Efficiency of the U-boat fleet

[wautd]

On another forum, someone mentioned that the recources spent on the larger ships of the German surface fleet, would have been more usefull when spent on building more U-boats instead. While I certainly agree with the first part, wouldn't it have been more usefull to divert those recources in the Luftwaffe and/or Wehrmach rather than building more U-Boats? As far as I know, U-boats were only efficient for a short period during the war but I might be mistaken.

So, which would give the most bang for buck? More U-boats or more land/airforce?

[Thanas]

Building more U-boote was a political impossibility.

That said, I am not sure how effective Dönitz' planned 300 boat navy was going to be, even if they were to be finished all before 1939.

[LaCroix]

The biggest problem was that once the detection technology was there, subs were unable to do much. I have a book about some notable submarine captains, and most of them were notable because they simply survived 'sorties' in the later years, even if the didn't sink anything.

The only way a sub could sink something was to sit in the water, waiting for a convoy to run into it. But as soon as they moved or fired, they were toast.

[Spoonist]

So, which would give the most bang for buck? More U-boats or more land/airforce?

For which purpose? Theatre? Ideology?

They would have needed more planes to fight the battle of britain.
They would have needed more uboats to 'isolate' british industry.
They would have needed more tanks for barbarossa.

However its only with the power of hindsight that we can tell that the Kriegsmarine had limited use of the bigger ships.

Also even if they had had the resources they would probably have spent it on another miracle-weapon instead of massproduction of something useful.

[Simon_Jester]

On another forum, someone mentioned that the recources spent on the larger ships of the German surface fleet, would have been more usefull when spent on building more U-boats instead. While I certainly agree with the first part, wouldn't it have been more usefull to divert those recources in the Luftwaffe and/or Wehrmach rather than building more U-Boats? As far as I know, U-boats were only efficient for a short period during the war but I might be mistaken.

So, which would give the most bang for buck? More U-boats or more land/airforce?

Keep in mind that this kind of production isn't infinitely fungible: you might be able to take the steel you would have spent building a battleship and use it to make a few dozen U-boats, but you certainly can't make it into a few thousand planes. Different weapons use different parts of the country's industrial capacity, so building less of X doesn't always mean being able to build more of Y. Even if it does, that doesn't mean the same equation will work the other way around.

That leads to a general question:
Might there have been logistics issues with building more U-boats, because of building slip availability or the U-boats needing materials that were vitally needed for programs other than capital ship construction? Stuart's recent posts on heavy tank production have got me wondering about that sort of issue...

Building more U-boote was a political impossibility.

What political factors were in the way? Internal Kriegsmarine issues coming from a misplaced nostalgia for the days of the High Seas Fleet, or something more general?

That said, I am not sure how effective Dönitz' planned 300 boat navy was going to be, even if they were to be finished all before 1939.

Could we take the per-boat effectiveness of the historical U-boat fleet and multiply, or would that be wrong for some reason?

However its only with the power of hindsight that we can tell that the Kriegsmarine had limited use of the bigger ships.

I don't know. Taking for granted that we know in advance that the war will begin in 1939-40, before Germany has time to really build up the capital ship force, I think it could be predicted in advance that the capital ships they did have wouldn't do them much good. When you only have two or four battleships against an enemy that has fifteen or twenty, the ones you do have are obviously going to be at a serious disadvantage.

[Spoonist]

However its only with the power of hindsight that we can tell that the Kriegsmarine had limited use of the bigger ships.

I don't know. Taking for granted that we know in advance that the war will begin in 1939-40, before Germany has time to really build up the capital ship force, I think it could be predicted in advance that the capital ships they did have wouldn't do them much good. When you only have two or four battleships against an enemy that has fifteen or twenty, the ones you do have are obviously going to be at a serious disadvantage.

[urlhttp://en.wikipedia.org/wiki/Plan_Z]Plan Z[/url]

[Marcus Aurelius]

I don't know. Taking for granted that we know in advance that the war will begin in 1939-40, before Germany has time to really build up the capital ship force, I think it could be predicted in advance that the capital ships they did have wouldn't do them much good. When you only have two or four battleships against an enemy that has fifteen or twenty, the ones you do have are obviously going to be at a serious disadvantage.

The "fleet in being" effect is often overlooked when discussing this matter. It means that the few German battleships forced the RN to build more battleships in order to stay on top of the capital ship race. If Germany had not built any battleships and heavy cruisers, the British could have built more destroyers to counter the increased U-boat threat instead of BBs and cruisers.

