156 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. -17. 11 



outer nulius of the vortex core iiiul tlio inner 

 radius of the vortex coil. 



Prciliction of the existence of a hub cavity or 

 swirl core and its size by the root-vortex metliod 

 may be found considerably more difficult. It 

 involves a knowledge — or an estimate — of the 

 circulation at the root sections or of the pressure 

 difTerences between the face and back of a blade. 

 It shoulii include any additional elTect of inter- 

 ference between adjacent blades. 



Although the data on the longitudinal extent of 

 a swirl core are somewhat meager it may be 

 assumetl from past observations on models and 

 ships that the core persists for at least several 

 propeller diameters. This means that it will be 

 present in the vicinity of any other part of the 

 ship likely to be placed abaft the shaft axis and 

 the propeller. 



The swirl core usually comes o(T the hub fairing 

 in line with the propeller axis but it Ls often otT.set 

 from the trailing end of a blunt hull. Whatever 

 may be its exact position, it swings rapidly into 

 the line of the aiijacent flow when this is not 

 parallel in direction to the shaft axis. The diameter 

 of the swirl core is increased if air gets into it. 



47.11 Prediction of Cavitation Erosion. The 

 results of metallurgical in\'e.stigations and tests 

 to date indicate that the materials which are 

 most resistant to ca\'itation erosion are solid- 

 solution alloys which have, in metallurgical 

 parlance, only a single phase. Single-pha.se alloys 

 have much narrower grain boundaries than 

 polypha,se alloys. Many corrosion-resisting steels 

 are austenitic alloys of this type; among them the 

 13Cr-87Fe alloy is a good example. 



A metal intended to resist erosion by cavitation 

 should have high resistance to lifiuid corrosion, 

 under the conditions in which it is to be used, as 

 well a.s high fatigue strength [Boetcher, H. N., 

 "Failure of Metals Due to Cavitation Under 

 Experimental Conditions," Trans. AS.MI], II V!';- 

 .'>8-l, Vol. 58, Jul ID.iO, pp. \]-)-i-m)\. 



Numerous tests of a wide varictj' of materials 

 have indicated a definite superiority in resistance 

 to cavitation erosion on the part of certain alloys. 

 A brief list of the.se allf)ys, with references to the 

 pubhshed test data, is givrjii in Sec. 70.45. In 

 one series of tests (Stewart, W. C, and Williams, 

 W. L., "Investigation of Materials for Marine 

 Propellers," ASTM, 1910, Vol. 4f), pp. 8:i(3~8l5] 

 the best all-around material was found to be a 

 0(5.14 nickel- 28.10 co[)per- -'{.05 silicon alloy. 



There has recently been i.ssued a paper entitled 



"A Review of Published Information on Cavita- 

 tion Erosion," (Admiralty Corrosion Committee 

 Rep. ACC/2C/54, N151/54, N316/54 (C.M.L. 

 Report RBS)], stamped 8 October 1954. On pages 

 25-27 there are listed 56 references in the technical 

 literature. The report is in five parts: 



I Historical Introduction 



II Theories of Cavitation lOrosion 



I I I Factors AffectingCavitaf ion Erosion Intensity 

 1\' Methotis of Testing 



V Erosion Resistances of Various Materials. 



Tables VIII and IX list many alloys in a scale 

 of relative resistance to cavitation erosion in 

 fresh and sea water, respectively. 



Cast-steel blades with corrosion-resisting steel 

 cladding have been used with success in propeller- 

 type turbines of water-power plants [.\SNE, Nov 

 1940, pp. 547-549( but this type of construction 

 has been found not too successful on ship append- 

 ages, perhaps becau.se of inadecjuate attachment 

 to the ferrous material underneath, and perhaps 

 because of harmful galvanic action between the 

 steel cladding and adjacent bronze propellers. 



Good design and construction requires the 

 greatest practicable initial smoothness of all 

 surfaces likelj' to be exposed to cavitation attack. 

 Any projecting irregularity, including definite 

 waviness of a surface, may be expected to initiate 

 cavitation if the pressures in the region approach 

 the vapor pressure of water. This is one important 

 reason why the curved backs of propeller blades 

 and hydrofoil surfaces .should, if anything, be 

 more regular and more smootii than the faces. 

 Erosion of the surface may be expected to occur 

 well downstream from a pronounced change in 

 shape of the solid surface. Nicks and turned-over 

 regions along a leading edge are notorious offenders 

 in this respect. A sharp, deep nick in the leading 

 edge may leave a trail of erosion at the radius of 

 the nick, extending irregularly all the way acro.ss 

 the blatlc. 



Pits and depressions in a surface, not forming a 

 l)art of the waviness previou.sly mentioned, 

 appear to have much less initial effect than cor- 

 resiKinding projections. However, holes through 

 the blades, such as are sometimes drilled for 

 handling jiurpo.ses, permit jets of water to .squirt 

 through from the face to the back. The discon- 

 tinuities thus formed in the flow may often be as 

 damaging .'is a solid jirojection in place of the hole. 



47.12 Propeller Performance Under Super- 

 cavitation. The general as])ects of the piTfnrni- 



