Matrix Games Forums

December 10, 1941 Local Tokyo

Multiple rounds of seppuku have occurred as the Emperor invited most of the IJN and IJA senior procurement staff to “join their ancestors” after dismal briefings. Duplicate programs, lack of definition, lack of recent war experience incorporated into program goals, and just rampant ego was all discovered in almost every single area. There are some areas of excellence (A6M2 program), but these are far too few for the Empire to succeed and even these lack follow-on programs.

Herein an example of one of the “better” interview sessions.

SS Design and Production Review:
The Emperor examines the list of classes and boats. “There are almost as many classes as there are boats. Is every single one we build a new class?”
The IJN SS bureau chief (IJN): “Well we learn so much from each one, there is much to correct, and the designs are complicated” as visions of seppuku fill his mind.
Emperor: “OK, so what are our newest and best designs?”
IJN: “K6 a standard attack sub and the B3 a Glen aircraft equipped long range sub. But we also have plans already for a B4 which is the B3 enabled to carry an SSX as well as the aircraft hangar.”
Emperor looking at chart, “Ok, I see. So why do we need a sub that can go around the entire world?”
IJN:”Tokyo to LA is …”
Emperor: “5500 nm, yes. Double that is 11,000 nm. I still do not see 20,000 nm.”
IJN: “Uhhh”.
Emperor: “And we have radar, why do we need a plane?”
IJN: “Uhhh, the plane can perform effective recon”.
Emperor: “Ahh, yes, that is a valid point. Yes, very long range recon could be valuable. And the long range allows for long loiter times, fewer transits. Ok. Still, this plan is a mess and we need more clarity in short order. Tomorrow. Chamberlain will schedule you.

Cables had been previously sent to Europe embassies and axis partners to query direction for efforts with the following, almost immediate, responses:
1. Allied 4E bombers are terrifying in their size, loads, speed, and ability to absorb punishment.
2. Submarine warfare is devastating to merchant traffic
3. Radar is a crucial horizon extender technology
4. Air power makes every surface target vulnerable.

The Emperor had given the ever evolving and hopefully motivated, R&D – Procurement staffs 12 hours to present updated mission profiles and preliminary development programs for each service. These should have addressed traditional roles and these new roles identified in the cables above. However there continued to be disappointment, the weeding through of generals was almost done, now he was working through colonels; Ministers had been disposed of and he was rapidly going through sub/vice ministers. There was still a number of gaping holes in his "leadership" team, how far would he have to go to find what he needed? How very far his country had fallen under the junta in the last 15 years.

PaxMondo wrote: Thu May 29, 2025 11:54 am Banzai! Banzai! Banzai!

Tenno Heika!

PaxMondo wrote: Sun Jul 27, 2025 4:09 am Jiro is a bit shaky, how is he going to respond to the Emperor’s summons?

Why is Jiro so worked up about responding to the Emperor's summons? He should just tell him 'no'. Tell him that he's involved in another project and that now is not a convenient time to drop everything and reply to his summons. Who does this Emperor guy think he is, anyways? The living manifestation of God or something?

Chickenboy wrote: Sun Aug 10, 2025 5:00 pm

PaxMondo wrote: Sun Jul 27, 2025 4:09 am Jiro is a bit shaky, how is he going to respond to the Emperor’s summons?

Why is Jiro so worked up about responding to the Emperor's summons? He should just tell him 'no'. Tell him that he's involved in another project and that now is not a convenient time to drop everything and reply to his summons. Who does this Emperor guy think he is, anyways? The living manifestation of God or something?

That's the spirit!!! SEPPUKU forever!!!!

PaxMondo wrote: Sun Aug 10, 2025 5:31 pm

Chickenboy wrote: Sun Aug 10, 2025 5:00 pm

PaxMondo wrote: Sun Jul 27, 2025 4:09 am Jiro is a bit shaky, how is he going to respond to the Emperor’s summons?

Why is Jiro so worked up about responding to the Emperor's summons? He should just tell him 'no'. Tell him that he's involved in another project and that now is not a convenient time to drop everything and reply to his summons. Who does this Emperor guy think he is, anyways? The living manifestation of God or something?

That's the spirit!!! SEPPUKU forever!!!!

+1

The Emperor's new Imperial Symbol:

10 Dec 41 - Ryoichi Nakagawa (Nakajima Engine Plant, Musashino district of Tokyo) 1913–1998) graduated from Tokyo Imperial University in 1936 and joined Nakajima Aircraft Company in the same year.

