NANYANG TECHNOLOGICAL UNIVERSITY

HW210 TECHNICAL COMMUNICATION

ASSIGNMENT 2 - 2008/ 09 SEMESTER 1FINAL REPORT

EFFICIENT PROPULSION FOR SUBMERSIBLE PROPULSORS

Group Members : Chen Qing

Ng Kah Meng

Ong Teng Ho

Rohit Jha

Tran Tuan Anh

Tutorial Group : TB07

Abstract by Chen Qing

A housing is a structure which is normally enclosing the propeller and

functions to control the water flow through it. In order to identify the most

efficient underwater propulsion system, this research is to investigate the

effects of different housing designs on propulsion. The experiment was

conducted by attaching four housings of different designs to a model boat one

by one and calculating the speed of the boat to compare the propulsion

efficiency of the boat under various conditions. It was found that the speed of

the boat without a housing was faster than that with a housing due to the

significant drag force experienced by the housing. Another important finding

was that the speed of the boat with a housing enclosing its propeller was

faster than that with a housing attached to the underside of the boat while not

enclosing the housing. The research shows encouraging results in terms of

the capability of the housing to improve propulsion efficiency by means of

concentrating and accelerating the propulsive flow of water through it. Future

research should aim to design the housing which minimizes drag force

exerted by water and turbulent flow within it in order to further enhance the

propulsion efficiency.

i

Abstract by Ng Kah Meng

The project aimed at finding a way to improve the efficiency of the

propulsion system of marine vessels. It was solely focused on the

modification of the propulsion system by the additional of a housing enclosing

the propellers of the marine vessel. The concept of using the housing to

contain and focus the propulsive flow generated by the propeller to improve

propulsive thrust was experimented. A downscaled experiment using model

boats was done to replicate actual marine vessels. The experiment tested on

3 basic housing designs, a cylindrical, a semi-cylindrical and a conical shaped

housing to indentify its benefits. The speed of the boat was obtained from

measure of time taken by the boat to travel 4m. Experimental results showed

the benefit of wake ingestion due to the attachment of the housing. Yet, drag

force due to the housing slowed the boat more than the speed increment

gained from the housing. Consideration of drag force were not taken into

account originally, hence the housings were not design at reducing drag

forces. Better designed housing using sturdier materials can possibly benefit

the propulsion of marine vessels significantly. Conical shaped housing was

discovered to increase the possibility of cavitations effects and result in

turbulent flow of propulsive fluid within the housing. This project show the

possibility of the benefits of having a housing attachment to propellers and the

importance of the consideration of drag force, cavitations, propulsive flow

within the housing.

ii

Abstract by Ong Teng Ho

To achieve ideal propulsion underwater, structure like objects like

housings can be attached to propellers to improve its efficiency. This is

supported by the theories of Bernoulli’s equation by Daniel Bernoulli and

Continuity equation by Sir Issac Newton. The aim of the study is to prove that

various designs of housing attachment allow the fluid to be focused and thus

adhere to the above mentioned theories. In the experiment, the focus of fluid

which is termed wake ingestion is proven to improve the efficiency of the

propulsion. The main scope is to study the effects of housings on propulsion.

The initial assumption of the experiment is that the effect of drag force and

cavitations are negligible. A model boat and 4 uniquely designed housing

were used for the experiment. Speed was calculated through the average

time taken by model boat to travel across a distance of 4m with and without

the attachment of different designs of housing. The direct relationship

between the speed and efficiency provides the conclusion that greater speed

would yield greater efficiency. Further investigations were carried out to

determine the effects of drag force and cavitations by varying the positions at

which the housings are being attached. Through the further investigation,

wake ingestion was able to offset the drag force thus showing a faster speed

when the housing A was placed enclosing the propeller. However, cavitations

and drag forces affects the results significantly. The initial assumptions that

drag forces and cavitation were negligible failed to hold despite the

downscaling of this study. It is important for future researchers to investigate

the effect of drag forces and cavitation. The last part of the experiment was

inconclusive due to the inability to design a perfectly streamline housing to

minimize the effect of drag force. It is also important to focus on the

streamline shape of the housing or boat in future investigation.

iii

Abstract by Rohit Jha

Boats and submarines always require an effective propulsion system

such that less fuel and resources are used. Enclosing the propeller blades in

a housing may result in an increase in the efficiency of the propulsion system.

Our research aims at determining the effects of additional housing on

propulsion (excluding the effect of the ratio of the dimensions of the housing,

its weight, or the material used in its designing).A model boat, with an

attached propeller, is used with four housings of different radii and designs.

