In a 4- to 5-quart pan over high heat, stir bacon frequently until crisp, about 5 minutes. With a slotted spoon, transfer bacon to paper towels to drain. Discard drippings from pan or save for other uses.
Meanwhile, scrub and peel potatoes. Cut into 1/2-inch chunks.
In same pan, combine potatoes, rice, onion, 4 cups broth, and thyme. Bring to a boil over high heat; reduce heat, cover, and simmer until rice is tender to bite, 1 to 1 1/4 hours.
Add remaining 2 cups broth, sherry, and whipping cream. Bring to a boil over high heat, mashing any potato chunks against the side of the pan with a spoon. Reduce heat and simmer to blend flavors, stirring occasionally, about 10 minutes.
Ladle soup into bowls. Top equally with cooked bacon. Add salt and pepper to taste.