

In order to achieve this objective, it was necessary for naval armament preparations to receive priorities in regard to securing personnel and materials, and the Army was asked to make concessions in this regard.

In accordance with the above, the Navy Ministry decided to convert two of the three Taiho-class aircraft carriers into Hiryu-class vessels and to support the over-all plan of the Navy General Staff.

In July 1941, just when various plans were gradually taking shape after the basic plan of the Fifth Armament Replenishment Plan was informally decided, the United States suddenly announced the Stark Plan, with its astronomical naval armament plans. This clearly demonstrated the United States' determination to dominate the seas. The plan was a great shock to the Japanese Navy and a quick counter-plan was sought. The Navy Ministry and the Navy General Staff conferred in regard to the over-all adjustment of the Fifth and Sixth Armament Replenishment Plans, and, toward the end of October, the Navy General Staff formally presented the Fifth and Sixth Armament Replenishment Plans simultaneously to the Navy Ministry. The Navy Ministry replied that utmost efforts would be

-44-

made for the execution of both plans in accordance with the decision to declare war, although nothing definite could be said about the Sixth Armament Replenishment Plan. In the meantime, air armament and the completion of 32 submarines of the Fifth Replenishment Plan would be undertaken, while other armament preparations contained in the same plan would be conducted after the budget of the following fiscal year was approved.

Immediately after the outbreak of the Pacific War, successive war preparation projects were undertaken. Japan was faced with a basic revision of the ratio between air and sea strength. Equipment of large ships called for in the ship plan was suspended and efforts were concentrated on small vessels and local strength. The Fifth Armament Replenishment Plan was first changed to the Revised Fifth Armament Replenishment Plan and later to the Third Stage Revised Armament Production Plan. The details of this revision are given in Monograph No 169, "Situation from Outbreak of War to Revised Fifth Armament Naval Armament Replenishment Plan."

The strength requested by the Navy General Staff of the Navy Ministry for the Sixth Armament Replenishment Plan was:

Ship strength: 197 vessels totaling approximately 800,000 tons, including 4 battleships, 4 super cruisers, 3 aircraft carriers, 12 cruisers, 34 destroyers and 67 submarines.

Air strength: 68 operational air groups (making a total of 200 groups after completion.)

Despite the fact that nothing definite was established in regard to such matters as the budget, because details of strength

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had not been decided, the Navy Ministry acknowledged the Sixth Armament Replenishment Plan. When war broke out, however, through battle experiences, a complete revision of plans for shipbuilding was brought about and the Sixth Armament Replenishment Plan ended as a mere paper plan.

As a condition of going to war, the Navy requested 1,450,000 tons of steel (including the priority allotment of secondary materials) and received an allotment of 1,100,000 tons as compared to the Army's 700,000 tons. This was tantamount to national priority being granted to naval armament preparation, and greatly contributed to the acceleration of naval armament in preparation for war.

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