Desperate War of Japan in the Pacific
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 1. Escort warships for merchant ships
Escort warships for convoy

Torpedo boat8unknown
Escort aircraft carrier01
Destroyer escort0103
Jpan had to send back important resources, oil and bauxite from Indo china. The sea lane was more than 2,500nm distance. While distance between Qubec and Liverpool was 2,634km (1,420nm). USN had 280 warships to escort merchant ships. IJN had only 55 warships in June 1941.
Morimoto, p110-111

The table shows escort forces of IJN and USN for merchant ships at the Pacific War break[6][7]. The USN data was in June 1941. IJN had 4 destroyer escorts (kaiboukan) which were Shimushushu (), Kunashiri (), Hachijou (Ȭ) and Ishigaki (г). But they were originally for defence of fishery rights in the north. Shimushushu took part in Malay invasion operation[9]. The 4 destoryer escorts were to protect fishery boats in the North Pacific originally. The specifications were displacement 1,020t, speed 19.7kt, range 8,000nm at 16kt, armament 3 guns of 12cm and 12 depth charges. The destroyer escorts removed a paravane and increased 12 depth charges in May 1942. The number of depth charges was 60 in fall 1943.

The other 3 destroyer escorts engaged in defending Tsugaru Strait and escorting convoys for Chishima islands. The 5th destoryer escort was commisioned in March 1943.

Three IJN old destroyers were belonged Kure Naval Base. They were Nokaze (), Namikaze (), Numakaze ()[5]. The escort warships increased to 26[8] on 10 April 1942.

    Morimoto wrote different 55 IJN escort warships from Ooi's 22.
Escort warships in Apr 1942

Torpedo boat22
Aux Gun boat51
The foreces had to defend 2 main sea lanes, 2,500nm of Moji to Singapore and 1,800nm of Yokohama to Truk. Former Navy Minister Yoshida Zengo remarked how we could defend the sea lane against USN submarines.

IJN also had 24 submarine chasers which had 460t of displacement and 36 depth charges. But these were uncapable of running the open sea.

IJN destroyer escorts shipbuilding and loss
The defense forece was parted in 2 groups in April 1942 which had only 24 warships.
Ooi p.95-97

IJN planned to build 30 destroyer escorts in July 1941. But the destroyer escort #1 wast to launch in 1945 at first. The destroyer escort was laid down in 1942 because IJN had reported an optimistic merchant ships loss. But the loss increased in 1943. So IJNGS made a proposal for building 360 destroyer escorts. Itoh reported that 360 destroyer escorts and 2,000 aircraft were necessary to suppress merchant ships loss within 30,000 gross ton a month in a meeting with Government and IJA on September 1943. IJN admitted building them quickly. It planned 114 destroyer escorts in 1943, 188 in 1944 and 130 in 1945. The result was only 175 of 446. It was late to build and Japan had no power to build destroyer escorts enough to protect her own merchant ships after all. Japan failed to ASW completely. While USN built 426 destoryers and destroyer escorts in 1943[2]. IJN built 15 only. IJN thought USN would give up the Pacific War soon, didn't it?
Morimoto, p126
Oui, p111
Kansen, p143, No507

When WWII broke up, RN had 140 warships for escort. RN had built 79 corvettes, 26 sloops, 200 frigates and 88 destroyer escorts since 1941. The total were 393. US gave UK 50 old destroyers and 84 destroyer escorts. RN had 486 desstroyer escorts and 40 escort aircraft carriers in the end of WWII[3].

 2. Destroyer

 Fuel consumption rate
Fuel rate of Asashio class destroyer
Speed [kt] 10151824283034
2nd Range [nm] 5,7004,6234,0002,8042,0441,675 960
3rd Range [nm] 5,7004,3643,5122,1481,5401,310 960
2nd Fuel [nm/t]
3rd Fuel [nm/t]11.48.774.
IJN task force ( kido butai ) accomanied 9 destroyers in Pearl Habor operaion, which left Hitokkapu Bay on 23 November 1941. 7 of 9 destroyers were Kagero class. And the rest were Asashio class[2]. Range of Kagero class was 5,000 nm at 18 kt, otherwise Asashio class was 5,700 nm at 10 kt, 960 nm at 34kt[1]. The displacement of Asashio class was less than 120t compared with Kagero class. As fuel capcity of Asashio class was 500 t, I guess fuel capacity of Kagero class was 620 t. So, Kagero class spent fuel of rate 8.1 nm/t at 18kt.