The boat was made to cover a distance of 4 meters using different propellers

each time and was timed using a stopwatch so as to calculate the speed

attained by the boat each time. Without any housing the boat reached a

speed of 0.796ms-1.Surprisingly, the speed of the boat reduced as opposed to

the hypothesis of efficiency increase. With housing A, the speed of the boat

was 0.734ms-1 .With housing B, the speed of the boat was 0.513ms-1. With

housing C, the speed of the boat was 0.513ms-1 .And with housing A+B, the

speed of the boat was 0.686ms-1.This anomaly occurred due to factors like

cavitations, turbulent flow of water, bubble formation and drag experienced

due to the size of the housing attached. When performed under a controlled

environment, the original effects of wake ingestion resulting from Newton’s

Laws and Bernoulli’s Principle could be seen and the boat was showing

efficiency. Although the effect of wake ingestion was proved in the experiment

but it was overshadowed by the cavitations, turbulent flow of water inside the

housing, increased drag forces and imperfect streamlining of the housing.

iv

Abstract by Tran Tuan Anh

Over the past centuries, people had useful inventions such as: steam

engine, paddlewheel, screw propeller… to improve the efficiency for sea

transportation. Many variations of the screw propellers and other forms of

propulsion have been developed. Our research aims to identify the most

efficient propulsor system that provides the optimum propulsion with the

minimum energy used. In this project, the effects of additional housing

enclosing the propeller were investigated. The attachment of the housing

aims to concentrate, focus and accelerate the propulsive flow of the water

expelled from the propeller through the concept of Newton’s Law of continuity

and Bernoulli’s principles. Three different housing designs were chosen for

study. Additional housings were attached to the boat at different positions:

enclosing propeller, underside of boat and top of the boat. In our experiment,

the effects of drag force and additional weight due to additional housings play

a role in slowing down the speed of the boat. The speed of the boat without

the housing enclosing its propeller was clearly faster than the speed of the

boat with the attachment of the housings. Among the three housings, the

conical shape housing gave the most propulsion. With the cylindrical shape

housing attached at different positions, the boat that housing attached at top

of the boat has highest speed. The data obtained showed that the effect of

wake ingestion was beneficial to the efficient propulsion of the boat and that

our controlled experiment was a success. The expectation of the conical

housing providing the most propulsion was proven otherwise. Because of the

limitation of our research, the benefit of wage ingestion is being

overshadowed by the effects drag force. Further researches should aim to

design housing that reduces the drag forces and make the housing more

streamline to enjoy more benefit from the effects of wake ingestion.

v

Table of Content

Title Page

Abstract by Chen Qing i

Abstract by Ng Kah Meng ii

Abstract by Ong Teng Ho iii

Abstract by Rohit Jha iv

Abstract by Tran Tuan Anh v

Table of Contents vi

List of Illustrations vii

Glossary viii

1. Introduction

1.1.Historical Background 1

1.2.Rationale 1

1.3.Research Objectives 2

1.4.Scope of Research 3

2. Literature Review 3

3. Methods and Materials

3.1.Materials 6

3.2.Methods 7

3.2.1. Investigation of Housing A, B and C 8

3.2.2. Further Investigation of Effects of Housing A 8

3.2.3. Further Investigation of Effects of Housing (A+B) 9

4. Findings and Discussions 10

4.1.Further Discussions of Housing A 12

4.2.Further Discussions of Housing (A+B) 14

5. Conclusion 16

References 19

Appendix A

Appendix B

vi

List of Illustrations

Name of Illustrations Page

Fig 1.1a: Pump-Jet 1

Fig 1.1b: Kort Nozzle 1

Fig 2.1: Peter Woodford Propeller Design 3

Fig 2.2: Formation of Cavitation 5

Fig 2.3: Example of a Housing enclosing a screw propeller 5

Fig 3a: Cylindrical Shape Housing A 6

Fig 3b: Conical Shape Housing B 6

Fig 3c: Housing C 7

Fig 3d: Housing A+B 7

Fig 3.2: Positions of the Housings 8

Fig 4: Charts of Various Housings 10

Fig 4a: Phenomena of Cavitation 11

Fig 4.1: Chart of Housing A 12

Fig 4.2: Smooth Effect of Housing A+B 14

Fig 4.2a: Chart of Housing A+B 14

Figure A: Illustration of Continuity Equation Appendix A

Figure B: Illustration of Bernoulli’s Equation Appendix A

Figure Bi: Table for section 3.2.1 Appendix B

Figure Bii: Table for section 3.2.2 Appendix B

Figure Biii: Table for secion 3.2.3 Appendix B

vii

Glossary

D

Discharged cavitations: A form of cavitations, visible by the formation of

bubbles. It normally appears in marine propulsors

when pump discharge pressure is extremely high.

P

Propellers: A device having a revolving hub with radiating

blades, for propelling an airplane or ships.

Propulsors: A mechanical device that provides propulsion.

Often it is an improvised version of propellers with

housing enclosing the propellers, such as turbines

of airplanes or jet-pumps, impellers systems of

marine vessels.

T

Tubular Flow: The flow of fluid within a tube or cylindrical pipe.

