The screw propeller; and other competing instruments for marine propulsion

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. 1909 Excerpt: ...of the ship, tow rope, is that due only to the resistance of the ship to her passage through water, as is the case when being towed without a propeller by another ship at a considerable distance from it. Such resistance is measured by the tension of the tow rope, as was done by Dr William Froude in his experiments with H.M.S. "Greyhound." He found that the resistance of the "Greyhound " was 0-6 ton at 4 knots, 14 tons at 6 knots, 2 '5 tons at 8 knots, 47 tons at 10 knots; at 12 knots 9 0 tons, which was excessive, as this ship was designed for 10 knots only. A much larger ship, the " Merkara," which had a resistance of one ton at 4 knots, had only the 9 tons at 12 knots. The resistance per square foot of wetted skin was 1396 lbs. at 10 knots with the " Greyhound." With modern ships at 10 knots the resistance is 1 lb. per square foot of immersed skin when fresh painted to 14 lb. in ordinary clean condition. (Vide Table V., Chapter III.) Augment of resistance is the increased resistance caused, first of all, by the increase in velocity of the water past the hull by the suction of the propeller; and, secondly, by the diminution in hydraulic head or pressure at the stern due to the action of the screw. Screws of large diameter and fine pitch have a special tendency to aggravate this loss of "head." Total resistance of the ship is the resultant of the resisting forces thus created and made active; it is these that the propeller thrust has to overcome, and thrust is equal to them when the ship is moving at uniform speed. Resistance of propeller, if of no pitch, is that due to the skin friction on moving through the water and to the resistance due to the bluntness of the edge and the form of the body forced through...