Drag force is proportional to speed by Newton's law. The drag force is proportional to square of speed considering area effect of the body. I computed fuel comsumption rate of Asashio by 2nd order (square) interpolating function. Considering the work of engine, drag force D is,

    D = kv²
    Dds = kv²ds ( ds is infinitive length )
    dW = Dds
    dW/dt = kv²ds/dt ( dt is infinitive time )
    P = kv³ (dW/dt = P, ds/dt = v)
Power is proportional to cubic of speed. Pitch of propellar is not variable. Power is dependent on revolution of propellar. Fuel consumption may be proportional to the revolution of engine. Capacity of fuel is constant. So range is inverse proportional to cubic of speed. Interporation curve of range is a function of
    Range = k/(v³-h)
The result is as follows. Asashio class may run 4,000 nm at 18kt judging from the fuel capacity. This is shorter than 1,000 nm than Kagero. How much fuel did the destoryers spend? After refuel off 600 nm Hawaii, The KB ran. The KB stayed till 13:00 to receive aircraft[5]. Sunrise and sunset are 6:56 and 17:49 in Honolulu on 7 December[7]. The KB ordered to launch aircraft at 6:00.
    24 kt for 5 hours 120 nm 21 t
    28 kt for 5 hours 140 nm 34 t ( Kaga's max speed )
    ? 110 nm 14 t ( in case of 18 kt )
    26 kt for 5 hours 130 nm
    20 kt for 6 hours 120 nm
It looks very little. The KB might run zigzag. The destroyers always have to adjust output of engine to keep formation of the KB. In fact, some destoryers refueld 200 t in 30 hours depearture from Hitokkapu Bay[3]. The KB ran at 14 kt. The destroyers was to spend at the rate of 2.1 nm/t at 14 kt. 7 Oilers fueld destoryers on 28, 29, 30 November, 1, 2, 5, 6 and 7 December. I do not know how each destroyer refueld. The destroyers was short of fuel, if the incredable rate was true.

Light cruiser Abukuma refueld on 30 November, 1, 2, 5, 6 and 7 December. Why did Abukuma refuel with capacity of 1,260 t seven times in spite of 5,000 nm range at 14 kt? IJN range data of warships were planned values. The displacement of Abukuma increased 1,500 t comparing with the original 5,500 t[4]. The range might decrease 27 % at least. After all, speed of the KB was limited by aircraft carrier Kaga, and freedom of the operation was limited by the fuel capacity of Kagero class destroyers.

 Escort carriers
USN destroyers ratio per carrier
OperaionFleetCarriers DestroyersRatio
OkinawaTG58-1521 4.2
TG58-238 2.7
Ref. Yomitan history, ASW
IJN destroyers ratio per carrier
OperaionFleetCarriers DestroyersRatio
PearlKB69 (7) 1.5
Mariana1Sf313 4.3
Ref. Genda p.274
Only 9 destroyers had escorted 6 aircraft carriers in Pearl Harbor Operation. Two destroyeres Kasumi and Arare were seperated from KB to escort oilers. The both were Asashio class destroyers.

Though 2 battle cruisers, 2 heavy cruisers and 1 light cruiser also escorted, they had no depth charges for ASW. IJN thought that USN submarines were not threat because of unknown operation, didn't it? The table shows ratio of destroyers per carrier. Some historians criticizes that the KB should do the third air strike. Could only 7 destroyers shield 6 aircraft carriers effective?

 The biter is sometimes bit
CNO King of USN ordered submarines to sink destroyers than merchant ships at first on Apr 1944. IJN lost 11 destroyers, 4 destroyer escorts and 3 other escorts because of enemy submarines from April to Jun 15th.

IJN selected Tawi-tawi as anchorage port of 1st Kido Fleet. Saipan is nearer to Japan than Taui. The reasons were,

  • Gunreibu guessed USN would invade Palau at first
  • Shortage of oilers was to anchor near oil export

But Tawi-tawi was enroute that USN submarines ran from Freemantle to the South China Sea. And worse Tawi-tawi and Japan was the same distance from Kwajalein.

1st Kido Fleet came to Tawi-tawi on May 13th. It was necessary for pilots of carriers to train landing everyday etc. Carriers could not run to train pilots out of the port freely, becasue submarines were hiding. So 1st fleet ordered destroyers to sweep submarines. 4 destroyer were sunk one by one for 4 days. These were Minazuki, Hayanami, Kazakumo and Tanikaze. Ooi p.238-241

Sonars of the destroyers were worse than US submarines. I do not know how each destroyer repeated failure of anti submarine combat. The below is image of the Tawi-tawi Bay

 Depth charge
The table shows 3 types of discharges which IJN developed in 1937 to 1943.
Specifications of IJN depth charges
TypeYearDecending speedSet of Depth [m]ExplosiveWeightLengthDiameter
Type 9519371.9m/s30, 60100kg160kg775mm450mm
Type 219422.0m/s30, 60, 90, 120, 150100kg162kg775mm450mm
Type 319435.0m/s40, 80, 120, 160, 200100kg180kg1400mm450mm

The production of depth charges was as follows. Strangely production of 1943 decreased. Was production of Type 3 depth charge in 1944 actually?
Production of IJN Depth charges
Sekai no kansen, No.507
Gunzo, p5-11, No74
Bakurai ichiran
Goei heiki

