Author Paul Halpern reports that U-23 and her three sisters all entered service between August and November 1917 .
U-23 was launched on 5 January 1917 , but It is not known with certainty when U-23 was commissioned .
Author Paul Halpern reports that U-23 and U-23 's three sisters all entered service between August and November 1917 .
U-23 was launched on 5 January 1917 , but It is not known with certainty when U-23 was commissioned .