THE PUMP DREDGE.

Temuka Leader, Issue 2660, 17 May 1894, Page 3

THE PUMP DREDGE.

The following is the report of Mr Gr. M. Barr, C. E., on the Harbor Board's pump dredge, read at the board's mee'ing on Wednesday : ' " I have the honor to submit the following report upon the suction dredge plan*- recently imported on your account, as requested by letter and telegram from your secretary. " I visited Timaru on two different occasions, the first on the 30th March and the second on the 24th April, remaining till the 28th. On the first visit I had a general overlook of the plant with a special inspection of the pump propeller, at that time not closed in, and on the latter visit I was occupied with the various trials of the plant as hereinafter particularly described. " The first trial of dredging capacity was within the harbor to the north of the Moody wharf, where the material is a mixture of fine sand aud silt, and with the engines running 120 revolutions per minute and a steam pressure of 751 b, a condenser vacuum of 26 and a pipe vacuum of 89 inches, the water was delivered highly charged with solid matter. The engines were run for three-quarters of an hour in two heats, after which the hopper showed a deposit of 42 tons and the bottom was deepened from 6ft to 7ft. " The second trial was made on the following day (25th April), to tha eastward of the first position, in material of a more sandy character. Work was continued here for 1. hour 27 minutes when operations had to be susoended so as to avoid loosening the ground in the neighbourhood of the buoy chain. The result of solid matter left in the hopper was 48 tons.

" The trials were resumed on the next day, but a commencement was not made until 10.45 a.m., on account of the dredge being required to tow the Queen Mab to a berth at the wharf. At that hour she steamed outside the breakwater for the purpose of a trial in the shingle of the open sea, aud considerable progress was made in mooring, but a hasty return to shelter had to be made on account of the increasing swell. In the afternoon some work was done pretty close to the breakwater, from a point in line with the noeth wall and outwards, the material being fine sand, with a touch of small gravel and shells at the outer end of the breakwater. The result of this operation was an addition in the hopper of 74 tons in 59 minutes, and the water was deepened from 20ft 6iu to 30ft at one part, and from 19ft to 28ft 6iu at another. "On the 7th April a good start outwards was obtained, and the dredge was moored with her half-length in line of the breakwater about 80 yards ontside thereof, bow on to the south-west swell, which was then coming along in waves of about 2jft in height. The dredge rode very easily, and the pipe with its joints reponded promptly to each motion of the vessel, so that there did not appear to be any risk of breakages under the circumstances. After working for one hour and three-quarters the hopper showed an additional quantity to the amount of 310 tons —certainly a great improvement upon previous results. The material was fine saud up to coarse gravel, the largest atone visible being in size 8 J x 5 J x 3 inches. Depth increased from 22ft at start to 33ft at end.

''Being anxious to ascertain how far the unsatisfactory results were due to imperfections in fit of the hopper doors, I had 10 tous of clay filled into them, and then on the 28th April made a further trial m fine material, but while this succeeded in preventing leakage at the doors the result in solid matter was Only 53 tons in 1J hours. " Tho dredge was taken twice outside to clear the hoppers. On the first occasion .there was a slight difficulty in getting the after door quite close after the stuff* was discharged but on the second occasion less difficulty was experienced both in opening and closing these, and as tho men become more accustomed to the work and the different parts lose some of their stiffness I havp no doubt that their working will be quite satisfactory. The time occupied from the opening of the first door to tho closing of tho fourth was 17 minutes.

"On threo occasions when tho pump appeared to be doing its boat in tho fino material, I got a bucketful as it poured from tho delivery pipe, aud after allowing some time for settlement I found that the proportion of solid was from l/28th to 1/1 Bth of the whole—a proportion which is much below tho avorago performance of this class of plant at steady work, which usually gives from l/10th to l/7th of 3olid matter. This deficiency will doubtless bo remedied as tho oporators gain exporiencs; for results depend upon the manner in which tho vessel follows up the cut and tho tact with which tho nozzle of the suction pipe is kept in touch with tho material.

" Tho question now arises as to whether or not tho plant, as judged by these trials, is of a satisfactory character; for tho question as to whether or not it is according to contract has already been answered in tho affirmative by its acceptance in the Clyde by your agent, Mr John Darling. With regard to tho power to be provided according to the contract, tho ruling stipulation ia that the pump must In; capable of delivering 200!) cubic feet of pure water per minute, with from 130 to 100 revolutions, and, allowing for tho

| slower speed at which the engines were [ driven during the trials, this capacity has been demonstrated ; but no stipulation with regard to solid matter to be delivered has been made, as is frequently though not always, done in contracts for dredging plant. The engine was on no occasion run up to the extreme rate above mentioned ; occasionally it waß almost up the minimum, but the general run was from 120 to 125 revolutions per minute, and with the quantity thus raised and the proportion of solid as ascertained by the tests referred to, the deposit in the hoppers ought to have been much greater than was found. Why so little was secured iu these was simply because the agitation of the water was too great, and the time too short to allow the fine material to settle, and I am afraid that I cannot encourage the hope that any adaptation of perforated plates, or that by any other expedient, can this be much improved. A moment's consideration will show this. The water pours from the delivery pipe from a height of 4£ft above the combings and with its great momentum keeps the water in constant commotion, and as soon as the hooper is full of water the following discharge will immediately run overboard, without leaving any of the solid matter behind it. On one occasion I made two measurements of the stuff in the hopper, the second being 1£ hours of stillnesses in the water after'the first, and the result showed a very slight additional precipitation in the interval. On another occasion I compared the quantities after a night's rest, and found that the interval of quiet had allowed a greater quantity to precipitate, but of course it would be absurd to think of allowing the dredge to lie idle while the material had time to settle down. The experience in this case is what has always been recognised with regard to this kind of plant and that nature of material, which is that delivery into hoppers is not a success, and that if thi3 principle of dradge is to be employed for fine stuff it must make its delivery towards land, or at any rate into some space where the solid will have ample time to settle down. There can be little doubt, I think, that Messrs Lobnitz and Co. had this in view when constructing the dredge, and that such was their reason for placing the delivery pipe so much higher than is requisite for discharging into the hopper. " The results obtained outside the harbor in the shingle and sand were more satisfactory than anything among the finer material, and though the quantity deposited in the hopper was not up to what might have been expected under more favourable circumstances, allowance must be made for the range caused by the swell and uneven nature of the bottom. The trial, however, demonstrated that the plant is well fitted for this class of material. I have no doubt that after the men have had more practice the proportion of solid will be increased. As judged by the vacuum gauge during the trials, thejpump. was a great part of its time barely half loaded, and it was only at infrequent times and for a single instant that the gauge indicated the safe maximum of its power. [The remainder of this report will appear in our next issue].