HASRUL TAUFIK BIN MOHD ROBI

AM080217

890422-11-5367

3-SMK

DRAG, BOUNDARY LAYER AND HULL RUOGHNESS ON THE HULL SURFACE

27/09/2010

(Signature)

____________________________________

HASRUL TAUFIK BIN MOHD ROBI

INTRODUCTION

The ship needs power to move at the given speed. The power used to overcome the total

drag of the ship. So it is important to know the cause of the drag and how to determine the

value. The drag is the force required to tow the ship at given speed on calm and smooth

water. The total drag has two major components, the frictional drag and residuary drag.

Example of the frictional drag is skin friction and example of the residuary drag is wave

making resistance. The factor that influenced the total drag is the boundary layer, hull

roughness, pressure, shape of the hull and also the weather and route taken of the ship.

The boundary layers show the behaviour of the flow at the surface of the hull. There are 2

type boundary layer, laminar and turbulent flow. They depend mostly on the Reynold

number. The roughness of the hull also effects the formation of the layers.

The hull roughness is important parameter that should not be ignored. Hull roughness

measured by the device called hull roughness gauge. The roughness increase the total drag,

thus increase the cost. Normally the roughness of the ship is increase after the service. paint

coating, antifouling and cleaning at the dry dock can reduce the hull roughness.

DRAG

In fluid dynamic study, the drag or resistance of a moving body in a fluid is refers to the

forces that oppose the motion at the given velocity. The drag of a body can be divides into

two components. One of the components is the drag is always opposite to the movement of

the body and for the perpendicular component called lift and it is important in aerodynamic

study. For a body to move at the given speed, the power system need to be installed to give

the thrust which the force that supplied to overcome the drag of the body. The shape of the

movement body is important factor to determine the drag. There are various type of drag

and can be divided into categories:

Parasitic drag- form drag, skin friction, interference drag

Lift-induced drag

Wave drag

For the high velocity movement or at high Reynold number because from the Reynold

number equation: Rn=v ∙Lν

Therefore Reynold number is proportional to the velocity and the overall drag of the body

can be calculated by the drag equation

FD=12ρV 2CD A

Where FD is drag force,ρ is a density, V is velocity, A is area and CD is the drag coefficient.

The area, A area of the object which is perpendicular to the movement of the motion such

example if the object is sphere, the area is the cross sectional area, that is area of the circle

of radius r same with the radius of the sphere. The drag coefficient is the dimensionless

quantity that pictures the drag of the body. The higher value of the drag coefficient shows

higher the drag force. The drag coefficient also depends on the Reynold number of the

moving body.

The further study of the drag or resistance of the ship is called hydrodynamic study. Based

on the study, many ships can be design to move efficiently with using less power for the

given speed. The study is very important to know the behaviour of the flow and the total

drag of the ship. The result of the study is very important to predict the cost of building and

performance of the ship. Many engineer and ship owner wants to reduce the cost as low as

possible for their own benefit.

To move a ship at the given speed we need to know the effective power of the designed

ship. The effective power of the ship can be calculated by using the formula:

PE=RT ∙V where

RT is t he totaldrag∨resistance and V ist he velocity of t he sh ip .

The drag of the ship that moves at desired speed is the force need to tow the ship at the

given speed at the smooth water. It is assume that there is no interferences form at the

towing ship. But the condition is nearly impossible for the actual ship can move without any

form interferences and sail smoothly. So in order to get the total drag of the actual ship, we

need to do the model test at the towing tank and by using the information of the model

test, the actual resistance of the ship can be determine. The method to determine the

resistance of the ship based on the model test is called ITTC method. The method is firstly

introduce by William Fraude in 1863 where he come out with the derivation and

justification on to extrapolate the model test result to the actual full scaled ship.

The total drag of the ship is divided to two big components that are the drag occur due to

the viscosity of the fluid where the ship move called viscous drag or frictional resistance and

the other component is the drag that formed because the ship running at the surface of the

water and the examples are the wave-making resistance, eddy resistance and air resistance

are called the residuary resistance. Therefore the total drag is sum of all the component of

drag: RT=RF+RR.

At early stage, there is difficulties happens when the total resistance of the model is not

same with the actual scale although using the same formula. This happen because of the

behaviour of the model at the towing tank is not the same with the actual ship at the ship.

