Someone forgot to preface that estimate with "Under ideal conditions..." Implementation of new weapons system technology takes time. Inventors trying to get official interest in their developments emphasize the strengths and minimize the weaknesses. Sea return remained a problem for decades, so it was difficult to differentiate a periscope from nearby waves in anything but the calmest conditions. Operators needed to be trained in the use of new equipment, and watch officers needed training in interpretation of of their operator's observations. Such training was hampered by the military secrecy of RADAR development. Finally, a small escort ship was a difficult environment for electronic devices of that era, and operational reliability could be rather low under combat conditions in the North Atlantic. =AL=