Two 6 cylinder MAN diesel engines, with salt water cooling, are installed on the XXI.  The engines were originally highly supercharged and rated at 1970 HP and 520 RPM.  Their rating has subsequently been reduced to approximately 1200 HP and 470 RPM by the removal of the supercharger.  The engines are in most all respects similar to the 9 cylinder MAN engines on the IXC submarines.    
          The air induction on the port side is the same as that for the IXC vessel.  On the starboard side it has been combined with the ships ventilation and snorkel air intake.  The head and hull valves as well as the outboard and inboard piping runs are essentially similar to those on the type IX submarines.  
          The exhaust gas system has been simplified to some extent.  The muffler and spark arrestors have been eliminated and the exhaust discharges below the waterline.  Hinged type outboard and inboard exhaust valves with self grinding in features have been retained from the IXC installation.  Several changes have been made in their design however.  
          A telescopic type of snorkel mast has been installed on the type XXI vessels.  Both engines and their associated systems are designed for operation with the snorkel.  As this vessel was designed contemplating its installation the piping arrangements, etc. are more satisfactory than are those on type IX and XB German Submarines.  
July, 1946
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  REPORT 2G-21
  1.  Introduction  
          The diesel engine installation on the IX C is described in Report 2G-9C-S41-5.  Those parts of the XXI installation that are similar for IX C components will not be described in detail in this report.  Also, a limited description of the diesel engine will be given as the engine is on test at the Engineering Experiment Station and will be subject to a report from that activity.  Attached pumps are discussed under the S group for the systems they serve.  
  2.  General Description  
          The original diesel engine built for the XXI submarine was an attempt to obtain a high powered, light weight and small size diesel engine by removing three cylinders from the MAN engine used on type IX submarines, and adding a higher speed exhaust gas driven supercharger to obtain greater supercharging pressures.  Allowable exhaust gas temperatures were raised from 550° C. to 600° C. (1022° F to 1112° F.).  The engine as designed was rated at 1970 HP and 520 RPM.  However, on shipboard tests these maximum exhaust temperatures were reached between 490 and 500 RPM, which accordingly limited the output to a lower figure.  
          The blower was designed to operate at a top speed of 12,300 RPM, producing 99.2 C.F.S. of air supercharged 8.53 psi.  On test the blower met these specifications.  
          The vessels in U.S. custody as well as numerous other type XXI submarines had their supercharger removed.  The precise reason for its removal is not known.  Whatever it was that caused the Germans to take this drastic step brought about a serious decrease in diesel power available and a resultant serious unbalance in the machinery plant.  A possible reason lies in the damage that would be caused to the exhaust gas turbine should it receive a slug of water during snorkel operation.  However, the exhaust gas driven blower on the 9 cylinder MAN engine on type IX submarine was not removed when the snorkel was installed, so that this reason for its removal is not conclusive.  
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  REPORT 2G-21
          The original diesel engine was provided with an ahead and a snorkel cam but no reversing cam.  The ahead cam was not changed upon removal of the supercharger with the result that the engine could only be operated on the snorkel cam setting.  The designed rating of the engine operating in this manner is 1180 HP at 470 RPM.  This rating has been reached during trials of one of the vessels in US custody but in so doing exhaust temperatures exceed the allowable maximum.  
          The starting, throttling and cam shifting arrangements on the XXI diesel engine are similar to those on the 9 cylinder MAN on the type IX submarines.  Also, pistons, valves, ejectors, attached pumps, and associated gear are for the most part interchangeable with the corresponding 9 cylinder engine parts.  
          The air induction piping has little change from the IX C design.  The outboard main induction valves are placed within the after section of the faired conning tower and bridge structure.  Air intake slits and louvers are built into the faired structure to provide the necessary supply of air to the valve.  These louvers act as baffles to deflect any water taken on with the air downward and away from the air intake.  The port induction is used only for engine air supply when on the surface, while the starboard induction line is used for engine and ship's ventilation air supply when submerged.  A lead-off line to the ventilation air supply is located just below the ventilation fitting.  A slight change in design to both the outboard and hull induction valves has been made.  The changes in both cases have been on the hull fittings and not in the valve proper and are unimportant.  Although the sketch and the instruction books for the XXI indicate that the outboard valve is provided with hydraulic operation, it was not built into the vessel.  
