Chief of The Boat

The Chief of the Boat (COB) is an enlisted position aboard some starships, especially those with a larger enlisted complement relative to the overall size of the crew (i.e. ships where more than 50% of the crew is enlisted). The COB is usually (though not always) the most senior enlisted crewmember aboard, and is tasked with maintaining the ship and crew. In many cases, the COB is jokingly referred to as the crew's mother, in that the COB will act to improve crew morale and ensure the crew is properly trained and prepared for a given task. 
In order to hold the position of COB, the crewmember must be of a senior rank (relative to other enlisted, but typically at least Chief Petty Officer). Like the Second Officer, assignment of the COB is usually at the discretion of the Commanding Officer, and is outside the normal chain of command and not part of the Senior Staff.