Soak navy beans overnight or several days in 1 1/2 quarts water, changing water as necessary.
Melt butter in a stockpot; sauté onions and celery until slightly tender.
Add beans and beef stock; simmer partially covered, for about 1 hour or until beans are just tender, stirring occasionally.
Add ham, potatoes and carrots. Return to a simmer and cook until vegetables are almost tender.
Season with pepper and thyme; simmer, uncovered for 15 minutes.
Add water as necessary during cooking.
Serve hot. Makes 8 servings.