Hōkoku-Maru class Auxiliary Cruiser, Hōkoku-Maru

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報国丸型特設巡洋艦 報国丸

Hōkoku-Maru class Auxiliary Cruiser, Hōkoku-Maru

Hokoku-Maru at Seletar naval base, Singapore, September 1942


Hōkoku-Maru class Cargo Liner (報国丸型貨客船) was high performance merchant ship owned by O.S.K. Line (大阪商船, Ōsaka Shōsen Kaisha / Now “Mitsui O.S.K. Line”) for ocean line from Japan to South Africa.

OSK line build 3 ship of this class: Hōkoku-Maru, Aikoku-Maru and Gokoku-Maru (in early: Kōkoku-Maru). In before OSK line use

They build by “優秀船舶建造助成施設”(Excellent Ship Construction Subsidy). This is preparation for warfare that get high performance merchant ship able to use for military uses. In others, Nitta-Maru class cargo liners and Argentine-Maru class cargo liners (both modified to Taiyō class Aircraft Carrier) and Kawasaki-type tankers (also known Tōa-Maru class, diverted to fleet oiler) are well known.

Three Type A cargo ships, three Hōkoku-Maru class cargo liners, two Kiyosumi-Maru class cargo ships, two Bankoku-Maru class etc are modified to Auxiliary Cruiser (特設巡洋艦) for commerce raiding, escort convoy and patrol.

The Hōkoku-Maru was build by Tama shipyards. Hōkoku (報国) means “Patriotism / For the Country”. Contract signed in 1937 but by the start of Second Sino-Japanese war, project was delayed. Also for the Tama shipyards it was first try to build huge merchant ship so it was very hard challenge for them. At first all vessels planned to get maiden voyage in 1939 but only Hōkoku-Maru launched in 1939. She completed June 15, 1939 but by the times cruise to South-Africa was only once and after that used for line to Dalian. (Government order to them stay good ship around mainland of Japan.) But she was lucky: her sisters couldn’t do commercial voyage. Navy requisition Aikoku-Maru in next day of complement, and they requisition Gokoku-Maru before she launched.

Navy requisition Hōkoku-Maru in August 29, 1941 and modified to auxiliary cruiser at Mitsubishi Nagasaki shipyards. In this modification they equip catapult for seaplanes, Armstrong 15.2 cm naval guns taken from warships before WW1, 13.2 mm machine guns and two 533 mm twin torpedo tubes. In October 15 Hōkoku-Maru and Aikoku-Maru consist 24th Squadron for commerce riding ato middle area of Southern Pacific. In December 13 they fin American merchant ship Vincent. They sunk she and capture 38 crews but Vincent send SOS so they change seeking area. In January 1 she lost a recon seaplane but in next day other seaplane find American merchant ship Malama. Recon seaplane bomb Malama and squadron sunk it and capture crews. In February 4 returned to Truck base. After that returned to Japan for next operation. In March 24th squadron disassembled and they belong to 6th Fleet 8th Submarine Squadron to support commerce raiding at Indian Ocean. Also in that time they replace main gun from 15.2 cm to 14 cm 3rd Year Type and add equipment to supply fuels and torpedoes to submarines at Kure.

In April 12, 1942 Hōkoku-Maru and Aikoku-Maru departed Kure to Penang and start commerce riding. During the operation they capture or sunk some allied merchants and supply to submarines. After that operation they used for transportation between Singapore and Rabaul. In September, Hōkoku-Maru and Aikoku-Maru were modified. In this modification they cut mast about 7 meter and add narrow pillars on 2nd and 4th twin post for antenna. Also they request to add dummy funnel. It made with canvas that able to fold.

In November 1 they re start commerce riding at Indian ocean. in November 11 they find Dutch tanker Ondina and Indian minesweeper Bengal. Bengal was equip only one 75 mm single gun and machine guns but cover Ondina to defend. Also Ondina firing its 102 mm single gun while running away. Some 102 mm shell hit Hōkoku-Maru and rear part burned. Fire spread a minutes and torpedoes and seaplanes. They try to dispose torpedoes but fire spread torpedo storage for submarines. Explosion of torpedoes, at last she sunk. After minutes Aikoku-Maru arrived but she can do only rescue crews and drive enemy away.

Ondina was disposed by the burning, but crews that evacuated discover she and restored it able to return to port.


  • Designer: Watsuji Haruki (Chief of engineering department of OSK line)
  • Builder: Tama shipyards
  • Laid down: August 18, 1938
  • Launched: July 5, 1939
  • Completed: June 15, 1939
  • Requisition: August 29, 1941
  • Sunk: November 11, 1942


  • Gross-Tonnage: 10,439 t
  • Length overall: 160.8 m
  • Length between perpendicular: 152.25 m
  • Width: 20.20 m
  • Depth: 12.40 m
  • Draft: 8.8 m
  • Engine: 2 Diesel
  • Propeller: 2 shaft
  • Max Power: 19,427 BHP
  • Max speed: 21.148 kt

Armament (early)

  • 8x 40 Caliber Armstrong 15.2 cm Naval Guns
    • Placement will same as later
  • 2x Type 93 13.2 mm twin AA machine guns
    • Top of the bridge
  • 2x 6th Year Type 53.3 cm twin torpedo tubes
    • Between 3rd and 4th mast
  • 2x Type 94 Recon Seaplane (Kawanishi E7K “Alf”)
    • Rear deck

Armament (late)

  • 8x 3rd Year Type 14 cm Naval Guns
    • Bow mountings: 1
    • Both side of top of 2nd bulk: 2
    • Both side of shelter deck of front of bridge: 2
    • Both side of shelter deck on 4th bulk: 2
    • Stern mountings: 1
  • 2x Type 96 25 mm twin AA machine guns
    • Top of the bridge
  • 2x 6th Year Type 53,3 cm twin torpedo tubes
    • Between 3rd and 4th mast
  • 2x Type 0 Recon Seaplane (Aichi E13A “Jake”)
    • Rear deck


In old researches, this photo labeled “Aikoku-Maru”

In old researches, this photo labeled “Hokoku-Maru”

Hokoku-Maru at Penang island, May 1942

US drawing

Pros & Cons


  • Unique Design!
  • Good armaments: able to fire 5 main guns to side of ship
  • Seaplane!


  • Large hull is good target for enemy
  • Less anti-aircraft armaments
  • No armor


Fukui Shizuo “Stories of Japanese Auxiliary Vessels/ Collection of works of Fukui Shizuo vol.11” Kojinsha, pp.46-62

ONI 208-J “Japanese Merchant Ships Recognition Manual”


1 Like

Wow some decent armament for this type of ship