In the next section, Fig. 2, the Servia, which is built of steel, on the other hand, the bottom is built on the longitudinal cellular system, the first application, he believed, of this system to an Atlantic liner. The plating of the Servia is of the usual alternate outer and inner strake system, partly double; while the third section, the Oregon, approaches more nearly to the ordinary system of framing and plating usually adopted, but it will be seen that she was well tied in the bottom by very heavy intercostal and plate keelsons, as well as in the top by heavy stringers and sheer strakes, with much of her plating doubled, and heavy web frames inside. The author next considered the question of stability, and went on to deal with the subject of twin screws, and stated that the Barrow Shipbuilding Company has done more in the way of planning and designing for the adoption of twin screws lately than for any other mode of propulsion, and this chiefly for passenger steamers. He did not attach much importance to the particular form of the blade either in single or twin screws, as he believed so long as the disk area, the surface, and pitch were properly adjusted to the speed of the vessel, and to enable the engines to use, at the maximum speed, just the full quantity of steam that the boilers can make, we have got pretty nearly as far as we can get.
To fix these dimensions of the propeller accurately at the present time, and without further knowledge of the action of the screw on the water, was, he thought, impossible. All the rules and formulae are empirical. The best one he knew is given in Table IV., due to Mr. Thom, the head of the Barrow Company's engineering drawing office, and at present acting manager, who has used it for some years in practice. These formulae are based upon the assumption that the area of propeller disk should be proportional to the indicated horse power, divided by the cube of the speed, and the same with the projected area of the propeller and also the surface.
Particulars of Propellers and Constants.
|Ship.||Length of ship.||Disk constant.||Proj. surf. constant||Feet per minute.|
Speed of tips.
|City of Rome.||542||220||69||4,715|
|Twin screw Fenella||200||244||64||2,890|
|Twin screw H.M.S. Fearless1||220||277||67||5,022|
|Twin screw H.M.S. Iris||-||4545||1355||- -|
|Twin screw H.M.S. Iris2||300||412||221||- -|
|Twin screw H.M.S. Iris3||300||346||99||4,961|
|Twin screw H.M.S. Iris4||300||439||82||5,309|
1 Estimated with a speed of 17.5 knots and 3,370 I.H.P.
2 With the first propeller at the estimated speed of 17.5 knots and 7,000 I.H.P.
3 With four bladed modified Griffith's on actual trial.
4 With two bladed modified Griffith's on actual trial.
5 Constants obtained from first propeller calculated from a speed of 18.5 knots and 7,500 I.H.P.
|Disk constant =||Area of propeller disk × speed of ship in knots.³|
|Projected area of constants||Projected area of propeller × speed of ships in knots.³|
Expanded area constants may be obtained and used in the same way.
The discussion which followed was opened by Mr. Holt. He said that if they were to have greater speed on the Atlantic, there was one point which was not alluded to in the paper, and that was the total abolition of cargo on board the great passenger steamers. If vessels were built solely for passenger traffic, they would be able to insure greater speed by reason of the greater slightness in build and the additional space at the command of the designer. The existing Atlantic express steamer was far too heavy, and might, if cargo was dispensed with, be made with finer lines and more yacht-like. He looked on the proposition to fit such vessels with longitudinal bulkheads with great fear. If a collision took place - such, for example, as that which sunk the Oregon - water would get access to one side only of the ship, and it was not at all improbable that if a sea was on, she would turn right over. At all events, very serious risk would be involved.
Mr. W.H. White, Chief Constructor to the Admiralty, said the question of twin screw propulsion was one of special interest to himself, and had been so for many years. In 1878 he dealt with it as fully as he then could on the basis of the Admiralty data, and he then ventured to say everything in favor of twin screws that Mr. John had said in his paper. If greater power than that now used in such a ship as the Etruria, for example, were demanded, two screws must be used. Good as are the results obtained with the Etruria, it was by no means certain that still better might not be had. If she had been fitted with two screws instead of one, very great advantage would be gained by the greater submergence of the twin screws, as thus racing would be almost wholly prevented.
Mr. Calvert urged that more attention should be devoted to studying the relative values of different portions of the propeller.
The sitting was then suspended. In the afternoon, as we have already stated, the members visited the steamship Germanic on the invitation of Messrs. Ismay, Imrie & Co., subsequently proceeding to Messrs. Cope Brothers' tobacco works, and thence to the exhibition, where the dinner of the Institution took place in the evening.
On Friday morning no paper was read; some official business was transacted, and this being done, the discussion on Mr. John's paper was resumed.