Lastly William Fraude has come with the solution by introducing two laws that very

important to the study named the Reynold’s law and Fraude’s law. Other law that introduce

by Fraude is law of comparison which stated that:

RR proportional ¿∆whenV is proportional ¿ t he √L

Then the Fraude’s formulation to determine the frictional resistance:

RF=SV n

From Fraude Hypothesis, CD=CF+CR and CRS=C RM

Where CF=0.075

(log10RN−2 )2

CDS=CFS+CRS+allowances and RT=12V S2 ρSSSCDS

PE=V S ∙ RT

TYPE OF RESISTANCE AT HULL SURFACE

Friction drag

Friction drag occurs due to the friction of water with the surface of the hull. The friction drag

is depends on the roughness of the hull, the wetted surface area and the type of boundary

layer form along the hull. The water has the viscosity, when the ship is moving, the water at

the surface of the hull will attracted to the motion of the hull and form the sheer stress

along the body. The stress is opposing the hull to move freely. As the wetted surface area

increase, which mean the bigger the hull, the bigger the drag. This is because more water

molecule attracted to the hull surface creates higher shear stress. The type boundary layer

affected the frictional drag. Most of the ship has turbulent flow which is thicker than the

laminar flow. The turbulent flow tends to has larger frictional drag compare to the laminar

flow. The friction drag is higher at the high hull roughness because more water is dragged to

the hull. The situation is similar when we walk on the slippery floor compare to roughen

floor. The friction is higher at the roughen floor.

Pressure drag

The pressure drag is related to the condition of the streamline at the surface of the hul. If

the streamline nicely stream, the pressure resistance is low. If the streamline is break and

has a lot of disturbance, the pressure resistance is higher. The pressure resistance is

depends on the shape of the hull. For a hull surface, the streamline crowded at the middle

of the hull and the streamline will shift at the stern. So, the pressure resistance is higher at

the stern and lead to formation of eddies thus increasing the total drag. The pressure

resistance also depends on the flow characteristic. The pressure resistance is lower when

the flow is turbulent but for the hull this condition is can be true because of the size the

turbulent flow. Example of the application is on the golf ball. The golf ball is design to have

the turbulent flow to lower the pressure resistance so the ball can go further. The

application is not applied to the hull because it will increase the building cost.

Wave making resistance.

When the ship is running, there will be distribution on water level will increase the energy

power usage and fuel. Because of the shape of the hull, the hull will generate the pressure

to the water level when running. The pressure will cause the waves to disturb the

distribution of the pressure at the hull thus increasing the effective power. The wave making

is largely depends on the shape of the hull. There are devices that can help the hull

influenced the wave making resistance in favourable ways called bulbous bow. The bulbous

bow help to create good wave that disturb the wave system that create by the hull, thus it

can lower the wave making resistance and lower the cost.

Air resistance

Air resistance caused by the interaction between the air and the above water surface part

and the structure. The air resistance contribute 2% of the total resistance.

BOUNDARY LAYERS

Based on the fluid dynamic, every motion body in the fluid will behave the dynamic flow

behaviour with different characteristic. The flow behaviour that forms at the surface of the

motion body is called boundary layer. As for the hull surface of the ship, the boundary layer

will react at along the surface of the hull transversely depend on the speed of the motion

and viscosity of the fluid. The speed and the viscosity of the fluid can be showed by the

Reynold’s number (Rn). So, the type of layer at the surface of hull is strongly depends on the

Reynold’s number.

The flow will give the force to the body of the hull against the motion of the ship that is the

shear force. Every real fluid has the viscosity, due to the intermolecular force. The bigger the

intermolecular force, the bigger the viscosity. When the ship move at the constant speed,

the fluid molecule near the hull will carried away along with the surface and the fluid that

are close to the hull move in the same way of the direction of the hull. As a result, there will

be layer that act at the hull surface and it become thicker from stern to bow.

The layer become thicker because of the velocity of the fluid is varied alongside the hull. The

layer become thicker and thicker and lastly will leave the hull surface at the end of stern and

form the frictional wake which is one of the components in total ship resistance. If the hull is

a blunt shape at the end, the boundary layer will leave at the separation point which can

lead into the pattern of eddies at back thus further increase the total resistance of the ship.

There are 2 type of boundary layer consist of the laminar and the turbulent layer. The

phenomenon is depends on the speed of the hull surface or the Reynold number of the

fluid. At the slow speed which is almost zero, means at very low Reynold number, the flow

behaviour of the streamlines are laminar flow. At the flow, the thickness is very small and

smooth that gives relatively small resistance or shear stress to the body of the hull surface.

Usually the laminar flow can be found at the Reynold number of 4 to 5^10. As the speed of

the ship increase thus increases the Reynold number, the boundary layer become thicker

and will disturb the laminar flow cause it to break down and the and change to the

turbulent flow. There will be the transition layer in between the laminar and the turbulent

flow where the flow changes its formation called sub laminar layer. There is no specified

place taken of the flow transition. The turbulent flow has higher friction, thicker and gives

more resistance to the body of the hull resulting the increasing of the total resistance of the

ship.

Another reason that the turbulent can grow faster is the roughness of the hull, dirt, wave

current, and even weather. All the reason proves that for a ship to have laminar flow is

impossible. All the ship sailing at the sea experience the turbulent flow, so the model test

that carried at the towing tank need to experience the same as the actual ship in order to

apply the ITTC method for calculating total ship resistance. The turbulence inducer must be

installed to the model to ensure that the model run with the turbulent flow alongside the

hull surface. The installation is at the bow of the model and the example of the turbulence

inducer is trip-wire, sand roughness and stud. Mostly the stud is applied because it easy to

install.

