What is the title of the second in command?

Answer 1

The 2nd in command of any naval vessel or command is known as the Executive Officer, or XO for short. In the Army and Marines, it's typically the Chief of Staff or something similar, but XO is also used.

Rank doesn't make any difference - the title is given to whoever is the Commanding Officer's 2nd under him. This is true for both ships and shore installations.

Answer 2

Aboard ship it's the First Mate (or First Officer); on aircraft it's the co-pilot. In politics it could be anything from Vice President, Prince,

Answer 3

Th first in charge of a ship is obviously the Captain. The second is the

First Mate.