CHAPTER 1

INTRODUCTION

The Voith Schneider propeller (VSP), also known as a cycloidal drive is a

specialized marine propulsion system. It is highly maneuverable, being able to

change the direction of its thrust almost instantaneously. It is widely used on

tugs and ferries. From a circular plate, rotating around a vertical axis, a circular

array of vertical blades (in the shape of hydrofoils) protrudes out of the bottom

of the ship. Each blade can rotate itself around a vertical axis. The internal gear

changes the angle of attack of the blades in sync with the rotation of the plate,

so that each blade can provide thrust in any direction.

It is seen that a dolphin is able to achieve a dynamic lift with the force required

to change the direction of movement by the ‘to and fro’ motion of the tail. In

flow theory this is considered as ‘unstationary’ method of propulsion. Thus it is

seen that ‘to and fro’ motion of a dolphin is capable of producing a force along

to and perpendicular to the direction of motion of its tail.

To achieve ‘to and fro’ motion of the propeller blades, the individual blade is

made to rotate about its own axis, as all the blades rotate about the propeller

axis.

Figure 1.1 A tug using VSP system

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A ‘point’ taken on the surface of the propeller is observed to take a cycloidal

path. This is due to the rotary movement of the propeller is ‘dragged’ along a

straight line path. This type of motion, called cycloidal motion is the basis of

Voith Schneider Propeller (VSP).

The Voith Schneider Propeller is a unique propulsion system allowing the

control of thrust precisely and quickly in magnitude and direction. With the use

of these propellers in tugs, it is able to move ahead, astern (backwards),

sideways and even turn about its axis by the control of the blade movement and

without the help of any rudder.

Figure 1.2 A Voith Schneider Propeller unit

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1.1 Objective

The objective of this seminar is to briefly explain the innovative idea behind

the Voith Schneider Propulsion system. This seminar tries to explain why and

how the VSP system works. This seminar also highlights the advantages of a

vessel using VSP system to other conventional vessels.

1.2 Report Overview

This seminar deals with the cycloidal propulsion system used in water tractors.

The first chapter is the introduction. In the second chapter, the marine propeller

is defined. Certain terms used while discussing a propeller is covered in the

third chapter. Various types of marine propellers are discussed in the fourth

chapter. The constructional details, main components and operating concepts

are covered in the fifth chapter. Particulars of a VSP unit are mentioned in the

sixth chapter. Comparison of conventional tugs to VSP tugs are made in the

seventh chapter. The eighth chapter mentions the advantages and disadvantages

of VSP unit. Applications of VSP are mentioned in the ninth chapter. The

seminar is concluded in the tenth chapter. The different reference materials that

helped in preparing this seminar is also mentioned at the very end.

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CHAPTER 2

MARINE PROPULSION

2.1. Propeller

Figure 2.1 A conventional propeller

A propeller is a type of fan which transmits power by converting rotational

motion into thrust. It consists of two or more blades about a central shaft and

operates like a rotating screw. A pressure difference between the forward and

rear surfaces of the airfoil-shaped blade is produced and water accelerated

behind the blade. Propeller dynamics can be modeled by both Bernoulli's

principle and Newton's third law.

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A propeller is the most common propulsion on ships, imparting momentum to a

fluid which causes a force to act on the ship.

The drawback of conventional propeller is that it can be used only for the

solely designed purpose. For example, a vessel designed for speed purpose is

found ineffective and inefficient in towing purpose. Since the design of

propellers with different purposes are different. Some vessels like fishing boats,

tugs etc are in need of both speed and thrust will be required according to its

operating conditions.

A propeller that turns clockwise to produce forward thrust, when viewed from

aft, is called right-handed. One that turns anticlockwise is said to be left-

handed. Larger vessels often have twin screws to reduce heeling torque,

counter-rotating propellers, the starboard screw is usually right-handed and the

port left-handed, this is called outward turning. The opposite case is called

inward turning. Another possibility is contra-rotating propellers, where two

propellers rotate in opposing directions on a single shaft.

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CHAPTER 3

TERMS USED TO DISCUSS A PROPELLER

There are many factors to be taken into account in selecting a propeller, from

the practical to the very theoretical. The thing to remember is that there is no

known formula which will automatically give the ideal propeller size for a

given boat. One can merely approximate. The only true test is “trial and error”

method.

The major terms used when discussing a propeller are:

1. diameter

2. pitch

3. slip

4. pitch ratio

Diameter is twice the distance from the centerline of the propeller hub

to tips of the blade, or the diameter of the circle scribed by the tips of the

blade.