HULL ROUGHNESS OF SHIP HULL SURFACE

Hull roughness is one important part of study and research of marine engineers because it is

included as one of the factors that relate the total resistance of the ship. Mostly, the hull

roughness cause the boundary layer thickness increases and as the result, increasing the

frictional resistance of the ship thus increase the operating performance and fuel cost of the

ship to keep the desired speed of the ship. Another effect of the hull roughness is the

increasing of the cost to clean the hull and the cost of coating which is done at the dry dock

is expensive and consumes time.

Many engineers and ship owner want to keep the hull as smooth as possible to reduce the

fuel cost of the ship. According to research done by Royal Institute of Naval Architect, for

every 10 micrometers increases in hull surface roughness is an increase of 0.5-1% of the

power to maintain the same speed. There are two main cause of the hull roughness that is

physically and biologically. Examples of the physically hull roughness are corrosion, cracking,

detachment, cold flow, touch-up repair, welds, plate profile, mechanical damage and etc.

Most of the biologically factors is cause by fouling. There are two type of fouling, which are

micro and macro fouling. Macro fouling is caused by animal such as barnacles, tube worms,

and mussels which resulting 40% increase of the drag and by weed such as marine grass,

Ectocarpus and etc can increase up to 10% of the drag. The minor fouling mostly caused by

the slime and can increase 1-2 % of the drag. The fouling can be control by using the

antifouling and can keep the hull smooth as possible. There are various types of antifouling

and the usage are restricted under the law of antifouling system that organized by IMO.

HULL ROUGHNESS AFTER SERVICE

The hull surface will roughen after the service because of:

Mechanical damages

Corrosion

Fouling

Over coated fouling remnants

Coating defect such as flaking and blistering

Uneven areas due to build up of old coating residues at places during maintenance.

The roughness of the mechanical damages, corrosion, fouling, blistering and flaking is higher

than the roughness due to coating applications.

MEASUREMENT OF HULL ROUGHNESS

Hull roughness is measured by the mechanical-electronic device which called hull roughness

analyzer or gauge supplied by British Maritime Technology (BMT). The standard unit of the

hull roughness is the peak to trough height in microns per sample of 50 mm length of

underwater hull (Rt50).

Methods of Measurement:

The will be divided into 10 sections which is same length.

The measurement will be taken 10 times for each section, which is 5 at the port side

and 5 at the starboard side.

There will be 50 readings will be taken at each side, 30 at the vertical side and 20 at

the flat side.

The total 100 reading will be used to plot the roughness distribution graph. The

average hull roughness (AHR) will be calculated to indicate the roughness of the ship.

As mention above, the hull roughness is one of the factors of the frictional resistance of

the ship. The model test in tow tank was carried out at the smooth surface but for the

actual ship, there is no term of smooth hull surface. So, in the calculation of the total

ship resistance, there will be some modification to be made for the allowance of the

roughness. There will be 15% roughness allowance added to the Fraude formulation

which was recommended by NPL for the use in ITTC line.

CONCLUSION

As the conclusion, the drag, the boundary layer and the hull roughness on the hull

surface is related to each other. In order to run the ship at trial speed and service speed

we need the propulsion system that efficient to overcome the drag of the ship.

Therefore it is important to measure the total drag of the ship to estimate the efficient

power needed for the ship. By doing that, we can predict the most suitable cost for the

ship and lower cost can give more profit. In determining the total drag of the ship, it is

important to understand the behaviour of the flow that exists at hull surface and the

behaviour of the flow is showed by the boundary layer that has two components that is

laminar and turbulent flow. Mostly for the ship, the flow is turbulent which is become

the important factor for the total drag because it increase the frictional resistance. The

understanding of the boundary layer and the flow will help during the analyzing of the

model test that carried at the tow tank and transform it to full scale. Without the

understanding we will not get the means of the Fraude’ law, Reynold’s law, law of

similarity and law of comparison and cannot apply the Fraude formulation of total drag.

The roughness of the hull is also one of the factors for the total drag and increases the

cost and reduces the performance and also the factor for the thickness of the boundary

layer increases. So it is important to have smooth hull surface and better coating so the

ship can do her service without any trouble and problems.

REFERENCES

1. http://en.wikibooks.org/wiki/Ship_Resistance

2. http://www.brighthub.com/engineering/marine/articles/60585.aspx

3. http://silver-sails.com/Primer/ResistanceII/

4. http://en.wikipedia.org/wiki/Drag_%28physics%29

5. The Effect of Biocide Free Foul Release Systems on Vessel Performance by John

Willshere, International Paint Ltd., London/UK

6. http://www.answers.com/topic/boundary-layer

7. http://www.informaworld.com/smpp/content~db=all~content=a714006372

8. Ghani, D. M. Ship resistance, johor bharu: UTM.