Musing on Japanese Knee Mortars in Crossfire

In World War II the Japanese issued three 5cm Mortars to every Rifle Platoon. Standard Crossfire makes the Knee Mortar rather useless. John Moher reinstates them as an effective piece of combat kit. And I do the same, but in a much simpler fashion.


5cm Mortar = Knee Mortar

Technically speaking the Japanese 5cm Mortar was a grenade discharger. And there were two types: Type 10 grenade discharger (smoothbore) or Type 89 grenade discharger (rifled barrel and options of special ammunition).

Japanese 5cm Type 89 grenade discharger - Knee Mortar

Japanese 5cm Type 89 grenade discharger – Knee Mortar

The Americans called both types a “Knee Mortar”.

Due to a translation error, Type 10 was called the “knee mortar” by the Americans. The manual for the mortar instructed the troops to carry the mortar on the upper thigh, with the base plate attached to the belt and the barrel running down the thigh. It must be understood that it was not strapped or secured directly to the thigh, but hung from the belt. It was also carried strapped to the backpack. American troops on Guadalcanal became aware of the name “knee mortar” and thought the light design allowed it to be fired with the base plate resting on the thigh. (Wikipedia: Type 10 grenade discharger)


Why so many Knee Mortars?

The Japanese issued three 5cm Mortars to every Rifle Platoon. That is three times the number provided by the Germans, Soviets, British or Americans. The Japanese relied on the 50mm Mortar because they had relatively little indirect fire support coming from the Battalion. The other nations had 81mm (or 82mm or 3″) Mortars at battalion level but the Japanese had to make do with a small number 7cm Type 92 Infantry Guns for fire support. Few battalions had access to the 8cm medium mortar. Instead Japanese battalions had lots of 5cm mortars.


Knee Mortar in Standard Crossfire

The organisation for the Japanese Leg Infantry Battalion (1939-’42) in the Crossfire rule book gives the special rules for the Knee Mortar:

The Knee Mortar uses only 2 Dice with 1 Squad Kill potential; no ammo limit; no Smoke. It may move separately (i.e. it does not require a PC to move). Otherwise, it fights crew-served (-2). Only the appropriate PC function as its FO in addition to his other functions. (CF, p.24)

The standard rules in Crossfire make Knee Mortars pretty ineffective. 2D6 in open and 1D6 into cover. No smoke. The Japanese might get a lot of them but they are more or less useless.


John Moher’s house rules for the Knee Mortar

In his post Modelling Japanese in Crossfire, John Moher radically alters the way to simulate Knee Mortars in Crossfire. John knows his stuff and I think these suggestions are a big improvement.

John aligns the indirect fire capability of the Knee Mortar with other light mortars (50-60mm, 2″). So 3d6 into open and 2d6 into cover. Can fire smoke. Limited FM (or similar constraint) rather than unlimited FM. And, given they are on-table, they fire when they have LOS.

  • Japanese Knee Mortars fire with 3D and 0 kp in a ±45º arc as per other Light Mortars (not 2D + 1 kp as in the Rule Book which is effectively pointless since you need 3D for kp to count).
  • Consideration be given to limiting their ammunition either, (a) abstractly by them not being allowed an outright Kill with indirect HE (so treat 3 Hits as a Suppression); or (b) alternately by assigning 4-6 FMs per game (contrary to the Rule Book’s unlimited fire missions) and tracking FM expenditure as normal. I prefer the former (no paperwork and perhaps captures the general performance better – there HE bomb performance was variable and as noted ammunition supply was often limited).
  • Japanese Knee Mortars can fire Smoke (again contrary to the Rule Book), however their smoke was somewhat less effective (and sometimes in short supply) so either, (a) only allow them 1 stand of smoke not the 2 normal Light Mortars get, and/or (b) optionally roll 1D6 pre-game and on a 1-2 no smoke is available, or (c) alternately only allow each stand to fire smoke once each game – but getting a normal 2 stand screen – however this latter requires some tracking so in my opinion possible isn’t ideal. My current preference is just for option (a) on its own.
  • Japanese Knee Mortars must always have LOS to the target, contrary to the rules the Platoon PC may not act as a FO for them.

