The Marine Steam Engine; A Treatise for Engineering Students, Young Engineers, and Officers of the Royal Navy and Mercantile Marine (Paperback)

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. 1902 Excerpt: ... race.--The action of the screw-propeller is to drive sternward a cylindrical column of water, usually called the 'propeller race, ' and the thrust of the screw is measured by the sternward momentum generated in this race in a unit of time. The area of this race is approximately equal to that of the screw disc, less that of the boss of the screw, the race being in fact approximately a revolving annular column. In consequence of the obliquity of the propelling surfaces, the race receives a rotatory as well as a sternward motion, and this centrifugal action causes a certain loss of thrust. The race of a screw-propeller may be conceived to be a series of concentric cylinders of water moving sternward, and rotating at different velocities. It is evident that the thrust must be diminished both by the centrifugal motion and by the frictional action of the particles of water. Augmentation of resistance due to action of screw-propeller.--The most important feature in the action of a screw-propeller, as affecting its efficiency, is the effect it produces on the water under the stern of the ship. In the absence of the propeller the water displaced at the bow by the passage of a well formed ship, would close in under the stern and cause a forward pressure there. The action of the screw withdraws this water, and consequently diminishes the pressure of water under the stern, which is equivalent to increasing the resistance of the ship, as compared with the natural resistance, or the resistance experienced by the ship when towed at the same speed. The resistance augmentation varies exceedingly, depending on the shape of the stern of the ship and the size and position of the screws relatively to the vessel. The late Mr. Froude experimented, and considered about 40 per cent..