V

Venturi Effect: The fluid pressure that results when an

incompressible fluid flows across a pressure

gradient. This effect is derived from a combination

of Bernoulli’s equation and continuity equation.

W

Wake ingestion: Wake is a region of turbulent flow due to a body

moving relative to the fluid. To ingest the wake is

to contain the wake and focus its direct of flow.

viii

1. INTRODUCTION

1.1 Historical Background

Pioneering ships relied on oarsmen and sails for propulsion through the

sea. In 1769, Scottish inventor, James Watt’s development of the steam

engine provided an alternative source of energy for sea transportation. The

marriage of the steam engine and the paddlewheel provided ships with a new

source of propulsion that do not tire or rely on the direction of the wind.

Paddlewheels are the pioneer form of mechanical propulsion of ships. In

the latter half of the 19th century, the screw propellers succeeded the

paddlewheels as they provided a more efficient propulsion system.

Over the past centuries, many variations of the screw propellers and

other forms of propulsion have been developed. Variations of the propellers

such as the Pump-jets (Fig 1.1a) and Kort Nozzle (Fig 1.1b) propulsors

provided great benefits over the bare screw propellers [1] [2]. Pump-jet is a

ducted propeller system that creates jets of water for propulsion [3]. The

benefits provided vary from greater energy efficiency to noise reductions and

better maneuverability [1] [2].

1.2 Rationale

The development of engines has gone a long way from the steam engine

in the 18th century and it is evident in the ships of today. However, many ships

still use similar screw propeller designs that date back to the 19th century.

Fig 1.1b: Kort NozzleSource: http://en.wikipedia.org/wiki/Kort_nozzle

Fig 1.1a: Pump-JetSource: http://en.wikipedia.org/wiki/Pump-jet

1

Furthermore, the present bare screw propeller blades differ in shapes and

angles positioned. No universal rule has been determined for the design of

the propeller that gives the most efficient propulsion.

As the design of propellers seems to have reached a standstill, it is time

to look at other ways of enhancing the efficiency of marine propulsion system.

In the 1700s, Daniel Bernoulli did many researches on the forces present in

moving fluids. From his researches, he has many equations relating fluids

and forces named after him, e.g. Bernoulli’s equation. One of the simplified

versions of Bernoulli’s equation enables us to work upon enhancing the

efficiency of the propulsion underwater. It states the relationship between

kinetic energy of the fluid versus the pressure exerted by the fluid [4]. Another

important theory is the conservation of mass by Sir Issac Newton which yields

the equation of continuity also known as venturi effect [4]. A detailed

explanation on their equations can be obtained in the appendix A. Through

their equations, we can expect that by altering the speed of fluid flow using

various structures known as housings enclosing the propellers, we can

improve the efficiency of the propulsion underwater.

1.3 Research Objectives

Our research aims to identify the most efficient propulsor system that

provides the optimum propulsion with the minimum energy used. To achieve

this aim, the effect of wake ingestion providing more efficient propulsion plays

a significant role in the research [5], thus we have to identify the relationship

between propellers and different housings enclosing them. The concept of

having a propeller enclosed in a housing is similar to the designs of the pump

jet propulsors.

2

1.4 Scope of research

In the research, tests will be made to determine the effects of different

housing designs enclosing the propeller. This is to determine the benefits of

additional housing (similar to those on water jet) attachment on propulsion.

We will not be investigating the ratio of the housing relative to the size of the

boat, materials of the housing as well as the weight of the housing. The

project’s limitations are the inability to experimentally investigate the effects of

cavitations.

However, during the process of the experiment, the results were not as

expected. Thus further modification of the experiment procedures were done

to investigate the cause of such results. Due to the above reason, the effects

of drag force, weight and phenomenon of cavitations were briefly discussed in

the later section of the report.

2. Literature Review

This literature review will focus on the different studies relating to the

improvement of efficiency on submersible propulsion system. From the

various studies, we have identified that the efficiency of submersible

propulsion system has a dependent relationship with the design of the

propeller as well as the different housing attached to the propeller. Studies on

cavitations and drag force are included in this section to provide knowledge

on the cause and effect of such phenomena.

A marine propeller is essentially a type of fan

which transmits power by converting rotational

motion into thrust for propulsion. A propeller is made

up of two or more twisted blades about a central

shaft. The blades act as rotating wings and produce

force through application of both Bernoulli’s principle

and Newton’s third law. A new design of propeller was

Fig 2.1: Peter Woodford propeller design

Source: Peter Woodford[6]