[Sea Skimmer]

That leads to a general question:
Might there have been logistics issues with building more U-boats, because of building slip availability or the U-boats needing materials that were vitally needed for programs other than capital ship construction? Stuart's recent posts on heavy tank production have got me wondering about that sort of issue...

Certainly are. Surface ships are ton for ton cheaper then submarines, and submarines drew off what was mostly a different, more specialist industrial base which usually involved completely separate building yards, and higher ratios of certain key mineral resources like copper for all the electric motors and batteries. So converting surface ship tonnage into submarines will not produce particularly impressive results. Germany had limitations on raw materials even before WW2 broke out (because they had to pay to import them and Hitler was spending every dime already) that affected production of weapons as much as a lack of sufficient industry did. So steel for ships doesn’t turn into aluminum for planes. Gneisenau accomplished the impressive feat of TWICE donating her entire main armament for shore batteries.

You could have turn the surface ship tonnage into more tanks and land artillery with perhaps better results. A plant that makes armor for battleships can also turn around and make armor for tanks, and gun factories tend to be pretty adaptable. Tank production was already held back by lack of steel. As it was Germany got a pretty good value using surplus warship guns from all the projects it canceled and ships it disarmed for coastal defense. Germany had actual slim hope of winning a land war too, the naval situation was pretty hopeless when even France alone was building or planning a fleet on par with the German program.

[PeZook]

That said, I am not sure how effective Dönitz' planned 300 boat navy was going to be, even if they were to be finished all before 1939.

The kind of investment needed to support that kind of U-Boat fleet is another matter, too. Even disregarding notorious problems with copper Germany had to import and even outright rob from Allied freighters during the war, you'd need more dry docks to service the boats, more bunkers to shelter them from aircraft, more torpedoes (Wasn't there only one plant making torpedo batteries in the entire third reich? I'm not sure where I read this, though), optics, hydrophones, officers, etc.

Donitz certainly thought he'd have been able to accomplish much, much more if he had 300 boats to play with, and it makes sense: the question is, though, would he have been able to do enough to actually starve Britain? The British Isles were never in actual danger of starving or even running out of vital resources to supply their industry throughout the war. And if Germany can't force them to their knees by early 1943, they uboat fleet is fucked.

[K. A. Pital]

Clay Blair's U-Boat War is essential for understanding. Germany had the largest U-Boat fleet in the world at the time it was defeated. However, it's efficiencly was never enough to seriously harm the Allies.

The biggest "achievment" of U-Boats was the mass convoy system introduction itself, which made shipments come in convoys and thus achieved some strategic attrition. Other "achievements" when seen relative to the shipped bulk of goods are quite... modest to say the least.

I'll quote some outtakes from the book once I lay me hands on it.

[PeZook]

Clay Blair's U-Boat War is essential for understanding. Germany had the largest U-Boat fleet in the world at the time it was defeated. However, it's efficiencly was never enough to seriously harm the Allies.

The book doesn't really help to gauge the effectiveness of a vastly larger than historical sub fleet, since it includes so many factors as to be impossible without a really detailed analysis of possible patrol intensities, logistics, repair schedules, basing etc.

Of course, even if the u-boat war suddendly becomes an order of magnitude more damaging to the Allies, it still means they lose 5% instead of 0.5% of all transported goods

[Simon_Jester]

However its only with the power of hindsight that we can tell that the Kriegsmarine had limited use of the bigger ships.

I don't know. Taking for granted that we know in advance that the war will begin in 1939-40, before Germany has time to really build up the capital ship force, I think it could be predicted in advance that the capital ships they did have wouldn't do them much good. When you only have two or four battleships against an enemy that has fifteen or twenty, the ones you do have are obviously going to be at a serious disadvantage.

[urlhttp://en.wikipedia.org/wiki/Plan_Z]Plan Z[/url]

Yes. On the other hand, Hitler should really have seen the war coming, seeing as how he was specifically planning to invade Poland already. And that if he'd had his eye on the political scene in Britain and France, that they weren't going to take his invasion of Poland sitting down.

I don't think those are "only with the power of hindsight" observations. Hitler planned a surface fleet that wouldn't be ready until 1944, when he knew quite well that he was planning to commit acts of war against countries with much larger navies in 1939. Being realistic about when you need your weapons available is a very important part of having a good plan for what weapons to build.

