Why was Francis Scott Key on a British ship while writing the Star Spangled Banner?

Francis Scott Key wrote the poem Defence of Fort McHenry, which became the US national anthem, The Star-Spangled Banner.

On September 3, 1813, Key had traveled with mediator John Skinner to the British fleet off Baltimore to obtain the release of a friend being held there, Dr. William Beanes.

After Beanes was freed, the three were held overnight aboard a truce ship until the British concluded an attack on Fort McHenry in the harbor. The following morning, inspired by the sight of the US flag over the fort, Key wrote the poem about the battle. Some weeks later, it was popularly circulated, set to a well-known melody, and renamed. In 1931 it was named the national anthem.