most acts of sabotage occured while using conscripted workers and non ethnic german dock workers at bases that were in other nations like Norway and France. Since so many working men of Germany were working for the war effort there was a labor shortage especially later in the war, this meant that sub pens often had to rely on french craftsman, engineers, mechanics and the like to service the submarine fleet.
Though acts of sabotage hit some bases harder than others and the problem was not rampant... many boats were frequently sabotaged in some way or another. Hans Goebler's book "Steel boats Iron hearts" points out several cases of sabotage to his boat and others. From bearings and seals being soaked in acid, to small holes drilled in the fuel and oil tanks to dead dogs placed in the drinking water storage tanks etc.
Though some subs may have suffered from sabotage during their initial construction in Germany, most of it occured in the outlying bases.
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