A lieutenant could command a snow as master and commander

What rank would command a snow in the Royal Navy?

Usually a lieutenant acting in command of the vessel as master and commander, much like the commander of a brig.

A snow and a brig occupied broadly similar command territory in the Royal Navy. The officer would normally hold the substantive rank of lieutenant rather than midshipman. While commanding his own vessel, he acted in the command role associated with master and commander. The distinction separates an officer's permanent rank from the authority of the appointment he held aboard ship.