Coco Solo
Following several practice dives out of Coco Solo, she transited the Panama Canal on 27 January 1920 for drydock work at Balboa. Returning to Coco Solo, "R-21" continued to refine her diving, approach, and torpedo tactics through the spring and summer. During two trips to Almirante Bay, 30 March-2 April and 17–20 May, she practiced with other units of the first division.