A Treatise On The Principles And Practice Of Dock Engineering

92 DOCK ENGINEERING. Straight, or spiral blade milling cutters, mounted around and concentrically with the end of the suction pipe. They consist of a number of knives (from 10 to 15) united by suitable discs, or rings, at one or both ends. The whole cutter may be secured to the end of the suction pipe and rotary motion imparted to them together, or the cutter shaft may be journalled in a suitable bearing provided in the end of the suction pipe, which is then made stationary. The use of cutters is only practicable in fairly smooth water; in situations where there is much swell, other means must be found for loosening and disintegrating the material to be removed. One alternative expedient is the application of numerous water jets through a series of orifices, specially provided for the purpose in the bars which traverse the mouth of the drag-piece, and communicating by means of suitable ports with a pipe riinning along the front of the mouthpiece. This system of nozzles is supplied with water under pressure through a flexible pipe The result is much inferior to that attained by the action of cutters, and, in order to obtain the best eflect, it is necessary to concentrate the pressuré of the jets upon a small surface, and to direct the stream towards the intake pipe. The value of the cutter appliance in dealing with beds of hard sand has been abundantly demonstrated on the Mississippi, the Scheldt, and the Volga But after witnessing a number of triais of a similar type of dredger upon stiff clay, the writer is inclined to doubt the efficacy of the system&in dealing with material of an argillaceous character, though he is prepared to admit that much may depend upon the precise form of cutter adopted. In this view he is confirmed by some remarks made by Mr. J. H. Apjohn at a recent engineering conference, which, indeed, are worth quoting as demonstrating the scope existing for experimental investigation.* “The author’s experience of rotary cutters has been with a dredger designed for the purpose of excavating clay for dock extension. The clay being silty, it was thought it would be easily broken up by the cutter, but this was not the case. The cutter had fourteen straight knives, set at an angle of 26° to the tangent of the circle round which they were placed and overlapping each other to a slight extent. The dredger was first operated at a small depth where the soil was brittle and the cutter proved efficient, but when the clay was reached at a greater depth, the openings between the blades of the cutter clogged with the tenacious plastic clay, with the lesult that the proportion of clay found in the water discharged through the pipe-line was extremely small. The cutter was then unshipped, and a width of some inches was cut off the inner edge of each blade, so that the overlap was done away with, and at the same time the circular opening at the bottom of the cutter was reduced in area. When again tried °the cutter worked better, there being but little clogging between its blades, Apjohn on “Dredging with special reference to Rotary Cutters,” Proc Enn Conf., London, 1903.'6’