748 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 75.13 



this respect because they extend farther from 

 the sliip's side. 



(b) Avoid an.v semblance of a flat bottom forward 

 beneath which air may be caught and buried 

 under the hull when the forefoot emerges and 

 then plunges heavily during wavegoing. The air is 

 carried aft from this region, along the bottom of 

 the ship, by the relative motion of the water. 



(c) Wherever practicable, the water-injection 

 openings on the under side of a ship are to be 

 kept free of streams of air bubbles. Usually, these 

 are trapped by the ship waves in smooth-water 

 operation and carried along under the ship in 

 more-or-less well-traveled routes. The routes can 

 be determined reasonably well in a circulating- 

 water channel by injecting a small stream of air 

 at any point around the bow where water appears 

 to be falUng on itself (as in a breaking bow-wave 

 crest) and tracing the route of the bubbles 

 visually or photographically. 



Venting holes in keels, for the escape of air 

 trapped below them, are described in Sec. 73.19 

 and illustrated in Fig. 73.0. 



Air leakage to rudders, propellers, and the like 

 is prevented by overhanging portions of the stern, 

 by Jenney fins, and by similar devices, described 

 elsewhere in the book. 



75.13 Design Notes for Shallow Recesses. 

 A shallow recess, illustrated at 1 in Fig. 75.J, is 

 defined here as one which has a depth —h below 

 the fair solid surface less than either its breadth b 

 across the flow or its length e in the direction of 

 flow. It has a closed bottom, so there is no 

 auxiUary licjuid flow into or out of it. 



Judging principally by results of flow tests for 

 inlet openings in the shell, such as condenser 

 scoops, any shallow recess of appreciable length 

 in the direction of flow may be expected to have 

 a stagnation point on its downstream face, 

 illustrated at Q in diagram C of Fig. 7.J and at 

 Q in diagram 2 of Fig. 75.J. The presence of 

 ram pressure on such a surface, facing forward, 

 means large -|-Ap's and added drag. Other 

 features of the flow are discussed in Sec. 8.3. 



W. Froude found in the early 1870's, when 

 experimenting with a pressure speed log for ships 

 [Brit. Assn. Rep., 1874, p. 256], that if a circular 

 pipe as small as 0.04 ft in diameter was fitted 

 square to and terminated flush with a flat surface 

 parallel to the direction of liquid flow, there was 

 developed a small +Ap within the mouth of the 

 pipe, amounting to about 0.04 of the ram pressure 



q = 0.5pUl . This +Ap would undoubtedly have 

 been larger for an opening longer in the direction 

 of flow. In any case it would be sufficient to start 

 a flow into the opening if a circuit for the liquid 

 were provided. 



In the design of closed-bottom recesses to 

 create minimum drag, one obvious solution is to 

 offset the downstream edge inward, away from 

 the water flow, depicted at 3 in Fig. 75.J. S. F. 

 Hoerner indicates that a setback at this edge 

 equal to 8 per cent of the depth of the downstream 

 face, when combined with a square, flush corner 

 at the upstream edge of the recess, reduces the 

 overall drag by an appreciable amount [AD, 1951, 

 Fig. 4.22, p. 56]. Reheving the downstream edge 

 in this manner, for a hull opening, involves 

 special shaping of the shell plating and possibly 

 local trimming of framing members. This is an 

 expensive item on any kind of metal hull. If the 

 recesses are few, and the openings large, as for 

 the hopper-door recesses under a dredge, it may 

 be worth while to incorporate this setback in the 

 downstream edge of each of them. It is not recom- 



Direction of Flow Over Surfoce as a Whole. 



-*-J)epth - Vi I _ 



T^L"Ie"^qT hT~~ I ' Breodjh b^r Wdth 

 [""^i^Cj the Flow * [>^ '^'^'"°" **^* ^<^ 

 For the Lo'^OLft Shown; 



Recess or Gap Area is Thot of Reciancjle ABCD 

 Frontal Area of Downstream Face is AD&F 

 Aspect Ratio of Openinq is -§- 

 For Low Broq, Keep e Short and Ratio -g- Lorqe 



1 



V iscous /Mixing and Retardation 



+Ap Reqion ot and Near Stoqnation Point Q. 



Schematic Streamlines 



Fig. 75.J Definition and Flow Sketches fob 

 Shallow Recesses 