Yesterday, Nakagawa-san’s day had started very much like Jiro’s, the Imperial interview. I guess that is the best description of the meeting. He and most of the other Chief Engine designers were there and questioned on a number of topics. Ryoichi, next to last to be questioned, stated simply that induction was the major, across the board, limitation. Specifically 2- stage induction. It was holding up everything, with the Ha-34 for the Ki-49 being the best example of their issues.

30 minutes later he was named Sr. Chief Aeronautical Engine Designer. Mitsubishi, Kawasaki, all of them, the design teams now reported to him. Unlike Jiro though, Nakagawa was fully aware of the entire engine family structure (for one thing there aren’t as many engines as there are aircraft designs) and he is anxious to get things moving forward. He spent the rest of the day organizing his thoughts, or better to say, trying to organize his thoughts.

To start, water cooled engine development is cancelled. There is no debating the merits, it has been decided, and done with a chrysanthemum chop so there is no chance of changing. Fine. This simplifies many things. First was that two of his senior engine designers were now free of their projects: Hiroshi Takeshima and Yoshimasa Tanaka. From Hiro and Kawasaki, he had one poor team each. Hitachi had a good team led by Hiro Odorosima. And then, oh how they must be grinding their teeth, the Mitsubishi teams, 3 of them and all reporting to him. Oh ko how good fortune had shined upon him. 6 good engineering team leads, not counting himself, plus two other teams. Ok, resources look good.

What isn’t good is that there are SO MANY engines being supported. How that occurred is a function of history, but the end result is a mess. Compared to the US aviation which had just 4 major radial engines (Wright 1820 cyclone, PW 1830 Double Wasp, Wright 3350 Dual Cyclone, and PW 4360 Wasp Major), he had more than a dozen to support. A mess. At the first level he has the established 7 point star (Ha-3x ) and with the emerging 9 point star (Ha-4x) engine families. Both are dual row designs as everything started from a PW License in the early 30’s. Then there a large number of single row 7 and 9 point star designs that power older aircraft models that are still in production, many of which date back to a Wright License from the 20’s.

There is no point lamenting the fact that the allies are years ahead, they just introduced a 4-row design (PW 4360 in 1940) AND their designs are 10% (or more ) narrower diameter for the same power. This means that IJ aircraft will have a wider diameter cowling cross section than the allies for the same output which in turn is a higher drag co-efficient. Nothing to do there on his end, the aircraft designers will have to do their magic.

The 7 point star family is quite mature with a range of sizes, the 9 point star engine designs are essentially done and working fine. Sure, we have the normal hot spots to address with the manifolds, but these are all within our capability. However, none of the 9 point designs have been released to production … induction is not complete. Further, only single stage induction is available for the 7 Point family.


Induction
This is where we are ‘sucking gas’. He let out a long exhale, and it is worse than bad. In the last 3 years he has seen zero progress, literally. Worse, the Ha-34 was released with a 2 stage induction that gratuitously ‘runs sometimes’; the army took delivery and placed orders against the promise to get it working which has yet to occur. There are several teams working this and he intends to consolidate them today. That isn’t the real problem though. The issue is both design technology and materials. We need help, and we need it bad. We are currently stuck with single stage mechanical chargers (super-chargers). Sure, we can build them with multiple speeds, but they are still single stage and that has some inherent limits (compressing air heats it up, without the multi-stage intercooler, there is a very real limit to max boost). Our multi-stage designs simply fail, frequently quite spectacularly. Worse, we’re not really sure why . The new engine designs will never meet requirements (+2000 hp) until we get the induction issues fixed.

Fuel Octane
Tied to the above somewhat, but still independent is the fuel issue. Despite whatever rhetoric he hears, and there is a lot of it, Nakagawa refuses to believe that there will ever be enough “AvGas” grade fuel (+120 research octane). This means they need to move to ‘wet gas’ “injection”. Wet gas is straight forward, just not easy. Injection is where it gets a bit tricky and is going to take a lot of testing to get it right; but truthfully he isn’t really that worried about it. Why worry about something now that isn’t a problem until you get the induction issues resolved, Neh?

He can’t wait for his meetings with Jiro the other aircraft designers later this week. Today though, beginning afternoon and continuing until complete, he has meetings planned to review all engine development.

And then, ….

