518 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.13 



blisters or bulges on a ship. Sec. 76.9 discusses the 

 design of ship hulls with discontinuous sections 

 and includes a number of design rules. Certain of 

 these are applicable also to ships with blisters and 

 bulges. 



Manifestly, the addition of a blister to the 

 main hull or the swelling of a bulge on the bound- 

 ary of it comprises an integral part of the volume 

 which must be pushed through the water. The 

 form coefficients and parameters of the ship 

 carrying them apply therefore to its outside 

 dimensions, shape, and surface. A ship already 

 built, to which a bulge or blister of large size is 

 appUed, becomes in fact a new ship, with new 

 proportions, new parameters, new hull coefficients, 

 and a different shape. 



Assuming that the transverse section contours 

 of a bulged form are fair, the discussions of Sees. 

 24.10 and 25.8 show that neither the maximum- 

 section shape nor its fullness coefficient Cx have 

 an appreciable effect on the ship resistance. This 

 means that the bulge can be shaped and positioned 

 to suit other requirements. If it comes at the 

 designed waterUne, of course, it almost certainly 

 changes the Bx/H ratio, the angles of waterline 

 slope in the entrance and run, the waterplane 

 coefficient Cw , the transverse moment of area 

 coefficient C,t , and other parameters. If it 

 comes below the DWL it changes only the fatness 

 ratios A/(0.010L)' and ¥/(0.10L)\ There is 

 considerable latitude in these values for a good 

 design. 



67.13 General Arrangement of Single-Screw 

 Stem. The counter or fantail type of stern, with 

 its relatively thin and deep centerhne skeg, its 

 rather long fore-and-aft abovewater overhang, 

 and its wide upper decks, came down through the 

 sailing ships of the Middle and Modern Ages. 

 It persisted as the normal form of stern for 

 mechanically driven vessels until the 1930's and 

 beyond. In the two decades preceding the time of 

 writing (1955) it has, for reasons still rather 

 obscure, largely been replaced as the normal 

 single-screw stern by the whaleboat or canoe 

 (cruiser) type. The latter has the practical advan- 

 tage that it is probably less costly and less 

 difficult to build, that it affords better protection 

 to the rudder and propeller, and that, lacking 

 projections and discontinuities, it is not likely to 

 give trouble in a following sea. Hydrodynamically, 

 it usually offers better shielding of the propeller 

 against air leakage and, because of the greater 

 waterline length, it should result in smaller 

 surface waterliue slopes at the stern. Rarely, 



however, are these small enough to avoid separa- 

 tion at the DWL. 



What is really important is the underwater 

 hull shape forward of the propeller, as it effects 

 flow to the wheel, and the augment of resistance 

 due to the reduced-pressure field in the inflow jet. 

 What is equally important, not only for propulsion 

 but for propeller maintenance, is that the single 

 wheel be kept well submerged in all operating 

 conditions. With the large powers now being 

 put into single screws, of the order of 20,000 

 horses or more, and the likelihood of still greater 

 single-shaft powers in the future, as ship speeds 

 increase, it is imperative that the waterUne slopes 

 ahead of the propeller aperture be such as posi- 

 tively to avoid any liability of separation. 



At the level of the 0.7 to 0.9 radius on a propeller 

 blade in the 12 o'clock position, the skeg waterline 

 slopes just ahead of the aperture should not 

 exceed 15 deg for near-surface levels, or 18 to 20 

 deg for levels that are always well below the sur- 

 face in smooth-water running. This slope is to 

 be carried aft, as close as practicable to the aper- 

 ture, eliminating blunt endings on sternposts or 

 stern weldments, forgings, or castings. The 

 terminal radii should be no greater than necessary 

 to prevent corrosion on thin sections, say 0.1 ft 

 or less on large vessels. 



The aftfoot may be cut up in profile to meet 

 maneuvering requirements, to form what is 

 sometimes called a clear-water stern, provided 

 dynamic stability of route is assured, and the 

 necessary docking support remains along the 

 centerline keel. If it is known that the flow is aft 

 and upward in the cutaway region the level lines 

 in the skeg termination may be rather blunt. It 

 is the hull slope along the actual flowline that 

 counts, below as well as above the shaft axis. 



In some tugs the aftfoot is cut away, as in a 

 clear-water stern, to afford greater maneuver- 

 ability but the keel bar and the sternpost are 

 extended aft and downward, respectively, to 

 carry a rudder shoe and to act as a guard for 

 both propeller and rudder ["Kort Nozzle Tug 

 Maamal," SBSR, 20 Nov 1952, p. 676]. 



Freedom from objectionable stern vibration in 

 service is often a requirement that ranks in 

 importance \vith efficient propulsion. Meeting this 

 demand means, in addition to a fine skeg ending 

 ahead of the propeller, an adequate aperture 

 clearance ahead of the sweep lines of the propeller 

 blades. This must be enough to keep down to 

 acceptable limits the periodic lateral forces on 

 the skeg as each blade, with its circulation pattern 



