fifiS 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 72.11 



be swept aft as gently as possible below the load 

 waterline; the buttocks forward should be very 

 easy and the bilges rounded" [INA, Jul 1952, 

 p. 148]. This is done in the Hillman pushboat 

 previously referenced, as depicted in the profile 

 of Fig. 72. F. The forefoot is cut away in similar 

 fashion on the Mary Powell and the New York, 

 indicated by the lines drawings in the references 

 cited for those vessels. 



72.11 Slope and Curvature of Buttocks. In 

 the slow- and moderate-speed ranges, on a wide, 

 shallow craft, most if not very nearly all of the 

 flow passes under the bottom. The buttock slopes 

 and curvatures therefore demand the same atten- 

 tion as the waterlines near the surface in a deep- 

 water ship design. This calls for easy transitions 

 where the forward buttocks or forelmes curve 

 downward and aft and under the bottom. Ex- 

 amples of these are found m the keel profile and 

 the chine line at the bow of the Hillman design 

 ofFig.72.F. 



For drafts of the order of 8 to 6 ft or less, it is 

 well to hold the buttock slopes under the run to 

 a maximimi of 15 deg with the horizontal; a 

 maximum of 12 or 13 deg is better. 



Apparently to save time and labor and to 

 facilitate fabrication and erection, the buttocks 

 under the run of small shallow-draft vessels often 

 have a sharp knuckle where they leave the base- 

 plane. Abaft this knuckle they are straight, with 

 constant slope. If they are given reverse curvature, 

 so as to slope downward and aft behind the 

 propeller position, there is a second sharp knuckle 

 where this change is made. It is pointed out 

 repeatedly in Parts 1 and 2 of Volume I that 

 water resists this sudden change in direction, 

 often with objectionable consequences. It is 

 usually found, furthermore, that with some 

 planning and some alteration of the structural 

 plans, it is just as easy to provide a good path 

 for the M'ater flow as to make a poor one. 



72.12 Adequate Flow of Water to the Propul- 

 sion Devices. The provision of means whereby 

 water may have free and easy entry to the pro- 

 pulsion devices is much more important in a 

 shallow-water than in a deep-water design. The 

 use of side paddlewheels eliminates the necessity 

 of shaping the hull locally to suit some particular 

 kind of device, especially to msure good flow 

 to it. In fact, this problem almost solves itself, 

 except for the proper fore-and-aft position of the 

 wheels along the hull to bring them in the crest 

 of a transverse Velox wave for the speed of most 



efficient propulsion. Further comments concerning 

 this feature are included in Sec. 71.6. 



When hmitations on overall beam prevent the 

 use of side wheels, the paddlewheels can always 

 be placed at the stern. Here they may be either in 

 the form of a pair, one on each quarter with the 

 drive mechanism in between, or there may be 

 only a single wheel, extending all the way across 

 the ship. In either case it is by no means easy to 

 provide the necessary displacement volume at 

 the stern and at the same time to embody the 

 reverse curvature in the buttocks under the run 

 which will project the inflow to the stern paddle- 

 wheels in a direction that is roughly horizontal. 



The after termination of the hull, just ahead 

 of the stern wheel (s), should not present a surface 

 against which the outflow jet, when the wheels 

 are going astern, can be reflected aft from the hull. 

 If an appreciable part of it is so reflected, an 

 engine order for astern rotation is liable to be 

 followed by an ahead motion of the ship! 



If rotating-blade propellers with vertical axes 

 are fitted at the stern, or at both bow and stern, 

 one or both ends are cut up toward a flat bottom 

 region in the vicinity of each propeller. This 

 prevents the blade tips from extending below 

 the keel. The blades are approximately vertical 

 and the water coming from under the ship meets 

 them in a nearly horizontal direction. It may 

 be found advisable to fit several of these propellers 

 abreast because the draft limitations are certain 

 to restrict the lengths of the blades. 



A shallow-water craft of normal form, to be 

 driven by screw propeUers, generally requires at 

 least two screws if the draft is limited and the 

 speed is high enough to call for large powers. 

 In addition, twin screws are useful if not manda- 

 tory to overcome the sluggish maneuvering 

 quahties of any vessel under which the bed 

 clearance is small. To prevent any undue reduc- 

 tion in propeller diameter and disc area, with its 

 loss of efficiency and maneuvering power, designers 

 of years gone by resorted to arrangements which 

 are still used to advantage if the situation de- 

 mands: 



(a) Make the tip submergence slightly negative, 

 with the tips normally out of water, on the basis 

 that they will have adequate submergence in the 

 stern-wave crest. This is only feasible if there is 

 to be such a crest at the propefler position and if 

 the propellers are to be lightly loaded. 



(b) Place the propellers under a wide torpedoboat 