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  REPORT 2G-21
          The exhaust gas system has been changed to conform to a different operating concept with this type of vessel.  The importance of noise reduction during surface operation was evidently considered much reduced from previous concepts.  This naturally led to a simplification in the exhaust gas piping by the elimination of the muffler and spark arrestor, and by provision for underwater discharge of the gases.  A very slight, but negligible increase in back pressure is required to overcome the slight water head and care must be maintained when starting the engines to prevent flooding.  However, it is presently an operating practice on US submarines to delay opening the outboard exhaust valve until the engine has started so as to remove water from the exhaust line.  
          The inboard and outboard exhaust valves have some improvements in design over former types.  They both have independently removable discs and seats so that new disc and disc inserts can be installed without removing the valve from the vessel.  Also, the inboard valve operates about a 45° arc in each side of the vertical, and is no longer counterweighted; the outboard seats with the force of gravity instead of opposed to it as on the type IX vessels.  Both valves have the disc rotating arrangement used on earlier German designs.  However, in spite of their improvements, extensive overhaul of the exhaust valves on the U-2513 and U-3008 indicates that they are still far from satisfactory.  The high temperatures to which these valves have been subjected have not only brought about necessity for removal of disc and seat bearing surfaces, but also have caused an excessive amount of warping of the valve body.  
          The differences in snorkel installation on the XXI over that on type IX and XB vessels are for the most part related to the mast installations.  The telescopic mast on the XXI imposed different problems in design.  The teardrop cross section was dropped and independent but interconnected air intakes and exhaust cylinders were used.  The air intake part of the mast has a float type valve at the top and a gear rack attached to one side.  The rack meshes with a pinion that is, in turn, driven by an air motor within the vessel.  (The air motor is not satisfactory as an excessive amount of time is required to raise the mast.)  At the bottom  
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  REPORT 2G-21
  of the air mast is a machined cylinder of slightly greater diameter than the rest of the tube.  The air outlet opening is in the middle of this machined section.  When the mast is in the raised position two rubber gaskets (similar to those used for German periscope packing) with doughnut type inserts, form a seal on the machined surface above and below the air outlet.  The gaskets are attached to a housing built on to the top of the pressure hull through which the mast passes and are designed to be tight against external pressure.  Two bearings are in this housing to support the mast.  The exhaust cylinder on the mast slides within a fixed cylinder secured to and passing through the pressure hull.  A gas tight seal between the cylinders is provided by a rubber gasket that is secured to the flanged upper surface of the stationary tube.  The exhaust gas pipe from the engine room is secured to the stationary cylinder.  When the mast is raised the port formed by this fitting is uncovered.  The exhaust opening at the top of the snorkel mast has been considerably simplified from earlier designs.  The deflectors used to direct the gasses away from the intake have been eliminated so that the gasses pass directly out of the mast through elongated slits.  When the snorkel mast is lowered it houses in a free flooding well that passes through the pressure hull to the keel.  
  3.  Conclusions  
          The attempt by the Germans to provide a high powered, small size diesel engine for the proposed operation of the XXI submarines that could use interchangeable parts from the 9 cylinder MAN engine proved a failure and created a serious weakness in the vessel as finally delivered.  The forced removal of the exhaust gas driven supercharger from the engine decreased the useful output of the engine by nearly half the original designed rating.  Furthermore, the engine, on test, both with and without the supercharger, was limited in output below designed ratings by excessively high exhaust temperatures which exceed those permitted in US submarine practice by 350° F., have created trouble in all exhaust valves as they are not suitably designed to take this temperature.  
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  REPORT 2G-21
          The omission of the muffler from the exhaust system eliminates a prevalent source of trouble, both as regards maintenance and design, from the vessel.  This omission is considered sound practice for the XXI submarine inasmuch as this vessel was not designed with the concept  of any prolonged or high speed surface running when in enemy controlled waters.  
          At the present state of development it is believed that the folding type of snorkel holds a slight advantage over the telescopic type.  Although the former takes up considerable space within the superstructure when housed, it has a more satisfactory hoisting arrangement and has simpler and more satisfactory seals against salt water entrance.  The well within the ship on the XXI takes up valuable space and from a strength standpoint forms an undesirable type of fitting passing through the pressure hull.  However, numerous possibilities present themselves for improvement in the XXI design, either through changes in the present system wherein a free flooding well is used, or by the installation of a type that can be housed within the pressure hull without resort to a pressure proof well like a periscope.  
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