Types of Propellers

Introduction

A propeller is a fan like rotating structure generally at the aft of a ship imparting thrust to propel the ship. Most ships use the conventional screw propeller. But in some ships depending upon the need and requirement, different types of propellers are used.

The different types of propellers with varying characteristics are listed below.

Paddle Wheel

Image Courtesy- Google Images.

 It is a very simple type of propulsion system. It consists of wheels with paddles attached at its periphery. It has its axis of rotation about the transverse direction of the ship above the waterline. The paddles get immersed in water only when they are at the bottom of the wheel. As the paddles rotate in water, they accelerate it, experiencing a reactive thrust from the water which is transmitted to the ship.

 It is of two types

• FIXED: These are simple and easy to construct. In this type, the paddles are firmly attached to the wheel. This system requires wheels of large diameter so that they can enter the water at large angles. Large diameter means low rpm and more weight. This is the main drawback of this system.

• FEATHERING: In this system, the paddles are attached at the ends of the wheel in such a manner that the angle of entrance is much larger than that of fixed type.By this principle, the diameter of the wheel can be reduced.

Tandem propeller

Image Courtesy- Google Images.

In this arrangement, two propellers are mounted on a single shaft turning in the same direction. Tandem propellers are fixed so as to reduce loading on a single propeller as it can lead to cavitation. Here the thrust is divided between the two propellers.

In normal loading, they are not of much use but in heavy loading, they produce better loading than a single propeller.

Generally, size and number of blades are kept the same

Overlapping propeller

Image Courtesy- Google Images.

It has the same advantage as a tandem propeller as the load is divided between two propellers. There are two propellers with their shafts placed at a horizontal distance less than the diameter of either propeller.

They have higher hull efficiency because they work in a region of a higher wake. The advantage over twin screw is that no extra appendage is required to support and hence the resistance is reduced.

Sometimes the mutual interaction between the propellers may, however, result in more cavitation.

Controllable Pitch Propeller

Image Courtesy- Google Images.

In this type of propeller, the blades are not directly fixed to the boss but attached to separate spindles. The spindles can be turned about the axis and so the pitch of the propeller can be altered. These are mainly used in ships requiring full power at varying speeds and resistances.These are mainly used in tugs, ferries, icebreakers etc.

Some advantages over the conventional fixed propeller are

• They provide better acceleration, stopping and manoeuvring properties.
• Non reversing propulsion machinery may be used thereby reducing its cost, weight and space occupied.
• At all loading conditions, the full power of machinery can be used.
• The speed of the ship can be varied without altering the speed of the main engine.
• Speed can be directly controlled from navigation bridge.
• It is easy to replace damaged blades.

Some disadvantages are:

• The control mechanism is very complex.
• It has high initial cost.
• Maintainance costs are also high

Ducted Propeller

Image Courtesy- Google Images.

In this type a non -rotating duct surrounds a screw propeller.

The gap between the propeller and duct is very minute.

These are mainly of two types:

• ACCELERATING TYPE: They increase the velocity of flow of water to the propeller.

• DECELERATING TYPE: They decrease the velocity of flow of water to the propeller.

Some advantages of ducted propeller over normal propeller are:

• Better course stability
• Less effect of load and speed variation on efficiency
• Fewer chances of damage to the propeller
• Improved efficiency at high loading

Some disadvantages of the ducted propeller are

• More chances of cavitation
• Poor astern performance

Supercavitating Propeller

Image Courtesy- Google Images.

Supercavitating propellers are used when the design criteria of the propeller are such that cavitation cannot be mitigated. It can produce very high thrust at same efficiency without cavitation, corrosion and noise. Ships with high engine power, speed and rpm deploy such propellers.The back of the propeller blade is covered by vapour filled cavity.There is a separation of flow on the back at the leading and trailing edge.The main objective is that the back of the blade should not be in contact.

 These type of propellers have however less blade strength owing to a thin leading edge of the blade.

They also don’t work properly at low speeds.

Surface Piercing Propeller

Image Courtesy- Google Images.

These type of propellers are partly submerged in water.

These are fitted just at the end of the ship rather than under it. The propeller shaft is just above the water surface. Since no extra appendages are required the drag resistance is considerably reduced.

Some advantages of the surface piercing propeller are:

• It requires less power to achieve the same speed as compared to the fully submerged propeller.
• Cavitation is considerably reduced.
• Since it is not vulnerable to cavitation, they can have low blade area

Some disadvantages of the surface piercing propeller are:

• Since the blades enter and leave the surface at each revolution, it is subjected to periodic loading and can lead to fatigue.
• They have very poor astern performance.
• Difficult to operate at low speeds

Contra-rotating propeller

Image Courtesy- Google Images.

It uses two propellers placed on two coaxial shafts.The propellers rotate in opposite directions.It helps to reduce the rotational energy losses caused in the slipstream.

Some advantages of contra-rotating propellers are

• Loading is shared between two propellers and hence cavitation is reduced.
• Efficiency is higher than a single propeller.
• Less pressure fluctuations and noise

Some disadvantages are:

• Greater weight of machinery at the aft
• Complexity of gearing and coaxial shafts
• Sealing of the shafting system is difficult

Azimuth propeller

Image Courtesy- Google Images.

An azimuth propeller is a configuration of marine propellers placed in pods which can rotate at any horizontal angle.

