There is something to be said for large screws with a small proportion of blade area to disk. For instance, two bladed screws have frequently given better results than four bladed screws of smaller diameter, neglecting, of course, the question of vibrations. Twin screws, however, should, as a rule, be made as small as possible in diameter without loss of efficiency. The advantages of small twin screws are the shorter shaft tubes and stern brackets, deeper immersion, and less exposure as compared with large screws. The exposure of the screws is usually considered an objection, but, perhaps, too much has been made of it, for those well qualified to speak on the subject consider that careful handling of the ship would, in most cases, prevent damage to the screws, and that where the exposure is unusually great, effectual protection by portable protectors presents no insuperable difficulty. HIGH SPEED TWIN SCREWS. --------------------------------------------------------------------- |Ship A.|Ship B.|Ship C.|Ship D.|Ship E.|Ship F. --------------------------------------------------------------------- Length, ft. | 325 | 315 | 300 | 300 | 220 | 250 Breadth, ft. | 68 | 61 | 56 | 46 | 34 | 32½ | | | | | | Draught on trial, | 26 ft | 24 ft | | 15 ft | 12 ft | 13 ft forward. | 2 in | 6 in | .... | 6 in | 10 in | 1 in | | | | | | Draught on trial, | 27 ft | 25 ft | | 19 ft | 15 ft | 14 ft aft. | 3 in | 6 in | .... | 9 in | 2 in | 7 in Displacement, | | | | | | tons. | 9,690 | 7,645 | 5,000 | 3,584 | 1,560 | 1,544 I.M.S., sq. ft. | 1,560 | 1,287 | 1,000 | 744 | 438 | 392 Speed of ship, | | | | | | knots. | 16.92 | 17.21 | 18.75 | 18.18 | 16.91 | 17 I.H.P. |11,610 |10,180 | 8,500 | 6,160 | 3,115 | 3,045 Revolutions per | | | | | | minute. | 107.2 | 88 | 120 | 122.6 | 150.4 | 132.1 | | | | | | Pitch of | 19 ft | 22 ft | 18 ft | 17 ft | 12 ft | 14 ft screw. | 5 in | | 9 in | 6 in | 7½in | 9 in | | | | | | Slip. per cent | 17.6 | 10 | ... | 14.2 | 9.7 | 11.4 | | | | | | Diameter of | 15 ft | 18 ft | 14 ft | 13 ft | 10 ft | 11 ft screw. | 6 in | | 6 in | | 6 in | | | | | | | Diameter of | 4 ft | 4 ft | 3 ft | 3 ft | 2 ft | 2 ft boss. | 4 in | 11 in | 9 in | 5 in | 9 in | 10 in Number of blades | 4 | 4 | 3 | 3 | 3 | 3