British 1936-42 purchase options, logistics and export/import of military hardware

[yulzari]

Citing the Valentine as perfectly serviceable tank for 4 years is both setting the bar very low and showing how bad the rest of the British tanks were and how faulty their doctrine was.

Good thing the Valentine is cheap because you need more of them. But needed to build more runs the total cost back up.

They were slow and at times could not flank the Germans in NA. They also could not avoid being flanked (or had to depend on other, faster tanks to do the flanking maneuvers)
Valentines, like a number of other British tanks, had very poor vision when locked down. And the 2 man turret versions suffered from this. Once you start shooting the commander was loader and if he pops his head out, he isn't loading the gun.
Valentines had limited ammo capacity. And with the bigger guns it was not only main gun ammo but machine gun ammo. I don't know were the sweet spot is but under 2000 rounds of co-ax gun ammo was not it. British Challenger tanks in Iraq went through over 4000 rounds of co-ax ammo in some engagements and had to withdraw. They were using the co-ax guns to suppress anti-tank missile teams and RPG teams. The need for decent co-ax guns and large ammo supply has not gone away.

Not buying 2pdr HE ammo for over 3 years was just stupid.
Not buying 6pdr HE ammo in quantity was also stupid. In 1942 they manufactured 2.5 million AP rounds and only 396,000 HE rounds. How many they issued?
One wonders how many German 50mm HE rounds were built and fired off in 1942?

German tanks had a better chance of success because the commander could see better. Both closed down and unbuttoned.
The British had jumped right over the 20-25mph speed range and either built somewhat reliable tanks that topped out 15mph or unreliable tanks that went 27-31mph.
The Germans built two different classes of 20-25 ton tanks but they were both 'universal' tanks. The MK III was supposed to handle just about everything and got smoke and HE support from the MK IV, but they were supposed to go the same speed and cross the same obstacles (or close) so they could support each other.
The British built too many different tanks that operated at different speeds and had different obstacle capabilities.

The British could have done a lot better than they did. There are reasons that there are books about "The Great Tank Scandal".

The British should not have to wait for the Merlin to get a tank that could beat the MK III & IV.

I presented that Valentine as an easy improvement not a perfect one.

I confess I am influenced by an old acquaintance who was a Valentine tank commander in the Red Army and thought it quite the dog's bollocks and happier with it than a T34. Taking it all the way to Berlin.

 

[tomo pauk]

Britain did not have enough money/resources to get even most of what they wanted.
the BEF in France had only the above mentioned 13 6in guns and
13 8in howitzers (12 active and 1 spare) 10,430 meter range.
16 60pdr (5in) guns with 3 spares. 13,800 meters.
176 6in Howitzers with 45 in reserve. 10,430 meters range.

after that they were down to the 18/25lbs and 4.5in Howitzers (6040 meters).
Long range may have been in the British Army's vocabulary but it was not in the Army's capability and it would not be for several years.
The British 4.5in guns were not that great even when they showed up. (Cheap shells with low HE, what a surprise)
The 5.5in guns don't show up until 1942.

Artillery is probably the least sexy military hardware to discuss Despite the huge importance of it.
To me, it seems like the British were keen to switch the calibres across the board, leaving a lot of overlap in the process. Combine it with the late start with the modernization program of artillery, as well as the long drawn-out introduction of the 'proper' 25 pounder, and their artillery park of 1940 looked a lot like that of 1918.

I'd try and get better mileage from the 60 pdr before embarking on something bigger, and forget the 4.5in cannon. A more modern steel and construction plus a 42-43 cal barrel might've netted them a gun that is 90% of weight and range of the Soviet 122mm A-19 cannon, and at least as rangy with the modern, better streamlined shells?

As a next step, I don't see the niche for opting for an all-new caliber that 5.5 in was, so I'd suggest simply continuing with the 6in route, but with a new gun, that can again be reasonably light, while offering the range in-between the 6in howitzer and cannon, or perhaps tad better than the 15cm sFh18.
The new 6in gun and the 60 pdr cannon might have shared the carriage.

Both the 18 and 25 pdr can get an additional level of usability as the weapon of an SP artillery system. Granted, a suitable doctrine is needed here.

