The detailed information with regard to the design and application of auxiliary machinery is given in the applicable "S" group reports, German practices in sound isolation and shock mounting are all that is treated herein.  
          The extensive use of secondary means of accomplishing these ends on installations in the IXC vessel indicate the German's appreciation of the need for sound isolation and shock resistance.  
          In sharp contrast to USN practice, the Germans depend, in nearly all applications studied, upon the adhesion obtained at the fraying surfaces of the rubber and metal used in the mounts.  Locking or jacking features as a backup protection in event of failure of the mount are not employed.  
          Tests on representative mounts indicate that the adhesions between rubber and metal are not as good as those available commercially in U.S.  
          Results of sound tests conducted by USN personnel indicate that German auxiliary installations are generally quieter in operation than comparable USN installations.  
July, 1946
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A. Introduction
B. Descriptive
C. Conclusions
D. Plate #1
          Results of tests on a representative German rubber mount used for sound isolation.  
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  A.  Introduction  
          The scope of this report is to indicate the German methods and application of sound isolation and shock mounting employed in this type of vessel and to delineate the major differences between German and USN practices.  
          The overall German sound isolation and shock mounting program is excellently treated in detail, both general and technical, in Nav Tech Report No. 251-45 dated August 1945.  
          A sound test was conducted by the Base Sound Force, New London, Conn. in Gardner's Bay, Long Island, New York to determine the noise levels in water caused by the various German auxiliary machinery installed in U-858.  The results of the test are contained in report form and have been forwarded to the Chief of the Bureau of Ships by the Commanding Officer, Subbase, N.L. ltr, NB7/S68 E Serial E-C23 of 18 June, 1946.  
  B.  Descriptive  
          1.  Sound Isolation  
          The German's employed to a large degree a secondary means of sound isolation on practically all auxiliary machinery except the Junkers Air Compressor.  The wide-spread application of sound isolation, when taken together with the information contained in Nav Tech Report 251-45 pertaining to German conceptions of sound isolating main propulsion plants, leads on to believe the ultimate goal of the Germans was to attain a relatively quiet vessel even when snorkelling.  
          In all cases the German depended on the bond of rubber to metal, usually copper plated steel, for sufficient strength under shock conditions.  Built in or external locking or jacking devices are not installed.  
          The rubber mounts take various forms to suit different installations much the same as available commercially in U.S. today.  However, most mounts were presented to the shipyard in desired cross sections of approximately 2" x 2", 2" x 3", 2" x 4", and 2" x 5", the thickness being 2", in lengths of six feet.  These were cut in desired lengths and the metal drilled and tapped to suit the installation by the yard.  The lengths varied to suit the size of the auxiliary.  
          On the larger installations the application was as follows.  Two lengths were installed, on on either side at one end, being arranged parallel to each other fore and  
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  aft.  A third length, usually the full width of the foundation or in some cases two lengths approximately 12 inches long, was arranged to run athwartships.  
          Plate #1 delineates the results of tests made on a representative German mount of the type described above.  Of particular interest is the relatively low adhesion strength between the rubber and metal, it being only 262 pounds per square inch.  It is understood that commercially, mounts of this type are available with adhesion strengths up to 500 pounds per square inch and that adhesion strengths approximating 1000 pounds per square inch have been attained in USN Laboratories.  
          In employing a single type mount to serve for both sound isolation and shock mounting, it is necessary to accept a compromise between the two requirements.  The German in using a bonded mount attained better sound isolation and due to the relatively poor adhesion poorer shock protection.  USN practice has been to emphasize the shock protection requirements and to accept somewhat poorer sound isolation characteristics in their mounts.  
          Nav Tech Report 251-45 contains considerable information with regard to the background used in designing and accepting the mounts employed as well as "future" design considerations.  In addition the text of the report is copiously cross referenced with numerous design calculations and diagrams as well as outline pictures of the mounts used.  Other means used by the German to obtain sound isolation and shock resistance are enumerated below and in some cases, comparisons with comparable USN installations are drawn:  
          (a)  Pipe couplings are installed in salt water systems similar to those installed in USN systems, but whose basic difference lies in the following.  
                  1)  They depend in part upon the bonded rubber for strength.  
                  2)  Only one coupling is used on either side of the given pump rather then the two required in USN practice.  
                  3)  Both rubber pieces are of relatively equal size and deflection area which is desirable from a vibration viewpoint.  USN couplings are provided with one thick and one thin rubber piece, the latter having no deflection are thus making the unit effective only over half a cycle.  
          (b)  All hydraulic piping is mounted on bonded rubber  
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  mounts which again depend upon adhesion of rubber to metal for shock protection.  This is understood to have been done because of inability to develop a satisfactory hydraulic hose connection.  Auxiliaries supplied by the system are not sound isolated.  
          (c)  Rubber ducts of suitable shape and size are inserted between the rubber mounted ventilation supply and exhaust blowers and the related ducts.  
          (d)  Refrigerant piping is not provided with flexible metal or rubber hose sections of any kind, dependence being placed on several turns of the piping coiled in loops of approximately 12 to 14 inches in diameter.  
