The were a

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

of twenty ''

kaibōkan or coastal defense ship was a type of naval ship used by the Imperial Japanese Navy during World War II for escort duty and coastal defense. The term escort ship was used by the United States Navy to describe this category of Japanese ships. ...

'' escort vessels built for the

Imperial Japanese Navy The Imperial Japanese Navy (IJN; Kyūjitai: Shinjitai: ' 'Navy of the Greater Japanese Empire', or ''Nippon Kaigun'', 'Japanese Navy') was the navy of the Empire of Japan from 1868 to 1945, when it was dissolved following Japan's surrend ...

during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...

.Worth p. 208 The class was also referred to by internal Japanese documents as the , and they were the fourth class of ''

''.

Background

The escort ship was developed after the start of the Pacific War, it became apparent that a design more capable of

anti-submarine warfare Anti-submarine warfare (ASW, or in older form A/S) is a branch of underwater warfare that uses surface warships, aircraft, submarines, or other platforms, to find, track, and deter, damage, or destroy enemy submarines. Such operations are t ...

than the previous and ''kaibōkan'' was needed. Despite being a simplified design, the ''Mikura''-class vessels still took too long to construct, and due to the high attrition of Japan's destroyer and escort ships, action needed to be urgently taken to produce more ships in a quicker time. Furthermore, operational experience had shown that the ''Mikura''-class was still very weak in its anti-aircraft capability. The first five of the new ''Ukuru''-class were authorized under the 1941 Rapid Naval Armaments Supplement Programme and an additional six in the 1942 Modified 5th Naval Armaments Supplement Programme and nine under the 1944 Wartime Naval Armaments Supplement Programme. Twenty vessels were commissioned; two more (''Urumi'' and ''Murotsu'') were launched by Uraga Dock and completed in August 1945 but were still uncommissioned by the war’s end. In addition to these, nine units and two additional unfinished ships belong to a sub-class called the ''Hiburi'' class and are included in the table below.

Description

The ''Ukuru''-class was a further simplification of the ''Mikura'' design. The hull was constructed using prefabricated sections which avoided the use of shaped steel or curved plates, which greatly reduced construction time. The curved plates on the bridge were also eliminated, and the smoke stacks were made of hexagonal elements instead of with a circular or oval cross-section. Internally, individual crew quarters were eliminated, becoming a communal area, and overall the construction was very spartan. These changes reduced construction time to under four months, although construction was often hindered by the lack of diesel engines. The

main battery A main battery is the primary weapon or group of weapons around which a warship is designed. As such, a main battery was historically a gun or group of guns, as in the broadsides of cannon on a ship of the line. Later, this came to be turreted ...

was the same as on the ''Mikura''-class, with three dual-purpose Type 10 120 mm AA guns one forward, and a twin mount aft, but the later ships in the class were fitted with modified gun shields. Anti-aircraft protection was by five triple-mount Type 96 anti-aircraft guns with two abreast the bridge, one of each side of the smokestack, and one aft on the deck house, along with a single-mount in front of the bridge. Some units received additional single-mount Type 96s, which were located on the forecastle. The ''Ukuru''-class was equipped with the Type 22 and Type 13 radar. The ''Ukuru'' class was initially armed with 120 depth charges with two Type 94 depth charge projectors, sixteen Type 3 depth charge throwers and two depth charge chutes at the stern. The ships were provided with a Model 93 sonar and a Type 93 hydrophone; later units received the Type 3 Model 2 sonar, and some would later receive an

trench mortar ''Minenwerfer'' ("mine launcher" or "mine thrower") is the German name for a class of short range mine shell launching mortars used extensively during the First World War by the Imperial German Army. The weapons were intended to be used by engin ...

. Initially, the class retained capacity as a minesweeper, and was equipped with two paravanes; however, this was removed soon after completion.

Operational service

Despite being easy to build, they proved quite durable, with 11 occurrences of the class striking mines and only 3 sinking, one of which was after the war. ''Ikuna'' survived being torpedoed by and striking a mine as well. The ''Ukuru'' vessels were used extensively on convoy escort assignment in the

South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by the shores of South China (hence the name), in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Phi ...

and East China Sea, where they frequently were attacked by Allied submarines or aircraft. However, despite their durability, they proved to be relatively ineffective against Allied submarines. ''Okinawa'' was the most successful ship of the class, helping to sink two US submarines, on April 14, 1945 with the ''kaibōkan'' ''CD-8'', ''CD-32'', and ''CD-52''; and on June 19, 1945 with ''kaibokan'' ''CD-63'', ''CD-75'', ''CD-158'', and ''CD-207''. Surviving ships were used in the immediate postwar period as minesweepers and for repatriation. Five vessels survived to return to Japanese control, and were used as weather survey ships or as patrol ships, with the last being retired in 1966.

Ships

Twelve other ships were cancelled in 1945 - numbers #4706, #4708, #4710, #4713 to #4721. These included ''Murotsu'' and ''Urumi'' (both launched but incomplete); also cancelled (unstarted) were #5261, #5262, #5267 to #5284 (all of the ''Yaku'' group) from the Modified 5th Naval Armaments Supplement Programme. Also cancelled incomplete were two of the ''Hiburi'' class - numbers #5265 (''Ōtsu'') and #5266 (''Tomoshiri'').

Notes

References