Principles of naval engineering - Historic Naval Ships Association

Chapter 5-FUNDAMENTALS OF SHIP PROPULSION AND STEERINGcannot be altered during operation; the pitch ofa controllable pitch propeller can be changedcontinuously, subject to bridge or engineroomcontrol. Most propellers in naval use are of thefixed pitch type, but some controllable pitch propellersare in service.A screw propeller consists of a hub and several(usually three or four) blades spaced atequal angles about the axis. Where the bladesare integral with the hub, the propeller is knownas a solid propeller . Where the blades are separatelycast and secured to the hub by means ofstuds and nuts, the propeller is referred to asa builtup propeller .Solid propellers may be further classified ashaving constant pitch or variable pitch . In a constantpitch propeller, the pitch of each radius isthe same. On a variable pitch propeller, the pitchat each radius may vary. Solid propellers of thevariable pitch type are the most commonly usedfor naval ships.Propellers are classified as being right-handor left-hand propellers, depending upon the directionof rotation. When viewed from astern,with the ship moving ahead, a right-hand propellerrotates in a clockwise direction and a lefthandpropeller rotates in a counterclockwisedirection. The great majority of single-screwships have right-hand propellers. Multiplescrewships have right hand propellers to port.Reversing the direction of rotation of a propellerreverses the direction of thrust and consequentlyreverses the direction of the ship's movement.Some of the terms used in connection withscrew propellers are identified in figure 5-3,The term face (or pressure face ) identifies theafter side of the blade, when the ship is movingahead. The term back (or suction back ) identi-,fies the surface opposite the face. As the propellerrotates, the face of the blade increasesthe pressure on the water near it and gives thewater a positive astern movement. The back ofthe blade creates a low pressure or suction areajust ahead of the blade. The overall thrust is derivedfrom the increased water velocity whichresults from the total pressure differential thuscreated.The tip of the blade is the point most distantfrom the~Tiub. The root of the blade is the areawhere the blade arm joins the hub. The leadingedge is the edge which first cuts the water whenthe ship is going ahead. The trailing edge (alsocalled the following edge ) is opposite the leadingedge. A rake angle exists when there is a rakeeither forward or aft—that is, when the blade isFigure 5-3.— Propeller blade.147.46not precisely perpendicular to the long axis ofthe shaft.Blade AngleThe blade angle (or pitch angle) of a propellermay be defined as the angle included betweenthe blade and a line perpendicular to theshaft centerline. If the blade angle were 0° , nopressure would be developed on the blade face.If the blade angle were 90° , the entire pressurewould be exerted sidewise and none of it aft.Within certain limits, the amount of reactivethrust developed by a blade is a function of theblade angle.Blade VelocityThe sternward velocity imparted to the waterby the rotationof the propeller blades is partiallya function of the speed at which the blades rotate.In general, the higher the speed, the greaterthe reactive thrust.However, every part of a rotating blade doesnot give equal velocity to the water unless theblade is specially designed to do this. For example,consider the flat blade shown in figure5-4. Points A and Z move about the shaft centerwith equal angular velocity (rpm) but with differentinstantaneous linear velocities. Point Zmust move farther than point A to complete onerevolution; hence the linear velocity at point Zmust be greater than at point A, With the samepitch angle, therefore, point Z will exert more89