Cover and cook the sweet potatoes in boiling salted water for 8 to 10 minutes or until sweet potatoes are just tender. Drain well and let cool.
In a large bowl, combine the cooled sweet potatoes, the celery, and the onion; set aside.
In a chilled bowl, combine whipping cream, nutmeg, dillweed, and pepper. Beat mixture with chilled beaters of an electric mixer on medium speed until soft peaks form (tips curl). Beat in the mayonnaise. Add the dressing to the sweet-potato mixture, tossing very gently to coat. Cover and chill salad up to 4 hours. Makes 8 side-dish servings.