Pitch is the angle the blade makes in relationship to the centerline of the

hub and is normally expressed as the distance in inches; the blade would

advance in one revolution, if the propeller were a screw working in a

solid substance.

Slip is the difference between the theoretical distance and the actual

distance covered in a given period of time. This relationship is usually

expressed in percentage.

Pitch ratio expresses the relationship between the diameter and the

pitch of the propeller. It is the ratio between pitch and the diameter.

3.1 Number of propeller blades

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Three bladed propellers are more efficient over a wide range of applications

than any other propellers. In theory, the propeller with the least number of

blades (i.e. two) is the most efficient. Diameter and technical difficulties in

most cases make a greater number of blades necessary.

An old water front rule for thumb for all propeller selection is:

“Tow boats – big wheel, small pitch”.

“Speed boats – little wheel, big pitch”.

All other applications can be shaded between these two statements of extremes.

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CHAPTER 4

VARIOUS TYPES OF PROPELLERS

4.1 Controllable pitch propeller

Figure 4.1 controllable pitch propeller

An alternative design is the controllable pitch propeller (CPP) or variable

pitch propeller, where the blades are rotated normal to the drive shaft by

additional machinery - usually hydraulics - at the hub and control linkages

running down the shaft. The pitch of the propeller can be varied by additional

machinery which is provided inside the hub of the propeller shaft. This allows

the drive machinery to operate at a constant speed while the propeller loading

is changed to match operating conditions. It also eliminates the need for a

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reversing gear and allows for more rapid change to thrust, as the revolutions are

constant.

This type of propeller is most common on tugs, fishing trawlers etc where there

can be enormous differences in propeller loading when towing compared to

running free, a change which could cause conventional propellers to lock up as

insufficient torque is generated. The downside of a CPP is the large hub which

decreases the torque required.

4.1.1 Advantages

This propeller has several advantages with ships. These advantages include: the

least drag depending on the speed used, the ability to move the sea vessel

backwards, and the ability to use the "vane"-stance, which gives the least water

resistance when not using the propeller (e.g. when the sails are used instead).

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4.2 Ducted propeller or Kort nozzle

Figure 4.2 a vessel using Kort nozzle propeller

A ducted propeller is a propeller fitted with a non-rotating nozzle. It is used to

improve the efficiency of the propeller and are especially used on heavily

loaded propellers or propellers with limited diameter. It was developed by

Luigi Stipa (1931) and Ludwig Kort (1934).

4.2.1 Advantages

Advantages are increased efficiency, better course stability and less

vulnerability to debris. Downsides are reduced efficiency and course stability

when sailing astern and increase of cavitation. Ducted propellers are also used

to replace rudders

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4.3 Azimuth thruster

Figure 4.3 Azimuth thrusters used in a ship

An azimuth thruster is a configuration of ship propellers placed in pods that can

be rotated in any horizontal direction, making a rudder unnecessary. These give

ships better maneuverability than a fixed propeller and rudder system. Primary

advantages are electrical efficiency, better use of ship space, and lower

maintenance costs. Ships with azimuth thrusters do not need tugs to dock,

though they still require tugs to maneuver in difficult places.

The effectiveness of Azimuth thrusters can be explained by the following

example: suppose I am holding a rubber ball in my hands and my intention is to

throw it at a wall, say wall A. Now, there are two ways for me to throw the ball

and reach its destination. One is throw the ball directly at the wall designated

A. the other method is to throw the ball at a wall designated B, and upon hitting

the wall B the ball ricochet off the wall B to hit its original target A. from this

example, one can clearly understand and appreciate the superiority of Azimuth

thrusters over normal propeller-rudder system.

4.4 L-drive

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Figure 4.4 L-drive propeller

L-drive is a type of azimuth thruster in which the pod-mounted propellers are

driven mechanically or electrically. Azimuth thruster pods can be rotated

through a full 360 degrees, allowing for rapid changes in thrust direction and

eliminating the need for a conventional rudder, thereby increasing the

maneuverability of the vessel. This form of power transmission is called L-

drive because the rotary motion has to make one right angle turn, thus looking

a bit like the letter "L". This name is used to make clear the arrangement of

drive is different from Z-drive

4.4.1 Applications

L drive is used in smaller launches, speed boats etc.

4.5 Z-drive

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Figure 4.5 schematic of a Z-drive propeller

Z-drive is a type of marine propulsion unit. Specifically, it is an azimuth

thruster. The pod can rotate 360 degrees allowing for rapid changes in thrust

direction and thus vessel direction. This eliminates the need for a conventional

rudder.