 

[Admiral Beez]

What of the thousands of 12 inch shells from the early dreadnoughts? Could these be modified for use as AP bombs?

 

[Shortround6]

A bit out of order. But let's remember that a lot of artillery calibers were kept around from before WW I to the end of WW II and in some cases till the 2000s.
It is not just the tooling but often there was a stock of hundreds of thousands (millions?) of old shell in storage. The was a great desire to use up at least some of the old shells.

Both the 18 and 25 pdr can get an additional level of usability as the weapon of an SP artillery system. Granted, a suitable doctrine is needed here.

Here we run into several problems. There were actually at least two different 18pdr guns. They used the same ammo (sort of) and things get complicated by several different carriages before we even get into rubber tires. Most of the 18pdrs built during WW I were worn out, destroyed, blown up by the end. The old pole carriage guns should have been retired. The 16 degrees of elevation restricted the range to 5,966 m unless the the crew dug a pit to hold the trail. Range went to 7,100 m. Problems with trying to use these old guns for sp guns is that you can keep the barrel and breech but you need to throw away everything else. 9 degrees of traverse and having to keep a chunk of the old axle was a major problem. So was the meter of recoil when they fired. The later guns (MK III & IV) got variable recoil and higher elevation and the IV had trails that spread and had much greater traverse.
The next problem is that a lot of the MK IIIs and IVs were re-tubed/barreled to the early 25pdrs and there weren't very many 'modern' 18pdrs left and after Dunkirk there were almost none. Pre-Dunkirk there weren't enough 25pdrs for the 10 divisions and some divisions had two regiments of 18pdrs or 18/25s and one regiment of the 4.5in howitzer.
From Wiki
"A total of 216 guns were lost in the 1940 campaign. This left the British Army with 126 guns in UK and 130 in the rest of the world, according to a stocktake in July 1940."
Any plans to arm tracked chassis should take this into account.

In typical British fashion the 4.5in howitzer only shared the bore size with 4.5 in gun and the ammo was definitely not interchangeable in anyway. This 1913-14 weapon was sturdy and well liked but it had rather short range. The Shell weight was only 15.7kg and it was planned to replace them with the 25pdr gun as soon as possible.

To me, it seems like the British were keen to switch the calibres across the board, leaving a lot of overlap in the process. Combine it with the late start with the modernization program of artillery, as well as the long drawn-out introduction of the 'proper' 25 pounder, and their artillery park of 1940 looked a lot like that of 1918.

Mostly covered above for the small guns.

I'd try and get better mileage from the 60 pdr before embarking on something bigger, and forget the 4.5in cannon.

Well, there were two distinctly different 60pdr although they could share ammo, mostly. The older 60pdr (1904-5) had limited elevation and limited traverse and had a pair of recoil cylinder on top of the barrel. The 1918 gun was pretty much brand new but would fire the old shells. But since these were peace time guns (war had ended before production geared up) there were not all that many and since 76 (all the carriages that were deemed suitable) of them were converted with new barrels to form the 4.5in MK I it would seem the British were trying to get more mileage out of them.
The old 60pdr WW I shells were blunt and had poor ballistics. They did make some shells with somewhat better streamlining but the 4.5in shells were much better and account for quite of bit of the range increase.
The 4.5in gun was the only really long range gun the British had. 18,750-19,200 meters But the crappy shell hindered it's effectiveness.

As a next step, I don't see the niche for opting for an all-new caliber that 5.5 in was, so I'd suggest simply continuing with the 6in route, but with a new gun, that can again be reasonably light, while offering the range in-between the 6in howitzer and cannon, or perhaps tad better than the 15cm sFh18.
The new 6in gun and the 60 pdr cannon might have shared the carriage.

