Dredges at Work

Sun (Auckland), Volume I, Issue 253, 16 January 1928, Page 8

Dredges at Work

SCRAPING THE HARBOUR FLOOR

Shallows Yield Their Secrets

EVERY time the dredge Hapai. now operating at Devonport, fills her 800-ton hopper, she steams over to bt. Marv’s Bav and returns the spoil to the harbour floor. The load is jettisoned in the neighbourhood ot the suc-tion-dredger, which proceeds to re-lift the material, and pump it ashore.

rpHIS seems a roundabout process, but the Harbour Board finds it more economical to lift the spoil twice than to send the Hapai out to sea to drop her unwanted cargo. Since both the suction dredger and the Hapai, with her buckets, have lately been working off the Devonport ferry wharf, North Shore residents have had an opportunity to study their methods. -Of the two, the suction dredger was the more spectacu lar. Through its massive discharge line, consisting of enormous flanged pipes bolted together, it poured a flood of sand and water. In the case

of the bucket-dredge, the plant is selfcontained. Hoisted in an endless chain of prosaic-looking buckets, instruments in an altogether prosaic business, the spoil is relegated to the hopper in the hold of the ship. The chief outward indication of the dredge’s toil is the ceaseless grinding of the buckets, marching by like stage supprs on parade from wing to wing. AUCKLAND’S FOUR DREDGES Life half a hundred pigs, jogging unhappily to market, the buckets whine and squeal as they ride, and the bucolic visitor to Devonport could be forgiven, when he first heard the dredge, for assuming that the air still held a trace of familiar rural sound. The Auckland Harbour Board has had four dredges, of which two are now working the third is yet in good order; and the fourth has been sold. The first dredge, forerunner to the

Hapai, was built on the Clyde in 1885. She could carry 600 tons in her hopper, dredge 36 feet below waterline, and remove, with her 36 buckets (each of 12 cubic feet capacity, 400 tons of spoil in an hour. This 43-year-old veteran, engaged in reclamation, ploughed from the sea floor much of the material on which Auckland's tallest buildings have been raised. She was almost a pioneer of reclamation, and though now laid up. is in excellent order, and still capable of doing the same work. Successor to No. 121 was the more modern dredger Hapai. built at the same yards in 1909. Though she has only 34 buckets, compared with the 36 on No. 121, the Hapai has a much larger capacity, and can handle 800 tons an hour. Each bucket, of 28 cubic feet capacity, holds just over a yard of spoil, and with such weapons the Hapai can soon alter the contours on the bottom of the Waitemata. She is useful in both sand and stone. Nearly all last year she was working in the basin between Prince’s and Western Wharves, in an area where the material handled was what engineers rather naively term “maiden” sandstone. It was the original sea-bottom, and every inch the Hapai shifted represented a permanent improvement in depth. CURIOS FROM THE DEPTHS Although there are so many arms and tributaries, the W-aitemata is a harbour where little maintenance dredging is needed. The contributing streams are' small, sluggish, and do not bounce down from hilly country, so that they carry very little silt. Once deepened, an area in the Waitemata generally stays clear for a long time. A good instance is the Bayswater ferry channel, cut by the Jlapai, which was able to cut her own flotation through the shallows approaching the point. Off Kauri Point, where there is a perpetual conflict of currents, is a very deep hole, perhaps the deepest part of the Waitemata, and from there to the Heads is a deep channel that allows ample water for all shipping. In such sandstone as the Hapai can excavate, the suction dredger would be practically helpless; but in sand, silt, or clay she is highly serviceable, with the salient advantage that, unlike the hopper dredges, she is always on the job. Into reclamations round the fringe of the Waitemata tons and tons of material have been pumped through the pipe lines of suctiondredgers. In construction these ships are little more than a pontoon on which is mounted pumping machinery so powerful that, as in the case of the Auckland dredge, spoil can be l delivered through a pipe-line half a mile long. Quaint fish, and curios ranging from coins ,to cannonballs, are aidong the material raised by the dredges, and there is always a chance that some rare and precious find may be hoisted from the depths.