Isn't the answer at the end of the cutting ... nothing to do with a screw on the vessel (which was dumb) but taking out or placing some sort of 'screw mooring', presumably one literally screwed into the river bed?
Thanks George, never occurred to me that you would screw a mooring into the river bed, but then they were banging piles into there so why not screwing things in as well. I live and learn. Unfortunately I forget more than I learn.