About an hour before his first planned meeting, his entire day changed. 7 large crates were delivered, courtesy of Intelligence Directorate. He is still speechless, the samples, drawings, and more have arrived fresh from Berlin. Over 3 TONS of materials on one unbelievably long range aircraft. The specifics of the flight he doesn’t know, and has been told to not ask. But the bounty …. The metallurgies are all within his capabilities. Drawings, tooling samples, fully assembled working samples, and assemblies for two more. 2 stage super and twin chargers both. What did we trade for this treasure? He rocks back on his heels …

The days meetings original agendas have been scrapped, instead everyone is digging through the treasure chests. By the end of the day, so much is now so much clearer. The senior design engineers have deciphered a lot and they can already see where they had been on the wrong track.

First; materials. They had gone with the wrong steel. They had chosen a steel that was easier to tool, but instead they needed to have gone with a more brittle steel that had better temperature characteristics. There had been some heated debate on this, but Ryoichi had stifled the resistance with a simple statement: those who wished to proceed with the old metallurgies were free to do so, he would organize them at the Hiro design center, their progress would be reported separately, and they could enjoy the responsibility of those results. The “volunteers” for this failed to appear.

Second, and this is the big one: residence time. This is a simple, yet incredibly complex concept with multi-stage induction (and if rumors are true, the Americans have already demonstrated 3 stage induction). It has to do with the amount of time that they air spends between the induction compression stages and the between the final induction stage and the engine cylinder. This is further complicated by the necessity of intercoolers; compressing air heats it up, so you have to cool it back down. Failure to get this correct causes the pressures within the system to start to cycle (bounce) and then ultimately the system fails; generally quite spectacularly. They had known the importance, but they lacked the empirical data to frame a working space. They had some of that data now, more than they had previously had, and with working parts they could take measurements and get more data. Ryoichi knew that there was still going to be some iterative designing for each implementation, but he was also confident that they could now deliver.

They had ended the day with the following design tasks assigned. There were still a number of projects to yet work out, but this was the start:

Design Team Leads:
Hiroshi Takeshima – Nakajima
Yoshimasa Tanaka – Nakajima
Hiro Odorosima - Hitachi
Ichiji Dobashi – Mitsubishi
Sadashichi Doi – Mitsubishi
Ueno Funabashi – Mitsubishi

Projects:
9 star Engines Ha-4x – (Ryoichi Nakagawa:induction, Ichiji Dobashi:engine)
Ha-35 – (Hiroshi Takeshima) + 2S (Hiro Odorosima, Yoshimasa Tanaka)
Ha-33 – (Sadashichi Doi)
Ha-32 – (Ueno Funabashi)
Older – only retro-fits. No new builds – (Hiro Odorosima, Yoshimasa Tanaka)
Ha-31
Kotobuki
Early Hitachi
Early Kawasaki
Hikari

Exact development direction would come after the meeting the Jiro and Kiro. He needed to know what the aircraft designers needed/wanted.

10 Dec 41 - Takijiro Ohnishi, Chief of the Naval Aviation Development Division in the Munitions Ministry

Well, some things become fairly easy with an imperial edict, Ohnishi “Taki” Takijiro mused. And that is essentially what he held. He was to get things, in this case machine guns, standardized across the entire armed forces of Japan and that was to happen ‘soon’. The definition of ‘soon’ was quite apparent from the imperial gardens. Even though he was within the Naval Aviation Ministry, his ‘reach’ had been extended outwards …

Funny how things work out. In this case, he never could have foreseen this and he still didn’t quite believe it. Essentially, HIE had decreed that standardization had to be put in place for all devices. Initially there had been the age old argument between the Army and Navy which had been jointly settled in the garden by all parties. The replacements had been more receptive, but it had taken the Chamberlain to come up with the deciding criteria: Who used the most machine guns? Surprisingly, it had turned out to be the Navy, in particular the Naval Air. It wasn’t that the Navy had more planes, they didn’t. They had FAR more guns per plane though. Further, when looking at heavy machine guns the Army had almost none and the few they had were in limited production. The “one” they had was a water cooled 7.7mm, no other service in the world considered a 7.7mm weapon to be “heavy”. The Navy on the other hand had both a 13mm and a 20mm in volume production, with the 20mm Type 99 in high volume for Japan.

Once the Navy was assigned leadership, he was put into the position of all machine guns. The IJA team had been seconded to him on paper, in practice he really just had his team which was fine. The IJA team would just handle deployment which most consisted of training.