The "fleet in being" effect is often overlooked when discussing this matter. It means that the few German battleships forced the RN to build more battleships in order to stay on top of the capital ship race. If Germany had not built any battleships and heavy cruisers, the British could have built more destroyers to counter the increased U-boat threat instead of BBs and cruisers.

Did they really build that many more heavy ships during the war, though? My impression was that they more or less made do with the heavy capital ships they'd laid down before the war and used their production capacity almost entirely for destroyers and lighter vessels during the war anyway... in which case the German fleet-in-being failed its objective in WWII, unlike the High Seas Fleet of WWI.

The book doesn't really help to gauge the effectiveness of a vastly larger than historical sub fleet, since it includes so many factors as to be impossible without a really detailed analysis of possible patrol intensities, logistics, repair schedules, basing etc.

Of course, even if the u-boat war suddendly becomes an order of magnitude more damaging to the Allies, it still means they lose 5% instead of 0.5% of all transported goods

The only reference I have handy is a questionable one: Churchill's biography/history of the war. Does anyone know whether that can be deemed a worthwhile source on the 1940-era U-boat war specifically, even if not on other issues?

[PeZook]

The only reference I have handy is a questionable one: Churchill's biography/history of the war. Does anyone know whether that can be deemed a worthwhile source on the 1940-era U-boat war specifically, even if not on other issues?

I don't think so ; Churchill was prone to exagerrations when it came to the war. It worked well to motivate people, but rarely reflected reality ; For example, he used to say in 1941 (and 1942, too) that the Allies were close to losing the battle of the atlantic - in reality, Allied domination of the Atlantic was never seriously contested, and Churchill had to know that.

He just liked to present the war as a mighty struggle against overwhelming odds some of the time. Which particular biography is that, BTW?

[Sea Skimmer]

The kind of investment needed to support that kind of U-Boat fleet is another matter, too. Even disregarding notorious problems with copper Germany had to import and even outright rob from Allied freighters during the war, you'd need more dry docks to service the boats, more bunkers to shelter them from aircraft, more torpedoes (Wasn't there only one plant making torpedo batteries in the entire third reich? I'm not sure where I read this, though), optics, hydrophones, officers, etc.

I don’t know about torpedo batteries specifically, but that wouldn’t be surprising nor is it that huge a limitation. Making the batteries would likely be held back by raw materials more then anything else since you need so much lead for each one. The Germans had redundancy from producing two different types of torpedo. They had the conventional wet heater powered G7a T1, and the battery powered G7e T2 for submarines as standard in 1939. The former was also used by surface ships, homing torpedoes and aerial torpedo production only began latter. The wet heater type was much cheaper, needed less maintenance and ran almost 50% faster. U-boat commanders just preferred the electric eels because they left no wake.

[Simon_Jester]

I don't think so ; Churchill was prone to exagerrations when it came to the war. It worked well to motivate people, but rarely reflected reality ; For example, he used to say in 1941 (and 1942, too) that the Allies were close to losing the battle of the atlantic - in reality, Allied domination of the Atlantic was never seriously contested, and Churchill had to know that.

He just liked to present the war as a mighty struggle against overwhelming odds some of the time. Which particular biography is that, BTW?

His. There are some substantial chunks of the work written as autobiography, which makes sense since he was one of the top ten or so people involved in the war.

Do you have any specific examples of how Churchill exaggerated the effect of the U-boats?

[atg]

One thing to remember as well is that most of the German surface fleet was built or very near completion during the late '30s. Scrapping or not building that fleet then isn't going to help much if the production is switched to building Mk1 Panzers.

[Steve]

I must finish Clay Blair's 2nd volume sometime, sadly the library system in this county doesn't have a copy of Volume 2 like Seminole County did.

IIRC, this was the nadir of the sub; when detection technologies made them easier to spot and kill while they still labored under the same old problems of being submersibles more than truly submarine vessels, unable to travel at high speeds underwater due to not being able to run their main engines and having a limited amount of time when they could be submerged. Then came the new techniques to sequester CO2 from a sub's atmosphere and to run engines for longer periods underwater (and of course the development of nuclear-powered subs) and everything shifted; the subs were suddenly lethal and potent and, IIRC, the best defense against a sub is now considered to be another sub.

[PeZook]

Do you have any specific examples of how Churchill exaggerated the effect of the U-boats?

There's mentions of it in Hitler's U-Boat War, but I will have to dig through the book, so it will take some time

I'll post citations as soon as I get them.

