If I remember my local history/stories right...
Many years ago (when horse boats were still common) one man used to walk his horse over the tunnel and leave it in some grazing for the night, go back to his boat, wait until about midnight, rig up a sort of sail using the cabin shafts and some of the cloths, shove his boat into the tunnel mouth and then go to bed, relying on the draft to propel his boat through.
Apparently this worked quite well, until morning an FMC steamer and her butty were out very early and hit the boat.. No-one was hurt, but the make-shift rigging was completely annihilted and some choice expletives were heard echoing through the tunnel