mikemike
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ORIGINAL: Nikademus
This is interesting and provides an interesting contrast to Clay Blair's analysis of the Type XXI. I got the impression that it was a bit on the harsh side (as are many of his viewpoints in his two book series. Here's what he said about the Type XXI's flaws based on a US analysis of the U-2513. (highlights)
1) Poor Structural Integrity
due to hurriedly prefabrication of hull sections from up to 32 different factories that had little experience in building submarines. He claims each section was crudely made and didn't fit well together therefore led to weak pressure hulls.
he cites a German report that simulated tests failed at 900 feet. A British report - 800 feet. I'm guessing the "poor" rating comes from Blair's concluding statement that the 800 foot rating was less than the failure rate of a conventional uboat.
2) Underpowered diesel engines.
Blair says the superchargers of the new model six cylinder diesels were so poorly designed that they could not be used. The alleged failure reduced HP to 1200 leaving the Type XXI underpowered.
3) Impractical Hydraulic system.
(I'll quote this in full given the detailed info you've provided)
The Main lines, accumulators, cylinders, and pistons of the hydraulic gear for operating the diving planes, rudders, torpedo tube outer doors, and anti-aircraft gun turrets on the bridge were too complex and delicate and located "outside the pressure hull." The gear was therefore subject to saltwater leakage, corrosion, and enemy weaponry. It could not be repaired from inside the pressure hull. (obviously..... )
4) Imperfect and Hazardous Snorkel.
This is a long standing Blair jump up and down point. He thinks the device was overblown in it's potential.
specifically, it dunked often, even in moderate seas which in turn caused auto closure of intake exhaust ports. This caused Carbon Monoxide buildup in the pressure hull which led to sickness and ill health effects. (headaches, eye discomfort etc) While closed the diesels would also suck internal air from the boat. He calls using the snorkel on a type XXI a "nightmarish experience" (or in any other U-boat for that matter)
Ironically, in the next paragraph he does admit that the US Navy "did" in fact incorporate "some" of the features of the Type XXI electro boat for it's new sub designs in the immediate postwar years.
I'd be interested in your opinions on Blair's analysis. (Tironu feel free to jump in too) I've always wondered if there were more than a little sour grapes in the above. His viewpoint on Germany's uboat force was very different at the time he wrote Silent Victory.
My comments:
1) Poor Structural Integrity
Essentially true. The Type XXI consisted of eight sections plus the conning tower, whose basic structure was manufactured by steelbuilding companies in the interior, none of whom had shipbuilding experience. They were unable to stay within the originally demanded tolerances, so these had to be downgraded to +/- 2,5 mm in diameter or +/- 5 mm in circumference. However, even when nominally staying within tolerance, nobody seems to have compensated for thermal expansion/contraction. These "raw sections" were delivered to eleven shipyards where they were completed and then transported to the three assembly yards. It was there that mismatches in diameter had to be "fixed". It is safe to assume that the quality of the pressure hull varied between boats depending on how well the sections matched. Sections were built for specific boats but I guess could have been selected for compatibility to a degree; they were exchanged anyway when damaged/destroyed by bombs. I don't think the sections as such were crudely made, the essential welds were made by automatic welding machines and checked by x-raying, and all the companies involved were quite capable of quality welding, the trouble came when assembling the sections to a complete boat.
Concerning the strength of the pressure hull, it was apparently impossible to precisely calculate the strength of the double-bubble hull at the time, so the lower lobe of the hull was more or less designed by educated guess. Model tests in a pressure tank could only be performed in January/February, 1945, and showed that the lower lobe started to deform at 30 atmospheres or about 300 metres of water column (about 985 feet) and failed at 31,5 atm (315 metres or 1030 feet), this was 10% below specification but still better than most other subs at the time. Deep-diving trials with actual boats could only be done off Norway and were performed in April 1945 to a maximum depth of 220 metres/720 feet; they were terminated because pressure-tight containers for inflatable boats under the upper deck imploded. Further tests were prevented by the German capitulation.
2) Underpowered diesel engines.
The diesels were turbocharged. They worked fine when running on the surface, developing 2000 hp/1760 shp, but were so choked by insufficient induction/exhaust line cross-sections when snorkeling that the supercharger was practically ineffective, so the supercharger was not fitted to later boats, reducing output to 1400 hp/1230 shp on the surface and 1200 hp/1030shp when snorkeling. This was quite sufficient to fully recharge the batteries when snorkeling in 6.2 hours. As the boats were expected to operate mainly submerged, surface performance was irrelevant.
3) Impractical Hydraulic system.
This was the first type using hydraulics, earlier boats were mainly electric, but the higher rudder forces at higher underwater speeds could better be handled by hydraulic actuators; in addition, this saved copper. The system was overly complex at first, but was considerably simplified following trials and gave no trouble afterwards. The only essential part outside the pressure hull was the actuating system for the forward dive planes; these were, another first for German subs, retractable, and for simplicity the whole mechanism was copied from the Dutch O25 class. Rudder/Stern planes were actuated by hydraulic rams inside the pressure hull via rods. I respectfully suggest that the system may have seemed "too complex and delicate" to US crews of the time where engineering officers didn't always have engineering backgrounds but may not have posed any particular problems to German engineering staff.
4) Imperfect and Hazardous Snorkel.
The snorkel was an afterthought on the Type XXI and the installation was not optimal, maximum snorkeling speed was six knots as the snorkel started to vibrate strongly at higher speeds, not being streamlined. The system was designed to suck air from the interior of the boat when the snorkel was dunked, that was what made it practicable as otherwise the engines would either have aspirated water or come to an immediate stop. Exhaust ports were never closed when snorkeling. Exhaust gas leakage was a problem on older boats retrofitted with snorkels, but the Type XXIs had an exhaust system designed with snorkeling in mind and had air conditioning anyway. Maybe the system was imperfect but it was better than the USN system, which was nonexistent, or anybody else's, for that matter.
There was a marked tendency immediately postwar for US analysts to declare that nothing of value could be learned from German or Japanese equipment which was patently untrue in many aspects. Probably a case of "Not Invented Here".
(This led, amongst other consequences, to a scrapping of most captured enemy equipment. The USA had three or four specimens of every aircraft type that reached combat status in the Axis countries, and a number of prototypes in addition, and there are hardly any original aircraft left today.)
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