570 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 69.4 



that Ps is 16,000 horses and D is 20 ft, the esti- 

 mated rpm are about 118. The rates of rotation 

 derived by the use of several propeller charts in 

 Sec. 70.6 vary from about 104 to about 110 rpm. 

 69.4 Effect of Type and Design of Propelling 

 Machinery. The general design of propulsion 

 devices is never dissociated entii'ely from the 

 type and design of propelling machinery because 

 these units or systems are, always figuratively 

 and generally literally, at opposite ends of the 

 same shaft. Nevertheless it may not be amiss to 

 point out in this book that the greatest freedom 

 of choice for the design, construction, and position 

 in the ship for both the propulsion-device and 

 the propelling-plant systems is afforded by a 

 suitable combination of the following elements: 



(a) A type of power-generating plant which 

 need not be located in the ship in some specific 

 region, dictated by the position of the propulsion 

 device (s), but which can be placed where it best 

 satisfies ship operating conditions. It should not 

 be forced into a certain — and not always de- 

 sirable — position because the propulsion-device 

 design requires compliance with a completely 

 different set of conditions, nor should it be such 

 that it will fit into only one position in the ship. 



(b) A completely flexible power-transmission 

 system by which a power-generating plant or 

 driving member can be connected to a driven 

 member on the propulsion-device shaft with 

 freedom of direction of rotation, direction of 

 shaft axes, relative position in the vessel, and 

 distance between the two. Freedom of this type 

 is afforded by electric-wiring or piping systems. 



(c) A propulsion-drive unit which is small, light, 

 compact, and adaptable as to location, requiring 

 a minimum of maintenance 



(d) A propulsion-drive speed changer which 

 permits use of the optimum speeds for both the 

 propulsion-drive unit and the propulsion device. 



At the time of writing (1955) these requirements 

 are met for a wide range of powers by the following 

 units grouped in one machinery plant. The import- 

 ant matters of space, weight, and cost are not 

 disregarded but are for the moment considered 

 secondary to flexibility. The specific mention here 

 of an installation resembUng an electric-drive 

 plant is intended solely as an example and not as 

 indicating the best or the ultimate achievement 

 to fulfill the requirements of the preceding 

 paragraph. Such an installation comprises: 



(1) An electric generating plant, driven by a 

 steam, internal-combustion, or gas-turbine engine, 

 or a plant of some type unknown at present but 

 which may be found feasible in the future. 

 This plant is to be in units of sizes and powers 

 which lend themselves readily to manufacture, 

 to mstallation in the most advantageous posi- 

 tion (s) in the ship, to economical and efficient 

 operation, and to progressive maintenance. 



(2) An electric transmission system, either AC 

 or DC, with the necessary safety and control 

 devices 



(3) A reasonably high-speed rugged electric 

 motor or motors, suitably cooled and protected 

 from dirt, moisture, spray, and liquid. The 

 modern railway traction motor fulfills all these 

 requirements. 



(4) Speed-changing gearing of the single- or 

 double-reduction or epicycUc type, utilizing 

 wherever practicable the so-called flexible con- 

 struction which provides uniform load distribution 

 along the gear faces and consequent maximum 

 loading on the teeth. 



Practically all the elements required by the 

 foregoing, although some of them in limited 

 powers only, are available and have proved 

 themselves in severe service afloat [Lisle, 'T. O., 

 Motorship, New York, Mar 1953, p. 36], often 

 where weight and space are at a great premium. 

 It is to be hoped, for the sake of the ship designer 

 who is keenly interested in the hydrodynamics 

 of his propulsion device (s), that active develop- 

 ment along these lines will continue. 



69.5 Number and Position of the Engines. 

 ParalleUng the comment in items (a) through (g) 

 of Sec. 69.2, on the number of propulsion devices, 

 it may be said that: 



(a) A single machinery unit is lighter, more 

 compact in total volume occupied, more cheaply 

 and easily installed and maintained, and cheaper 

 to run than several units of the same total power. 

 Aside from only one engine there is only one 

 thrust bearing, one line shaft, one propeller or 

 stern-tube shaft, and one set of shaft bearings 

 and hull stuffing boxes. The space and weight 

 of the fuel saved may be devoted to other useful 

 items. 



(b) A single unit requires fewer operating per- 

 sonnel. This in turn reduces the space and weight 

 devoted to crew accommodations and required 

 for crew's stores. 



(c) Modern machinery may not yet be sufficiently 



