Rn vs IJN

The frequently quoted figure of 24 USN and Allied warships sunk is normally for the whole Guadalcanal Campaign and comprises Wasp & Hornet (carriers), Canberra, Astoria, Quincy, Vincennes, Northampton & Chicago (heavy cruisers), Atlanta & Juneau (light cruisers), Jarvis, Blue, O'Brien, Duncan, Meredith, Porter, Cushing, Laffey, Barton, Monssen, Walke, Preston, Benham & DeHaven.

The IJN lost Ryujo (carrier), Hiei & Kirishima (battleships), Kako, Furutaka & Kinugusa (heavy cruisers), Yura (light cruiser), Mutsuki, Asagiri, Fubuki, Murakumo, Natsugumo, Akatsuki, Yudachi, Ayanami, Takanami, Terutsuki & Makigumo (destroyers), I-172, I-1, I-3 & I-18 (submarines).

The US submarine effort in the Pacific was pretty dismal for just about a year. There were variety of reasons. Which meant that the Japanese ASW effort didn't have to very good.

In Dec and Jan the US made 64 attacks, fired 144 torpedoes, sunk 10 ships of all types and lost 3 subs, not all due to enemy action.
Feb saw the US subs sink two ships and lose another 2, again perhaps not to enemy action?

at the end of 1942 the last 3 months saw the US subs sink more tonnage than they had in the previous 10 months.
Things got better in 1943 but it was 1944 that was the real "happy time" for the US subs. In 1945 targets were getting scarce.

For this thread against the RN we can expect the British to have torpedoes that actually work. A more realistic tactical (attack and defense) mindset. Subs in somewhat better shape, some of the US subs were in pretty dismal shape. Having the Japanese bomb the navy yard and take out around 60% of the spare torpedoes didn't help and the loss of many spare parts for the boats didn't help either.

This means that while 15 or so British subs would not stop the Japanese it also means that such a force could achieve a significantly better outcome/results in first few weeks or months than the US did. If this is enough to slow down or stop at the mid point/s some of the Japanese advances is a question.
The US Subs, after the war, were confirmed to have killed only one ship in the first two weeks in 18 attacks.

If the British could sink 100,000 tons of shipping in the first 2 months could this have slowed the Japanese progress in the Dutch East Indies and bought more time?
How many troop ships?

Shortround6 said:

The US submarine effort in the Pacific was pretty dismal for just about a year. There were variety of reasons. Which meant that the Japanese ASW effort didn't have to very good.

In Dec and Jan the US made 64 attacks, fired 144 torpedoes, sunk 10 ships of all types and lost 3 subs, not all due to enemy action.
Feb saw the US subs sink two ships and lose another 2, again perhaps not to enemy action?

at the end of 1942 the last 3 months saw the US subs sink more tonnage than they had in the previous 10 months.
Things got better in 1943 but it was 1944 that was the real "happy time" for the US subs. In 1945 targets were getting scarce.

For this thread against the RN we can expect the British to have torpedoes that actually work. A more realistic tactical (attack and defense) mindset. Subs in somewhat better shape, some of the US subs were in pretty dismal shape. Having the Japanese bomb the navy yard and take out around 60% of the spare torpedoes didn't help and the loss of many spare parts for the boats didn't help either.

This means that while 15 or so British subs would not stop the Japanese it also means that such a force could achieve a significantly better outcome/results in first few weeks or months than the US did. If this is enough to slow down or stop at the mid point/s some of the Japanese advances is a question.
The US Subs, after the war, were confirmed to have killed only one ship in the first two weeks in 18 attacks.

If the British could sink 100,000 tons of shipping in the first 2 months could this have slowed the Japanese progress in the Dutch East Indies and bought more time?
How many troop ships?

Click to expand...

We know that the entire Japanese offensive was run on a shoestring. Relatively minor losses of shipping during the invasion of Lea and Salamua in March '42 created significant delays in their timetable to go after Port Moresby.
So, yes. If the Japanese are losing a higher percentage of their shipping, it's going to have an effect.

Earlier USN assumptions played a part as well. In the 1930's when exercises were carried out, any sub that was detected or sighted
was deemed to be automatically sunk due to a belief that subs were so vulnerable to attack they couldn't possibly survive.

This was where the other belief that USN sub commanders were not aggressive enough came from. Can't really blame them when they were trained
to not go into any situation where they might be detected. That changed pretty quickly with the example shown by U-boats in the Atlantic (although
the experience of WWI should have shown that too - maybe it was assumed ASW was already that much better).

