Escalate things upwards size-wise - shafts on large ships are hollow as well. Even if the shaft were solid, the torque is transmitted by the outer layer of metal only, so there’s no point wasting fuel turning something that adds nothing. Torque is carefully measured and monitored on a ship, particularly warships, as they can apply enough torque to seriously damage the shaft and bearings. Not sure how big a ship would need to be before they have hollow shafts though 