Sec. 70.43 



SCREW-PROPELLER DESIGN 



633 



70.43 Mechanical Construction ; Type of Hub ; 

 Shaping and Finish of Blades. The mechanical 

 design and the details of construction of screw 

 propellers, of both the solid and the built-up 

 types, have become rather well standardized in 

 the past century. They are described and illus- 

 trated by R. H. Tingey [ME, 1942, Vol. I, pp. 

 267-293, esp. pp. 291-293] and by the authors 

 of up-to-date handbooks on marine engineering. 



The choice of whether a particular design of 

 propeller is to be of the solid or built-up type is 

 usually made by the owner and operator, often 

 based upon considerations far removed from 

 hydrodynamics. The marine architect is called 

 upon only to state how much reduction in effi- 

 ciency is involved if the wheel is built up. This 

 depends upon the ultimate size and shape of the 

 hub, including the flanges at the roots of the 

 blades, the fairing of the bolts and nuts for these 

 flanges, the fairing of the whole hub into the 

 hull, and other features. The probable reduction 

 in efficiency on the ship, mentioned in Sec. 70.14, 

 is of the order of 2 or 3 per cent. If a solid pro- 

 peller has an efficiency tjo of 0.70 the equivalent 

 built-up propeller may have an ijo of (0.70) (0.97) 

 = 0.679. In what are known as points, often used 

 by marine engineers to indicate a change in 

 percentage numerals, this is a reduction of 

 (70 - 67.9) = 2.1 points. 



As a means of reducing this loss and retaining 

 the demountable-blade advantage, there are 

 several possibilities which call for comment: 



(1) The built-up propeller without adjustable 

 features. In the orthodox design, followed for 

 many decades, the base or bolting flange of each 

 blade is circular. This permits some adjustment in 

 geometric pitch when the blade is bolted firmly 

 in place on the hub. However, if the blade is 

 shifted in this process, the shift must be a constant 

 angle d(j> for all blade sections. This is by no 

 means equivalent to a constant change in linear 

 pitch P, or even to a constant percentage change 

 in P, because P = 2:rfl tan <^ and the distance 

 2tR changes with radius. The great number of 

 solid propellers in use indicates a small need for 

 the adjustable feature and for changing pitch in 

 service. By eliminating the adjustment in angle, 

 and with it the need for the circular base flange on 

 each blade, it is possible to make the blades 

 detachable or demountable without greatly in- 

 creasing the size of the hub. 



(2) It appears almost certain that in the not- 



distant future there will be developed a strong, 

 not-too-expensive, corrosion-resisting, weldable 

 ferrous alloy for propeller blades which will 

 permit the separate blades of a screw propeller 

 to be cast individually with specially shaped root 

 palms and welded to a steel hub, or to an enlarge- 

 ment on a short stub shaft. An arrangement 

 diagram of the latter scheme, for the arch-stern 

 ABC ship, is sketched in Fig. 74.L. 

 (3) The availability of a strong, rigid, ferrous 

 alloy will, among other things: 



(a) Enable the larger propellers, whose ship- 

 ment is expensive and inconvenient, to have their 

 hubs and blades assembled by welding at the 

 yard where the ship is built. Annealing of the 

 welds is possible by induction heating. 



(b) Eliminate the trouble, expense, and vul- 

 nerability of tapered fits, keyways, keys, screw 

 threads, and nuts necessary to attach the present 

 propellers to their shafts. Bolted flanges are 

 much simpler and more reliable. 



(c) Eliminate the need for galvanic-action 

 protectors in the neighborhood of bronze pro- 

 pellers, with their never-ending added drag and 

 continual expense 



(d) By the use of plated chromium or some 

 other material, such as on the stub shaft project- 

 ing abaft the ABC ship propeller in Fig. 74. L, 

 eliminate the need for bronze bearing sleeves on 

 steel shafts. 



Constant pressure and attention applied to 

 ■the manufacture of more accurate propellers for 

 the past two or three decades, that is, propellers 

 conforming more nearly to the design drawings, 

 has produced valuable results. Tolerances of 

 plus or minus 1/4 per cent in mean face pitch 

 over a blade, and of plus and minus 1/2 per cent 

 in local pitch variation, are now being approached 

 or exceeded. Blade thicknesses in important posi- 

 tions are being specified and measured. This is 

 excellent as far as it goes, but it still leaves as un- 

 specified the shape of the entire back or —Ap 

 surface of the blade. However, there is a growing 

 appreciation of the importance of back shape 

 among owners and operators as well as among pro- 

 peller designers. Lack of suitable equipment to 

 make measurements on large propellers, both 

 during manufacture and inspection, is delaying 

 progress along this line. 



Edge shapes of propeller blades are important. 

 These require delineation on the drawings by 

 large-scale details or by geometric dimensions. 



