Papers and discussions on screw propulsion; Reprinted from Transactions of the Institution of naval architects

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1883 Excerpt: ...of the mercantile marine, the progressive trials of which were analyzed by Mr. Froude and explained at the meeting of 1876. In these he found the average value of this loss to be about 13 per cent. of the indicated horse-power, and he assigns this as the probably general value. I come now to the question why was the performance of No. 1 screws with four blades so much inferior to the performances of all the others? Authorities on the subject of screw propulsion teach us that the effective thrust or efficiency of a screw propeller depends upon the quantity of water acted on in a given time, and the stern ward velocity impressed upon it, and that it is always preferable to make the sternward velocity of the current small, by adopting a form of screw which will act on the largest possible quantities of water. Indeed, it has been said that if it were not for the objectionable element " surface friction," there would theoretically be no objection to an indefinite extension of blade area. In short, it has been always considered as essential to the efficient performance of screws generally, that tbeir blade area should be large, and that the water must be permitted to flow freely to them. Now, No. 1 screws with four blades appear to fulfull the above conditions very completely; for the blade area is undoubtedly very large, and as the run of the Iris is very fine, there is no obstruction to an ample supply of water for the screws. Notwithstanding this, however, the results of the trials appear to show that on account of the large diameter of the screws, the shape. and the large area of the blades, combined with a fine pitch and consequent high rotary velocity, an extraordinarily large amount of power was absorbed in the surface friction of the blades. But...