U-boats also operated from bases into the South West Pacific and Indian oceans - group Monsoon (14 I think mostly operating from
Penang but not until late 1942) . Would these be used in this scenario ?

Lastly, by 1943 the Japanese had learned to use spotter aircraft to liaise with naval vessels in submarine hunting which increased their efficiency
somewhat. Subs could be spotted by plane in daylight down to depths of 100 feet or so due to the clarity of the water, especially compared to the Atlantic.
This was the reason why Atlantic subs ended up being being painted in lighter colours than Pacific subs.

Initial U-boat operations in 1942/43 to South African and IO waters were run from French bases.

Only in Dec 1942 did the Japanese suggest that the Kriegsmarine create a sub base in the IO. It took some time for this to come to fruition as supplies needed to be shipped out to allow its creation. The first operational, as opposed to transport, U-boat didn't sail East until May 1943 and was destined for handover to the IJN. The Monsun boats left Jun/Jul 1943 with a second wave Oct/Nov and more in 1944.

The Wiki article covers a lot of the set up and transport operations and draws heavily on the U-boat.net site.




Their IO operations were curtailed after the RN sank the last of their supply ships Jan-March 1944. Charlotte Schliemann & then Brake.

From Oct 1943 the RN began deploying escort carriers to the IO with AS squadrons, starting with Battler and then, after the Feb-Apr panic of the IJN redeploying its main fleet units to Singapore, Begum, Shah & Ameer from mid-1944. There was also a Librator GR.VI squadron redeployed from West Africa in early 1944.

Edit - two of the Type VIIF torpedo transport U-boats finally found a use.

Edit 2 - only 1 U-boat operated into Pacific waters. U-862 1 patrol Nov 1944-Feb 1945. Sank one ship off Western Australia and another off New South Wales. Travelled as far as New Zealand.

It is interesting that Doenitz first looked at sending U-boats to the Indian ocean area from the cape of Good Hope
to as far as Singapore. this was first talked about as early as one week after Germany had attacked Poland.

Other factors kept getting in the way which slowed the progress of this plan.

The other part of the equation comes about from a Japanese sortie from Malaya which started in April 1942. Five
I boats with midget subs headed to Diego Suarez where the midget subs severely damaged the battleship HMS Ramillies
and sank a tanker. After this they headed North towards the Red Sea where they sank 22 allied supply ships at a total of
over 100,000 tons.

The Japanese report given to the German command showed a large number of transport vessels operating around the African coast to supply
8th army in Egypt plus India.

Doenitz was having success across the Atlantic including the East coast of the USA at this time and German supplies of important materials
were still getting through from the Asian region so nothing happened on the East coast of Africa. Japanese subs could have continued to
attack Allied shipping. Losses would have been critical to the 8th army as at the time there was little to no escort shipping available
so the convoy system was not in use.

The obvious opportunity escaped both Axis powers with the end result being El Alamein 2 and the Axis loss of North Africa.

Strategically, this was a monumental error by both Germany and Japan. By the time Doenitz had decided to push U-boat operations
to the East it was too late. Allied escort / ASW capability (including aircraft) built up quickly and the opportunity to decimate supplies
to North Africa as well as India was gone.

Geography plays its part. Fortunately!

It is interesting to follow the course of the U-boat war via the merchant ship sinkings month by month. How from 1939 it spread westwards into the Atlantic as bases in France became available, then times like Operation Drumbeat, then the move to South African waters etc.

Operation Drumbeat sent U-boats across the Atlantic to operate off the US and Caribbean coasts in early 1942. A distance of 3,600-4,500 miles. This was a journey that was within the capabilties of the not just the long range Type IX U-boats but also the Type VII U-boats which formed the bulk of the U-boat fleet in WW2 (over 700 completions).

MId-1942 did see the Kriegsmarine tackling shipping traffic in the Central Atlantic and around Freetown. Freetown was the big convoy terminus in the region. Ships stopped to rebunker and re-store. It was here they joined convoys to sail north or convoys broke up to allow ships to sail independently either south or across to the South American coast. At least one of the big southbound WS troop convoys took a 300 mile diversion out into the Atlantic in autumn 1942 after leaving Freetown to avoid an area of U-boat activity.