I am not sure quite what the thinking was behind the 5.5 in except that the 5.5in could fire a 100lb/44.5kg shell at 1675fps for 16,200yds range and the 4.5 in barrel would fit on the same carriage and recoil system for a 55lb shell at 2250fps and 20,500yds range. They even used the same breech mechanism.
The 4.5in only had a HE shell. No smoke or other specialty ammunition.
When the British finally woke up (1943) and designed a light 5.5in shell with better steel and a more streamline shape the 5.5in gun hit 1950fps and 18,100yds range.
To show the importance of artillery the British 5.5in guns of 21st army group in NW Europe fire 2,610,747 rounds between D-Day and VE Day. using 80lbs (and many were not) per round that is 104,429 tons of 5.5in artillery ammunition fired. Or about 17,400 4 engine bomber loads at 6 tons per load.
The British 21st Army also fired 159,898 rounds of 7.2in howitzer shells (200lbs each) 15,989 tons.
I don't have the figures for the smaller guns or the Lend Lease guns.

BTW the 5.5in barrel/breech weighed 4120lbs, the old 6in WW I gun barrel/breech weighed 10,248lbs. The US 155mm M1 howitzer barrel/breech weighed 3,825lbs, the American 155mm M1 gun (Long Tom) barrel/breech was 9595lbs. This was the gun that could fire a 95lb shell 25,395 yds. But the weight in action was 30,600lbs. Powerful, long range 6in guns were heavy and required special tractors/tow Vehicles.

 

[Shortround6]

What of the thousands of 12 inch shells from the early dreadnoughts? Could these be modified for use as AP bombs?

Why?
The old 12in AP shells weighed 850lbs to 1038lbs depending on Navy and the heavier ones tended to be for navies that never developed carrier aviation.

Old battleship shells require a lot of labor to turn into bombs. British shells were usually 850lbs. and held about 20-27lbs of HE. If you are happy with that design and build a tail and fins to suit and start dropping trials. If you want more HE you have to take out the old HE, chuck the over 800lb chunk of steel in a large lathe and start boring out the body of the shell. And then put everything back together.
Britain had decent manufacturing base for making forged large artillery shell bodies or bombs. They seemed to be able to make what the RAF or Navy wanted with little trouble rather than tying up machinery on makeshifts.

You also have to figure out if the shell/bomb will do what you want. The British 12in/45 shells usually landed at around 1100fps at long range.
How high up do you have to be get the velocity you want for the amount of armor you are trying to deal with.
In the Late 20s the British were testing a 1500lb AP bomb (this was before they canceled all the big bombs) and they found that the 1500lb bomb would defeat 3 in armor when dropped from 3000ft (level bomb). Now a 12in naval shell might have to dropped from a lot higher up to penetrate 3in and accuracy starts to get rather iffy. Dive bombing helps but now you need a dive bomber that can carry a 850-1000lb bomb.

 

[z42]

As a next step, I don't see the niche for opting for an all-new caliber that 5.5 in was, so I'd suggest simply continuing with the 6in route, but with a new gun, that can again be reasonably light, while offering the range in-between the 6in howitzer and cannon, or perhaps tad better than the 15cm sFh18.

Instead of developing the 5.5", they could have adopted the French 155mm system, like the US already had. Make a howitzer similar to the US M1, and a long range cannon like the US Long Tom, firing the same shells.

As a bonus, this would have given them a heavy artillery park of the right caliber once NATO standardized on 155mm post war.

 

[Shortround6]

Instead of developing the 5.5", they could have adopted the French 155mm system, like the US already had.

OK 5 reasons the British would not do that.
1. It's French
2. It's French!
3. It's French!!!
4. Not invented here
5. It's French!!!!

Make a howitzer similar to the US M1, and a long range cannon like the US Long Tom, firing the same shells.

US in a strange position in 1918-1919. They had a lot of factories tooled up to make assorted French and British guns and ammo. And not much tooling for anything of their own.
US also screwed themselves with the 75mm "crime of 1916" gun which forced adoption of a bastard 75mm gun. A British 18pdr gun rifled and chambered for the French 75mm ammo.
The US in 1917-18 just picked through the guns/ammo being made in the US so they would have something to send to France for the later battles in 1918 and 1919 offensive.
That is how the US got the 155mm 1917 & 1918 Howitzers and the 155mm 1918 gun
The US 155 M1 Howitzer wasn't standardized until May of 1941.

