566 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 6R.16 



John Johnson two decades ago, are still worthy 

 of serious consideration b}^ the ship designer 

 [Thomas Lowe Gray lecture, IME, 10 Jan 1936; 

 SBSR, 16 Jan 1936, p. 74]. 



68.16 Reducing the Wind Drag of the Masts, 

 Spars, and Rigging. Tall, raked masts on a 

 self-propelled vessel are undoubtedly reUcs of 

 saiUng-ship days. They serve well for the flying 

 of flags, the carrying of navigation lights, and 

 the working of long derrick booms. Nevertheless, 

 with their attendant rigging they are subject to 

 heavy loads when rolling and they represent 

 wind resistance out of all proportion to their 

 usefulness. A tall vertical pole or post, properly 

 attached to the ship structure and without 

 benefit of stays, can mount a radar antenna, 

 hold up one end of a radio antenna, support a 

 crow's nest, carry a range or masthead light, and 

 serve as an air intake or exhaust, all at the same 

 time. It can not easily be streamlined for flow 

 on all bearings in which high relative winds are 

 encountered but its wind drag is at least justified 

 by the number of functions which it performs 

 simultaneously. 



Masts intended to be used as ventilators 

 appeared as far back as 1860 on the steamer 

 Ly-ee-Moon [SBSR, 9 Nov 1939, p. 507]. It is 

 possible that these in turn were rehcs of the early 

 smoke stacks which were extremely tall in order 

 to obtain good natural draft. 



Self-supporting masts and derrick posts, with 

 all stays eliminated to reduce wind drag, inter- 

 ference, and expense, are coming rather rapidly 

 into use [SBSR, 8 Oct 1953, pp. 484-485; MENA, 

 Dec 1953, p. 53; SBSR, 24 Jun 1954, pp. 800-801]. 

 On the Uner Orsova the running rigging for the 

 derricks is carried entirely inside the posts 

 [SBSR, 20 May 1954, p. 645]. 



68.17 Consideration of Increased Draft 

 Through the Years. Before leaving the pre- 

 hminary design of the abovewater body as finished 

 it is to be remembered that, as the result of a 

 continual series of modifications and changes in 

 the course of its life, most of which act to increase 

 the weight, the ship sinks slowly but steadily 

 deeper in the water. This is specially true for a 

 combatant vessel or for a merchant vessel which 

 is likely to be converted to a naval auxiliary in 

 time of emergency. A heavy or pronounced flare 

 or a sharp knuckle may in this way be brought 

 too close to the designed waterline. A flat counter 

 may be lowered to the point where it is subject 

 to frequent sea slap and slamming from under- 



neath. A transom stern may have its immersed 

 draft increased to the point where the transom 

 does not clear at the designed speed. Finally, a 

 propeller designed for the original displacement 

 may be heavily overloaded at a later one, when 

 the average draft has increased. 



68.18 Preparation of Hull Lines for Model 

 Tests. It is recalled that the design rules for 

 screw-propeller apertures and clearances em- 

 bodied in Sec. 67.24 are in many cases based upon 

 propeller-blade widths, upon the thicknesses of 

 rudder posts, and so on. To determine whether 

 there is room for the propulsion devices on the 

 outside and, in the proper places, for the propelhng 

 machinery within the ship, it is necessary to run 

 through a prehminary design of these devices, in 

 the manner outlined in Chaps. 67, 69, 70, and 71. 



Before attempting to delineate the whole ship 

 hull on a single drawing, the fixed and movable 

 appendages need to be roughed out and checked 

 for position, shape, and dimensions. This pro- 

 cedure is described in Chaps. 73 and 74. Further- 

 more, the hull-and-appendage combination re- 

 quires design attention and checking to insure 

 that shallow-water maneuvering, wavegoing, and 

 other requirements are met. These matters are 

 covered in Chap. 72 and in Parts 5 and 6 of 

 Volume III. Simultaneously the preliminary 

 design needs working over for arrangement, 

 volumes, capacities, strength, metacentric sta- 

 bihty, damage control, and other non-hydro- 

 dynamic requirements. 



With this work accomplished, and with the 

 shape and principal features of the underwater 

 and abovewater hulls, propulsion devices, and 

 appendages worked out, a set of lines for the hull 

 as a whole is drawn. This is in no sense a set of 

 lines to which the ship is to be built. For the 

 hydrodynamic design, its principal function is to 

 guide the building of a model or models for towing, 

 self-propulsion, maneuvering, shallow- water, wave- 

 going, and other tests. 



The preparation of lines to embody all the 

 features developed in the foregoing sections is 

 largely a matter of drafting, except as mathe- 

 matical processes such as those discussed in 

 Chap. 49 may be used to calculate the offsets 

 for drawing (and fairing) these lines. 



A high degree of precision at this stage is not 

 called for. The fines should be to a sufficiently 

 large scale, not only for construction of the 

 model, but for a fairly detailed design of all the 

 appendages which are eventually to be added to it. 



