636 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 70.46 



hardness number and (2) the erosion-fatigue resistance 

 expressed in tons per in- for 50 million cyeles of reverse 

 bending. The more highly resistant alloys have values in 

 excess of SOO, and in descending order of merit they include : 



(a) Austenitic stainless steels 



(b) Aluminum bronzes, with or without nickel additions 



(c) Low-nickel stainless steels 



(d) Silicon monel 



(e) Monel metal 



(f) High-tensile bronze 



(g) Turbadium bronze. 



"Below 800 are placed the normal: 



(h) Manganese bronzes 



(i) Silicon bronzes 



(j) Phosphor bronzes 



(k) Gun metals 



(1) Cast irons 



(m) Aluminum alloj's. 



"This does not imply that the manganese bronzes 

 which have given such good servide have poor resistance, 

 but they are used purely as a basis of comparison." 



Designs and techniques have been evolved, and 

 are in use on the blades of propeller-type turbines 

 in hydroelectric plants [ASNE, Nov 1946, pp. 

 547-549; Maritime Reporter, 1 Mar 1955, p. 17], 

 whereby a thin corrosion-resisting steel cladding 

 is applied to a cast-steel blade by welding, either 

 in the form of a multitude of welding beads from 

 corrosion-resistant rod or a thin sheet of rolled 

 corrosion-resisting metal. 



It is entirely possible that some form of plastic 

 coating may be evolved which will protect a 

 blade from minor mechanical damage. This 

 coating might resist the action of sand and mud, 

 prevent corrosion, resist erosion by cavitation, 

 and serve as an insulating layer over a copper 

 alloy so that no galvanic-action protectors would 

 be needed on the adjacent steel surfaces of the 

 hull and the appendages. To make such a coating 

 reliable and successful, however, will undoubtedly 

 call for a special application procedure, involving 

 the following operations: 



(1) Heating the entire propeller in an oven for a 

 considerable period, to drive out all the moisture 

 in the interstices between the metallic crystals 



(2) Subjecting the heated propeller to an almost 

 complete vacuum, to pull out not only all the 

 moisture but all the gases to be found within the 

 metallic structure 



(3) Application of a sealing compound while the 

 propeller is still heated and under a vacuum, to 

 fill up all the internal crevices and pockets where 

 gases or moisture might otherwise collect 



(4) Application and curing of a durable plastic 

 coating to all the external surfaces except those 

 which have to be in metal-to-metal contact with 

 the propeller shaft. 



It is beheved that the blistering and other 

 difficulties with plastic and similar propeller 

 coatings in the past have arisen from the presence 

 of moisture and gases within the metal proper, 

 underneath the coating. The procedure outlined 

 in the foregoing is similar to that which was found 

 necessary and which has been employed success- 

 fully for many years in vacuum-impregnating the 

 wound coils of electric motors and other electric 

 devices. 



70.46 Prevention of Singing and Vibration. 

 It is assumed in Sec. 23.7 that the singing of 

 propeller blades is due to the alternating circula- 

 tion component around a blade section, and the 

 periodic variation in lift, accompanying the shed- 

 ding of alternate vortexes in a vortex street or 

 trail. It is possible to relate the frequency of 

 lateral vibration with the blade velocity, the 

 Strouhal number S„ , and the Reynolds number 

 Rn provided the diameter or transverse dimension 

 of the vibrating (and eddy-creating) body is 

 known. This transverse dimension corresponds 

 generally with the thickness t in Fig. 70. P, 



Anqle at E, Separation 



Should Be Point 



18 deq or More, E 

 With Respect to 



Tanqent at E, 



Ton 



NOTE! Points Ei.Eg, ond Ej Must 8e Shorp to be 

 Effective 

 Estimated General Direction of Flow 

 in Vicinity of Trailinq Edqe 



Not Less Than 165 d 



Fig. 70.P Sketch of Chisel Type of Trailing Edge 

 To eUminate singing effectively it is most important that 

 the corners at Ei , E2 , and E3 be sharp, to create definite, 

 fixed separation points there. 



roughly abreast the average position of the 

 separation points E on one or both sides of the 

 blade. If the trailing edge of the section were 

 approximately semi-circular it would be easy to 

 estimate the value of /, but difficult to confirm it. 



