Instead of having either screw or paddle wheels, both of which would, by their wash, damage the banks of the canal, the engine turns a large " sprocket" wheel amidships.
These machines are chain-boats.  Instead of having either screw or paddle wheels, both of which would, by their wash, damage the banks of the canal, the engine turns a large " sprocket" wheel amidships.  A chain, which is laid the whole length of the canal, is brought to this wheel, and the tug winds herself along, much after the fashion of the floating bridge at Portsmouth.