SCREW PROPELLER- PART 1

INTRODUCTION

Screw propeller is the hydrodynamic device that rotates in a hydrodynamic medium due to a torque (Q) provided by the main engine of the ship. Due to this rotation, a thrust (T) is produced by the propeller and this thrust is transmitted from the propeller blades, through the propeller shaft, finally to the thrust block of the ship that transfers the load to the entire hull of the ship, which pushes the ship forward.

Image Courtesy: Google Images.

In spite of several advancements in the marine propulsion system in the recent past, screw propellers are still the most widely used propulsion system, generally fitted to almost every displacement vessels. In this article, I am going to explain the geometry of screw propeller and various terminologies associated with screw propellers.

SCREW PROPELLER- VARIOUS PARTS

Screw propeller consists of a number of blades that are attached to a paraboloid structure called the boss. Generally, the connection is done by welding the blades with the boss for a fixed pitch propeller. Whereas for a controllable pitch propeller where the blade can change its angle of orientation, a special mechanical arrangement is provided that assists smooth rotation of the propeller blades along with maintaining the water tightness of the whole system.

          The side of the propeller blade seen when one observes the propeller from behind the ship is called the face of the propeller. The other side of the propeller blade is called the back of the propeller.

          The narrow end of the boss creates a lot of disturbance as it rotates in the hydrodynamic medium leading to flow turbulence, separations and even hub vortex cavitations. Thus in order to smoothen the flow around the hub tip, a smoothly streamlined cap is provided called the boss cap.

          The axis passing through the centreline of the propeller shaft is called the propeller axis. The part of the propeller blade farthest from the axis is termed as the propeller tip. In the propeller blade, the side that pierces the water first as per the direction of rotation is called the leading edge of the propeller, and the other side that follows the leading edge is called the trailing edge of the propeller.

Image Courtesy: Team LSD.

SCREW PROPELLER- RIGHT AND LEFT-HAND SCREW

Screw propeller whose direction of rotation if is represented by curling of our fingers of our right hand produces thrust along the direction of the thumb finger, then it is called right-hand screw propeller. If a propeller exhibits the same phenomenon for the left hand, then it is called left-hand screw propeller.

          Generally, in most of the ship where single screw propeller is provided, a right-hand screw propeller is used. There is no such reason behind this. It may be considered as a conventional practice. On the other hand for twin screw propellers one of the propellers is right hand and another is the left-hand screw. This is done to negate the circulation produced by one propeller to disturb the flow onto another.

Image Courtesy: Google Images.

SCREW PROPELLER- THE SLIP PHENOMENON

Screw propeller works with the basic principle of a screw, such that with every complete rotation of the screw, it advances by 1 pitch. The only difference being that in a screw and nut pair due to the presence of a solid thread, the nut exactly advances by 1 pitch on one complete rotation of the screw. In case of propellers, the situation is analogous but as the propeller works in a hydrodynamic medium where there is no thread, so the propeller slips. Thus on every rotation, the propeller advances by a distance less than the pitch of the propeller.

Image Courtesy: Google Images.

SCREW PROPELLER- TERMINOLOGIES

To understand screw propellers we must understand some of the characteristic terminologies associated with the geometry of the propeller.

RAKE

We know that propeller is made up of different radial sections joined together and the face of the radial section lies on a helicoidal surface. If the line generating this helicoidal surface is perpendicular to the propeller axis then the propeller blade is said to have no rake.

           Rake is provided to increase the clearance between the stern frame and the propeller blade tip such that the pressure pulses emitted by the propeller blade due to the rotation in the hydrodynamic medium don’t impart vibrations in the hull.

Image Courtesy: Google Images.

SKEW

In the projection of the propeller blade on a plane perpendicular to the propeller axis (projected view) a line is drawn joining the midpoints of the projected sections. If this line is perpendicular to the propeller axis then the propeller is said to have no skew.

           Skew is provided in a propeller to increase its expanded area so that loading on the propeller (Burrill Cavitation Criteria) is less and thus cavitation is reduced. Also due to skew the different radial sections of the propeller don't pass simultaneously along the stern counter of the hull where the disturbances in the wake are high. thus pressure fluctuations are not over a large area and the peak is reduced. Thus skewed propeller performs better in a hydrodynamic medium. 

Image Courtesy: Google Images.

 
PITCH AND PITCH ANGLE

Pitch is defined as the distance the propeller would advance by one complete rotation of the propeller. For a radial, section pitch may be same or different.

Pitch angle (∅) is defined as

            Where,

            P= Pitch.

            r= radial section radius.

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CONCLUSION

In this article, I have tried to introduce the readers to the basic terminologies of a screw propeller. The geometrical parameters of a screw propeller will be discussed in the upcoming article. 

This is an informative video of various Marine Propellers prepared by Team LearnShipDesign. Hope you all like it.

Article by- Rijay Majee.