634 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 70.44 



described in Sec. 70.19. They are rather easily 

 checked b.v small full-scale templates. 



Considering that the surfaces of a screw-pro- 

 peller blade almost always travel through the 

 water faster than the ship, and the surfaces of 

 the outer portions often several times as fast, 

 the blade surfaces should be as smooth as modern 

 tools and techniques can make them. Indeed, in 

 keeping with the necessity for smaller roughness 

 tolerances on a large ship than on its model, to 

 make the two surfaces hydrodynamically smooth, 

 the full-scale propeller surface should actually 

 be smoother than that of its model. 



In particular, no lifting holes should be drilled 

 through the blades, contrary to the design shown 

 by R. H. Tingey [ME, 1942, Vol. I, Fig. 1, p. 268], 

 nor should nicks and bent-over portions of the 

 edges be permitted to remain after the first 

 opportunity to repair them. It often happens that, 

 if the dock trials are run with the ship's own 

 propellers in place, pieces of wire rope and other 

 debris which have been dropped overboard at 

 a fitting-out dock may be picked up by the pro- 

 pellers, with damaging effects. If a ship's propellers 

 have been so menaced, it is wise to have all blade 

 edges examined by a diver after the vessel is in 

 clear water, before it is permitted to undertake 

 standardization and acceptance trials. 



In this connection the following is quoted from 

 the Conclusions of the Sixth International Con- 

 ference of Ship Tank Superintendents, 1951, 

 page 10: 



"6. The Conference re-emphasizes Decision 2 of the 1948 

 Conference on this subject, which stated that 'It is neces- 

 sary that the model propellers should be made to a. high 

 degree of precision and in all published work the measured 

 tolerances and the quality of the surface finish should be 

 stated.' " 



How to keep this ship-propeller surface smooth, 

 even on the "stainless" metals which are essen- 

 tially resistant to corrosion and erosion, is still 

 a problem, but one for metallurgists rather than 

 marine engineers. 



70.44 Blade Strength and Deformation. On 

 many if not most screw propellers it is necessary 

 to shape the root sections, and possibly also 

 some others, to give the necessary strength and 

 rigidity to the blades. On icebreakers and ice- 

 ships, structural considerations may outweigh 

 those of hydrodynamics. However, it is not 

 possible in this book to devote space to this 

 feature, other than has already been done in 

 Sees. 70.19 and 70.30. 



Rather complete procedures for determining 

 blade thicknesses adequate for strength and 

 rigidity, and for calculating stresses in the blade 

 material, are found in the following references: 



(a) Schoenherr, K. E., SNAME, 1934, pp. 113-114 



(b) Schoenherr, K. E., PNA, 1939, Vol. 11, p. 1.57 



(c) Taylor, D. W., S and P, 1943, Chap. 29.3, pp. 127-141 



(d) Tingey, R. H., ME, 1942, Vol. I, pp. 281-291 



(e) Van Lammeren, W. P. A., RPSS, 1948, pp. 269-273 



(f) Hecking, J., "Strength of Propellers: Analysis Made in 



Connection with Classification Rules at the Ameri- 

 can Bureau of Shipping," MESA, Oct 1921, pp. 

 762-767. 



It may very well be that elastic deformation 

 under heavy load of the blades of a nearly perfect 

 "static" design will modify rather appreciably 

 the shape and the performance contemplated by 

 the designer. This is a manifestation of hydro- 

 elasticity, described in Sec. 21.5 and mentioned 

 in Sec. 70.13. Furthermore, this deformation 

 may take place periodically and in varying 

 amounts as a blade rotates through a complete 

 revolution, leading to blade vibration and other 

 objectionable results. 



It is interesting to note in this connection the 

 comments made by Dixon Kemp in the late 

 1890's in his treatise on yacht design ["Yacht 

 Architecture," Cox, London, 1897, 3rd ed., p. 284]: 



"There would seem to be some advantage if the blades 

 are elastic, and bend whilst revolving, especially in the 

 case of small vessels; and Messrs. Yarrow have recorded a 

 case within their e.xperience of torpedo boat propulsion 

 where, by submitting a thin elastic blade for a perfectly 

 rigid one, the speed was altered from n\ knots to 19 knots." 



It is unfortunate that no record has yet been 

 found of the shape and materials of these thin, 

 elastic blades, nor an explanation of their superior 

 performance. 



To return to a consideration of modern wheels, 

 the propeller designer is advised to sketch a 

 so-called deformation diagram of his propeller, 

 in which the estimated deformations under thrust 

 load are greatly exaggerated for emphasis. The 

 aim is to delineate the shape of the propeller 

 blade under load and to determine changes in 

 pitch at various radii. Procedures for constructing 

 these diagrams are as yet not well formulated 

 but a few hints may be helpful for guidance: 



(a) The thrust forces are exerted roughly normal 

 to the line joining the nose and tail of each section. 

 The drag force may be neglected because it acts 

 generally in line with the chord of each section. 



(b) At the effective angles of attack normally 