US had adopted the British 8in Howitzer which is how the US wound up with 8in caliber even if the US didn't use any old 8in howitzers in combat in WW II.
The US wound up with a bunch of British 6in guns on 8in Howitzer chassis and after WW I a bunch of them were emplaced as coast defense weapons in some areas. By the early 30s they were in storage (no more ammo) and in 1940 they were taken out, dusted off and handed over to Brazil (99 guns and some(?) spare barrels) as part of a military aid package in a swap for bases. US helped Brazil get ammo made or helped Brazil start their own production facility?

Heavy artillery is/was expensive and long lasting (or rebuildable) and caliber change was not taken lightly.

As a bonus, this would have given them a heavy artillery park of the right caliber once NATO standardized on 155mm post war.

Kind of out the era and requires tremendous foresight.
The 5.5 stayed in British service, at least with the territorial army, until 1980.
Granted the British just adopted (were given? reduce prices?) the 155mm US guns for the Forces stationed in Europe.
The 5.5 was used in Korea, South Arabia and Borneo by the British. It lasted a lot longer in some ex colonies. A few showed up 1999 with the Pakistani forces fighting India in Kashmir.
Not too shabby for 50 year old guns.

Again this speaks to the longevity of many artillery systems/calibers.

 

[Shortround6]

Make a howitzer similar to the US M1, and a long range cannon like the US Long Tom, firing the same shells.

This may not have been quite as simple as it appears. Or maybe it is?
The shells for the 1917/1918 howitzers had one set of model numbers. The M1 howitzer had different model numbers and the long guns had a different set of numbers.
They often weighed the same and held the same amount of HE or other filling.
But the shells of the old howitzers was was only supposed to be used in the modern M1 Howitzer in a dire emergency. The old Howitzer shells used a narrow driving band.
I don't know if there were any restrictions with the newer guns and shells. No other 155 projectiles from other guns were supposed to be used in the 1917/18 howitzers.
The old Howitzers used a max 8.09lb propelling charge, the M1 Howitzer used a max 13.26lb charge. The long range M1 gun used max 30.74lb charge. Perhaps there differences in alloy/heat treatment or driving band?

 

[Howard Gibson]

Merlin XX fitted on the Spitfires, too.
In 1940.

Panic was due to the Bf 109E having a 30-40 mph advantage over the Hurricane, thus Hurricane with Merlin XX. Merlin 45 inherited the impeller from the Mk.X (slightly improved vs. what the previous marks had) and elbow intake from the Merlin XX (much improved


Yes.

You are referring to the Spitfire MkIII, which did not reach service. The Merlins 32/45/55 were ugly compromises to get the new impeller into action against the Germans ASAP. The Spitfire MkVs and all the Merlin Seafires had single speed superchargers.

 

[Elan Vital]

Possibly the British could have picked up on Szydlowski's and Planiol's work on the swirl throttle supercharger, which was mentionned as early as 1936 in the French press. This would allow further optimization of the single speed superchargers to improve low altitude performance without sacrificing high altitude power, or to have even higher altitude versions.

On the 2-speed ones, the benefit would be smaller but not insignificant.

 

[tomo pauk]

You are referring to the Spitfire MkIII, which did not reach service.

Spitfire III prototypes started their lives on the production lines as Spitfire I, and were tsken away as unfinished aircraft from there to be modified to the Mk.III standard, that included the installation of the Merlin XX in the nose.
Further, a number of Spitfire II were retrofitted with the Merlin XX.
Last but not least, the even larger 2-stage 2-speed Merlins and 1-stage Griffons were retrofitted by hundreds on the Spitfires that started out with 1-speed 1-stage Merlins.

The Merlins 32/45/55 were ugly compromises to get the new impeller into action against the Germans ASAP.

These engines were good.
What was not good on the Spitfire V were the fit & finish (up to 15 mph loss), external BP glass (minus 5-6 mph), draggy exhausts (7 mph vs. the small individual ones), float-type carb (10 mph deficit vs. the 'fuel pump' type), stone/ice guard in front of the ram air intake (8 mph), no retractable tailwheel, no wheel well covers - so we have a potentially 400 mph fighter doing 375 mph on a good day, and further 5-10 mph less for many examples tested.

The Spitfire MkVs and all the Merlin Seafires had single speed superchargers.

That is not disputed.

