Sec. 6634 



STEPS IN PRELIMINARY DESIGN 



501 



propeller. In the design stage, consideration 

 siiould be given to increasing the capacity of the 

 after ballast tanks of cargo vessels to permit fully 

 submerged propeller operation." 



66.33 First Approximation of Steering, Maneu- 

 vering, and Shallow-Water Behavior. It is 



difficult to obtain an idea of the steering, maneu- 

 vering, and shallow-water characteristics of the 

 hull as roughed out at this early stage of the 

 design. Nevertheless, something should be known 

 of them before the preliminary design proceeds 

 much further. 



Good steerability, as pointed out in Part 5 of 

 Volume III, is largely a matter of avoiding 

 excessive dynamic instability of route and provid- 

 ing sufficient swinging moment through the use 

 of the rudder. The former is largely a function 

 of the amount and position of the fixed and 

 movable fin area at the stern, comprising the 

 rudder, horn, skeg, and the like. It may depend 

 somewhat on the shape and proportions of the 

 hull. In the ABC design there is considerable 

 latitude in the rudder and skeg areas, especially 

 because the aftfoot is to be cut away by a "suit- 

 able" amount. The swinging moment is a function 

 first, of the shape and area of the rudder, and 

 second, the magnitude of the lateral forces which 

 may be expected on a rudder horn, if fitted, and 

 on the adjacent portions of the main hull. The 

 amount of the swinging moment, in turn, depends 

 on the ease — or difficulty — with which the ship 

 is swung to a drift angle that generates sufficient 

 inward lift to balance the centrifugal force. 



The ABC ship must traverse rather consider- 

 able distances with relatively small clearances 

 under the bottom. Time lost in these inland 

 waters means just as many hours to make up as 

 the same time lost in the open sea. Furthermore, 

 the maximum speed in the long, fresh-water 

 river, especially on the downstream trip with the 

 ship heavily loaded, may be limited as much by 

 the ability to steer the ship and to turn it around 

 bends in the river as by the action of the shallow 

 and restricted waters in augmenting the resistance 

 or causing unfavorable changes in its running 

 attitude. 



The turns in the river leading from Port Correo 

 and in the canal leading to Port Amalo are known 

 to be of sufficiently large radius to enable the 

 average cargo vessel to negotiate them without 

 difficulty. However, the ABC design is expected 

 to average nearly as high a speed in these waters, 



at least in the river below Port Correo, as many 

 cargo vessels make in the open sea. Furthermore, 

 traveling at these higher speeds means more 

 rapid response of the ship for an equal degree of 

 safety. 



Considering the single-screw, centerline-skeg 

 design of stern depicted in Figs. 66.P, 66. Q, and 

 67. R, the single rudder is directly behind the 

 propeller, where it works in the outflow jet. It is 

 far enough abaft the propeller to take advantage 

 of some augmented outflow-jet velocity abaft 

 the disc. It is as close as possible to the stern so 

 that the arm of its swinging moment is large. 

 There is good opportunity to tailor the movable 

 and the fixed areas to suit all the steering require- 

 ments. Adequate and possibly superior steering 

 may therefore be predicted at this stage, leaving 

 the detailed design until later. 



Considering next the maneuvering require- 

 ments. Fig. 64.B indicates that when backing out 

 of the slip at Port Bacine the ship must turn with 

 a minimum radius of about 1,090 ft. When going 

 ahead, out of the harbor, the minimum radius is 

 about 1,490 ft. The latter represents a steady- 

 turning diameter of 2 (1, 490) /5 10, or a little over 

 5.8 lengths. This must be accomplished at a 

 relatively slow speed since the ship is at a stand- 

 still at the inner end of the "Y" in the harbor. 

 When backing, the steady-turning radius is 

 equivalent to about 2(1,090)/510, or some 4.27 

 lengths, likewise at a relatively slow speed. 



It is manifest without making any calculations 

 that the ship's propeller will have to supplement 

 the normal rudder forces to create the necessary 

 swinging moments for making these turns. 

 Indeed, it is possible that both the ahead and 

 the astern turns must be effected more by swinging 

 the ship on its vertical axis than by changing its 

 heading through fore-and-aft motion. Normally, 

 this situation would point up the need of twin 

 screws. However, as the turns are to be made in 

 a counter-clockwise direction, bow to port and 

 stern to starboard, in which a vessel with a 

 single right-handed propeller normally swings, it 

 is possible that the necessary augmentation can 

 be obtained with a single propeller. 



66.34 Preparation of Alternative Preliminary 

 Designs. Often the ship for which the design is 

 being prepared is a large or important one. 

 Possibly a number of vessels are to be built to 

 the same design, or major decisions may depend 

 upon the performance of one ship. Wisdom then 

 dictates that several studies be made in the pre- 