The Z-drive" is so coined because of the appearance (in cross-section) of the

mechanical driveshaft or transmission configuration used to connect the

mechanically-supplied driving energy to the Z-Drive azimuth thruster device.

This form of power transmission is called a Z-drive because the rotary motion

has to make two right angle turns, thus resembling the letter "Z". This name is

used to differentiate the arrangement of drive to that of the L-drive. The Z-drive

is unique in that it uses a mechanical device to drive the propellers rather than

via electrical motor.

4.5.1 Applications

This type of propulsions are used in small and medium sized cruise vessels,

barges etc.

CHAPTER 5

13

VOITH SCHNEIDER PROPULSION SYSTEM

5.1 Constructional Details

The VSP is a vertical axis propeller, i.e. the shaft to the propeller disc is

vertical and the plane of the propeller disc is horizontal. The propeller blades

are project from the circular rotor casing about 1/3 rd length from forward and

rotor casing is made flush with the ship’s bottom. The individual blades can

rotate about their own axis. Water tightness of all these joints is made by oil

seals.

The in-built reduction gear and other working mechanisms of the propeller are

mounted on top of the rotor casing. This unit is placed within the ship’s engine

room. Only the propeller blades are seen from the ship’s hull which projects

vertically downwards outside the hull and into the water.

The blades are interconnected by means of linkages inside the rotor casing. The

different linkages from all individual blade shafts are commonly linked to a

point called steering center. This steering center can be moved from the center

of the propeller disc within a smaller circular area. The movement of the

steering centre plays a pivotal role in the thrust generation of propellers.

1. The Voith Schneider Propeller is arranged in the fore body, with free

inflow and outflow in all directions. The thrust forces act ahead of the

pivot point of the vessel.

2. The hull should ideally serve only as a support for the propulsion system

and towing gear. For this reason it is desirable to obtain the smallest

dimensions necessary for the task to be performed.

3. Fitted underneath the propeller is a nozzle plate whose nozzle effect

increases the propeller thrust. It also protects the propeller against

grounding and supports the vessel in the dock.

4. The tug is controlled from the bridge. The control stand is located at the

wheel house. One or more control stands may be connected in tandem

according to the design of the vessel. It is usually connected

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mechanically to the propellers by a control rod system. This ensures hat

the captain has direct control over the vessel.

5.2 Main Components

5.2.1 Propeller

In the Voith-Schneider Propeller (VSP), the blades projecting at right angles

from the rotor rotate around a vertical axis. An oscillating movement of each

individual blade around its own axis is superimposed to this uniform rotor

movement, each of this movement has its own cause. First is the swiveling or

oscillating movement of the blades.

The second is the rotation of the rotor. Compared to a screw propeller, where

the propulsion action is relatively easy to comprehend, the propulsion method

of the Voith-Schneider propeller may appear somewhat unusual. As a matter of

fact the origin of the propeller thrust is far less easily illustrated than in screw

propeller.

The illustration is made easier when the path of a VSP blade is drawn omitting

those blade positions where no or only very little thrust is generated. As the

figure shows, essentially the to and fro motion of the blade remains, consisting

of a so called advance and return at the circumference of the blade circle.

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Figure 5.1 cycloid path traced by a VSP blade

The to and fro movement of VSP blades – which is an unstationary means of

propulsion in terms of fluid mechanics – is a movement preferred by nature to

generate propulsion. The fin movement of fish and the wing movement of birds

are based on similar movements.

While in the screw propeller each point of the screw surface moves along a

cylindrical screw path, VSP blades describe a cycloidal path. For this reason,

the VSP is also called cycloid propeller.

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5.2.2 Rotor

Figure 5.2 cross sectional view of a VSP unit

“Rotor” is that part of the propeller which is driven by the motor and rotates. In

the rotor, the blades and blade bearings, and the blade actuating linkages is

installed which also rotates. When the rotor is set in motion, the blades move

tangentially around the propeller axis – initially without producing thrust. Only

when the rotor is turning and the blade actuating device is adjusted, the blades

produce a thrust.

5.2.3 Stator

Stator is the portion of the VSP which remains stationary, without movement.

Stator is attached to the hull of the vessel. Stator consists of propeller housing,

housing cover and the control rod support.

