Beatty as the longest-serving First Sea Lord

Who was the longest-serving naval chief, and how long did he serve?

David Beatty is identified in the discussion's Royal Navy context, although the answer does not give an exact tenure length.

David Beatty is initially offered as the likely answer because of the number of years he remained in charge. A check during the stream confirms the identification. The exchange points to Beatty's unusually extended tenure at the head of the Royal Navy after the First World War. The exact length of that tenure is not stated in the answer.