PRINCIPLES OF NAVAL ENGINEERING 



147.47 



Figure 5-4.— Linear velocity 

 and reactive thrust. 



pressure on the water and so develop more re- 

 active thrust than point A. The higher the linear 

 velocity of any part of a blade, the greater will 

 be the reactive thrust. 



Real propeller blades are not flat but are de- 

 signed with complex surfaces (approximately 

 helicoidal) to permit every infinitesimal area to 

 produce equal thrust. Since point Z has a higher 

 linear velocity than point A, the thrust at point 

 Z must be decreased by decreasing the pitch 

 angle at point Z. Point M, lying between points 

 Z and A, would have (on a flat blade) a linear 

 velocity less than Z but greater than A. In a real 

 propeller, then, point M must be set at a pitch 

 angle which is greater than the pitch angle at 

 point Z but less than the pitch angle at point A. 

 Since the linear velocity of the parts of a blade 

 varies from root to tip, and since it is desired 

 to have every infinitesimal area of the blade pro- 

 duce equal thrust, it is apparent that a real pro- 

 peller must vary the pitch angle from root to tip. 



Propeller Size 



The size of a propeller— that is, the size of 

 the area swept by the blades— has a definite ef- 

 fect on the total thrust that can be developed on 



the propeller. Within certain limits, the thrust 

 that can be developed increases as the diameter 

 and the total blade area increase. Since it is 

 impracticable to increase propeller diameter 

 beyond a certain point, propeller blade area is 

 usually made as great as possible by using as 

 many blades as are feasible under the circum- 

 stances. Three-bladed and four-bladed marine 

 propellers are commonly used. 



Thrust Deduction 



Because of the friction between the hull and 

 the water, water is carried forward with the hull 

 and is given a forward velocity. This movement 

 of adjacent water is called the wake . Since the 

 propeller revolves in this body of forward mov- 

 ing water, the sternward velocity given to the 

 propeller is less than if there were no wake. 

 Since the wake is traveling with the ship, the 

 speed of advance over the ground is greater than 

 the speed through the wake. 



At the same time, a propeller draws water 

 from under the stern of the ship, thus creating a 

 suction which tends to keep the ship from going 

 ahead. The increase in resistance that occurs 

 because of this suction is known as thrust de- 

 duction. 



Number and Location 

 of Propellers 



A single propeller is located on the ship's 

 centerline as far aft as possible to minimize the 

 thrust deduction factor. Vertically, the propeller 

 must be located deep enough so that instill water 

 the blades do not draw in air but high enough so 

 that it can benefit from the wake. The propeller 

 must not be located so high that it will be likely 

 to break the surface in rough weather, since this 

 would lead to racing and perhaps a broken shaft. 



A twin-screw ship has the propellers located 

 one on each side, well aft, with sufficient tip 

 clearance to limit thrust deduction. 



A quadruple-screw ship has the outboard 

 propellers located forward of and above the in- 

 board propellers, to avoid propeller stream in- 

 terference. 



Controllable Pitch Propellers 



As previously noted, controllable pitch pro- 

 pellers are in use on some naval ships. Con- 

 trollable pitch propellers give a ship excellent 

 maneuverability and allow the propellers to de- 

 velop maximum thrust at any given engine rpm. 



90 