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5.2.4 Blade actuating mechanism

Figure 5.3 blade actuating mechanism in a VSP unit

This mechanism includes those assemblies within the propeller which perform

the adjustment of blades in the rotor. The force of the blade adjustment is

generated by the two servo motors. The servo motors are controlled by means

of electrical directional control valves, which receive a digital control signal

from the control. The servo motors are supplied with oil pressure by oil supply

system.

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5.2.5 Servo motors

Both servo motors (hydraulic cylinders) are installed at an angle of 90 ۫ relative

to each other and bolted to the housing top. The cylinders have spherical

bearings at both ends so that they can follow the movements of the servo

motors.

The servo motors produce the force required to adjust the blade actuating

linkage to point N and to keep it in set position. The pressure of adjustment of

servo motors is given by oil supply system.

5.2.6 Control rod

The control rod is arranged in the longitudinal propeller axis and installed in

the central ball. It is a large lever which transmits the adjustments of the two

servo motors to the blade actuating linkages in the rotor. The bearing points of

the control rod are spherical bearings and allow control rod adjustment in all

directions. Three bearings are at the top, center and at the bottom.

The control rod has a hole through its length. The hole admits the oil from the

oil circuit into the rotor.

5.2.7 Blade actuating linkage

The blade actuating linkage is installed in the propeller rotor and rotates around

the propeller axis with it. It consists of a total of 4 identical linkages which

have a common bearing in the bottom of ball of the control rod. If the control

of the propeller is adjusted from O to N, each linkage moves around the

common control point N and executes the swiveling motion of the blades.

5.2.8 Blade bearings

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Figure 5.4 Longitudinal section of propeller with blades and bearings

The VSP is equipped with 4 blades which installed vertically in the rotor. The

part of the blade outside the rotor case and producing the thrust is named the

leaf and the part inside the rotor case shaft. The shaft has radial bearings. In

axial direction, it is guided both at the bearing race in the blade bearing cover

and at the bottom bearing bush. The bearings have adequate lubrication by the

cooling oil in the rotor body. Two shaft sealing rings are arranged in such a

neither manner that neither oil can leak from nor seawater enters the rotor

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body. The actuating lever is firmly attached to the blade shaft and coupled to

the blade actuating linkage.

5.2.9 Rotor casing

Figure 5.4 Rotor casing

The rotor casing is designed as a stiff disk which is connected to the driving

sleeve via a bolt connection. Arranged on the blade orbit diameter – depending

on the design – are 4 or 5 pockets in which the blades are supported. The blade

actuating gear, which initiates the blade movement upon adjustment of the

control rod, is also arranged in the rotor casing

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5.2.10 Corrosion Protection

Figure 5.5 sacrificial anodes

VSP are generally equipped with highly effective sacrificial anodes for

corrosion protection. The sacrificial anodes are welded to the rotor bottom in

the blade area at regular intervals. However, additional anodes should be

installed in the ship’s hull.

5.3 Fundamental Working Concept

A fish’s fin or a bird’s wing action not only produces a force in the direction of

motion but simultaneously also produces a force normal to its direction.

By superimposing the rotary movement of the propeller on a straight line

perpendicular to the rotational axis (which represents the movement of the

vessel), the blade of the Voith Schneider propeller follows a cycloid. The

rolling radius of the cycloid is equal to “λ* D/2” and forward motion of the

propeller during one revolution is therefore λ*D*π.

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To generate thrust the blade profile must be turned against the blade path by the

angle α by moving the steering centre from O to S. The ratio OS to D/2 = λ is

the pitch ratio of a Voith Schneider propeller.

O- Propeller centre

S- Steering centre

Figure 5.6 fundamental working concept

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5.4 Operating Concept

The propeller blades perform an oscillating movement about their axis and a

propeller jet is produced with thrust acting in the opposite direction.

In idling condition, when no thrust is required the position of S is made such

that it coincides with the centre of the circle O. the point S is called steering

centre, where the links from all the blade shafts meet the blades remain

tangential during revolution and therefore no thrust is developed.

Figure 5.7 Idling condition

The leading edge of the blade is directed outwards in the first half circle and

directed inwards in the next half of the circle. Thus in the first half, the water is

thrown into the blade orbit, and in the next half, water is thrown away from the

blade orbit, though in the same direction. In this way a water jet is produced,

and as a result a reaction force is produced which provides with a propeller

thrust.

Figure 5.8 Direction of thrust

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It is seen that, the thrust produced is at a direction normal to the line ON. It is

also noted that the magnitude of thrust is directly proportional to the length of

the line ON. Thus, a change in the position of the steering centre N brings forth

a proportional change in the magnitude and direction of the thrust produced.