Fig 3.Peter Woodford’s Propeller Design

Casing for the propeller

Propeller blade

3

invented by Peter Woodford using the concept of minimizing the slippage

between the central hub and the outer edge of blades [6]. He also claimed

that ideal propulsion occurs when the fluid displacement ratio between the

input and output per revolution of the propeller is one [6]. From these

concepts, he designs his propeller by reducing the exposure of blades. He

supported his design with 16 claims covering the purpose and the mechanism

of the propeller. However, without statistical results, it is rather hard to justify

that his design is workable. Despite that, his work is relevant to this research

as it highlights the significance of slippage in ideal submersible propulsion

system. It is important to note that the reduction of the exposure of blades

does improve the efficiency of submersible propulsion

Another theory that has dependent relationship on governing the

efficiency of propulsion underwater is the pitch of the propeller. The

propulsion system is claimed to be at its maximum power output only when it

is at its optimized pitch [7]. The deforming effect of composite materials

causing the pitch of propeller to vary has significant effect of the propulsion of

the propellers [7]. Ching-Chieh Lin, Ya-Jung Lee and Chu-Sung Hung did an

experiment on verifying the effects of pitch of propellers with regards to its

respective efficiency. However, in their experiment, small models of propellers

were used. This leads to slight inaccuracy in their experimental results as

compared to the theoretical results. From their work, we can infer that the

propellers used in our field of study must be at their optimized pitch. Due to

the shortcoming in their experiment, we would be using a propeller that has a

comparable relative size to the model of boats used in our own field of study.

This is to ensure fairness and accuracy in our experiment conducted.

Cavitation is a phenomenon which occurs when marine propeller blades

are rotated at a sufficiently high speed developing very low pressures along

the curved section side or reverse of each blade[8]. The cause of cavitations

are also very sensitive to the formation and transportation of vapor bubbles,

4

the turbulent fluctuations of pressure and velocity, and the magnitude of

noncondensible gases, which are dissolved or ingested in the operating

liquid[9]. The flow over a marine propeller blade is inherently unsteady due to

the rotation of the blade through the spatially varying wakes of the hull and/or

appendages [10]. In conventional propellers, the efficiency is around 70%

[10]. Of the 30% lost efficiency,15% is approximately attributed to induced

drag, while the remaining 15% is

lost due to viscous or frictional

drag[10]. This is shown in Fig

2.2. Housing is a protective cover

designed to contain or support a

mechanical component. A boat

propeller mounting and steering mechanism including an upper housing

adapted to be mounted inside the hull of a boat and serving as a mounting

and support means for a relatively rotatable lower housing[11]. With the

attachment of an additional housing to the boat propeller blades results in an

additional increase in cavitation and frictional drag. The additional housing

results in the net increase in the weight of the boat and thereby increasing the

drag forces.

Our experiment deals with the effect on the efficiency of propulsion by

housing the propeller blades. Housing is a kind of a funnel-shaped cylindrical

hollow protective cover designed to contain or support a mechanical

component, which in our case are

the propeller blades. A housing for

the propeller of a conventional

gasoline or diesel powered marine

engine is provided attachable to a

bracket mount on the engine [11].

The housing completely encloses

the engine's propeller and drive shaft

Fig 2.3: housing enclosing a screw propellerSource: G. P. C. Canevari, NJ, "Efficiency ship Propeller ” [11]

Fig 2.2: Formation of cavitationsSource: Mathematical Basis and Validation of the Full Cavitation Model

5

and defines a tubular flow path for the water engaged by the propeller with

the flow path being generally coaxial with respect to the propeller's rotation

axis [13]. Thus, the experiment would be looking at the aspect of increasing

efficiency by means of using 4 different designs of housing enclosing the

propellers.

3. Research Methods and materials

3.1 Materials

This experiment will be conducted at a swimming pool and the following

materials are required:

a) A motorized model boat with an exposed propeller. This is to allow for

the housing to be easily attached and enclosing the propeller as required

for the experiment.

b) Four housings of different designs are made using plastic and

represented as housing A, B, C and (A+B). Housing A shown in figure

3a, is a normal cylindrical shaped like housing whose radius remains the

same along its central axis.

c) On the contrary, radius of housing B, as shown in figure 3b, is not

constant and decreases along its central axis which resembles a conical

flask. Housing C, shown in figure 3c, is a combination of two curved

inward pieces each attached on one side of the propeller. Housing (A+B)

as shown in figure 3d is a combination of housing A and B by connecting

Fig 3a: Cylindrical shape housing A Fig 3b: Conical shape housing B

6

housing B at the end of housing A. All these housings were attached

using a combination of sticky tack and superglue.

d)

d)

Measuring tape is required to measure and mark down 4m distance

travelled by the boat. The distance 4m was chosen after several tests by

comparing the travel time of the boat without housing attached against

the different distance. It was determined that 4m is the shortest distance

for a clear distinction between the timings taken with and without the

housings. A shorter distance reduce the possibility of experiment errors

taken while the boat is in motion

e) Stopwatch was used to measure the time taken for the boat to travel the

4m distance.

3.2 Methods

The basic method of this experiment is to record down the time taken for

the model boat to travel across a distance of 4m. With the time taken, we can

then calculate the average time and the speed of the boat. This allows us to

identify the propulsion efficiency of the boat under various conditions.

