ELECTRIC DREDGE.

Hauraki Plains Gazette, Volume XXXVI, Issue 4859, 31 July 1925, Page 2

ELECTRIC DREDGE.

SUCCESSFUL TRIAL RUN. WORK ON THE WAIHOU RIVER. In company with the resident engineer and officials of the Public Works Department a representative of this paper had an opportunity of attending the official trial run on Tuesday of the newly-constructed 12-inch electric suction dredge at present located on the Waihou River near, Laughlin’s Bend. Although electricity has been used with great success in the United States and on the Continent of Europe, to apply this power to such a machine in New Zealand opens up a new avenue for in the drainage and flood protection or vast areas of land. The distance over which the machine will operate on the Waihou River depends very’largely on economic conditions, and the success met with. A number f factors have to be considered before the ultimate success of the dredging scheme can be gauged. In dredging a navigation channel two factors have to be kept in mind. The waters- of a normal flood tend to; keep to the outside of bends, while a heavy flood has a tendency to keep imre to the centre of the river. These conditions present a serious proposition in dredging a defined channel. , A visit to the interior of the dredge' reveals an amazing collection of machinery, each section having its uses, and the whole mass co-operating and working by the electric system. A 12-inch gravel pump is- directly coupled to a 250-h.p. alternating current motor. For the purpose of driving the two service pumps and winches there are also two 25-h.p. A.C. motors installed. As a stand-by in case of failure of the electric current a 30-h.p. Anderson oil engine is. installed amidships, this engine being capable of driving the service pumps- and winches necessary to alter the position of the dredge. Huge flood-lights and small electric lights are provided on various parts of the machine by an independent oil-driven D.C. dynamo.

Probably the most interesting and the most complicated part of the mar chinery is to be found on the starboard side of the dredge. Surrounded by a massive iron grill are thes witching and metering cubicles, which afford automatic protection to the system. The doors of these cubicles are automatically locked and cannot lie opened while the current is on. In case of trouble the service line is "tripped out” automatically. The 11,000-volt current passes from the heavily-insulated service lines into the cubicle, and from there on into a transformer, where the current is reduced from 11,060 to 400 volts. It is the fact that such high voltage can be reduced on a movable plant without interruption to the service lines that its the outstanding feature of the system, and is an epoch-marking event in the uses of hydro-electricity. The fundamental idea underlying the system is to use high -pressure .for economic transmission, stepping down the pressure to a suitable value for use on the motors.

From the traisnformer the current passes to the low tension cubicle, whence it is distributed to one 250h.p. and two 20-h.p. motors. Each •motor is protected against mistake in operation, and also against any abnormal condition that might .arise. The whole system is interlocked with the main switches to effect this result, thus making it practically foolproof. Situated in the bow of the dredge is the pivotted cutter-arm, which is lowered into the bed of the river. Above the .arm is the inlet end of the suction pipe and cutter head. The duty of the cutter head, the blades cf which are manganese steel, is to cut through the sand and gravel and mix it with a proportion of water to enable the spoil to more easily pass tnrougn the pipes. Metal up to nine inches in size will pass through the pipes without undue risk of a blockage.

On the upper deck are situated the engineers’ quartens, comprising sleeping accommodation for three men, and replete with cooking range, lockers, tables, chairs, etc. Attached to the yardarms are various signals for indicating to vessels navigating the river on which side tc pass and where the cables are laid.

On the main deck are situated the huge warping drums, which can be put into instant operation and the dredge moved in the event of anything going wrong.

The supply of electricity is obtained by tapping the Thames Valley Power Board’s linos. The tapping line comes through Laughlin’s, and the high tension lines have been erected by the department along the left bank of the river. To guard against mistakes or accidents the lines are earthed in several places. The current is taken on board from the shore line by means of a heavily insulated threecore armed cable.

Connected to the stern’ of the dredge are the huge pontoons, constructed on steel, floats, to carry the discharge pipe-line from the dredge to the stop-bank. On each pipe there is a ball-joint coupling of. hard steel to permit of flexibility when dredging, and also to enable the dredge to be moved in any direction without risk of the pipes becoming disjointed. A patent quick-release coupling is fitted to save time and to simplify matters in the event of it being necessary to lengthen or shorten the pipe line. At regular Intervals through the pipeline are situated trapdoors, through which the pipes can be quickly cleared in the event of a blockage taking place. One of the most difficult problems which beset the engineers- was how to overcome the difficulty of lengthening and shortening the high voltage cable while both ends of it were fixtures. Happily this was solved and an ingenious method has been adopted.

Once this machine gets into proper working order a staff of three is all that will be required to manipulate it. An electrical engineer will have

charge of the high tension system, and a dredge engineer with a winchman will operate the mechanical side. The steel hull, which is 70ft long by 28ft wide, was built' in the department’g workshops at Tauranga and towed from there to Ngahina. The whole of the electrical and mechanical installation was carried out by the Public Works Department engineers at the Puke workshops, Paeroa.

The primary object in view is to utilise this dredge to complete the existing preliminary stop-banks on the lower Waihou River to the final level; The material required to do this work is to be dredged from the bed of the river, and this work will be carried out in such a manner as to ensure the maximum profitable benefit to navigation.