IIRC, this was the nadir of the sub; when detection technologies made them easier to spot and kill while they still labored under the same old problems of being submersibles more than truly submarine vessels, unable to travel at high speeds underwater due to not being able to run their main engines and having a limited amount of time when they could be submerged.

The funny thing is that all the technologies used for the Type XXI subs (the first "real" SSKs) existed well before the war: hell, it even used Type IX batteries! The "revolutionary" solutions on the Type XXI were basically: a hydrodynamic hull optimized for underwater performance, large battery compartment and snorkels. Some other cool kit it had was pretty much optional to those three: the streamlined hull alone allowed the Type XXI to move faster underwater than most convoy escorts could without losing the ability to use their sonar.

[Stuart]

The funny thing is that all the technologies used for the Type XXI subs (the first "real" SSKs) existed well before the war: hell, it even used Type IX batteries! The "revolutionary" solutions on the Type XXI were basically: a hydrodynamic hull optimized for underwater performance, large battery compartment and snorkels. Some other cool kit it had was pretty much optional to those three: the streamlined hull alone allowed the Type XXI to move faster underwater than most convoy escorts could without losing the ability to use their sonar.

It's worse that that; the streamlined hull idea went back to the British R class submarines that were being built at the end of WW1 (in fact, when the technical details of the Type XXI submarines became available to the allies in 1943, the first thing the RN did was to pull the design cover for the R class). However, the design wasn't original even then, the basic idea of a submarine being streamlined for underwater speed went back to the first days of submarine warfare. Then, submarines were seen as being for coastal (and in particular harbor) defense. They would stay in port, then when an enemy blockading fleet appeared, go out, attack and return. Very much as the Hunley did in fact. What they needed was underwater speed so the boats were built relatively streamlined. When subs turned into commerce destroyers, they needed the paraphenalia associated with that mission and that took priority over underwater speed.

The underwater speed of the Klasse XXI is also misleading. Yes, the type could do around 16 knots underwater but in doing so she would run her batteries flat in around 45 minutes. That's true today by the way; a modern diesel-electric SSK running at top speed underwater will deplete her batteries in about the same length of time. Also, a surface ship trying to keep track of her will indeed be outrunning her own ability to use sonar but that doesn't actually mean too much. Surface ships worked in teams with some ships drifting to keep sonar contact, others moving at high speed into new tactical positions. This was - and is - called "sprint and drift". The interesting bit is that of the Klasse XXI has the same shortcoming; if she ran flat-out she would be sonar-blind also so her situational awareness drops to zero. The true importance of the Klasse XXIs underwater speed isn't that she can outrun surface ships, she can't. What she can do is use her speed to slide between the beams of the sonars being used to track her, in other words evade rather than outrun. This was countered by a modified sonar system that used three beams rather than one.

What the Klasse XXI fans don't tell you is that by 1944 the Allies knew all about the design, what it was capable of doing and how to counter it. They'd had the design specs for the Klasse XXI and had modified two British S-class submarines to simulate Klasse XXIs (in fact, ironically enough, the British were operating submarines with Klasse XXI performance before the Germans). They learned a lot from those experimental boats. One lesson was that, in fact, Klasse XXI was a pretty bad design. Amongst other things, it was extremely sensitive to depth control. A combination of high underwater speed and that sensitivity meant she could dive out of control without warning and go through her maximum safe depth before the crew could correct the dive. Another was that the "figure of eight" hull design was unnecessarily complex and hard to build. Finally, the Allies learned that the standard Allied submarines, the Balao/Tench class for the US and the T/A class for the UK, could be modified to equal or exceed Klasse XXI performance at relatively little cost. That gave rise to the post-war Guppy programs.

In terms of countermeasures, the plans for what to do in the event of a major Klasse XXI offensive were in place. Perfectly adequate weapons already existed; the Hedgehog and Squid ahead-throwing ASW launchers were already in service and perfectly adequate. The next-generation UK Limbo and US Weapon Able were already advanced in development (Weapon Able became Weapon Alpha and was disappointing; Limbo in contrast was a truly deadly submarine-killer. As to platforms, what Klasse XXI did was make obsolete the slow escorts, the British Flower class corvettes and the US 19-knot DEs. Note how quickly they all went postwar. However, the US had already shifted production to the 24-knot DE and the British to new classes of frigates. In addition, the plans were for the mass conversion of older destroyers to ASW configuration. Essentially, the US Navy planned to convert all of its destroyers (up to and including the Fletcher Class) to ASW configuration with three Hedgehogs per ship (A big trainable one in place of B gun and two smaller fixed mounts in the waist). The British planned to mount two Squids per destroyer.