The boat travelled 4m and the timing was recorded. This was repeated 3

time to obtain the average timing. The average speed of the boat was

calculated and used as a reference for comparisons in later sections.

3.2.1 Investigation of housing A, B and C

Fig 3d: Housing A+BFig 3c: Housing C

7

The attachment of housing A or C enables control of water flow such that

water flow within the housing would be likely to behave streamline, whereas

housing B exercises focusing effect on the flow of water as it passes through

the housing.

Housing A was attached to the propeller, and similarly, the time taken by

the model boat to travel a distance of 4m was recorded. A set of 3 readings

were recorded and the average of these 3 readings and the speed were

calculated for analysis purposes. The same procedures also applied to

housing B and C.

Due to inability to yield the expected result, further investigations were

carried out to analyse on the cause of such experimental results.

3.2.2 Further investigation of effects of housing A

Three considerable effects might be exerted by housing A on the

propulsion efficiency of the boat, which are wake ingestion, drag force and

additional weight of the boat due to housing A. In order to analyze these three

effects separately, housing A was successively attached to three different

positions on the boat as shown in

Figure 3.2 and these positions are

denoted as position X, Y and Z.

(i) With housing A attached enclosing the propeller (position X), all three

considerable effects, wake ingestion, drag force and additional weight were

introduced. The boats travelled 4m for three times and three timings were

Position Z: Top of boat

Position Y: Underside of boat

Position X: Enclosing Propeller

Fig 3.2: Positions of the Housing

8

recorded, followed by calculation of the average timing and the speed of the

boat.

(ii) The attachment of housing A to the underside of the boat while not

enclosing the propeller (position Y) led to additional drag force and additional

weight of the boat due to the housing while excluding the effect of wake

ingestion.

(iii) Placing housing A on top of the boat (position Z) resulted in only

additional weight due to the housing while excluding the effect of additional

drag force and wake ingestion.

The above 3 cases were carried out to investigate the effect of wake

ingestion, drag force as well as weight of the additional housing on the

efficiency of the boat’s propulsion underwater.

3.2.3 Further Investigation of effects of housing (A+B)

The function of housing A is to control the flow of water in hopes of

obtaining streamline water flow before focusing it using housing B. The same

procedures adopted for investigation of housing A was also applied to

housing (A+B).

The data obtained from above procedures 3.2.1, 3.2.2 and 3.2.3 are

presented in Appendix B. Findings and discussions derived from the data are

elaborated in details in the next section.

4. Findings and Discussions

The findings are recorded in the form of timings. Through equation 1

listed below, it was converted to speed to further aid the discussions. Speed

9

Fig 1.1

has a direct relationship with efficiency. Thus a higher speed would mean that

the efficiency is greater.

Speed = Distance ÷ Time -------------- (Equation 1)

Speed of the boat wi th vari ous housi ngs attached

0. 796 0. 734

0. 5130. 597

0. 000

0. 200

0. 400

0. 600

0. 800

1. 000

Non A B CType of Housi ng

spee

d(m/

s)

speed of the boat

The speed of the model boat to travel a distance of 4m with and without a

housing attached to its propeller is shown in Fig 4. From Fig 4, the speed of

the boat without the housing enclosing its propeller is 0.796ms-1, clearly faster

than the speed of the boat with the attachment of the housings. This result

differs from the original expectation of the outcome; it is expected that the

housing would focus flow of the propulsion and thus increase the speed of the

boat. Not only did the speed of the boat slow down with the housings

attached, the expectation of housing B giving the most propulsion among the

3 housings was also proven otherwise as shown from the experiment.

The unexpected result is possibly due to the over-simplification of the

experiment. The initial assumption was that the focusing of the propulsive

stream would significantly increase the speed of the boat relative to the drag

force acting on the housing. Instead, the drag force acting on the housings

played a significant role in slowing down the speed of the boat. This resulted

A: Cylindrical shapeB: Conical shapeC: 2 piece housing

Fig 4: Chart of various housings

10

Fig 4a: Phenomena of Cavitation

in the decreased speed of the boat when the housing is attached to the

propeller. The effect of the drag forces will be further elaborated in the later

section and shown with experimental data.

Yet, the drag forces cannot account for the slower speed of 0.513 ms-1

obtained from using housing B as compared to a speed of 0.734 ms -1 by

housing A as shown in Fig 4. The concept of the Venturi effect where fluid

flowing through a constricted space will result in a gain of velocity is

challenged. Once again, it is

important to go in depth to account

for this unexpected result. The

possibility of a turbulent flow of the

water within the housing was

explored and suggested from the

slower speed obtained from housing

B. Furthermore, under deeper

investigation of housing B, sign of

cavitation (Fig 4a) occurring within the housing was seen. Initial suggestions

were that cavitation was unlikely to occur due to the slower speed of the

propulsion stream compared to commercial propellers of actual marine

vessels. However, the decreasing radius of the housing creates a high

pressure condition within the housing that results in the turbulent flow of water

that circulated inside the housing instead of dispelling it through the narrow

end. This high pressure also causes a condition termed discharge cavitation

that can be observed from the creation of the bubbles in the housing as

shown in Fig 4a. The above mention reasons thus account for the slower

speed resulting from the using of housing B as compared to housing A.

