Dredging.

My attention has been directed for some time back to the subject of dredging the Dunedin Harbour, and about a year ago I procured, through the kindness of a Dunedin merchant who was visiting England, some reliable information relating to the very latest improvements in dredging machinery.

The most recent invention in dredging plant is a Combined Screw Hopper Dredge, patented by Messrs. Simons and Co., London Works, Renfrew; and in reply to a memorandum of mine, this firm state "that there will be no difficulty in one of our Patent Combined Hopper Dredges accomplishing the voyage to New Zealand under steam and canvas, so that there would be no necessity for constructing it in pieces.

"The last one we made for the Canadian Government steamed over the Atlantic to Halifax in 17 days, without coaling anywhere, and had 50 tons of coal left in her bunkers, and so satisfied are they with this vessel, that they have last mail written us about building a duplicate vessel twice the size.

"When working in free-lifting ground, in less than 20 feet depth of water, 500 tons lifted and conveyed about 10 miles off per day, or 3,000 tons weekly may be expected in ordinary circumstances. A small vessel, to lift 100 to 150 tons in an hour and a half, will cost £6,500. The first-mentioned vessel would be quite fit to make the voyage out to New Zealand."

The advantages claimed for this new system of dredging apparatus are, that the work of raising the material and conveying it to the place of discharge is performed by the same vessel, engines, and crew. That the stuff can be delivered at the level of a few feet above the deck, thus avoiding the usual long bucket ladder, and high level of discharge required for delivering into punts or barges moored alongside the dredge. And, lastly, the Patentees claim a considerable saving of expense, not only in dredging, but in depositing the stuff at a distant point from where it is excavated.

The chief recommendation of the new Hopper Dredger for the work proposed to be carried out in Otago Harbour, is the fact that the whole of the dredging operations can be worked with one crew, whereas the old system of dredging would require at least four steam Hopper Barges to keep one large fixed Dredger in constant work, if the stuff had to be conveyed by barges outside the Heads. Each of these steam barges would require separate crews, coals, and stores, and would cost when new about £5,000 each. From a study of these particulars, I am inclined to think that the best system to adopt for deepening our Harbour would be to get two powerful Combined Hopper Dredges, similar to or even larger than the "Canada," which will cost less than one ordinary dredger and the necessary Hopper barges, and it will be obvious that the annual working expenses will be very much less, for there will only be two crews instead of five, and only two vessels to supply with coals and stores instead of five.

It may be said that a large quantity of the dredged stuff from the channel could be deposited behind the circular pier in front of Dunedin, and reclaim valuable land, at the same time; also, in filling up Pelichet and Anderson's Bays, and other convenient spots on both sides of the Upper Harbour, thus obviating the necessity of carrying the stuff out to sea.

At first sight this plan would seem to recommend itself in preference to any other; but from a financial point of view it would be found by experience that it would be cheaper to carry the stuff 20 miles than deposit it on the foreshore of the Harbour, in such positions where it would be safe from the influence of the tides, and kept from being drawn back again into the dredged channel.

The Clyde experience is that it costs about an average of 1s. 3d. per cubic yard to deposit the stuff on the alvens of the river from the punts, whereas it is deposited in Loch Long, 27 miles from Glasgow, by steam Hopper barges, at a cost of 5.46 pence per cubic yard.

The chief reason why the latter system is so much cheaper than the former is, that once the stuff is delivered into the vessel that carries it, there is no more handling required until it is dropped out at sea; but to deposit the stuff on shore requires frequent handling, as well as numerous punts for carrying it—hence the difference in cost. Another system has been adopted recently for depositing dredged mud by means of a variety of pumps, forcing the stuff through wooden tubing in a semi-fluid state. But this can only be carried out economically when the ground to be reclaimed is situated close to the dredger.

The wharf is calculated for a length of 3,000 feet, but of course a much shorter length would be sufficient for some time to come; and, in connection with this wharf, very valuable sites for warehouses and manufactories would be reclaimed in the centre of the city. It is superfluous for me to point out to the Committee that every extension of this wharf, both north and south, would reclaim valuable areas of land in proportion to the length of extension, and, therefore, the length proposed to be finished at the outset is purely arbitrary; because it might be found that the demand for the sites behind the pier would justify a much greater length being carried out as the large ships began to come up to Dunedin.

It is also very probable that much shorter guiding walls than I have shown on plan for the new channel would be sufficient; but this could only be determined as the channel was being dredged out, and would depend very much on the nature of the stuff cut through.

If less walling would maintain the channel, then, of course, my estimate of cost would be so much less, but I think it better to make a liberal provision for this item in case it might be required.

In conclusion, I think I should state that I believe dredging plant was never applied to an easier piece of work than deepening the Harbour of Dunedin, and I am of opinion that the largest vessels that enter Otago Heads can be brought up and berthed alongside of Dunedin wharves within two years from the date of commencing dredging operations; and if the scheme I have suggested were carried out, Dunedin would possess one of the finest commercial harbours in New Zealand.