Chapter 5-FUNDAMENTALS OF SHIP PROPULSION AND STEERING 



cannot be altered during operation; the pitch of 

 a controllable pitch propeller can be changed 

 continuously, subject to bridge or engineroom 

 control. Most propellers in naval use are of the 

 fixed pitch type, but some controllable pitch pro- 

 pellers are in service. 



A screw propeller consists of a hub and sev- 

 eral (usually three or four) blades spaced at 

 equal angles about the axis. Where the blades 

 are integral with the hub, the propeller is known 

 as a solid propeller . Where the blades are sep- 

 arately cast and secured to the hub by means of 

 studs and nuts, the propeller is referred to as 

 a builtup propeller . 



Solid propellers may be further classified as 

 having constant pitch or variable pitch . In a con- 

 stant pitch propeller, the pitch of each radius is 

 the same. On a variable pitch propeller, the pitch 

 at each radius may vary. Solid propellers of the 

 variable pitch type are the most commonly used 

 for naval ships. 



Propellers are classified as being right-hand 

 or left-hand propellers, depending upon the di- 

 rection of rotation. When viewed from astern, 

 with the ship moving ahead, a right-hand pro- 

 peller rotates in a clockwise direction and a left- 

 hand propeller rotates in a counterclockwise 

 direction. The great majority of single-screw 

 ships have right-hand propellers. Multiple- 

 screw ships have right hand propellers to port. 

 Reversing the direction of rotation of a propeller 

 reverses the direction of thrust and consequently 

 reverses the direction of the ship's movement. 



Some of the terms used in connection with 

 screw propellers are identified in figure 5-3, 

 The term face (or pressure face ) identifies the 

 after side of the blade, when the ship is moving 

 ahead. The term back , (or suction back ) identi- 

 fies the surface opposite the face. As the pro- 

 peller rotates, the face of the blade increases 

 the pressure on the water near it and gives the 

 water a positive astern movement. The back of 

 the blade creates a low pressure or suction area 

 just ahead of the blade. The overall thrust is de- 

 rived from the increased water velocity which 

 results from the total pressure differential thus 

 created. 



The tip of the blade is the point most distant 

 from the~Tiub. The root of the blade is the area 

 where the blade arm joins the hub. The leading 

 edge is the edge which first cuts the water when 

 the ship is going ahead. The trailing edge (also 

 called the following edge ) is opposite the leading 

 edge. A rake angle exists when there is a rake 

 either forward or aft— that is, when the blade is 



147.46 



Figure 5-3.— Propeller blade. 



not precisely perpendicular to the long axis of 

 the shaft. 



Blade Angle 



The blade angle (or pitch angle) of a pro- 

 peller may be defined as the angle included be- 

 tween the blade and a line perpendicular to the 

 shaft centerline. If the blade angle were 0° , no 

 pressure would be developed on the blade face. 

 If the blade angle were 90° , the entire pressure 

 would be exerted sidewise and none of it aft. 

 Within certain limits, the amount of reactive 

 thrust developed by a blade is a function of the 

 blade angle. 



Blade Velocity 



The sternward velocity imparted to the water 

 by the rotationof the propeller blades is partially 

 a function of the speed at which the blades ro- 

 tate. In general, the higher the speed, the greater 

 the reactive thrust. 



However, every part of a rotating blade does 

 not give equal velocity to the water unless the 

 blade is specially designed to do this. For ex- 

 ample, consider the flat blade shown in figure 

 5-4. Points A and Z move about the shaft center 

 with equal angular velocity (rpm) but with dif- 

 ferent instantaneous linear velocities. Point Z 

 must move farther than point A to complete one 

 revolution; hence the linear velocity at point Z 

 must be greater than at point A, With the same 

 pitch angle, therefore, point Z will exert more 



89 