Possibly the British could have picked up on Szydlowski's and Planiol's work on the swirl throttle supercharger, which was mentionned as early as 1936 in the French press.

The earliest I know of is the article in the 'L'Aeronautique' from Dec 1938. Do you know where it was mentioned earlier?

 

[Elan Vital]

The earliest I know of is the article in the 'L'Aeronautique' from Dec 1938. Do you know where it was mentioned earlier?

Louis de Monge, Deux années de progrès technique, L'aéro, 11-12-36, p.5 and M. Victor, Un compresseur à haut rendement, Les Ailes, 11-11-37, p. 8 discuss it.

First source directly below, second one on this link :Les Ailes : journal hebdomadaire de la locomotion aérienne / directeur, rédacteur en chef, Georges Houard | 1937-11-11 | Gallica

The first source just says that after 8 years of research, Szydlowski-Planiol obtained a high performance supercharger which could do 80k rpms instead of 25k rpm of normal SCs of the day. The 1937 article is quite detailed.

 

[Shortround6]

British Army had a number of problems in the 1930s.
Some are doctrine, some a budget. Some are ?????

But they did have a real lack of HE support for the ground troops/tanks.
RAF wanted to do as little as possible for Army support/ground attack. This waned and waxed during the years but until late 41/42 and local commanders, France 1940 was a high point which shows how bad things were.
We have gone over some the Army's heavy artillery options (or lack of them) and while the Army was working on designs actual production was always a low priority until 1939-40-41 and it took a while to catch up. This left the British army with the field artillery and there wasn't enough of them and the HE shells were not good. The older British guns were short ranged and had limited traverse meaning they could not shift targets easily. British shells often held 6-10% HE. Most other nations held 9% and up. Larger shells carried a larger percentage.
So the infantry had lack of air support, less than average number of guns tubes and lower % of HE per shell.

And then we get to the tanks. The tanks, as we know, had no HE of any substance. Two tanks out of 16 that were mostly smoke filled are not HE support.
The rest of the world accepted, even if they didn't like it, 37mm-47mm HE shells fired out of tanks/AT/infantry guns. Only the British adapted the NO HE policy.

The British turned this around from 1942 in air support. More Artillery tubes, somewhat better HE in a few cases. And with the introduction of the 75mm guns into tanks they finally got HE into the tanks. Note that the adoption of the US 105mm Howitzers (M 7 Priests) upped the HE support somewhat. A US 105 howitzer shell held 2.66 times the amount of HE that the 25pder did. That is an extreme cases but it does show that were large differences in even common guns. Now start figuring in supply and things get a lot more complicated.

This does offer up a host of things that could be have done differently.

 

[yulzari]

Possibly the British could have picked up on Szydlowski's and Planiol's work on the swirl throttle supercharger, which was mentionned as early as 1936 in the French press. This would allow further optimization of the single speed superchargers to improve low altitude performance without sacrificing high altitude power, or to have even higher altitude versions.

On the 2-speed ones, the benefit would be smaller but not insignificant.

Rolls Royce were due to make the swirl throttle as an update addition to the post war BRM V16 Formula One car but the supercharged 1,500cc for,Ila fell into disuse before it was tried out. The purpose was to cope with the use of a centrifugal supercharger which, with its conventional throttle, had the power and torque climb exponentially as the revs went up which was hard to control. A gentle bit more throttle and suddenly you get twice the power………

 

[Shortround6]

British Artillery was underfunded during the 1930s.
British got strangely lucky at Dunkirk in some senses. They lost a lot obsolete stuff which forced a bit faster change over. Unfortunately it did leave them short of anything that would actually shoot. Losses in Greece added to this. British got a bit of break In Africa as the Italians and early Germans didn't have a lot of good guns either. Things swapped back a forth as both sides had supply difficulties.

