A NEW STEAMERSHIP PROPELLER.

Globe, Volume XXI, Issue 1791, 17 November 1879, Page 3

A NEW STEAMERSHIP PROPELLER.

Last week (says a Home paper) a number of officers of the Boynl Navy and Mercantile Marine and many scientific gentlemen attended at Gravesend for the purpose of taking a trial trip on board the steam yacht Jolair, with the object of testing the advantages of a newly-invented propeller called " De Bays's Patent Direct Acting Propellor," with which it has been fitted. So extraordinary a revolution is this invention that it is estimated to realise a saving of seven or eight millions per annum in the c<y% of material in the steam shipping of the country, and will tend to greatly accelerate the speed of every ship to which it is attached. In order to give some idea of its effects and to show the comparative power it possesses over that of the ordinary screw, it is necessary to mention that the Jolair, which was built by Scott and Co., of Greenwich, in 1876, is in length 81ft. 6in. t breadth 12ft., and depth Bft. 2in.; she has two engines of 20-horse power nominal, and registers forty tons. Her original screw had a diameter of 6ft. 2in., with a 6ft. 4in. pitch, whilst her new "De Bays" propeller has but a diameter of 4ft Sin, and a pitch of 7ft 6in. Its construction is of a very peculiar character, and consists of two screws of the same pitch, one, the larger, with four, and the other with five blades ; portions of the blades of each are cut out. The screws are fixed on two shafts, one tubular, the other solid and rotating within it, each screw being made to revolve in opposite directions, the solid portions of each blade of one screw passing through the gaps cut out in the other, and vice versa. The effect of this, in the first place, is to prevent the tremendous rush of water flying about in all directions from the ordinary screw, and to drive the greatest weight of water astern in a line with the keel of the ship, thus entirely obviating that waste of power which is the consequence of the effects of the ordinary screw. The influence of one screw tending to deflect the water moving astern to the right is counteracted by the other screw, which tends to send it to the left. The company were met on board the Jolair by Mr De Bays, and having had the construction explained to them, the trial of the tests was made, and compared with the last trial trip of the yacht with the old propeller. The number of revolutions made per minute with the former was 124, whilst with the new propeller they were 136, the steam pressure on the former being 74 51b., while with the latter it was 771 b., the vacuum of the former being 15 - 39, and the latter 1825. In time, the mile was run with tide by the old propeller in 6min. 53sec, whilst the time occupied by the new (propeller was smin. 30sec, against tide, old propeller, 9min. 57sec. ; new propeller, 9tnin. 32sec. The whole of the experiments took place passing down the river in the direction of the Nore, and in making a complete circle the time occupied was 2min. 36aec. In ord< r to test the powers of the new propeller, and its efficacy in reversing and going astern, some teed covers of wine bottles were thrown from he stern at a moment when the yacht was going at the rate of eleven knots an hour, with tide. The order was given to reverse the engines, and the result was that the time occupied in reaching the floating covers was but fifty-two seconds, and that straight aßtem, without any divergence of the head of the vessel, considered a most important feature in the avoidance of collisions or rocks or other impediments ahead. It was suggested by Admiral Brooker that the complex machinery of the screw might cause it to be damaged by ropes or other substances getting into it, and the test was applied by throwing out ropes, which for a long time were regulated by the screw, till at last one of the men called out that the rope had caught, but there was no jerk, and in an instant it was' loose, when it was discovered that the rope was completely cut by the flanges of the screw —a result considered most satisfactory. Another important feature connected with the test was to show that even in the smallest rivers and canals there was no sidewash to destroy banks, &c. The general results were considered to be most satisfactory. It is stated that upon the results of this experiment depends the adoption of "De Bays's" principle by some of our largest steamship companies.