His initial thoughts were fairly straightforward:
• Light machine guns would continue to be the 7.7 mm currently being rolled out. This would transition from universal application to eventually being only used as platoon level support.
• Initial heavy machine gun would be the 13mm Type 2 IJN weapon. Air cooled, flexible mount. Canister feed. This would be used in HMG units and light AFV’s.
• This would be followed by the 20mm Type 99 and the subsequent 20mm MG 151/20 (design and samples coming from Berlin, reputedly before the end of the week). Both of these were flexible mount. These would be used in heavier AFV’s. Currently the Type 99 was canister feed, a belt feed would need to be developed. The MG151/20 would be developed with belt feed.

OK, fine. Now he needed to take this preliminary plan out to all of the other users and see where he ended up in terms of a deployment schedule.

BBfanboy wrote: Sun Aug 10, 2025 7:11 pm The Emperor's new Imperial Symbol:

seppuku.jpg

Yeah, he really know how to "move" things along.


Kinda like ol' King Henry: "Off with his head!!"

Type 2 12 cm mortar is ready for production and should be an organic infantry weapon over 70mm Type 92.

Seacat54 wrote: Wed Aug 13, 2025 3:05 am Type 2 12 cm mortar is ready for production and should be an organic infantry weapon over 70mm Type 92.

Yes, and no.

The Type 2 120mm mortar was an effective weapon, but it had one serious drawback: weight. Both the weapon and the ammo were heavy, neither of which was man portable (reputedly 260kg and each round was another 12kg). The IJA suffered from lack of transport the entire war. Thus, the 120mm was not historically introduced into service until '43, and then only into defensively positioned units with relatively permanent positions. In the early war, where mobility was a crucial factor, the Type 2 simply could not be moved.

This led to them continuing to use the Type 97 81mm mortar almost the entire war. In contrast the Type 97 was man portable, although it took a squad to operate and transport one mortar. Coming in at about 66kg with only 3kg per round, this one could be lugged around the battle field.

The 70mm Type 92 was actually a howitzer, not a mortar, even though it did look like a wheeled mortar with its very short barrel and its effective range being very comparable to a mortar. Again, the reason for its use throughout the war was that it was man portable. Weighing about 200kg, but with wheels, it could be towed by a 2 man team in a harness. Each round was less than 1kg each, so again, weight was a deciding issue. It had one other advantage, it could, and was, used as an AT weapon effectively beginning in the Philippines, able to pierce up to 70mm of armor.

What you will see in my mod is that the Type 2 is available to Class A ID's in an upgrade with enough Mot Support devices. Same with Mortar battalions/regiments, they upgrade to contain Mot Support allowing them to support the Type 2. Catch is, you can't upgrade until you have enough VEH production, and as IJ that is always in short supply. The Player in my mod has to decide whether you want to spend your VEH on Mot support for the Type 2 or get Type 1 Med Tanks to replace the POS Type 97/89A Med tanks that you start with ....

PS: One last note: the weight of the unit/ammo is reflected in its load cost, which impacts supply usage. Another thing which the IJ is always short of and something that also needs to be considered before upgrading this device. You want more firepower when facing US army/marine troops, but against the CHI forces your firepower is already much higher than theirs.

PaxMondo wrote: Wed Aug 13, 2025 11:21 am

Seacat54 wrote: Wed Aug 13, 2025 3:05 am Type 2 12 cm mortar is ready for production and should be an organic infantry weapon over 70mm Type 92.

Yes, and no.

The Type 2 120mm mortar was an effective weapon, but it had one serious drawback: weight. Both the weapon and the ammo were heavy, neither of which was man portable (reputedly 260kg and each round was another 12kg). The IJA suffered from lack of transport the entire war. Thus, the 120mm was not historically introduced into service until '43, and then only into defensively positioned units with relatively permanent positions. In the early war, where mobility was a crucial factor, the Type 2 simply could not be moved.

This led to them continuing to use the Type 97 81mm mortar almost the entire war. In contrast the Type 97 was man portable, although it took a squad to operate and transport one mortar. Coming in at about 66kg with only 3kg per round, this one could be lugged around the battle field.

The 70mm Type 92 was actually a howitzer, not a mortar, even though it did look like a wheeled mortar with its very short barrel and its effective range being very comparable to a mortar. Again, the reason for its use throughout the war was that it was man portable. Weighing about 200kg, but with wheels, it could be towed by a 2 man team in a harness. Each round was less than 1kg each, so again, weight was a deciding issue. It had one other advantage, it could, and was, used as an AT weapon effectively beginning in the Philippines, able to pierce up to 70mm of armor.

