Stir in broth, water, thyme, and bay leaf. Bring to boiling; reduce heat. Simmer, covered, about 1 1/4 hours or until meat is nearly tender. Discard thyme and bay leaf. Stir in potatoes, turnips, and sweet potato. Bring to boiling; reduce heat. Simmer, covered, for 15 minutes more or until meat and vegetables are tender. If desired, serve the soup with crackers.