List of British guns..................
guns.................................shell weigh lbt............HE lb...........range yds
18pdr..................................18.5...............................1.1..............11,100
25pdr...................................25.................................1.8...............13,400
3.7in mountain................20...................................2....................6,800
4.5in How..........................35..................................4.3..................6,600
4.5in gun...........................55...................................3.9...............20,500
60pdr..................................56...................................3.9...............15,100
5.5in gun..........................100................................10.5..............16,200
5.5in gun............................80..................................12................18,100
6in How.............................86.2..............................10.8..............11,400
7.2in How.........................202..................................28...............16,900

US aid
105 How...........................33..................................4.8.................12,205
155 1917 How................95.3............................15.13...............12,400
155 M1 How...................95.3............................15.13...............16,355

A lot of the max ranges were done with supercharges and/or the most modern shells/versions.
British got a number of heavier guns during the later periods and some obsolete stuff early in the war, used mostly for training.

British guns were often good weapons, let down by poor quality ammo. A lot of the older guns were good guns but they were of the previous generation/s and were not a good match for modern warfare.

 

[z42]

OK 5 reasons the British would not do that.
1. It's French
2. It's French!
3. It's French!!!
4. Not invented here
5. It's French!!!!

I would have reversed #2 and #3, but yes. Still, having ammunition commonality with both the French and the US should have counted for something? If you're going to develop a new gun in a new caliber, as long as there isn't anything fundamentally wrong with 155mm, it would have made sense to go with that.

Heavy artillery is/was expensive and long lasting (or rebuildable) and caliber change was not taken lightly.

Kind of out the era and requires tremendous foresight.
The 5.5 stayed in British service, at least with the territorial army, until 1980.
Granted the British just adopted (were given? reduce prices?) the 155mm US guns for the Forces stationed in Europe.
The 5.5 was used in Korea, South Arabia and Borneo by the British. It lasted a lot longer in some ex colonies. A few showed up 1999 with the Pakistani forces fighting India in Kashmir.
Not too shabby for 50 year old guns.

Again this speaks to the longevity of many artillery systems/calibers.

Not saying there was anything fundamentally wrong with the 5.5" howitzer. As you mention it served for a long time after the war. It was just a suggestion what could have been done if for whatever reasons they wouldn't have gone ahead with the 5.5.

And yes, changing artillery caliber is a tedious business logistically. You give the latest and greatest kit to the front line troops, and then the old guns can be used for territorial troops, coast defense and whatnot. But then those old guns won't be used very much, so you're not shooting up the barrel life or consuming those old ammo stockpiles you want to get rid of. Rinse and repeat when the next generation kit shows up, shove the previous gen kit to the territorials, but they haven't yet worn out their previous kit either so keep that as well. And then eventually you end up with a plethora of artillery systems all in different calibers.

Also, of course I wasn't suggesting that the British should have chosen the 155mm based on seeing it becoming the NATO standard post-war. Obviously nobody could have foreseen that with any certainty in the 1930'ies. Just that, had they made that choice, it would later turn out to have benefits long after the war.

 

[z42]

And then we get to the tanks. The tanks, as we know, had no HE of any substance. Two tanks out of 16 that were mostly smoke filled are not HE support.
The rest of the world accepted, even if they didn't like it, 37mm-47mm HE shells fired out of tanks/AT/infantry guns. Only the British adapted the NO HE policy.

Hence the suggestions presented earlier in this thread to adopt some kind of 6pdr gun with modest MV. About equal AP performance compared to the historical 2pdr, but can be provided with a half decent HE load too. Obviously not as good as a 75mm, but better than what they had historically (aka nothing). And still small and light enough to fit into early war tanks.

Note that the adoption of the US 105mm Howitzers (M 7 Priests) upped the HE support somewhat. A US 105 howitzer shell held 2.66 times the amount of HE that the 25pder did. That is an extreme cases but it does show that were large differences in even common guns. Now start figuring in supply and things get a lot more complicated.

Wikipedia mentions the Priest was more or less a stopgap solution in British service, replaced with the Sexton once those started showing up in order to simplify ammunition logistics. Sexton was a very Priest-like vehicle made in Canada based on M3/M4 chassis but mounting the 25pdr. So it used the same ammunition as the 25pdr the rest of the army was already using. The 105mm was probably a better gun with substantially more punch as you point out, but logistics matter, it seems.

 

[tomo pauk]

Louis de Monge, Deux années de progrès technique, L'aéro, 11-12-36, p.5 and M. Victor, Un compresseur à haut rendement, Les Ailes, 11-11-37, p. 8 discuss it.

