Engineering Wonders of the World
Volume I

DESIGNING A SHIP. 357 Fig. 12.—THE NEW MODEL TESTING TANK AT WASHINGTON. The Model is attached to the electrically moved Carriage spanning the tank. given out by the engines, known as the indicated horse-power. Its value rarely ex- ceeds *5, or, in other words, the effective horse-power (e.h.p.) is seldom more than half the indicated horse-power (i.h.p.), as the i.h.p. must include, besides the e.h.p., the power necessary to overcome the friction of the engines and that absorbed by the pro- peller, mainly owing to the “ slip of the latter. By “ slip ” is meant that the pro- peller, owing to its not working in a solid medium, moves at a greater speed than that at which the ship is driven forward, and consequently imparts sternward velocity to a large quantity of water, with a necessary absorption of power in the process. The design of a suitable screw propeller, if a high speed is desired, is a very difficult problem. This, again, is best attacked with the aid of the experimental tank, although a disadvantage of tank work is that the model and the screw must be tested separately. For this reason it was considered advisable, when designing the Cunard liner Mauretania, to construct a self-propelled electric model launch, 47 J feet long, having the exact form of the ship, and driven by model propellers. But for the expense involved, this certainly ap- pears to be the most reliable method yet devised. With regard to rolling in a seaway, the naval architect, after satisfying the claims already referred to, lias generally little lati- tude to effect changes in the form or dimensions in order to Rolling reduce rolling, and must rely a J Seaway. mainly on external appliances, such as a centre keel, bilge keels, etc. Steadi- ness at sea is, of course, highly desirable in the case of passenger vessels, in order to reduce