The same, low capacity distilling unit installed on the IX-C submarine has been adopted on the type XXI submarine.  Only minor changes have been made in the external piping to the unit.  However, because of the larger battery, the total capacity of the battery water tanks has been materially increased.  Ten individual tanks, four for the after battery and six in the forward battery, are installed.  No ready means for filling the tanks is provided.  
July, 1946
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  REPORT 2G-21
  1.  Introduction  
          The distilling plant on the IX-C submarine is discussed in detail in Report 2G-9C-S58.  Only the differences that exist on the XXI will be included in this section.  
  2.  General Description  
          The 63.5 gallon per day distilling plant on the type XXI submarine has to furnish replacement fresh water to a crew of approximately 57 men and battery water for the 372 battery cells. From a design as well as heat standpoint, the still should be operated only on the surface.  The feed water for the still comes directly from the circulating water main.  During submergence the latter is at submergence pressure so the feed water must be throttled in order to prevent rupture of the light tanks and piping leading up to the distiller.  
          The XXI installation has been simplified slightly from the IX-C arrangement by the elimination of the gravity distillate tank.  Also, in lieu of a portable hose connection to the battery water tanks, a simple spigot arrangement on the main distillate tank is used.  It is necessary to transport the water in containers to the individual battery water tanks.  
          Each of the ten battery water tanks holds 150 liters (39.6 gals.) of distilled water.  They are situated between the batteries on the upper battery flats, 4 in the after battery space and 6 forward.  
  3. Conclusions  
          The same comments made on the IX-C report in regard to the low standard distiller capacity adopted by the Germans apply to a more pronounced degree on the XXI.  
          Little reliance can be placed on the distiller to furnish replacement water both because of its low capacity and because of the limited time when on patrol that the vessel would be on the surface to operate the unit without hazard.  
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  REPORT 2G-21

This limitation in distiller capacity has far reaching effects on the vessel's operation.  Even though it is assumed that none of the replacement water would be made available to the crew, it still is not sufficient for the batteries for any normal length of patrol.  The total battery water carried is limited to approximately 400 gallons; this reserve would soon be used even in mild climates if U.S. practices were followed (500 gals. every 4 to 10 days are used during patrols, dependent on conditions).  The need for conservation is thus drastic.  The baffle arrangement in the individual cell ventilation has some effect on limiting the amount of carry over during battery ventilation.  However, to reduce the evaporation to required levels, battery ventilation had to be reduced to dangerous levels and the use of ship's ventilation had to be limited.  Extreme hardship on the crew would surely result during operation of the vessel in tropical waters and the duration of stay would be limited by the high rates of evaporation from the cells.

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