Propulsion and Speeds  
   
          Speed is rarely noted in U-boat KTBs.  Rather, U-boat COs referred to the setting and configuration of the engines.  Unless otherwise noted, it may be assumed that both engines were running together.  Most often the CO would enter the abbreviation for the telegraph setting.  The table below gives the propeller revolutions per minute and resulting speeds for Type VII and Type IX U-boats for various telegraph settings.  
   
 
 
Surfaced
Type VII
Type IX
German Abbreviation English R.P.M. Speed R.P.M. Speed
Kleine Fahrt K.F. dead slow 161 6.1* 160 7
Langsame Fahrt L.F. slow 263 9.63 240 11
Halbe Fahrt H.F. half speed 310 10.98 300 15
2x H.F. 3/5 speed 404 14.24 400 17.5
Grosse Fahrt G.F. 3/4 speed 469 16.73 460 19
2x G.F. 4/5 full speed
Äusserste Kraft A.K. emergency speed 485 17.29 480 20
Dreimal A.K. 3x A.K. utmost speed
*Speeds on two engines in calm water
For a short burst of high speed on surface, if the state of the batteries permitted, speed could be increased about half a knot by running the electric motors and Diesels simultaneously. 
U-boats sometimes used "Diesel-Electric" (dieselelektrisch) propulsion, running only one Diesel; it drives its own shaft and motor which, acting as a generator, supplies power to the motor on the other shaft.  The electrically-driven shaft makes about half the revolutions of the Diesel-driven shaft.  Economical Diesel-electric speed is about 6 knots.  It has the advantage that the idle engine can be serviced, and that the operating Diesel engine can run at its most efficient speed. 
       
     
Submerged
     
Type VII
Type IX
Kleine Fahrt K.F. dead slow 60 1.5 60  
Langsame Fahrt L.F. slow 90 3.0 90  
Halbe Fahrt H.F. half speed 160 4.1 110  
  2x H.F. 3/5 speed 200 5.23 160  
Grosse Fahrt G.F. 3/4 full speed 233 6.1 180  
Äusserste Kraft A.K. emergency speed 280 7.4 210  
 
  *R.P.M. and Speeds above are from the British Cumulative Edition (CB 04051 103)  Those for the type VII were observed during testing of U-570.  
  Great stress is laid on finding the most silent running speed for each U-boat.  It is generally  about 90 r.p.m. and this "silent" speed is known as "Schleichfahrt".   
     
     

 


 

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