Sec. 70.45 



SCREW-PROPELLER DESIGN 



G35 



encountered, and with hydrofoil or airfoil section 

 shapes, the lift force may be expected to act at a 

 point between 0.25c and 0.40c abaft the nose of 

 each section 



(c) Tlie bending forward (in the direction of 

 thrust) of cantilevered screw-propeller blades due 

 to thrust load is usually not diminished as the 

 designed thrust-load factor decreases because the 

 blades are made thinner in an effort to increase 

 the efficiency 



(d) The centrifugal forces acting on raked blades 

 are functions only of the amount of actual rake, 

 taking deformation into account, the radii of 

 the sections involved, and the rate of rotation 

 CO (omega). 



The deformation of heavily loaded screw-pro- 

 peller blades is best counteracted, not by thicken- 

 ing the blades but by using materials of the highest 

 practicable modulus of elasticity. The nickel- 

 copper alloys and the corrosion-resisting chro- 

 mium-iron alloys are considerably superior to the 

 best bronzes in this respect, although accurate 

 data as to elastic moduluses are often difficult to 

 obtain. 



Screw-propeller blades which have long, thin 

 trailing overhangs such as those of the weedproof 

 type shown in diagram 12 of Fig. 32. L and those 

 fitted on the liner Normandie in the early 1940's, 

 almost certainly suffer some bending of the 

 overhang in an ahead direction. This reduces the 

 geometric pitch angle 4>, straightens out the mean- 

 line, and diminishes the blade camber in that 

 region. The net effect is to reduce not only the 

 local but the overall lift of the blade sections at 

 those radii. A slight additional camber of the 

 overhung portions may well be introduced to 

 overcome this deformation and to make all the 

 blade area work effectively. 



On a propeller which is loaded moderately or 

 heavily, the blades are never skewed to an appre- 

 ciable extent in the forward or ahead direction. 

 Were this done, the centers of pressure on the 

 outer elements, lying near the forward quarter- 

 or third-points of these elements, would be much 

 farther ahead of the torsion axis in the root 

 sections of the blade than they are abaft that 

 axis in a blade swept or skewed normally aft. 

 This would mean a greater twisting moment 

 forward, and a greater increase in the geometric 

 pitch angle than the reduction in that angle 

 when the blades are swept back. This increment 

 A0 increases the effective angle of attack «/ , 



increases the lift on the outer elements, and in- 

 creases the twist deformation. The effective angle 

 of attack «; thereupon becomes still greater. A 

 vicious cycle continues until the vessel speeds up 

 to match the increased thrust, the engine slows 

 down because of the increased torque, or the 

 blade takes a permanent set in twist. 



A sequence of events of this kind is encountered 

 when a heavily loaded propeller with swept-back 

 blades has its direction of rotation reversed, as 

 during a crash-back maneuver. The greatly dis- 

 turbed condition of the water around it probably 

 saves the propeller but at least one case is on 

 record where blades have been bent in a sudden 

 high-power reversal. 



70.45 Propeller Materials and Coatings to 

 Resist Erosion. One of the most satisfactory 

 materials now known for use in screw propellers 

 which must resist corrosion, erosion, and impact 

 from sand, ice, and the Uke is an alloy composed 

 of approximately 14 per cent chromium and 86 

 per cent iron. This alloy is capable of heat treat- 

 ment to give yield points of the order of 70,000 

 lb per in^. As for other iron alloys, the yield 

 point is fairly definite; this is not the case for the 

 bronzes and brasses containing large quantities 

 of copper. The proper kinds of corrosion-resisting 

 irons and steels have proved in practice their 

 ability to withstand severe usage on vessels which 

 must work in the ice. 



A corrosion-resistant alloy of austenitic charac- 

 teristics was employed by the Germans some 

 years ago for the propellers of destroyers and 

 similar naval vessels. This is an alloy containing 

 22 per cent chromium and 1 1 per cent nickel, with 

 most of the remainder composed of iron. Screw 

 propellers of this alloy require special casting 

 techniques, speci^-l cutting tools, and tedious 

 machining procedures but once fabricated they 

 stand up well in service. Examples are the two 

 corrosion-resisting steel propellers removed from 

 the German destroyer Z37 and now (1954) on 

 exhibition at the Engineering Experiment Station 

 at Annapolis, Maryland. 



Some comments on cavitation erosion and 

 means of preventing it on marine propellers and 

 other appendages are given by S. F. Dorey 

 [Jour. Inst. Metals, Great Britain, Jul 1954; 

 ASNE, Feb 1955, pp. 94-96]. On page 95 of the 

 latter reference appears the following, modified 

 slightly for emphasis: 



"A reasonable assessment of the erosion-resistance of an 

 alloy is given by the product of (1) the suifaoe Brinell 