Bordeaux (the southernmost U-boat base in France) to Cape Town is 5,800 miles while to Durban on the IO side it is 6,600 miles. The vast majority of the U-boats sent to the IO area were the long range Type IX (a couple of Type X boats and 2 Type VIIF torpedo transports were also despatched). At the outbreak of WW2 only about 7 of the 56 U-boats in service were of that type. Total completions of various models of that Type only reached 193 by the time the last one commissioned in May 1944. The range of these began at 10,500 miles and was gradually increased to c13,500 miles in the Type IXC (141 completed from 1941) to 23,700 miles in the Type IXD (30 completed from early 1942). So allowing for losses, refits, work up times etc as well as the long journey times there were never huge numbers available to send to the South Atlantic & IO.

Britain became very concerned about the Axis setting up a submarine base on Madagascar following the Japanese occupation of southern French Indochina at the end of July 1941. Not necessarily by a Japanese invasion but also by Vichy French acquiessence in allowing either Japan or Germany use of harbour facilities such as existed at Diego Suarez. By Nov 1941 Britain was planning the invasion of Madagascar. That plan was temporarily put on the back burner after Pearl Harbor only to be resurrected in early 1942 and carried out from May to Nov 1942 starting with Operation Ironclad. It would however have taken a lot of resource to set up such a base and then to defend it.

Britain set up an extensive system of bases on the various island chains in the IO to allow active hunting for both surface raiders and submarines, both German & Japanese. So from the Indian coast up near Calcutta, down through Madras, Ceylon, the Maldives (Addu Atoll), Diego Garcia (Chagos Archipeligo), Mauritius, Seychelles to Madagascar (after May 1942) to the South African coast then up through Dar es Salaam (Tanganika), Zanzibar, Mombasa (Kenya, home to the Eastern Fleet from mid-1942 to the end of 1943) to Massawa & Port Sudan on the Red Sea. Then across the Arabian Sea to places like Khormaksar (Yemen), Masirah (an island off Oman), Bombay on the west coast of India.The work to do that began before the outbreak of war with Japan and evolved to the end of 1944.

By Sept 1942 there were for example 4 Catalina squadrons operating across the region. By June 1943 it was 6 squadrons. And more General Recce aircraft continued to arrive into 1944. These squadrons would send detachments from their home bases to patrol the vast expanse of ocean as required. And as I noted in my last post the escort carrier Battler entered the arena in Oct 1943 followed by others in 1944 until the threat had reduced so far that they could be deployed on other duties in 1945.

As well as the shipping traffic to Egypt and India there was also the increasing Lend Lease deliveries from late 1942 to the Persian Gulf. Eventually something like nearly 25% of supplies to the USSR went via that route.

The problem got worse for the Germans after the Med was reopened to through traffic from May 1943 and it was once again able to use the Suez Canal. The amount of traffic routing via the Cape to the MIddle East & India reduced significantly and was then mostly in the northern Indian Ocean / Arabian Sea Area. Meanwhile the German subs couldn't make use of the Suez Canal and actually had an even longer journey to make. That is when a U-boat base at Penang really came into its own. It has just dawned on me how Monsun dovetails with the reopening of the Med. You can see the change in pattern by comparing the locations of the monthly sinkings.

The Japanese anti-shipping campaign in 1942 was unusual for them, as they saw attacking enemy fleets as the main target of their subs.

So while it may seem like a strategic mistake, it was one that Donitz could really do little about in the early war years. Too few Type IX U-boats early on. The opportunity to use a base in the DEI didn't arise until mid-1942, and by mid-1943 much of the opportunity had evaporated to a large extent with the re-opening of the Med. Once a decision was taken to open a Far East U-boat base the problem was then one of keeping it supplied with torpedoes & other technical eqpt. Pretty much everything except oil still had to come from Germany. For torpedoes the Type VIIF U-boat would have been a big help but only 4 were built.

EwenS said:

So while it may seem like a strategic mistake, it was one that Donitz could really do little about in the early war years. Too few Type IX U-boats early on. The opportunity to use a base in the DEI didn't arise until mid-1942, and by mid-1943 much of the opportunity had evaporated to a large extent with the re-opening of the Med. Once a decision was taken to open a Far East U-boat base the problem was then one of keeping it supplied with torpedoes & other technical eqpt. Pretty much everything except oil still had to come from Germany. For torpedoes the Type VIIF U-boat would have been a big help but only 4 were built.

Click to expand...