First source directly below, second one on this link :Les Ailes : journal hebdomadaire de la locomotion aérienne / directeur, rédacteur en chef, Georges Houard | 1937-11-11 | Gallica
The first source just says that after 8 years of research, Szydlowski-Planiol obtained a high performance supercharger which could do 80k rpms instead of 25k rpm of normal SCs of the day. The 1937 article is quite detailed.

Thank you for the excerpt and the link.
The '80 k rpm' figure is very ... curious, to say at least. Impeller of the probably best S/C of 1940, that on Merlin XX, was rotating at 3000 x 9.45= 28350 rpm (max engine rpm, FS gear). Tip speed was 1273 fps (= 388 m/s, ie. well above speed of sound at 15°C and sea level); 26 cm impeller. The V-1710 impellers at 9:60:1 step-up ratio, with the 24cm impeller were turning at 3000 x 9.6 = 28800 rpm; tip speed of 1194 fps (364 m/s).
The S-P supercharger that was actually installed on the 12Y-45 was geared to the 10:1 step-up ratio, meaning the impeller is turning at 2400 x 10= 24000 rpm; tip speed of about 1000 fps (305 m/s).

Having a 24cm impeller turning at 80000 rpm will push the tip speed to close to Mach 3? Seems like that special impeller was the diameter of a stopwatch and half, ie. under 10 cm?

When we are debating the merits of the S-P S/C, several things should be noted. The excerpt from the 'Les Ailes' magazine doesn't show the same S/C as it was installed on the -45 engine. That S/C is described, for example, here. Further, we have two distinct 'things' that made the S/P system better than the HS supercharger. 1st is that the modern impeller was with three small impellers before the big impeller of 24 cm. All on the same shaft, one right behind another. That set-up was indeed much better than the legacy HS impeller, even if not that better than what the V-1710 or DB 601A had (and definitely no better than what the Merlin X and XX had) - capable for 880 mm Hg (~34.65 in Hg, or about +2.5 psi) at 4100m.
Another thing was the system of throttling, that employed the movable vanes. Lower losses (mostly due to the lower temperature of the compressed air) when the vanes were suitably angled contributed a lot to the engine loosing less power when throttled, with net effect that the power under the rated altitude was greater than it would've been the case with the classic throttles.

How much all of that would've been applicable as plug & play to the British engines, and for what gains?
On the Merlins and Griffons, it might mean that RR makes the two-sided intake elbow, with fuel being injected either in the eye of supercharger, or that fuel injection is used, while the vanes take all the throttling tasks. If successful, this will mean no need for the Spitfires with LF suffixes, that used cropped impellers in order to gain the low-altitude power, while loosing the high-altitude power. It will be also a good fit on the Merlin 46/47, that were disliked because the low-alt power was meagre. Getting rid of the float-type carb = better power at all altitudes.
On the (Bristol) radials - a good improvement across the board.

Ironically, the BMW radials would've benefited the most here with S/P system. BMW 801 is already with the two-sided elbow, already the fuel injection is there, and the increase of power under the rated altitude will be achieved even without the high-octane fuel. Mating the small 'pre-impellers' on the existing big impeller of the 801 might not be a long shot.

 

[Shortround6]

In superchargers what often matters is the tip speed of the impellers, NOT the rpm as Tomo has pointed out.
Due to the pressures and temperature inside the supercharger tip speed can exceed the speed of sound, but not by much.
If tip speed exceeds the speed of sound you get major shockwaves (sonic booms) inside the supercharger and actual airflow through the supercharger goes to hell.
Modern car turbos use very high rpm but they also use small impellers. They also operate near sea level (mostly) in dense air. At high altitude in thin air you have to to move a lot of volume of air to actually move mass/weight needed to make big power.

 

[tomo pauk]

At high altitude in thin air you have to to move a lot of volume of air to actually move mass/weight needed to make big power.

Big superchargers seem to be better in moving/compressing the thin air than the small superchargers. Even if the tip speed was similar (= slower-turning big impeller vs. fast-turning small impeller). Perhaps less losses due to the tip speed not going much beyond Mach 1, keeping the efficiency in the favorable range?