Sec. 63 A 



RASTC DESIGN FACTORS 



443 



in the preceding chapters, it gives a practical 

 example of the design of a modern ship. It stresses 

 the specified Umitations and requirements, the 

 free or open choices available to the designer, and 

 the compromises that are unavoidable in anything 

 put together by one man to satisfy the conflicting 

 wishes of other men. 



No ship design can be carried very far without 

 considering the primary features involving hydro- 

 statics. Among these are displacement and trim, 

 metacentric stability, floodability, subdivision, 

 and damage control. They are treated extensively 

 in textbooks and other references [PNA, 1939, 

 Vol. I], and require no further elaboration here. 

 They are brought in as necessary, without detail 

 consideration, in the ship-design example which 

 follows. 



That phase of ship stability generally known as 

 dynamic stabihty but defined here as dynamic 

 metacentric stability is, however, very definitely a 

 problem involving ship and liquid motion. It is 

 accordingly included in Part 6 of Volume III, 

 under the chapters relating to wavegoing charac- 

 teristics. 



63.3 General Assumptions as to Propelling 

 Machinery. With a few exceptions which are 

 noted at the proper places, there is no need to 

 consider the type of propelling machinery in any 

 phase of the hydrodynamic design. It is taken for 

 granted here that the machinery is adapted to 

 the most efficient rate of rotation for the selected 

 type of propulsion device, although too often the 

 reverse is true. For propelling the ship, the pro- 

 pulsion device can be driven by a turbine, a 

 reciprocating engine, an electric or hydraulic 

 motor, or a hand crank, as long as the desired 

 torque is apphed, the requisite rate of rotation is 

 attained, or the necessary power is delivered. , 



An exception to this rule is the case of the ship, 

 mounting two or more propdevs on opposite 

 sides of the centerplane, which is called upon to 

 make frequent turns and maneuvers at relatively 

 high speeds and powers. Here the port and star- 

 board propdevs operate in liquid streams moving 

 at different velocities with respect to the ship. 

 The type of propulsion machinery almost certainly 

 affects the rates of rotation and the powers 

 delivered and absorbed on the two sides. Another 

 exception is the case of the ship which must 

 maneuver rapidly, and in which the maneuver- 

 abihty is in direct proportion to the promptness 

 with which the propelling machinery responds to 

 its own controls. 



There arc cases also where the type or form 

 of the propelling machinery affects the declivity 

 and the parallelism or divergence of the screw- 

 propeller shafts, just as the position of the 

 machinery almost invariably determines the 

 position of one end of the propeller shaft. On 

 some high-powered vessels, the necessary clear- 

 ances for machinery inside the vessel may prevent 

 shaping or fining the hull where this procedure 

 would otherwise be desirable. 



The matter of locating the propelHng machinery 

 in the stern or in other fore-and-aft positions is 

 really not one for discussion in this book, except 

 that: 



(a) The location of the machinery aft may affect 

 the shape of the stern and the position of the shaft 

 carrying a screw propeller, because of the clear- 

 ances required inside the vessel 



(b) The machinery weight assists in trimming the 

 vessel by the stern and pushing the screw pro- 

 peller down under water. 



The matter of the absolute speed of the ship, 

 and of its actual size, is usually determined by 

 economic, military, or other considerations beyond 

 the control of the designer. He must, however, 

 be prepared to predict the results of variations in 

 these factors, and of each upon the other, so that 

 when a final design decision must be reached, it 

 can be based upon sound and accurate premises. 



63.4 The Fundamental Requirements for 

 Every Ship. Every ship designer, no matter how 

 logical and realistic he may be, needs to get back 

 to first principles every so often in his search for 

 the best way to make nature serve him. He need 

 not think it in the least beneath his dignity or 

 intelUgence to write down, in a few lines, as did 

 the renowned Rankine many years ago [STP, 

 1866], the following simple requirements for every 

 ship: 



(a) To float on or in water 



(b) To move itself or to be moved with handiness, 

 in any manner desired 



(c) To transport passengers or cargo, or other 

 useful load, from one place to another 



(d) To steer and to turn, in all kinds of waters 



(e) To be safe, strong, and comfortable in waves 



(f) To travel or to be towed swiftly and econom- 

 ically, under control at all times 



(g) To remain afloat and upright when not too 

 severely damaged. 