          (e)  Rubber collar type couplings which depend upon the adhesion of rubber to metal are used extensively between motors and auxiliaries, even in such vital installations as the rudder and plane operating gear.  Locking devices are not employed.  
          (f)  Considerable attention has been devoted to torpedo handling gear, the loading rack being mounted on pads of rubber, thru bolts being employed in this case.  A noteworthy feature is the chain fall arrangement which provides ecliptically shaped rubber cushions molded at approximately five inch spacing on the chain.  These rubber cushions engage in the jaws provided in the drive wheel much the same as anchor chain in a "wildcat".  
          (g)  Cables to motors and controllers are looped to provide reasonable freedom of motion between the auxiliary and the first strap hanger.  Sheet rubber is not employed between the cable and the strap hanger.  
          The following practices in equipment design with their attendant effects on noise reduction are enumerated.  
          (a)  Valve designs show a trend toward avoidance of sharp edges and corners in order to decrease "flow" noise and cavitation.  
          (b)  While the Nav Tech Report indicates the Germans had considered building up small back pressures in liquid lines to reduce cavitation and venting sounds, no installations of this type have been observed in this class vessel.  
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          (c)  Roller and ball bearings are used in the majority of auxiliaries, however, information brought back by Nav Tech Miss Europe personnel indicate that the German was aware of the desirability of using sleeve bearings with their attendant improvement in sound isolation.  
          (d)  German auxiliary motors are very compact, carefully designed to minimize abrupt changes in way of ventilation flow, and, more important, spacing between field pole iron is arranged to provide minimum space between adjacent poles thus in effect increasing the mass of the frame, and thereby minimizing the possibility of vibrational disturbance.  Armature slots are kept as narrow as possible.  
          (e)  Satisfactory grease and oil cup fittings for inboard parts, and greasing manifolds for outboard installations are provided.  
          (f)  Increase in foundation mass to attenuate vibration has not been considered.  
          (g)  I.C. transmitter indicator systems are mounted on bonded rubber mounts in compression, tension and shear or sound isolation and shock protection.  
                I.C. systems which are provided with audible alarms are also provided with a transfer switch to change to flashing light signals when operating under evasive tactics.  
          (h)  The shaft R.P.M. transmitter is coupled to the shaft by means of V-belts or in some cases with sprocket chain.  In both cases the transmitter shaft is coupled to the pulley shaft thru bonded rubber mounts.  
          Careful consideration had been given each installation.  Observed practices are reasonably uniform and sound "short circuits" have not been observed.  
          As nearly as can be determined the results of any sound tests made by the German's on the installed auxiliary machinery were not distributed in report form to the vessel tested.  
          No means of self monitoring noise levels of ships auxiliaries is provided.  
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          2.  Shock Mounting  
          External shock mounting is provided in nearly all cases as compared to the USN practice of designing hi-shock components.  
          The mounts provided for sound isolation of auxiliary machinery also served for shock-mounting.  
          Items in the categories indicated below are mounted on bonded rubber mounts in compression tension or shear for shock protection.  
          a)  Main control cubicles.  
          b)  Switchboards.  
          c)  I.C. Systems.  
          d)  Gauges and gaugeboards.  
          e)   Electronic equipments.  
          Neither felt nor any material such as "Fabreeka" has been observed in use as shock mounting.  
          Lighting fixtures are not shock mounted nor have the lamps been designed with high shock features incorporated in USN lamps.  
  C.  Conclusions  
          The German approached the problem of sound isolation and shock mounting more elaborately from the secondary viewpoint than is current in USN Practice.  However, USN developments along these lines from the primary viewpoint (redesign of equipments as required) are believed to be in advance of German practices due to the fact that his awareness of these changes had not been incorporated in his equipment.  
          In spite of the different emphasis in this approach to the matter of sound isolation, the sound test results on the U-858 indicate that a very quiet vessel had been achieved.  
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  TEST NO:  ___9033___________________  DATE:  _______8 Nov. 1945___________  
  MATERIAL:  ____Rubber Motor Mount____  NO. OF SAMPLES:  ____one__________  
  SOURCE:  _____German Submarine_______  AUTHORITY:  ___Mr. Galle___________  
  DIMENSIONS OF RUBBER:  2 inches thick, 2 inches by 4 inches in cross-section.  
  TYPE OF RUBBER:  Chemical tests indicated that rubber was Buna S.  
Volume swell in SR-6 immersion media )par. F9 of Navy Dept. Spec. 337d dated 1 Aug. 1944), % 281
Required by Spec. 33G7d. max. % 95
Hardness, Shore (type A) durometer 50
Adhesion, rubber to metal, psi. 262
    (Plates separated at rate of 1.0 inch/min.  Plates had been brass or copper plated before bonding to rubber)  
Load - Deflection relationship (Compression Test)  
Load (total)
          Above Load - Deflection values were the average of 2 readings taken at machine speed of approx. 0.4 inch per minute after mount had been compressed to 1/4 inch deflection and relaxed 8 times.  



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