In 1771, the owners of the ironworks near Ringwood, New Jersey, tapped Erskine to replace Peter Hasenclever as iromaster after Hasenclever's profligate spending nearly bankrupted the operation.
Erskine immediately set about trying to make the operation profitable. His efforts were cut short by the American Revolutionary War. Erskine was sympathetic to the American cause, but worried that might lose his workers to the army. He organized them into a militia and was appointed a militia captain in August 1775.
Once the war broke out in earnest, there was concern among the rebels that the British warships would use the Hudson river to attack northern forts and separate New England from the rest of the colonies. Erskine, ever the engineer, designed a tetrahedron-shaped marine Chevaux-de-Frise -- essentially a barrier that would keep warships from moving upriver.