But John gives Knee Mortars some special abilities similar to Rifle Squads. Direct Fire with rifles with 2d6. Close combat as a Rifle Squad not a heavy weapon

  • As per the Rule Book & Japanese HMGs they can also initiate Close Combat and count as normal Infantry Stands for Close Combat (not Crew Served Weapons as stated in the Rule Book).
  • Additionally Japanese Light Mortar Squads (in 1941-43) should also be permitted to fire direct fire as a reduced Rifle Squad with 2D to represent the relatively large number of accompanying Riflemen as ammo bearers (on paper 13 men manning 3 x 5cm Light Mortars). Note: Once a Battalion has been in combat (or on campaign) for some time (typically 1943, e.g. actively on campaign and involved in multiple battles or on Garrison duty overseas for 2 years plus) this added function should be dropped to represent the stripping of surplus manpower from the Squad as rifleman replacements.

Lastly John gives some rules for how many platoons get Knee Mortars depending on various factors. The official orbat has one Knee Mortar squad per rifle platoon but campaigning took their toll as the rifle squads replaced casualties from the Knee Mortar squad. John suggests throwing 1d6 for each Rifle Platoon:

1 – Light Mortar Squad is removed if unit being modelled had previously been in action in the campaign the scenario is set in.

2-3 – Light Mortar is removed if unit being modelled has been on campaign for some time or has suffered heavy casualties.

4-6 – Light Mortar Squad is always present.

But in the late war John says the depleted Knee Mortar squad would lose its close combat special abilities.

In 1944-45, or when units have been in heavy combat and are depleted (as above), the Light Mortars that are present will likely have minimal manpower, so reinstate treating them as Crew Served Weapons (as per the Rule Book) for Close Combat (however they may still initiate it as per HMGs).


Steven’s Knee Mortar House Rules

I think the whole knee mortar situation can be much simpler. Both how to simulate them and how to add them to an order of battle.

How to simulate knee mortars in Crossfire

I think, on balance, it is much simpler to just treat the Knee Mortar Squad as an on-table light mortar, just like all my other light mortars. In my house rules, on-table light mortars are identical to an FO. Job done.

This ignores John’s suggestions around direct fire with rifles and close combat ability. Both suggestions presuppose that the mortar men think like riflemen although tasked with manning an indirect fire weapon. I suspect this was not the case. Certainly it wasn’t for other nations. Treating the Knee Mortar Squads as an FO leaves them more vulnerable to close combat than even the standard Crossfire rule which treats them as a heavy weapon. That is the draw back of my simple on-table mortar house rule; 13 men fight like two in close combat. Personally I think it is worth it for the simplicity.

How many knee mortars to use in a game

John’s point about reduced numbers of Knee Mortars after hard campaigning is illuminating and sound. For me the number of stands of a particular type is a Crossfire scenario design issue. So I would go a different way to John and remove the random element of John’s proposal for assignment of knee mortars.

I think the strength of the platoon and company should influence the number of knee mortars to assign – this already indicates if the unit is battle worn.

So here are my guidelines for assigning Knee Mortars when preparing the order of battle for a scenario:

  • Assign a Knee Mortar Squad to a Rifle Platoon if the platoon has three Rifle Squads and is considered fresh
  • Do not assign a Knee Mortar Squad to a Rifle Platoon if either of these apply:
    • The Rifle Platoon has one or two Rifle Squads
    • The Rifle Platoon has three Rifle Squads but the platoon has seen recent and serious campaigning so has had to draw upon the Mortar men to keep the Rifle Squads at full strength
  • A Rifle Company containing two or three Rifle Platoons without a Knee Mortar Squad (because of the above) gets a Knee Mortar Squad at Company level

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