[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 I.H.P. |
| Projected area of constants | Projected area of propeller × speed of ships in knots.³ I.H.P. |
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.
Mr. Biles remarked that there were many advantages in the use of twin screws which had not been sufficiently taken into account. When a ship with twin screws was being handled in dock there was greater maneuvering power, and therefore less liability for the ship to come in contact with the walls, although, if she did so, there would be greater probability of damage to the propellers. He thought means could be easily devised of protecting the screws when the ship was in dock. Another of the incidental advantages connected with twin screws was that smaller engines and smaller propellers were required, and therefore they might run them at a higher speed. They would also get lighter machinery with twin screws, and there would be less liability to have bad castings and forgings in the smaller engines, and of course the cost would be less.
With respect to the question of the middle line bulkheads, he could not quite agree with Mr. John as to the great advantages of them in a big passenger steamer. He thought there would be greater difficulty in managing a ship so built if she was in danger of sinking. Increased subdivision in a longitudinal direction was a very desirable thing, and almost necessary for a condition of immunity from sinking. In future Atlantic steamers longitudinal bulkheads should be placed not in the middle line, but nearer the sides of the ships, and they should recognize the fact that they had engines and boilers in different compartments, and make arrangements whereby the ship would still float, although the doors in these compartments were kept open. The proper way to arrive at that was to have a ship with great beam, and to have two longitudinal bulkheads at considerable distances from the sides of the ship, subdivided as completely as possible, both under and above water, so that, even supposing they got water into the space between one bulkhead and the side of the ship, they would have sufficient buoyancy in the other parts of the ship to keep her afloat. Broad ships must necessarily mean deep ships, in order to have comfort at sea. They were limited in length, and first came the question how many passengers they wanted to carry. The experience of a ship like the America—which was only 400 ft. in length—showed it was not necessary to go to great length to have great speed. A ship of 400 ft. to 430 ft. in length, 65 ft. of beam, and with a depth of 45 ft., would be a ship of proper dimensions for the Atlantic trade, and he believed it quite possible to build a vessel of special construction of about 7,000 tons gross register which should steam with less consumption of coal than the Umbria and Etruria at a rate of 22 knots, crossing the Atlantic from Liverpool to New York in six days. He thought that was likely to be the vessel of the future, and that it would be quite as commercially successful as the Umbria or Etruria.