For a DEI base it seems obvious to get fuel from one of the DEI refineries the Japanese conquered, but where and how did u-boats otherwise refuel on such long missions? For the Arabian sea and the Red Sea, guess it's not too far from DEI, but what about, say, the South Atlantic? It's not like they could waltz into a neutral harbor with a u-boat and buy some diesel? Or to the extent that is possible, surely the Allies had agents in every harbor which could relay a message to a ASW hunter group that happened to be in the vicinity? Have a supply tanker that can travel to the DEI to fill up, and then loiter around in the South Atlantic waiting to be contacted by a sub that needs to refuel?

Great thread!

z42 said:

For a DEI base it seems obvious to get fuel from one of the DEI refineries the Japanese conquered, but where and how did u-boats otherwise refuel on such long missions? For the Arabian sea and the Red Sea, guess it's not too far from DEI, but what about, say, the South Atlantic? It's not like they could waltz into a neutral harbor with a u-boat and buy some diesel? Or to the extent that is possible, surely the Allies had agents in every harbor which could relay a message to a ASW hunter group that happened to be in the vicinity? Have a supply tanker that can travel to the DEI to fill up, and then loiter around in the South Atlantic waiting to be contacted by a sub that needs to refuel?

Click to expand...

It was the big long range U-boats that were sent to the South Atlantic & IO areas.

A Type IX sub wouldn't need to refuel en route Bordeaux to Penang. Approximate distances by sea - Bordeaux to Cape Town (see my last post) = 5,800 miles. Cape Town to Penang = 5,300 miles. Total distance 11,100 nautical miles. The most common U-boat sent to the IO area was, as I noted, the Type IX. The range of the Type IXC onwards (the most produced versions of that U-boat Type) was about 13,500 nautical miles at economical speeds, with the bigger IXD having a range of 23,700 nautical miles. So no refuelling required en route. It is only about 3,500 miles from Penang to the Arabian Sea/Gulf of Aden area.

For Caribbean & Central Atlantic operations, to extend the range of the mainly Type VII U-boats there were the 10 Type XIV Milch Cow resupply U-boats. Most of these were sunk in 1943 as the mid-Atlantic Gap was closed. Only 1 was sent to aid the Monsun Group in the IO in 1944, U-490, but it was sunk in mid-Atlantic while en route.

And see my post #865. There were German resupply ships operating in support of Kriegsmarine U-boat and surface raider operations in the South Atlantic & IO areas. The last of these, Brake, was not sunk until March 1944. These ships were operating out of Japanese ports.

Of the 6 foreign subs handed over to the IJN in May 1945, 2 were of Italian origin, 2 were Type IXD2, 1 was a Type IXD1, with the last being a Type XB minelayer (another long range type capable of 18,450 miles).

This provides some background to German U-boat operations in the IO

The last of the Kriegsmarine raiders to sail from France was the Michel in March 1942. She ended her first voyage in Japan in March 1943. Sailing again in May 1943 her second voyage took her to the eastern part of the IO then south around Australia & then across the Pacific to South America, before being sunk by a US sub 50 miles of the coast of Japan in Oct 1943. Stier had been sunk in the South Atlantic in Sept 1942 by the merchantman Stephen Hopkins.

For some context the size of some of the boats.

MK IXC was 1120tons surfaced and 1132/1540tons submerged and was 76.76 meters long.

British T class was 1330tons surfaced 1585 tons submerged and was 83.6 meters long.

US Gato Class was 1825tons surfaced 2410 tons submerged and was 91.6 meters long.

Japanese Junsen type 2135-2525tons surfaced 2791-3535 tons submerged and was 97.5-109.3 meters long. 8 built

Japanese Kaidai type 1705-1833tons surfaced, 2300-2602 tons submerged and were 97.-105 meters long. 33 built

Japanese Kiachu type 940-1200tons surfaced, 1115-1447 tons submerged and were 73-80.5 meters long. 20 built

Japanese A type 2919-2934tons surfaced, 4150-4172 tons submerged and were 113.7 meters long. 4 built

Japanese B type 2,589-2,624tons surfaced, 3,654-3700tons submerged and were 108.7 meters long. 29 built

Japanese C type 2,554-2,564tons surfaced, 3,561-3,644 tons submerged and were 108.7-109.3 meters long. 11 built

and the fan favorite,
Japanese Sen-Toko type 5,223 tons surfaced, 6,560 tons submerged and were 122 meters long. 4 built, the ones with 3 seaplanes, a number of the earlier boats carried 1 or two planes.

