NEW HARBOUR DREDGE.

Otago Witness, Issue 3806, 22 February 1927, Page 5

NEW HARBOUR DREDGE.

• ENGINEER’S REPORT'ADOPTED 'As the result of the recommendations of the Harbour Board Engineer (Mr J. ‘M G. Wilkie) the Otago Harbour Board has decided to build a self-propelled bucket hopper dredge, 265 feet in length, draught 16ft to 16ft 6in, and an estimated hopper Capacity of 2000 tons. The building will be supervised by Mr Robertson, the assistant engineer, who will have the assistance of h naval architect for the hull and stability trials. decision arises from the adoption Of u-L cn S’neer’s report on the subject ■which was considered at a special meeting of the board on Tuesday, attended by Messrs J. M. Dickson, M.P. (chairman), H C Campbell, H. M. Driver, W. Gow ‘ D Larnach, J. Loudon, IL E. Moller, K. S ■Ramsay, D. F. H. Sharpe W. Wilkinson, and Captain Coll M'Donald. The Chairman said that he was sure that those who had read Mr Wilkie’s report. must have become very interested. Iheie was a vast amount of information in it which could not, but be of value to the board, and also to the board’s engineers, both present and future. There were two reports, one on the harbours Which the engineer had visited, and also a special report dealing with the dredge. It was for members to say whether the discussion,of these reports should be taken in ■open board or in committee. « Mr Moller moved that the board go into conimiftee. He took it that the desire of members "would be to get as much information as possible, and that could best be obtained by a discussion in committee. Mr Campbell said he seconded the X motion. . Mr Ramsay said he was opposed to going into committee. Those outside the board were Wondering what the members were doing, and they desired enlightenment. Mr Gow said that if the subject were to be discussed in open board, he thought the board should first of all discuss tho matter committee and then, if thought desirable, it should go into open board. Ihe motion was carried, six voting in favour and four against. After emerging from committee it was reported that the chairman and Messrs Gow and Loudon were appointed a Finance Committee to arrange and report on finance in connection with the building of the dredge. It was also reported that the chairman, Messrs Cable, Moller, and Wilkinson had been appointed a committee to go into the question of the specifications of the dredge. ENGINEER’S REPORT. Ihe report of the engineer stated inter aha

After careful study-of the dredges and u the conditions of work in London, Liverpool, Glasgow, Hull, Newcastle, and Melbourne, I am of the opinion that the dredger best suited for the board’s requirements;, and the one calculated to give the most • economical results, is a selfpropelled, self-contained hopper dredger, with a theoretical loadirig rate of 960 yards per hour. In my opinion, the length should not exceed 265 ft between perpendiculars. With a reasonable width and on a draft or from 16ft to 16ft ’iin Bitch a dredge should have a hopper capacity of about 2000 tons. If fitted with buckets of one yard-capacity, this dredge should do nearly three times the work of dredge 222, at a little over half the >ost per yard (working expenses only, not including interest and depreciation) Mr Robertson has scrutinised the machinery and hull specification, prepared by the naval architects, and with the alterations and additions suggested by him, this is now submitted as a detailed description of the type of dredger recommended and can easily be worded to suit any length of bucket dredge decided upon. ■ l n Ml the ports visited in Britain the barge loading dredger was employed, but m all cases the distance to the depositing ground was very considerable, undoubtedly warranting this method of dredging lo be economical a plant of this description must work at least two shifts, and preferably three shifts, and therefore the annual expenditure is high. The quantity of material removed in a given time is very.much larger than that removed by a self-contained hopper dredge, which leaves her moorings and conveys the spoil to the dumping ground, but I maintain that within certain limits of transport lower costs per yard are obtained with the latter type of dredger. In a combined suction and bucket dredge it is usual to place the suction pipe over the side of the vessel. In this position it is considered impracticable to support tho P'pe upon a ladder of sufficient dimensions <f. nd if a cutter is fitted it is-supported on the suction pipe itself. Without the suction pipe being mounted on a heavy ladder with adequate cutter gear it is considered that a suction dredger would not be an economical success. It is-true that the silt from certain areas at the top end of the harbour could be easily handled through a suction pipe over the side but when working in sand better results would be obtained with a bucket dredger than wit.i a suction dredger of the proposed type; I consider that the use. likely to be made of an overside suction equipment does not warrant the cost of installation on a bucket dredger. One firm of dredger builders suggested that it might be possible to design a combined suction and bucket dredge with the suction pipe on a ladder working in the bucket well. This design would give good results but would necessitate the docking of the dredge and the removal of the suction pipa and ladder, whenever a change over- to bucket dredging was required. I do not, therefore, advocate to combine bucket and suction dredger for use in Otago Harbour. I feel confident of producing satisfactory results with the length of dredge I have recommended, but fear that delays in manoeuvring a larger vessel would largely detract from her utility, and, therefore. I advocate a length, of 265 ft. A dredge 330 ft long, as far as is known, would be tne largest bucket dredger afloat, but in fairness to the advocates _of the larger vessel I will say that, provided she can be manoeuvred as satisfactorily as say, dredge 222 all the advantages claimed for the smaller vessel would apply to the larger one with the additional advantage of doing more work m the same time. Judging from my experience of the working of dredge 222

I am strongly of the opinion that less frequent delays will be occasioned with the shorter dredge. In conclusion, my considered opinion is that dredging in Otago harbour can bo most economically accomplished by a tool of the following brief description:— Type: Self-propelled bucket hopper Length, 265 ft, draught 16ft to 16ft 6in. Estimated hopper 'capacity, 2000 tons. Should the board decide to build a dredge I still recommend that Mr Robertson bo appointed to supervise the building, and that he have the assistance of a naval architect for the hull and stability trials.