The soft plastic bottle used to create housing C resulted in some

discrepancy in the timing taken for the boat to travel the distance of 4m. The

speed of the model boat when housing C is attached is slower compared to

Signs of Cavitation

(Bubble Formation)

11

Speed of the boat wi th housi ng A at tached at di ff erent posi t i on.

0. 796 0. 734 0. 6930. 792

0. 000

0. 200

0. 400

0. 600

0. 800

1. 000

Non X Y Z

Posi t i ons of housi ng

spee

d(m/

s)

Speed of the boat

the model boat without any housing attached. Bernoulli’s principle states that

fast moving fluid will create an area of lower pressure that will result in

suction. The result of housing C conforms to Bernoulli’s principle as the

housing is often being pulled closer toward the propeller when the boat is in

motion. This results in vibrations and deformation of the original shape of the

housing when the boat is in motion. The vibration and inconsistence shape of

the housing were the main cause inconsistent timings for housing C. On a

similar note, an online account [1] of a homemade Kort Nozzle propeller

resulted in the warping of the material used when the engine is pushed into

higher speed. The housing warped sufficient enough for it to be caught by the

propeller and be smashed to bits. For future consideration of housings for

propeller, it is important to determine the rigidness of the material used for

each propeller speed. This is to ensure that the vibrations caused will not

decrease the efficient of the propulsor significantly or that it will not damage

the housing or propeller.

4.1 Further Discussions of Housing A

With the unsatisfactory results obtained from the housings, it is necessary

to further investigate the reasons for the slowing of the boat to show the effect

of wake ingestion.

Fig 4.1 shows the average speed for the different test conditions to

determine the effects of drag forces and additional weight of the boat due to

Fig 4.1: Chart of housing A

X: enclosing propellerY: underside of boatZ: Top of the boat

12

housing A. The housing contributed additional weight to the model boat as

well as caused additional drag force when the boat was in motion.

The attachment of housing A to the underside of the boat at position Y

while not enclosing the propeller will provide the condition for which the boat

experiences the additional drag forces due to housing A without the effect of

wake ingestion. From Fig 4.1, it can been seen that with the housing attached

to the underside of the boat, the boat is slowed down to a speed of 0.693ms -1

as compared to the original speed of 0.796ms-1 without the housing. This

showed that the effect of drag forces caused by the housing significantly

slowed the boat.

The other consideration of the slower speed of the boat with the housing

was the weight gained due to the housing. Having the housing place on top of

the boat at position Z gave the additional weight due to the housing while

excluding the effect of the additional drag forces as well as wake ingestion.

The average reading obtained was quite close to those of the boat without the

housing, with the speed of the boat with the additional weight being 0.792ms -1

as compare to the original of 0.796ms-1. Therefore, the weight of the housing

did not contribute to the slowing of the boat significantly.

With the housing enclosing the propeller, the boat reached an average

speed of 0.734ms-1, faster than the speed reached when it was not enclosing

the propeller. This result shows the effect of wake ingestion providing more

efficient propulsion. Yet, the effect of the wake ingestion was unable to offset

and overcome the additional drag forces in this experiment, resulting in an

overall slower speed of the boat as compared to without the housing.

However, since effect of wake ingestion is proven to increase efficiency when

housings are attached, this conforms to our initial hypothesis that this is

another scope which is worth exploring upon looking for sources to obtain

ideal propulsion. In our experiment, due to the lack of funds, the housing is

13

Speed of the boat wi th housi ng A+B at tached at di ff erent posi t i ons.

0. 796

0. 5990. 671

0. 788

0. 000

0. 200

0. 400

0. 600

0. 800

1. 000

Non X Y Z

Posi t i ons of housi ng

spee

d(m/

s)

Speed of the boat

crudely design without the consideration of the streamline shape to reduce

drag forces. Proportion of the housing to the size of the boat is also inaccurate

as compare to actual marine vessels as a model boat is employed for our

experiment. These 2 reasons could possible attribute to a greater drag force

that dwarfs the additional benefits of wake ingestion. A more accurate reading

can only be obtained from the use of a properly design housing and well fund

project that employs the use of an actual marine vessel.

