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

The machine gives also the important advantages of enabling faster progress to be made, and economising time, as none is lost in disputes with the men ; the cost of excavation can therefore be estimated with greater accuracy, and when once it has been determined for a given cutting, the rate may be relied upon till its completion. The contractor therefore is practically independent of strikes in this department of his business ; and is further relieved of the trouble of getting together so large a body of men, and of housing and providing for them in out-of the-way districts.

Subaqueous Work. —The navvy can also be worked for excavations under water, for which purpose it is mounted on a pontoon, as shown in Fig. 10.

Figure 10 - Dunbar and Ruston's Steam Navvy - Subaqueous View.

The pontoon is about 50 ft. long by 22 ft. wide, and draws 24 ft. of water ; it is built specially to receive the machine, and is most conveniently made of iron ; but where timber is on the spot and plentiful, it may be of course substituted with advantage. For better distributing the weight in the pontoon, the frame of the navvy is divided transversely, and a distance-piece is bolted in between for completing the continuity.

Extra stays are carried back from the top of the tower down to the engine end ; and an extra roller is put in, to take the sag out of the main chain. Rollers are also provided under the step of the jib for the drawing back chain that winds upon the drum on the second shaft. The bucket arm has of course to be lengthened, according to the depth to be taken out ; and the bucket is made with rather more taper, for the stuff to clear itself, and is pierced with holes to let out the water. The action is similar to that on land, except that the resistance of the water somewhat impedes the back swing of the bucket, necessitating the use of the drawback chain.