The USS Potomac was FDR’s “Floating White House” — the boat where he ran the country, secretly slipped away in the fog to meet Winston Churchill mid-ocean, and could move freely without the world seeing that he couldn’t walk.

But after his death in 1945, it fell into decades of neglect (save for a brief moment when Elvis Presley bought it) before it was left to rot and eventually sank in the San Francisco Bay.

We spent the day aboard with Ford Roosevelt, FDR’s grandson and executive director of the USS Potomac Association, who walked us through the ship his family spent 15 years fighting to save — down to the elongated fantail chair built for his grandfather’s leg braces and the crew’s cramped “hot bunks” belowdecks.

Elvis bought the Potomac at auction in 1964 and donated it to charity, but the ship kept changing hands after that until it sank and the Port of Oakland bought the wreck for $15,000.

The restoration that followed dragged on for a decade and a half until FDR’s son James lobbied President Reagan, who earmarked $2.5 million to help finish the job, matched by another $2.5 million in private donations and thousands of volunteer hours. Ford was in the White House the day the Reagan deal came together, and he shared that story with us firsthand.

Along the way, he pointed out the small, personal details that don’t make it into the history books: the custom bathtub FDR lowered himself into by hand, the radio room where he delivered his fireside chats, the sons he’d grip for support to fool the country into thinking he could walk.

Docked today at Jack London Square in Oakland, CA, the Potomac is a National Historic Landmark — a small wooden boat that carried an outsized piece of American history, and somehow survived to tell it.