4.2 Further Discussions of Housing (A+B)

With the concept of the turbulent flow of water within housing B in mind,

an on-site modification was made to the

housing. To reduce the turbulent formation of

water, it was suggested that the usage of

housing A can control the flow of the water

before focusing it using housing B. Hence,

housing A & B is combined to obtain the

smooth effect of fluid flow from the propeller as

shown in Fig 4.2. Fig 4.2: Smooth effect of Housing A+B

X: enclosing propellerY: underside of boatZ: Top of the boat

Fig 4.2a: Chart of Housing A+B

14

The time taken by the boat to travel 4m is recorded for the cases of the

housing enclosing the propeller, not enclosing the propeller and the housing

being place above water. Fig 4.2a shows the speed of the boat for each case,

respectively. Unlike the results obtain from housing A, the addition of the

housing still provides a slower speed with the drag forces due to the housing

factored in. However, the implementation of housing A together with housing

B provided a faster speed as compared to the use of housing B alone.

Furthermore, there were no visual signs of cavitation throughout the

experimentation with housing A+B.

The use of housing A reduced the turbulent flow of water forming at the

propeller, prevented cavitation and improved the speed of the boat. However,

due to the last minute design of the housing, there were plenty of the

imperfections within the housing that can result in a turbulent flow of water,

especially at the joint of housing A & B. As a result, the analysis using

Bernoulli’s equation and the continuity equation would not hold for this case.

Thus the readings are unable to give us a conclusion on the effects on a

narrowing housing providing for a better efficiency of propulsion. An in depth

study into the flow of the water within the housing is required in further

research to answer this question.

5. Conclusion

The research started off with the aim of improving propulsive efficiency of

submerged propulsion system with the concept of modifications made to the

propulsion system through the use of a housing enclosing the propeller of

marine vessels. The attachment of the housing aims to concentrate, focus and

accelerate the propulsive flow of the water expelled from the propeller through

the concept of Newton’s Law of continuity and Bernoulli’s principles. It aimed

to prove the benefits of wake ingestion.

15

The experiment was carried out in a simplified method through the use of

a model boat so as to focus mainly on the benefits of wake ingestion. At a

slower speed, the effect of drag forces or cavitations was assumed to be

negligible or absent. Having the benefits of wake ingestion proven,

consideration for more intrinsic and detailed design of an actual marine

propulsion system can be researched on. However during the experiment, the

discovery was made that even in a downscaled model, many factors affect the

efficient propulsion of the boat and they cannot be view independently.

With a crudely designed housing attached to the boat, the speed of the

boat is greatly reduced in all 3 cases of different housing attachment. Due to

the non-streamline flow of the fluid around the housing, the drag forces acting

against the boat is greatly increased. A controlled experiment of the speed of

boat due the drag force acting against the housing was carried out. Its aim

was to eliminate the drag forces present and concentrate the effects of wake

ingestion. The data obtained showed that the effect of wake ingestion was

beneficial to the efficient propulsion of the boat and that our controlled

experiment was a success.

The expectation of the conical housing providing the most propulsion was

proven otherwise. The results seem to contradict the theories of Sir Issac

Newton and Daniel Bernoulli. Further investigation showed that the flow of the

water within the housing was far from being steady. The sign of cavitations

indicated that the flow of the water within the housing was extremely turbulent.

Thus, the use of Bernoulli’s equation is no longer applicable. Furthermore, the

turbulent flow of water meant that the propulsion flow is being circulated within

the housing instead of being expelled out. This results in a significant

decrease in the efficiency of the propulsion.

Further experimentation using the combined housings of A and B was

used to reduce the turbulent flow within the housing. Though the speed of the

16

boat was increased compared to the attachment of housing B alone, it was

still significantly slower as compared to the case when no housing was

enclosing the propeller. This gave rise to the realization of the importance of

the study of the tubular flow of the water in the housing. The experiment was

unable to show the benefits of a decreasing radius housing providing an

accelerated propulsion flow due to the interference of turbulence. Future

researches should investigate the tubular flow within the housing to provide a

streamline flow such as to investigate this benefit.

The benefit of wake ingestion was proven in the experiment, yet it is

being overshadowed by the effects drag force. Limitations of funds and time

resulted in housings that are not proportional to the size of the boat and this

resulted in a significantly greater drag force faced. Working models of

propulsion system with a housing enclosing the propeller such as the Kort

Nozzle system are found on the marine vessels of today. They employ the

concept of wake ingestion to give greater propulsion. As drag forces are

related to the individual housing, a general relation between the benefits of

wake ingestion and the drag force cannot be easily deduced. Such

relationship will differ from one housing to another and should be investigated

on individually. Further researches should aim to design housing that reduces

the drag forces and make the housing more streamline to enjoy more benefit

from the effects of wake ingestion.

17

References

1. Gearhart, W. S., and Henderson, R. E., "Selection of a Propulsor for a

Submersible System," Journal of Aircraft, Vol. 3, No. 1, pp. 84-90, Jan.-

Feb. 1966

2. Leroy H. Smith Jr., “Wake Ingestion Propulsion benefits”, Journal of

Propuslion and Power, vol 9, no. 1, pp. 74-82, Jan.-Feb. 1993.

