Question

How did the French contribute to the American victory at Yorktown

Answer 1

French fleets of ships in the harbor had blocked British reinforcements from coming to the aid of the failing British soldiers.
I hope this helps.

Answer 2

In order to stop the British troops arrival to Yorktown at the entrance to Chesapeake Bay in Virginia, the French forces carried out a strategic plan of attack. After cutting off supplies to the British by blocking Chesapeake Bay with a fleet of 28 ships, French and American soldiers mobilized on ground against British troops. As a result, British were forced to surrender on October 17th, 1781.