What you will see in my mod is that the Type 2 is available to Class A ID's in an upgrade with enough Mot Support devices. Same with Mortar battalions/regiments, they upgrade to contain Mot Support allowing them to support the Type 2. Catch is, you can't upgrade until you have enough VEH production, and as IJ that is always in short supply. The Player in my mod has to decide whether you want to spend your VEH on Mot support for the Type 2 or get Type 1 Med Tanks to replace the POS Type 97/89A Med tanks that you start with ....

PS: One last note: the weight of the unit/ammo is reflected in its load cost, which impacts supply usage. Another thing which the IJ is always short of and something that also needs to be considered before upgrading this device. You want more firepower when facing US army/marine troops, but against the CHI forces your firepower is already much higher than theirs.

All good points and I can understand why IJA liked the Type 92 but acutely lacking in artillery firepower Type 2 was clearly the way to go much sooner.
Possibly a mix of both would have been a possible solution if light artillery in general had been looked at critically, Type 2 should have been the preferred indirect weapon of the existing light systems in places like Solomons and Burma.

Type 92 should fit nicely in some of the landing craft types for example, do you know if it was used in assaults?

Seacat54 wrote: Wed Aug 13, 2025 4:05 pm
All good points and I can understand why IJA liked the Type 92 but acutely lacking in artillery firepower Type 2 was clearly the way to go much sooner.

Only If you can move it, which they couldn't. They needed more trucks, which they didn't have.

Seacat54 wrote: Wed Aug 13, 2025 4:05 pm Possibly a mix of both would have been a possible solution if light artillery in general had been looked at critically, Type 2 should have been the preferred indirect weapon of the existing light systems in places like Solomons and Burma.

I don't agree, but just my opinion.

Range of only 4.2km is rather small for something that requires a truck to move. If I have to have a truck to move something, I'd rather have a 75mm Type 90 field gun myself, much bigger bang and range 15. Even a 75mm T94 Mtn Gun would be preferable. That's just me.

Seacat54 wrote: Wed Aug 13, 2025 4:05 pm Type 92 should fit nicely in some of the landing craft types for example, do you know if it was used in assaults?

I doubt it. IJ LB's are only 8 tons displacement ... that's pretty small. But I'm not a naval weapons officer, need someone with that type of rating to evaluate this. However, the IJ only mounted a pair of 25mm Guns which is a LOT smaller than 120mm. So, in my mod, the answer is no.

Nice thing about mods, you can do what you want.

That small a vessels firing an indirect fire weapon? I would not want to be downrange of the vessel firing!

PaxMondo wrote: Wed Aug 13, 2025 7:44 pm

Seacat54 wrote: Wed Aug 13, 2025 4:05 pm
All good points and I can understand why IJA liked the Type 92 but acutely lacking in artillery firepower Type 2 was clearly the way to go much sooner.

Only If you can move it, which they couldn't. They needed more trucks, which they didn't have.

Seacat54 wrote: Wed Aug 13, 2025 4:05 pm Possibly a mix of both would have been a possible solution if light artillery in general had been looked at critically, Type 2 should have been the preferred indirect weapon of the existing light systems in places like Solomons and Burma.

I don't agree, but just my opinion.

Range of only 4.2km is rather small for something that requires a truck to move. If I have to have a truck to move something, I'd rather have a 75mm Type 90 field gun myself, much bigger bang and range 15. Even a 75mm T94 Mtn Gun would be preferable. That's just me.

Seacat54 wrote: Wed Aug 13, 2025 4:05 pm Type 92 should fit nicely in some of the landing craft types for example, do you know if it was used in assaults?

I doubt it. IJ LB's are only 8 tons displacement ... that's pretty small. But I'm not a naval weapons officer, need someone with that type of rating to evaluate this. However, the IJ only mounted a pair of 25mm Guns which is a LOT smaller than 120mm. So, in my mod, the answer is no.

Nice thing about mods, you can do what you want.

120mm mortar is composed of three main parts and the total weight is still much less than any of the 75mm pieces, it is easier to move than any of the light guns or howitzers.

All the 75mm have inferior indirect projectile and all of the 75-105mm guns and howitzers are inferior in elevation making the range a lot less relevant.

It is many times easier to find an indirect firing position and fire for effect with the 120mm mortar, especially when you have a canopy and defilades.

I am not talking in game terms but in IRL, this might not come in to play in game.

RangerJoe wrote: Wed Aug 13, 2025 8:33 pm That small a vessels firing an indirect fire weapon? I would not want to be downrange of the vessel firing!