3. Stan Zimmerman, “Submarine Technology for the 21st Century”, 2nd

Edition. Canada: Trafford Publishing, pp. 112-114, 2000.

4. Tom Benson, “Bernoulli and Newton”, 2008 Available: http://www.grc.

nasa.gov/WWW/K-12/airplane/bernnew.html, [Accessed: Sept 30, 2008]

5. Luigi Stipa, NASA Technical Report Server, 93R23681, 1932. Available:

http://ntrs.nasa.gov/search.jsp?N=0&Ntk=AuthorList&Ntx=mode

%20matchall&ntt=stipa, [Accessed: Aug 29, 2008]

6. Peter Woodford, UK Patent Application, GB 2419861, 10/05/2006.

7. Lin, C-C., Lee, Y-J., Hung, C-S., “Optimization and Experiment of

Composite Marine Propellers”, Composite Structures (2008),

20/07/2008.

8. Clark, J. D. T. R., Algoa, TX, “Marine propeller housing”, 1995.

9. W. S. Vorus, Kress, Robert Frederick, "Marine propeller”, ATTWOOD

CORP (US), 1989.

10.A. K. Singhal, M. M. Athavale, H. Li, and Y. Jiang, "Mathematical Basis

and Validation of the Full Cavitation Model", Journal of Fluids

Engineering, vol. 124, 01/09/2002.

11.G. P. C. Canevari, NJ, "Efficiency ship propeller," United States: Exxon

Research & Engineering Co. (Florham Park, NJ), 1982.

12.Ross, Robertson, "Boat propeller mounting and steering mechanism",

United States: Ross, Robertson (Seattle, WA), 1975.

18

Appendix A

Sir Issac Newton discovered the theory behind conservation of mass.

Through this conservation of mass, the continuity equation is derived. This

continuity equation is essential to our experiment as one of our housing,

housing B, is design based on this equation. The continuity equation is listed

below,

A1U1 = A2U2

where A denotes the cross sectional area and U denotes the velocity at which

the fluids are being displaced.

Figure A is a simple model that is widely used to illustrate the

continuity equation. Since the cross sectional area at A1 is larger than A2, we

can conclude from the continuity equation that the velocity U1 would be

smaller as compared to U2.

The simplified version of bernoulli’s equation for horizontal tube is

stated below.

P1 + ½ρU12 = P2 + ½ρU2

2

where P denotes the pressure of the fluid, ρ denotes the density of the fluid

and U denotes the velocity of the fluid.

U2U1

A2A1

Figure A: Illustration of continuity equation

In figure B, using the continuity equation, we know that U2 is higher compare

to U1. It is important to note that the fluid across this conical shape structure is

the same, which implies that the density, ρ, is constant. Placing these known

variables into the simplified version of Bernoulli’s equation, we can conclude

that the pressure at P2 is much lower than the pressure at P1. This further

implies that there will be a suction effect at P2 due to lower pressure.

However, there are some restrictions towards the application of Bernoulli’s

equation. These restrictions include steady flow of fluids, streamline fluids,

friction losses are negligible and density of the fluid is constant. Thus if the

flow of fluids are turbulent, the Bernoulli’s equation would be unable to hold

true.

P1

U2U1

A2A1

P2

Figure B: Illustration of Bernoulli’s equation

Appendix B

In section 3.2.1 investigations of housing A, B and C, the raw data are listed

in the table below. The travel time was taken for a distance of 4m travelled.

Type of

housing

1st travel

time (s)

2nd travel

time (s)

3rd travel

time (s)

Average travel

time(s)

Speed

(m/s)

Non 4.99 4.98 5.1 5.023 0.796

A 5.49 5.37 5.48 5.447 0.734

B 7.92 7.8 7.68 7.800 0.513

C 6.25 5.93 7.92 6.700 0.597

In section 3.2.2 further investigations of housing A, the raw data are listed in

the table below. The travel time was taken for a distance of 4m travelled.

Positions of

housing

1st travel

time (s)

2nd travel

time (s)

3rd travel

time (s)

Average travel

time(s)

Speed

(m/s)

X 5.49 5.37 5.48 5.447 0.734

Y 5.81 5.79 5.72 5.773 0.693

Z 5.06 4.98 5.11 5.050 0.792

In section 3.2.3 investigations of housing A+B, the raw data are listed in the

table below. The travel time was taken for a distance of 4m travelled.

Type of

housing

1st travel

time (s)

2nd travel

time (s)

3rd travel

time (s)

Average travel

time(s)

Speed

(m/s)

A 4.99 4.98 5.1 5.023 0.796

B 6.73 6.79 6.51 6.677 0.599

C 5.81 6.13 5.94 5.960 0.671

Figure Bi: Table for section 3.2.1

Figure Bii: Table for section 3.2.2

Figure Biii: Table for section 3.2.3