The Type 89 Light Mortar, Type 89 Grenade Discharger, or "Knee Mortar" was a small knee mortar used by Japan during World War II.


It fired 50mm Type 89 Shells and could even be fitted to fire the Type 91 Grenade. The rate of fire for the mortar was about twenty-five rounds per minute.

The Type 91 Grenade was primarily used in the jungle where there was a significant chance that any mortar round would hit something on its way to the target. This is where the seven to eight second delay came into action. The firing pin was also adjustable so the range could be set; the higher the firing pin was set, the lesser the distance the shell would go.[1]

There were no sights for aiming, only a line on the barrel and the effective range was about 120 meters. Due to bad translation, many American soldiers thought the weapons were called the "Knee Mortar". Thusly, there were numerous instances of American soldiers losing or hurting their legs because of this. In reality, the mortar was designed to have its spade like plate propped against a hard surface. The spade-type support at the back of the Type 89 was meant to be angled at about forty-five degrees and sometimes it could be positioned pointing straight against a tree.

Firing from the knee would cause several injuries including broken bones.[2] The empty weight of the mortar was 4.7 kilograms and the length was 61 centimetres. The Type 89 was also capable of being disassembled into three main components and being reassembled at the desired location. There were no variants of the Type 89 and it was made as better version of the earlier Type 10 Mortar.


The Type 89 Light Mortar was developed in 1929, and it was used in many different places, such as in the Pacific Theater. Some of these include China, Burma, and some other Pacific islands. It had a certain psychological effect on Allied troops and some learned to recognize the Type 89 Mortar by its "pop" sound when firing.[3] The effectiveness was mixed, but overall it was fairly good.



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