3 US destroyers were sunk by a storm off the west of Philippines on December 18th 1944. The above diagram shows a loci of the fleet TF38. The loci is a flag warship, USS New Jersey.
Weather, p
Typhoon Cobra(1944)

RN took a note to shipbuild small warships for ASW on February 7th 1939. RN orderd development of a modified catcher boat on 24th. The first ship was launched on April 6th 1940. After all, UK and Canada shipbuilt 143 and 122 each. The diagram shows the layout of the Flower class. A sonar called ASDIC 123 equipped at first. The ASDIC was installed in a section shown encircled number 6. While destroyer escorts of IJN were not equipped with a sonar in the Pacific War.
Gunzo, p22-27 No94

[1] Jentschura p.147
[2] Koei p.20
[3] Sensou to sekiyu (4)
Nagara gata no kindaika
[5] Shinjyu-wan kougeki
[6] Sekai no hinode-hinoiri

 3. Warship parity
Warship balance of IJN vs USN
numbertonnumber ton
1941Dec08237 1,001,0003451,439,000
Dec10236 1,000,0003411,313,000
1942May31235 1,100,0003681,471,000
Jun07230 1,004,0003661,449,000
Jul31232 1,030,0003931,595,000
1943Feb08212 1,007,0004571,810,000
1944Jan08208 996,0006612,850,000
May31186 982,0007343,188,000
Jun21182 902,0007343,188,000
Sep31165 879,0007913,522,000
IJN had seeked sea power parity 70% of USN. However, the parity 70% had continued till May 1942 only. The parity fell down 25% in the end. The table of sea power balance of IJN vs USN does not include small warships for example, marine chaser and torpedo boat.
Morimoto p.77

Warships of IJN increased a few 9.9% at peak weight ton in 1942. While USN increased 145% since breaking the war! IJN warship power peak was 1,100,000t on 31 May 1942. USN warships increased more than 1,000,000t from 8 February 1943 to 8 January 1944 which was equivalent to all the IJN warships.

IJN armament
Japan declared denunciation of London Naval Treaty in January 1936. It meant Japanese Government would compete naval armament with US and UK. The table, Tbl-106 shows all the warships that IJN built from 1938 to 1945. IJN shipbuilt the peak warships of 468,404t in 1944. Otherwise USN was the peak shipbuilding in 1943, as I showed as above. Why was Japan late to shipbuild warships than US, though many books wrote tha Japan prepared the war? Government discussed shares of steel between IJN and IJA in a meeting on 1 November 1941. In the meeting, IJN demanded 1.35 million ton in 1941, 1.45 million in 1942 and 1.20 million in 1943. IJN requested 300 k ton of IJA share in 1941 and 1942.[10] IJN endured decreased steel in 1943. This meant that IJN would end the war in 1944 at first. Even Germany did not declare war on France and Russia. Japan had spent much money and steel in China. IJN was afraid of decrease of navy budget. If IJN refuse the war, IJA would get much budget and steel, and begin a new war against Russia at the same time. IJN agreed with the war because of fear of decreased budget. This is true how bureaucrats make to get budget whether the budget would effective or not for Japan's future.

Japanese military or economic bureaucratic leaders had thought Japan could begin a new war against Russia or US and UK without cease fire against China at the same time in 1941. How did Japan bring up such leaders?

IJN building plan
DatePlan noVesselsDisplacements
1933Dec248137 kton
1936Dec366270 kton
1938Dec480321 kton
If IJN built destroyer escorts instead of 2 battle ships (128,000t), 7 cruisers (48,900t) and sucide boats (10,508t). IJN might have additional 250 destroyer escorts of 745t. It would be required number nearly. In order to do so, IJN had to give up that Yamato's keel was laid down on 4 November 1937. In fact, IJN sea power could not defend an oil lane from Java while defending Truk, Rabaul and Palau.

IJN did not accounted properly for the battleship to Imperial Diet. It approved of building two battleships of 35,000 ton in December 1936. So USN built more economical and faster Iowa class battleships than Yamato class ironically.

USN building plan
1934Mar17Vinson-Trammell Act94330
1938May17Naval Expansion Act72650
1940Jul19Two-Ocean Navy Act25715,000
US Congress increased USN budget $900 million from $644 million in 1939. The Congress passed Two-Ocean Navy Act on 10 July 1940. US decided to prepare war against Germany and Japan at the same time. How did IJN think about the act? I have not read any comments from IJN bureaucrats. Ambassdor Nomura had informed of the USN armament plan on 8 May. The act planned 15,000 aircraft from 650 (3,000). Although USN had thought conventional armament in 1938, aircraft was main sea power in 1940 plan. What happened in USN doctroine?

[1] Ouchi, p105
[2] Kansen Seizou-ryou ni miru Nichibei-sa
[3] Ouchi, p106
[4] Ouchi, p248
[5] Minekaze gata
[6] Oui, p102
[7] Oui, p96
[8] Oui, p98
[9] Shimushu-gata Kaibou-kan
[10] wo meburu Φ no tunahiki

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