I have left out some of the real early boats built in the 1920s and new classes/types started after 1940-41 except for the Sen-Tako. or a few that they only built a very small number of.

The Junsen turned into the type A's and Kaidai type turned into the Type Cs.

Japanese boats were generally very big.
Usually very fast, some could make 23.6 kts.
Long ranged, as suited the Pacific except that most of the Kiachu class could only do about 5,000 miles at 16 kts but might do much better at slower speeds.

However most did not dive fast, did not turn fast, did not maneuver well under water, max dive depth was usually 100 meters at best, and they had poor electronics and with their size easier to spot (either eye or electronically).

The lost strategic opportunity in mid 1942 is not as much to do with U-boat availability in the Indian Ocean but more to do with
the lack of follow through from the Japanese submarine force.

As I posted five I subs sank over 100,000 tons of allied merchant shipping in one run. Twenty or more sent to follow up against
virtually unprotected shipping and you have a mass of easy targets. The loss of many of these to such an attack would be
inevitable under those circumstances, with the resultant loss of supplies of all types to 8th army being in the hundreds of thousands
of tons range.

Supplies to India would also have suffered badly.

Strategically this keeps North Africa in dispute (if not worse) for a longer period and weakens the defence of India. The situation from The Indian
Ocean right across to the West coast of the USA was pretty close to desperate for the allies in 1942. So much so that US Admiral Land
oversaw the building of 5,600 merchant vessels during the war to redress the weakness. 30% of the US resource expenditure during
the war went to ship building of all types. Fortunately Admiral Land had already started the programs for supply shipping before the US
entered the war.

The other interesting factor is the tenuous "cooperation" between the Axis countries. Although Germany kept requesting the japan do more
against mercantile shipping Japanese sub command simply didn't. The usual counter argument was that the subs needed to be in reserve
to attack the enemy fleet (US). At the same time it was argued that merchant vessels could be easily replaced so it was logical to go after
carriers etc.

As to Japanese command requirements when sinking enemy shipping, including merchants, the following was the method to be employed;

"Do not stop with the sinking of enemy ships and cargoes at the same time you will carry out the complete destruction of the crew of the enemy's ships."

In other words - don't worry about prisoners, kill everyone.

Shortround6 said:

The US submarine effort in the Pacific was pretty dismal for just about a year. There were variety of reasons. Which meant that the Japanese ASW effort didn't have to very good.

In Dec and Jan the US made 64 attacks, fired 144 torpedoes, sunk 10 ships of all types and lost 3 subs, not all due to enemy action.
Feb saw the US subs sink two ships and lose another 2, again perhaps not to enemy action?

at the end of 1942 the last 3 months saw the US subs sink more tonnage than they had in the previous 10 months.
Things got better in 1943 but it was 1944 that was the real "happy time" for the US subs. In 1945 targets were getting scarce.

For this thread against the RN we can expect the British to have torpedoes that actually work. A more realistic tactical (attack and defense) mindset. Subs in somewhat better shape, some of the US subs were in pretty dismal shape. Having the Japanese bomb the navy yard and take out around 60% of the spare torpedoes didn't help and the loss of many spare parts for the boats didn't help either.

This means that while 15 or so British subs would not stop the Japanese it also means that such a force could achieve a significantly better outcome/results in first few weeks or months than the US did. If this is enough to slow down or stop at the mid point/s some of the Japanese advances is a question.
The US Subs, after the war, were confirmed to have killed only one ship in the first two weeks in 18 attacks.

If the British could sink 100,000 tons of shipping in the first 2 months could this have slowed the Japanese progress in the Dutch East Indies and bought more time?
How many troop ships?

Click to expand...

USN submarine losses 1941 and 42.

Interesting that 4 were own goals, 1 was bombed in drydock and 1 was sunk by the merchant vessel it was trying to attack. Not a stellar performance.

As you note there were a lot more problems than crappy torpedos (By the way the Mark 10 used by the S boats was not as flawed as the Mark 14 but was still a poor performer). The S boats were the oldest submanies of any navy to see combat in WWII. They were a WWI design that should have been relegated to training years before. On Dec 7, 1941 the Asiatic Fleet had 6 S class, 7 P class, 6 Salmons and 10 Sargoes. This was the first line of defense against Japanese aggression. Elsewhere in the Pacific were 6 more S Class, the 9 V Class, 3 Ps and 6 new Tautogs. The 6 Gars were finishing their shakedowns. The Tautogs were the first truly satisfactory class of USN submarines. The Vs were not a true class but rather a hodgepodge of unsuccessful designs. The first 3 never saw combat and the 3 survivors were retired as soon as enough Gatos were available. The surviving P classes were all rengined in the second half of 1942 (including the Winton 201A engined ones) as were several of the Salmons and Sargos.