Most of this stuff can be found in Norman Friedman's "Illustrated Design Histories" of US Navy destroyers and submarines. It doesn;t set out to do so but as a side-efefct , the books destroy the myth of the Klasse XXI quite effectively.

[PeZook]

The underwater speed of the Klasse XXI is also misleading. Yes, the type could do around 16 knots underwater but in doing so she would run her batteries flat in around 45 minutes. That's true today by the way; a modern diesel-electric SSK running at top speed underwater will deplete her batteries in about the same length of time.

Well, yes. The XXI's capabilities would only be decisive in combat during the early war, when ASW tactics were in their infancy and sonar-equipped escorts were few and far between. They were marginal by the time when the XXI was actually introduced and would have to face heavily escorted convoys with experienced crews.

Also, a surface ship trying to keep track of her will indeed be outrunning her own ability to use sonar but that doesn't actually mean too much. Surface ships worked in teams with some ships drifting to keep sonar contact, others moving at high speed into new tactical positions.

That started inning its way into operations after a couple of years, though.

What the Klasse XXI fans don't tell you is that by 1944 the Allies knew all about the design, what it was capable of doing and how to counter it. They'd had the design specs for the Klasse XXI and had modified two British S-class submarines to simulate Klasse XXIs (in fact, ironically enough, the British were operating submarines with Klasse XXI performance before the Germans).

Well, that I didn't know.

One lesson was that, in fact, Klasse XXI was a pretty bad design. Amongst other things, it was extremely sensitive to depth control. A combination of high underwater speed and that sensitivity meant she could dive out of control without warning and go through her maximum safe depth before the crew could correct the dive. Another was that the "figure of eight" hull design was unnecessarily complex and hard to build. Finally, the Allies learned that the standard Allied submarines, the Balao/Tench class for the US and the T/A class for the UK, could be modified to equal or exceed Klasse XXI performance at relatively little cost. That gave rise to the post-war Guppy programs.

Blair also mentions in his book it had other deficiencies: one, the snorkel had a faulty floater blocking the air intakes from taking in sea water. It had a tendency to get stuck in the "closed" position, and there was no cutoff for the diesels, so, uh, you can see the problem

There were also problems with ball bearings and the modular construction done by companies that never built ships before, but those were not endemic to the design.

In terms of countermeasures, the plans for what to do in the event of a major Klasse XXI offensive were in place. Perfectly adequate weapons already existed; the Hedgehog and Squid ahead-throwing ASW launchers were already in service and perfectly adequate. The next-generation UK Limbo and US Weapon Able were already advanced in development (Weapon Able became Weapon Alpha and was disappointing; Limbo in contrast was a truly deadly submarine-killer.

Wasn't one of the only two Type XXIs sent to Norway to begin combat operations sank by an airplane while snorkeling? That alone proved it was far from invincible, especially in light of swarms of Allied ASW airplanes.

Most of this stuff can be found in Norman Friedman's "Illustrated Design Histories" of US Navy destroyers and submarines. It doesn;t set out to do so but as a side-efefct , the books destroy the myth of the Klasse XXI quite effectively.

Blair's "Hitler's U-Boat War" does that, too, especially with anecdotes about the Type XXI trying to kill its own crews in various creative ways

[CaptHawkeye]

The Type XXI's abilities were pretty much moot in every way once it entered service. It didn't matter if Germany could build another 300 of them. By 1944 most German submarines were being sunk before they could even leave port. Even if they could, Allied Radar sensitivity and roaming hunter killer groups made performing so much as a sortie an act of suicide. The Type XXI's snorkel could actually be detected on radar, defeating the purpose of the whole system!

Also, what PeZook said did most likely happen. Submarines traveling at periscope depth can be seen by aircraft in calm waters. Conveniently the only time during which the snorkel could even operate.

The Type XXI just never really stood a chance in the Atlantic. Once 1943 was over Allied shipping was more or less untouchable. The only thing the Kriegsmarine could hope to do by that point was harass any Allied ship that strayed too close to Germany's coastline.

[Simon_Jester]

All right. With all that established: why weren't those submarine technologies introduced sooner by someone, given that most of them seem to have been around well before 1943? Was it simply a case of "we can't design and prototype this thing fast enough to get it into service when it will do some good," or something more subtle?

