Original Pinner said: Over 150 years old, the Laxey Wheel, a.k.a Lady Isabella, is the largest working waterwheel in the world at 72 feet in diameter. As an American, I have seen many water-powered grist mills on the small wooded streams of the Appalachian Mountains and Piedmont. But these working-collar wheels seem crude compared to the beautifully engineered backshot Laxey Wheel on the Isle of Man.