The last 3 months of 1942 were when the Gatos had taken over much of the duties with better results although the torpedo problems still remained. Incidentally the H-O-R engined Gatos also suffered engine problems and were eventually reengined.

RN Submarines were quite successful in 1941 and 1942. Their torpedos worked well.

The Japanese had a problem in that their subs were not well designed for a war of attrition.
They were very large and very, very expensive.

MK IXC was 1120tons surfaced and 4400hp diesels

British T class was 1330tons and 4500hp diesels

US Gato Class was 1825tons surfaced and 5400hp diesels

Japanese large submarines, several classes 12,400hp diesels.

They needed size and speed to get to the Allied supply routes, but the size and speed required meant they were twice (or more) expensive than the Allied or German type IX classes which meant they couldn't build enough of them.

Shortround6 said:

They needed size and speed to get to the Allied supply routes, but the size and speed required meant they were twice (or more) expensive than the Allied or German type IX classes which meant they couldn't build enough of them.

Click to expand...

Attacking supply routes wasn't priority, according to what I've read and watched. They did so every so often, but IJN sub skippers were very much tasked with attriting combat units as a priority.

Thumpalumpacus said:

Attacking supply routes wasn't priority, according to what I've read and watched. They did so every so often, but IJN sub skippers were very much tasked with attriting combat units as a priority.

Click to expand...

You are right, but it sort of fits together. The Japanese had built a number of subs for the fleet battle role. But they couldn't build enough for a wide spread commerce raiding role.
And they could not afford to trade their large, expensive boats for a small number (each) of cheap freighters/merchantmen or they would not have enough subs for big battles.

In the end the Japanese sub fleet didn't achieve much at all, mostly due to the expectation of a big fleet battle
which wasn't the way things went. Life is something that gets in the way of plans I suppose.

One problem with designing submarines to take part in a decisive battle was that the IJN wanted their submarines to be able to get to the area of the battle quickly. The German Type 1XB had a range of 12,000 nmi at 10 knots and a maximum speed of 18.2 knots surfaced whilst the USN's Gato class had a range of 11,000 nmi at 10 knots and a maximum speed of 21 knots. By contrast, I-9 (a Cruiser submarine Type-A) had a range of 16,000 nmi at 16 knots and could make 23.5 knots surfaced. Smaller, cheaper and less vulnerable submarines would have demanded that the IJN accept that submarines had to be pre-positioned for a battle or used to attack commerce.

cherry blossom said:

One problem with designing submarines to take part in a decisive battle was that the IJN wanted their submarines to be able to get to the area of the battle quickly. The German Type 1XB had a range of 12,000 nmi at 10 knots and a maximum speed of 18.2 knots surfaced whilst the USN's Gato class had a range of 11,000 nmi at 10 knots and a maximum speed of 21 knots. By contrast, I-9 (a Cruiser submarine Type-A) had a range of 16,000 nmi at 16 knots and could make 23.5 knots surfaced. Smaller, cheaper and less vulnerable submarines would have demanded that the IJN accept that submarines had to be pre-positioned for a battle or used to attack commerce.

Click to expand...

Part of the problem is the distances the Japanese had to travel to get into shipping lanes. The US - Australia trade was now where as big as the US-Europe trade and that was thousands of miles away from any exist Japanese base. The US west coastal trade was over 5,000 miles from Japan and the Japanese home islands were closer than any of their bases.

For perspective it is about 50% further to get from Taiwan to India - Perth shipping lane that it would be for a Sub to go from Leningrad to the North West Approaches to England.
The Japanese cannot project power (even than token Submarine patrols) into British or American trade routes without getting bases thousands of miles closer or using large fast submarines or a crap load of smaller, slow subs that spend a large amount of time traveling back and forth.

And once you go to the large, expensive and fast subs you have subs that don't operate well against good AS escorts. But the Japanese did not know that, at least not until 1942-43.

Shortround6 said:

And once you go to the large, expensive and fast subs you have subs that don't operate well against good AS escorts. But the Japanese did not know that, at least not until 1942-43

Click to expand...

For which we can be grateful.