Commandants to man the following:
An Admiral to man a fleet.
Rear-Admiral to man a fleet or squadron.
A Captain to man a division, or ship of the first or second rating or a ship not rated.
Commander to man a division or a ship of the second or third rating or ship not rated.
Lieutenant-Commander to man a ship of the third or fourth rating or a ship not rated.
A Lieutenant to man a ship of the fourth rating; a torpedo boat destroyer, torpedo boat, tug, tender or a ship not rated.
A Lieutenant, junior grade, to command a torpedo boat, tug, tender or ship not rated.
An Ensign to man a torpedo boat, tug or ship not rated.
William McKinley, Executive Order Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/205639