He was the son of Alexander Schank of Castlerigg, Fife, Scotland. He entered the Royal Navy when young. His remarkable skills at mechanical design earned him the nickname "Old Purchase" following his design and construction of a cot fitted with pulleys that allowed it to be adjusted by the person lying in it. As a lieutenant in 1776, he was placed in charge of assembling ships to battle the American Revolutionaries on Lake Champlain. In less than six weeks, he constructed HMS Inflexible, which he then commanded as part of a fleet that defeated General Benedict Arnold's fleet in October 1776. His talents as an engineer were applied in General John Burgoyne's expedition to the building of floating bridges.