Type 92 could fire direct or indirect fire and I did not suggest it would be permanently mounted on the craft.

Seacat54 wrote: Wed Aug 13, 2025 8:39 pm
120mm mortar is composed of three main parts and the total weight is still much less than any of the 75mm pieces, it is easier to move than any of the light guns or howitzers.

All the 75mm have inferior indirect projectile and all of the 75-105mm guns and howitzers are inferior in elevation making the range a lot less relevant.

It is many times easier to find an indirect firing position and fire for effect with the 120mm mortar, especially when you have a canopy and defilades.

I am not talking in game terms but in IRL, this might not come in to play in game.

You are still missing the point. I will try again. Real life or game is the same answer.

The 3 individual pieces were roughly 90kg each. It isn't about anything else than that. 90kg is NOT man portable. That means a truck. The truck did NOT exist. How do you get it to where you want it to be? Sitting on the loading dock at the factory does not make it an effective weapon.

In your mod, you can do whatever you want. But, if you want to pass a reality check on it, remember the truck and the lack of VEH points that the IJ has, particularly early war when she needs EVERYTHING and has almost NOTHING.

A slight digression ...

Since we got sidetracked with mortars, I thought I'd bring up the famous "knee mortar" of the IJ Army.

What the IJ did use was the Type 89 grenade discharger, known by the allies as the Type 89 50mm Knee Mortar. This isn't modelled in the game as a separate weapon, but is built into the IJ squad device. It was a VERY effective indirect fire weapon weighing only 4.7kg with the ammo coming in at 0.9kg each with a range of up to 670m. This was a squad level weapon, and eminently man portable. {Yes, a 10 lb mortar. Think about that. 80 years ago, they had that.}

Throughout the war, but particularly the early war when it was first being encountered, this weapon was a terror to the allies. As seacat notes, indirect fire is particularly effective in difficult terrain and this weapon was designed for exactly that. The IJ built and deployed almost 20,000 of these in the war. It was the definitive wwii IJ squad weapon that the allies never really came up with a good response to. The best counter the allies had for these, that I am aware of, was air support, but that meant the ground troops had to withstand 10 - 30 minutes of the Type 89 before the cavalry showed up. {the most common relief was actually that the IJ ran low on ammo ... that comes up time and again in the books}


Sources:
Guadalcanal Diary - Richard Tregaskis
The Battle for Guadalcanal - Samuel B. Griffith II

Both of these are first person narratives, both of the authors were actually there during the critical early months of the battle. The perspective is decidedly US, of course, but nonetheless you can easily parse what was effective and what wasn't in the early ground war. These two books, along with Japanese Destroyer Captain are among my favorites from this era.

RangerJoe wrote: Wed Aug 13, 2025 8:33 pm That small a vessels firing an indirect fire weapon? I would not want to be downrange of the vessel firing!

I wouldn't want to be IN the vessel firing it.

Maybe empty it might be ok (maybe capsize?), but if full of landing troops, I'm pretty sure firing a 120mm mortar would sink the ship (Newton: for every action there is an equal reaction; propelling the charge up is a large thrust down, so either hole in boat bottom or simply the boat goes awash, either way you are swimming!)

PaxMondo wrote: Wed Aug 13, 2025 10:40 pm

RangerJoe wrote: Wed Aug 13, 2025 8:33 pm That small a vessels firing an indirect fire weapon? I would not want to be downrange of the vessel firing!

I wouldn't want to be IN the vessel firing it.

Maybe empty it might be ok (maybe capsize?), but if full of landing troops, I'm pretty sure firing a 120mm mortar would sink the ship (Newton: for every action there is an equal reaction; propelling the charge up is a large thrust down, so either hole in boat bottom or simply the boat goes awash, either way you are swimming!)

I was thinking of the choppy wave action and the boat moving about. With no gyroscope, the rounds could land anywhere. Now think if that was your fire support and the enemy was very close. Not to mention the pounding that the hull would be taking as you pointed out or firing during when a wave hits the boat broadside . . .

Yes, the 90 kgs per part with three parts (plus any supporting equipment, firing tables and such) would be too heavy to be man portable. Maybe it could be carried by horse/mule pack trains, but then those need fodder and water with their own medical needs as well. The American .50 M2 HB is at the limits of being man portable for a 4 man crew. But then the crew would then have "male Tupperware" for storing things! Plus the weight of all of the additional ammo needed . . . .

I consider those very light mortars to be more like a grenade launcher, with a warhead just a little bit larger than the M79/M240 used by the United States of America.