[Thanas]

^At the start of the war, the decision was made to focus on mass production of existing types, considering how few submarines Germany had available. There were months where less than ten submarines in total were on patrol, so the decision made sense then.

[Stuart]

All right. With all that established: why weren't those submarine technologies introduced sooner by someone, given that most of them seem to have been around well before 1943? Was it simply a case of "we can't design and prototype this thing fast enough to get it into service when it will do some good," or something more subtle?

The simple answer is that they weren't needed. From about 1915 onwards, the submarine was seen as a commerce destroyer. That meant it needed to have radios so that it could communicate with home base (to get the latest data on convoy routings etc and to report results), the ability to search wide areas to locate targets - implying relatively tall masts and spots for lookouts, a pentiful supply of torpedoes to kill said merchant ships and a gun (or more) to finish off targets not worth a(nother) torpedo. The target was a fleet of merchant ships that thumped along at 6 - 8 knots tops. Subs had to run on the surface to find their targets (and, later that exposed them to aircraft requiring more guns to fight off said aircraft) so the design stressed surface speed and surface seakindliness. Long underwater endurance, high underwater speed etc simply weren't needed anb that meant those characteristics could be sacrificed to get the other requirements.

It's not often realized the extent to which submarines - up to 1945 at least - fought on the surface. Typically they would run into a convoy at night, on the surface, make their attack on the surface and run on the surface. Diving was pretty much a last resort. Once they were down, they were very slow, and had a limited life. After 1943 there were so many allied escorts that they could afford to detach some to sit on each submarine until its batteries went flat (which is still a viable tactic by the way, it's called hold-down). The Klasse XXI was an effort to design a submarine that could evade the air patrols and escorts to make its attacks. That was a desperation resort and represented a sacrifice of commerce destroyer characteristics in favor of survival. In other words, Klasse XXI represented the decision that it was better to have a poor commerce destroyer that could survive rather than an excellent commerce destroyer that couldn't.

The Type XXI's abilities were pretty much moot in every way once it entered service. It didn't matter if Germany could build another 300 of them. By 1944 most German submarines were being sunk before they could even leave port. Even if they could, Allied Radar sensitivity and roaming hunter killer groups made performing so much as a sortie an act of suicide. The Type XXI's snorkel could actually be detected on radar, defeating the purpose of the whole system! Also, what PeZook said did most likely happen. Submarines traveling at periscope depth can be seen by aircraft in calm waters. Conveniently the only time during which the snorkel could even operate. The Type XXI just never really stood a chance in the Atlantic. Once 1943 was over Allied shipping was more or less untouchable. The only thing the Kriegsmarine could hope to do by that point was harass any Allied ship that strayed too close to Germany's coastline.

That's overstating things a little bit. Pre 1943 and pre-snort (by the way, the snort was a Dutch invention; pre-war Dutch submarines serving in the DEI had snorts, the Germans copied the idea from a captured Dutch O-boat) a submarine running on the surface could be spotted from literally hundreds of miles away. So, once there were enough MPAs to cover the Atlantic, teh U-boats were history. What the snort did was cut down that radius of search from around 200 miles to about 20. In other words the huge area search capability of MPAs had gone. That gave the U-boats the ability to go back into the Atlantic provided they didn't actually do anything. If the captain was so ill-advised as to want to do something, he would have to give his position away to do it. That's the beauty of convoys by the way, they eliminate the step of finding the enemy. They force the enemy to find you and tell you he's around. So, a Klasse XXI out in the Atlantic was probably quite safe as long as he stayed away from convoys. Or anything else. But, as soon as the boat made an attack, its a target (the sinking merchant ship is called a flaming datum). This is where the other characteristic of the Klasse XXI comes in, it's fast enough to get away from the flaming datum. This process is called Clearing Datum. Something nuke boats are very good at, diesel-electrics not so much. So, given a flaming datum, a Klasse VII or Klasse IX boat is dead meat, a Klasse XXI has some chance of getting clear. The improvement is there but its incremental rather than revolutionary. And, of course, like every other incremental improvement, teh advantages it brings are transient.

[Simon_Jester]

MPA: Something Patrol Aircraft?

Stuart, would you then say that the Klasse XXI was a step down in commerce raiding effectiveness compared to the VII and IX, with that sacrifice being accepted for greater evasive ability?