762 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.5 



single propeller. On ships of the maximum length 

 permitted by turning basins and pier facilities, 

 all this length could be put into the waterline. 



76.5 The Design of Dry-Cargo Vessels with 

 Box-Shaped Holds. Somewhat similar to the 

 long, narrow, blunt-ended vessel, and to the canal 

 boat, and not unlike the box-shaped watercraft 

 of Sec. 76.15, is the vessel which is required to 

 house rectangular, box-shaped, dry-cargo spaces 

 within the boundary of a ship-shaped hull. In 

 some respects the design problem resembles that 

 of a newsprint carrier in which the storage space 

 is required, almost literally, to be larger than the 

 dimensions of the outer shell! 



Vessels of the rectangular- or box-hold type 

 include: 



(a) Seatrains, in which railway cars are stowed 

 on tracks at levels from the bottom of the holds 

 to and including the upper deck [Burrill, L. C, 

 "The Design and Construction of the Rail-Car 

 Carrying Steamship Seatrain," NECI, 1929-1930, 

 Vol. XL VI, pp. 179-204 and Pis. VI, VII; MESR, 

 Aug 1952, front cover] 



(b) Cargo-container craft, in which the cargo 

 consists almost exclusively of metal shipping 

 containers in the form of cubes and parallele- 

 pipeds. For these vessels the cargo, when loaded 

 and unloaded, as well as when transported, may 

 be stowed instead on pallet boards suitable for 

 handling by fork-lift trucks. The combination of 

 pallet and material carried by it is usually of box 

 shape. 



(c) So-called trailer ships, in which the cargo 

 consists of the box or body portions of trailer 

 trucks, minus the tractor portions [Mar. Eng'g., 

 Apr 1954, pp. 48-49, 60; Jan 1955, pp. 57-58, 74]. 



These ships are designed on the principle that 

 all holds are to be truly rectangular, with flat 

 floors or decks and vertical sides and bulkheads. 

 The boundaries are all to be flush on the hold 

 side and to stand at right angles to each other. 

 Actually, instead of designing a ship hull of normal 

 form and arranging cargo holds inside it, the 

 holds are laid out first and the ship envelope is 

 drawn around them. 



The hydrodynamic design problem becomes one 

 of fashioning such an envelope so that it has the 

 greatest practicable fullness coefficient but is not 

 too difficult to drive. The box-shaped holds can 

 not be raised appreciably to help with the hull 

 shape because of the consequent loss of volume 

 and the raising of the center of gravity of the 



cargo. No design rules are available for develop- 

 ing an acceptable underwater form under these 

 conditions except the general principles of achiev- 

 ing reasonably good flow around all parts of the 

 hull and a reasonably uniform pressure distribu- 

 tion. The comments in Chaps. 8, 27, and 28 

 relative to the flow around and the behavior of 

 straight-element and discontinuous-section forms 

 are of value here. 



Achieving practically a rectangular maximum 

 transverse section presents no problem because 

 successful ships are now built with Cx values of 

 0.99 or more. By using a flat inner bottom and 

 placing a vertical wing-tank bulkhead inboard of 

 the shell on each side, the transverse section of the 

 hold is completely rectangular. 



To obtain the greatest length of this full rec- 

 tangular section^ the cargo holds must occupy 

 all of the fullest portions of the length, leaving 

 the tapered portions at the ends to accommodate 

 the propelling and auxiliary machinery and mis- 

 cellaneous equipment. 



It is by no means necessary to break up or 

 interfere with the best hold space by the installa- 

 tion of a midship deckhouse, as has been the 

 practice on tankers and ocean-going ore ships for 

 many decades. The whole central portion of the 

 vessel may be kept clear for hatches, cargo- 

 handhng gear, and deck loads, if desired, by 

 moving the machinery and accommodations all 

 the way aft. This was done on the large Swedish 

 tanker Oceanus [SBSR, 16 Dec 1954, p. 20 of 

 advt.; Motor Ship, London, Jan 1955, p. 460] 

 and is shown by J. J. Henry in his "Artist's (and 

 presumably Naval Architect's) Conception of a 

 Modern Ore Carrier" [SNAME, 1955, p. 57]. 



76.6 The Design of Straight-Element Hulls. 

 Other than to obtain the special V-bottom 

 shapes for high-speed motorboats and other 

 planing craft, the straight-element form defined 

 in Sec. 27.1 is used to facihtate hull construction. 

 This involves not only reductions in cost, time, 

 and effort but in the amount and kind of fabri- 

 cating equipment required. Depending upon the 

 structure of the boat or ship the transverse frame 

 members may be made up of straight segments 

 around the periphery, the planking or shell 

 plating may be made developable, embod3ang 

 single curvature only, or there may be a combina- 

 tion of both. If ease of construction is a pre- 

 dominating factor, the shape of the underwater 

 hull, and perhaps also that of the abovewater 

 hull, must conform to the limitations of the 