Some advantages of azimuth propellers are:

• Rudder use is not required
• Excellent manoeuvrability
• Good astern performance
• Good speed control
• Vibrations are less

This is a video made by Team LearnShipDesign on some basic type of propellers. Hope you all like it.

Article By- Anil Kumar Singh

SCREW PROPELLER- PART 2

INTRODUCTION

In the previous article on screw propellers, I acquainted the readers with the basic terminologies related to screw propeller geometry and its slip phenomenon. In this article, I will impart an insight into the core helicoidal geometry of screw propeller emphasising on the different views of the propeller.

SCREW PROPELLER- THE REFERENCE SYSTEM

To describe the propeller geometry we generally take reference of the cylindrical coordinate system. This is because in propeller we talk about radial sections and helix formed on a cylinder. Thus a cylindrical coordinate system is appropriate. The coordinate system is as shown.

The major planes in the coordinate system are z=0, Θ=0 and r=0. Any offset of the propeller blade is taken in 3-dimensional space with reference to these planes.

SCREW PROPELLER- THE GEOMETRY

To be specific the geometry of the propeller is a bit complex. I have tried to simplify the geometry as much as possible.

If we take a propeller and cut a radial section, then it will look as shown.

The face line of the radial section is a part of a helicoidal surface with some offset at the leading and trailing edge. What does this mean?

This means that suppose I have 2 sticks orthogonally arranged such that one stick is rotated about the second stick as an axis and the first stick advances along with rotation. Thus the loci of the tip of the stick as shown will trace out a helix on the imaginary cylinder as shown. The face of the propeller at a radial section is a part of the helix where the radius of the imaginary cylinder is the radial section. Also, the face offsets from the helix at the leading and trailing edge. The back surface of the propeller blade depends on the aerofoil section profile that each radial section of the propeller blade is made up of.

Image courtesy: Google Images.

Now if we cut the imaginary cylinder longitudinally and open the cylinder to form a rectangle, the helix forms the diagonal of the rectangle such that the face of the radial section lies on the diagonal as shown. 

Image courtesy: Google Images.

This denoted the actual section of the propeller. It can be thought that this actual section of the propeller is bent and giver a definite curvature and this is done for all radial sections. Then these sections are joined to form the complete propeller blade. This brings us to different views how we look at the propeller which will give us a firm idea about the various curvatures that are imparted to a plane aerofoil section to form a propeller radial section.

        PROPELLER VIEWS

Before we jump onto propeller views it must be noted that the propeller blade radial section has curvature in 2 planes. If we see the propeller blade face and just concentrate on the radial section then we can see one curvature (the radial curvature) as the section is a part of the circular arc cut off from the total blade. Also as the face is a part of the helix, so looking at the propeller blade from the top view and concentrating on the radial section cut off from the blade, the section has a second curvature due to the helicoidal surface as shown.

Thus if we take a radial section of the propeller and straighten the 2 curvature one by one then the different views of the propeller are formed.

PROJECTED VIEW

Suppose we have a propeller blade. This is made by various radial sections as told earlier. If we take the orthogonal projection of the blade radial sections on a plane perpendicular to the z-axis (z=0 plane), the view formed is called the projected view. These projections taken for all radial sections form the projected outline of the propeller blade. This can be understood by taking a propeller blade and projecting a light on the blade along the z-axis. The shadow cast on the wall which is perpendicular to the z-axis is called the projected view of the propeller. The area within the projected outline of the blade is called the projected area A_P.

Image Courtesy: Team Learn Ship Design.

DEVELOPED VIEW

This view is a bit difficult to understand and has to be concentrated. Now again we take a particular radial section of the propeller blade which has curvature in 2 planes. We have learned the definition of pitch and considering a propeller having uniform pitch distribution along the radius of the blades, we can infer that the pitch angles of all radial sections will be different that comes from simple calculations. We know that the face of each radial section is a part of the helix chord and thus it will have a midpoint (C say). By definition developed view of a propeller is the projection of the radial section on a plane which is through the point c and makes an angle equal to the pitch angle (∅ for that radial section) with z=0 plane.

This is similar to that of projected view but difference being, the light is projected at an angle equal to pitch angle for that radial section with the z-axis.

The physical significance of this view is that the helicoidal curvature of the radial section is straightened in this view. Thus unlike projected view, this view is not like a circular arc but has offsets laterally forming an ellipse due to the straightening of the helicoidal curvature.

This process done for all radial section gives the developed outline and the area within the developed outline is called the developed area A_D.

Image Courtesy: Google Images.

EXPANDED VIEW

This view is easy to understand. If we open up the imaginary cylinder for a particular radial section then actually both the helicoidal and radial curvature of the section is straightened and it gives a proper aerofoil section with no curvature along the diagonal of the rectangle as shown.

Image Courtesy: Google Images.

This aerofoil section makes an angle of pitch angle (∅ for that radial section) with Θ=0 plane. If we rotate it by the same pitch angle, the expanded section is obtained. This view done for all radial sections give us the expanded outline and the area within the expanded outline is called expanded area A_E. 

Image Courtesy: Team Learn Ship Design

It must be mentioned that these different views and areas are characteristics of a propeller and thus are very important to understand.

CONCLUSION

This article wraps up the screw propeller basics, giving useful insights into the reference coordinate system used to define a propeller, the helix concept and the various propeller views and how they characterise the geometry of a screw propeller. 

Article by: Rijay Majee.