Volume Eleven - Pre-Victorian to the Present Day - some more aspects - The Steam Navvy and the Dock Extension of 1883 . . .

The pontoon is steadied by three piles or legs, one at each front corner on either side of the jib, and one centrally behind the engine ; these are provided with racks and pauls, and are raised by hand levers ; two or three crabs, with guy ropes attached to anchors or otherwise, are also necessary.

The machine delivers into a barge moored on one side of the pontoon, a second empty barge being brought up on the other side ready to receive the stuff as soon as the first is full. When all within reach of the bucket has been excavated to the required depth, the piles are raised, and the pontoon hauled forwards by the guy ropes ; the piles are then dropped again, and the next piece in front is excavated.

The deeper the face, the better ; indeed a very shallow face can hardly be dealt with successfully. The water also should be comparatively still, as waves lift the pontoon and interfere with the bucket taking its proper cut ; the output can be averaged at about half what it would be on land.

The cost per cubic yard may be taken at about double that of a dry excavation with similar depth of face, up to say 6 ft. ; but it will also vary according to the depth below the water line.

As a dredger the machine has not been extensively tried. For mud and sand it is hardly considered as a competitor of the dredgers in ordinary use ; but where there is a very hard bottom and a fair face to work at, it offers without doubt very considerable advantages.