from The Century Dictionary and Cyclopedia
- n. In a screw steamer, a passageway along the shaft as far aft as the stern tubing, affording an opportunity for thorough examination of the shaft and its bearings: known in the United States as shaft-alley. Also shaft-tunnel.
Sorry, no etymologies found.
Notwithstanding my age, I was so curious to see an ironclad that I went all over the "Resistance," even to the engine-room and screw-alley.