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28 MARINE REVIEW. AND MARINE RECORD. [May 7, i Shallow-Draught Screw Vessels. | In a paper on "The Screw as a Means of Propulsion for Shallow-Draught Vessels," read at a recent meeting of the In- stitution of Naval Architects in London, Mr. A. F. Yarrow gives some important information regarding the experience of his firm, Yarrow & Co., in the construction of what are known as tunnel- screw steamers. [he introduction to Mr. Yarrow's paper re- views the history of shallow-draught vessels with stern paddle wheels, side wheels, etc., and refers also to some vessels con- structed years ago on the tunnel system. "Coming to the latest development of this type," said Mr. Yarrow, "I would refer to the line drawing which represents a section through one such as we frequently build. 'The upper part of the tunnel is considerably above the water line, thus enabling the diameter of the propeller to be greater than the draught of water. We adopt, for example, in a boat drawing a foot, pro- pellers of 2% ft. diameter and in a vessel drawing 2 ft. propellers of 4% ft. to 5 ft. diameter. When the vessel is at rest the water level inside the tunnel is naturally the same as it is on the out- side, but when the propeller begins to revolve the air which is enclosed in the upper part of the tunnel is forced out and re- placed by solid water. By this means it will be seen that a large propeller, capable of utilizing considerable power, can be used in It is, therefore, desirable to reduce the capacity of the tunnel to a minimum, as considerations of draught render it not always practicable to select a favorable inclination at the after part of the tunnel without causing losses, such as those alluded to above, which may be even greater than if a steeper inclination of tunnel be adopted. 'The tunnel is sealed on all sides, this being neces- sary, because when once the air is forced out it must never be allowed to pass in, or the propeller would not be working in solid water. As regards the forward part and the sides, there is no difficulty in sealing the tunnel satisfactorily, but the ex- treme after part of the tunnel should be arranged to come 2 or 3 in. below the water line; that is. sufficiently below the surface to exclude the air. 'Io make the operation clear I would draw your attention to the model, showing the after part of a launch. This, however, does: not demonstrate the system favorably be- cause the boat is at rest and the sweeping away of the bubbles of air is not so rapidly effected as when the boat is moving; in fact, in actual full-size examples the action of expelling the air is almost instantaneous. It will be seen that when the motor driving the screw is set in motion the air is driven out of the tunnel, and the propeller then works in solid water. De - "Above the screw at the highest part of the tunnel is a door POSITION OF FLAP WHEN LAUNCH 1S AT ITS LIGHTEST DRAUGHT OF I) INCHES. TiG. a SHALLOW DRAUGHT LAUNCH. ( Yarrow System) LenotH 75-0. Beam 9-3 moose o z 2 F 290 a ° > wu cc x ® 20, WW o 2 a a « APT A ot a gay ior' seid te % '9 10 Seeepo (Statute Mives Per Hour) Eve, 3: 7 " " ' - oo DHALLOW DrauGuT LAuNCcH'. ( Yarrow System) Lencru 75-0" Beam 9'-3" 40,00 8 / S - # 5 ne 2 30.0 a > Ww 7 a y x / ers LZ? me?' =e 40° = Xg° ° Ao 6 se u ae ZX "4 Zens eae oH ; 4 pv Aas s 5 7 8 . Seeep (Statute Mies Per Hour) Fie. 4 Section of Tunnel=-Screw Steamer built by Yarrow & Co. combination with a shallow draught. 'There will be an increased resistance to the forward motion of the vessel, due to the action of the screw in reducing the pressure of water at the inclined part of the tunnel forward of the propeller, and this increased resistance 1s common, more or less, to all screw ships, but it is probably proportionately greater in this class of vessel than in those where the propeller is in the usual position. 'There is also a loss of efficiency due to the resistance of the inclined surface of the tunnel aft of the propeller. "The inclined portion of the tunnel aft of the screw should be as nearly horizontal as possible, so as to diminish this resis- tance; but this would increase the length of tunnel and involve greater draught, because to augment the capacity of the tunnel below the water line is just so much loss of displacement, and the water in the tunnel above the water line is equivalent to just so "much load carried when the boat is at rest and the tunnel full. which enables access to be obtained to 1t with very little difficulty. On one occasion we tested the time taken in changing a pro- peller on one of our gun boats and it was removed and replaced by another in twenty minutes, when the vessel was afloat. 'The propeller, when working in a tunnel enveloped all round, is 'not so liable to damage as if it were at the stern in the usual place. One of the six vessels (Heron type) built by us on this plan for the admiralty a few years since is shown in this paper. 'They were 100 ft. in length by a beam of 20 ft., and the draught (carrying 25 tons), was 1 ft. 11 in., and the speed, with this load, was 10% miles an hour. One of two vessels, the Sheikh and Sultan which we built for the Nile for operating against Omdurman during the last expedition, is also shown in this paper. 'These vessels were 145 ft. by 24% ft. It has been stated that these ves- sels did not tow efficiently, and that the stern-wheelers, which we and other firms built for the previous Nile expedition, towed