Stories and Battle Histories of the IJN's Converted Netlayers

9 April 2019

By Berend van der Wal, Gilbert Casse and Peter Cundall

with Bob Hackett

Discussion & Questions

(KATSURA MARU under air attack 31 January 1944)

Prior to and during the Pacific War, the Imperial Japanese Navy enlisted some of Japan's merchant fleet's cargo, passenger-cargo ships and liners. They were converted to military use and some became Tokusetsu Kyusetsumokan, (Converted Net Layers). The main task of a net layer, also known as a net laying ship, net tender, gate ship or boom defence vessel is to lay and maintain steel anti-torpedo or anti-submarine nets. Nets can be laid around an individual ship at anchor, or around harbors or other anchorages. Between 1937 and 1945 fifty three merchant ships were used as Tokusetsu Kyusetsumokan by the IJN. Some were later released to their owners or reconverted to perform other military tasks.

The IJN divided its auxiliary net layers into three groups based on their allocated tasks. These were: Tokusetsu Kyusetsumokan, Tokusetsu Hokakumotei and Tokusetsu Bosemmotei.

Tokusetsu Kyusetsumokan were used to patrol and to lay anti-submarine nets capable of fast elongation. They were also used as motherships for auxiliary submarine chasers. Two merchants were converted to Kyusetsumokan.

Tokusetsu Hokakumotei were used to lay indicator nets, to patrol and to attack submarines. Forty four merchant ships were converted to Tokusetsu Hokakumotei.

Tokusetsu Bosemmotei were used to lay and to maintain anti-submarine nets with mines, to patrol and to attack submarines. Seven merchant ships were converted to Tokusetsu Bosemmotei.

All were also used to escort individual ships and convoys from one destination to another and to escort and to pilot individual ships and convoys into a harbor or out of a harbor.

Other ships were also requisitioned by the IJN but not enlisted. They were called Ippan Choyosen (General Requisitioned Ships) and manned by civilian crews. IJA and the civilian shipping administration requisitioned many other ships.

To summarize, during the war employment of ex-merchant ships was divided into three main administrative categories:
A = requisitioned and operated by the Japanese Army and named Rikugun Yusosen (IJA Transports). They include Cargos, Passenger-Cargos, Liners and Tankers.

B = requisitioned and operated by the Japanese Navy with civilian crew (captain often being Navy Reserve). These are named Ippan Choyosen.

C = controlled and operated by the Senpaku UnŽeikai (Civilian Shipping Administration).

X = requisitioned by the IJN and converted into armed Naval Auxiliaries with IJN crews (gunboats, minesweepers, etc.) and named Tokusetsu Kansen followed by another suffix for the ship’s use.

Beside the above categories, there were many shared employments Army/Civilian (A/C) and Navy/Civilian (B/C). These ships were respectively called Rikugun Haitosen and Kaigun Haitosen. Normally, this meant that the vessel on its outbound journey was under Army or Navy control carrying troops, military supplies, etc. and on its return voyage was carrying cargo for the benefit of the civilian owner of ship. While used by the Army, the Haitosen also received Army transport numbers like Army requisitioned vessels (A). Employments by Army or Navy could also be expanded, meaning that the ship was used by Army/Navy for one full turn (outward and homeward voyage) or even a series of voyages.

This page will cover the activities of the fifty three IJN Converted Auxiliary Net Layers.

Tabular Records of Movement (TROMs):
IJN Converted Kyusetsumokan :

(Classes link to specifications summaries)

KOTO MARU No. 2 Class

Sumanoura Maru
(posted 1 February 2018)


Seian Maru
(revised 3 July 2013)

Tabular Records of Movement (TROMs):
IJN Converted Hokakumotei

(Classes link to specifications summaries)


Kiku Maru
(revised 17 April 2017)


Nissho Maru No. 5
(posted 4 April 2015)

TOYO MARU No. 3 Class

Toyo Maru No. 3
(revised 23 September 2018)

HINO MARU No. 2 Class

Hino Maru No. 2
(revised 30 April 2016)

FUKUEI MARU No. 10 Class

Fukuei Maru No. 10
(revised 17 August 2018)

Fukuei Maru No. 15
(posted 5 March 2019)

Kashima Maru
(posted 9 April 2019)


Kashi Maru
(posted 2 April 2018)

Katsura Maru
(posted 1 November 2018)

Kokai Maru
(posted 3 December 2018)

Korei Maru
(posted 9 January 2019)

Shinto Maru No. 2
(posted 3 February 2019)


Kyosei Maru

Tabular Records of Movement (TROMs)
IJN Converted Bosemmotei

(Classes link to specifications summaries)


Agata Maru

(posted 4 October 2018)


Zuisho Maru
(posted 22 August 2018)


Iwato Maru
(posted 1 March 2015)

Bibliography of Sources

About the Authors

Mr. Berend van der Wal got interested in the Japanese Navy through his hobby ship modelling. He researches the Japanese Navy since 1978. He is an IT support engineer and resides in the Netherlands.

Mr. Gilbert Casse is a maritime historian and researcher. Retired from the aerospace industry, and later from managing Le Mans racing cars and drivers, his advocation is ship modelling, maritime photos and paintings. He resides on the SW coast of France.

Mr. Peter Cundall is a maritime historian and researcher who specializes in merchant ships and convoys. He resides in Australia and works in the maritime industry.

Mr. Bob Hackett is a military historian and researcher. Retired from the United States Air Force and later from the aerospace industry, he resides in Florida.

Questions to the authors concerning these TROMs should be posted on the Discussion and Questions board.