The position of N is controlled by mechanical links and gears, which can be

controlled from the steering position at the bridge.

Figure 5.9 maneuverability of a VSP tug

Figure 5.10 thrusts in different directions

CHAPTER 6

25

PARTICULARS OF A VSP UNIT

The main particulars of a VSP tug “W.T. VALLARPADAM” now in operation

at Cochin harbor is given below.

Tug

Length = 32 m

Breadth = 10.65 m

Depth = 4.74 m

Draft = 5.5 m

Bollard Pull = 45 tons

Speed = 12 knots (22 kmph)

Propeller

Blade Diameter = 2.8 m

Propeller height = 4.2 m

Input power = 1772 kW

Rotor speed = 757 rpm

Weight per VSP unit = 28600 kg

CHAPTER 7

COMPARISON OF PROPULSION SYSTEMS

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Ship handling tugs can be broadly classified according to their propulsion and

steering systems into two

1. conventional tugs - using twin screw propellers

2. tractor tugs – using cycloidal 360 degree steer propulsion system, i.e.

Voith Schneider Propeller (VSP)

7.1 Conventional tugs: - [using screw propellers]

These tugs are equipped with one or two propellers with rudder behind the

propellers. The propellers can be of fixed pitch and controllable pitch with or

without nozzles. These tugs can deliver thrust along the centerline fore and aft

only. When a change in thrust angle is desired she must change her hull

orientation or apply a fractional component of total thrust with rudder action.

An additional tug may be required to reduce this loss of time and low thrust

level of non-aligned tugs. Obviously a tug capable of high thrust levels and

rapid changes to any direction would be a welcome improvement for an

efficient and safe handling.

7.2 Tractor tugs: - [using VSP]

In a tractor tug the propulsion machinery is mounted about one-third length

from front part and consists of either cycloidal propellers or steerable

propulsion unit(s). These tugs can immediately produce their thrust in any

desired direction as per demand of the situation, which makes them so efficient.

It has the extra ordinary maneuverability which has never been achieved in any

other conventional system. These tugs can go forward and backward just the

same without recourse of reversing the engines during the maneuvers. These

tugs can also go sideward and their turning circle diameter is not more than

their own length.

CHAPTER 8

FEATURES OF VSP

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8.1 Advantages

1. Maneuverability

Controlling a tug using VSP is direct. The direction of ship movement is the

same as the direction of control transmitter movement on the bridge. The entire

engine power can be employed for control with the utmost precision and

sensitivity. Directional changes can be made quickly.

2. Safety

The Voith water tractor concept averts the risk of capsizing, which is a constant

dread with a conventional screw tug when operating on a towline.

3. Ship handling ability

VSP can tow and push ships with its bow, aft and both sides. Conventional

screw tugs cannot generate thrust sideways. Conventional screw tugs can push

with its bow only, apart from towing.

8.2. Disadvantages

1. The working mechanism inside the propeller is quite complicated and

hence the maintenance and servicing is expensive.

2. The vessels fitted with VSP require deeper draft for operation since the

propulsion system is fitted vertically downwards, the depth of which is

around two meters for a tug.

3. The vessel fitted with this propulsion system has a poor running speed

compared to a screw tug.

4. The fuel efficiency is low.

CHAPTER 9

APPLICATIONS OF VSP

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Vessels which require excellent maneuverability such as tugs or water

tractors, passenger vessels, ferries operating in heavy traffic region.

Vessels which require high accuracy of dynamic positioning such as

floating cranes, research and survey vessels, drill ships and buoy laying

ships.

Vessels involved in pollution control that is cleaning the water surface.

This mode of propulsion does not churn the water surface as done by a

screw propeller.

CHAPTER 10

CONCLUSION

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Nowadays the ship industry introduce very large vessels like ultra large crude

carriers with capacity of 3 lac tons and more, which require very efficient tugs

having great maneuverability to position them in the required place.

Increased off-shore activities like oil drilling, buoy laying etc which are very

sophisticated operations require tugs with high maneuverability and efficiency.

So the Voith Schneider propeller, which is considered to be the perfect

controllable pitch propeller, is the ideal propulsion system for all floating crafts

needing frequent and precise maneuvers when safety is a big issue and money

is not.

REFERENCE

1. www.voith-marinetechnology.com

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2. www.voith.com

3. www.cochinport.com

4. www.en.wikipedia.org

5. Operation manual of Voith Schneider propeller unit, obtained from

Cochin Port Trust.

6. PDF document of Voith Schneider propeller unit, obtained from Cochin

Port Trust.

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