Ship tecnic Sharif university Lecture 6
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Transcript of Ship tecnic Sharif university Lecture 6
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Chapter 6
Ship Resistance
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As a ship moves through the water, it experiences forces that work
against its forward movement. The sum of all these forces is the
- This is designated as RT- It is from this value that theEffective Horsepower, EHP, is calculated
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Resistance Values and Coefficients
Resistance values, denoted by R, are dimensionalvaluesRT = Total hull resistance is the sum of all resistance
RT = RAA + RW + RV
RAA = Resistance caused by calm air on the superstructure
RW = Resistance due to waves caused by the ship
- A function of beam to length ratio, displacement, hull shape &Froude number (ship length & speed)
RV = Viscous resistance (frictional resistance of water)
- A function of viscosity of water, speed, and wetted surface
area of ship
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Total Resistance and Relative Magnitude of Components
- At low speeds Rv dominates
- At higher speeds Rw is dominates
- Hump (Hollow)- location is function of ship length and speed
Viscous
Air Resistance
Wave-making
Speed (kts)
Res i s
t anc
e( lb)
Hum
p
Hollow
The amount of each resistance component will vary depending on speed:
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Speed-Power Trends EHP = (Resistance) x (Speed)
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Similar to the resistance components are the
- Resistance Coefficients, C, are dimensionless values of resistance- Allow the comparison of dissimilarly shaped vessels
- Used extensively in modeling
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Coefficients
CT = Coefficient oftotal hull resistance
CT = CV + CW
- CV = Coefficient ofviscous resistance over the wetted area of
the ship as it moves through the water
- CF = Tangential component (skin resistance)- KCF = Normal component (viscous pressure drag)
- CW = Coefficient ofwave-making resistance
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Coefficient of Viscous Resistance, CVLets look at the resistance due to the water, CV, first
- Consists of tangential and normal components
FF KC+C=+= normaltangentialV CCC
- Tangentialresistance, CF, is parallel to ships hull and causes a net forceSkin Friction opposing the motion by the water
-Normal resistance, KCF, is perpendicular to the ships hull. K is unique
to the hull form
flow shipbow sterntang
ential
normal
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Coefficient of Viscous Resistance, CV
Laminar Flow
Laminar flow - Fluid flows in layers that do not mix transversely but
slide over one another
Tangential Component, CF
Also called the hull frictional resistance, CF can be characterized by the fluid flowaround the hull:
Turbulent Flow
Turbulent flow -The flow is chaotic and mix transversely- Denoted by the Boundary Layer
- The boundary layer forms at the Transition point where flow changes from
laminar to turbulent
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Normal Component, KCF - Causes a pressure distribution along the underwater hull form of ship
- A high pressure is formed in the forward direction opposing the motion
and a lower pressure is formed aft
-Normal component generates the eddy behind the hull- Is affected by hull shape
Fuller shape ship has larger normal component than slender
ship
Full ship
Slender ship
large eddy
small eddy
Coefficient of Viscous Resistance, CV
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- The viscous resistance component CV can be related to another
common dimensionless coefficient, the Reynolds Number
Rn = L V
Reynolds Number
Coefficient of Viscous Resistance, CV
Laminar Flow Turbulent Flow
Rn < 5 x 105 Rn > 1 x 106
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How to Reduce the Viscous Resistance Coefficient
- For tangential component, increasing the length decreases the
skin resistance
- For normal component, a more slender ship decreases the pressure
drag on the hull
Very long, narrow, slender hull is favorable ( A slender hull form will createa smaller pressure difference between bow and stern)
Increase L while keeping the submerged volume constant
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Froude Number, Fn
The Froude Number is another dimensionless value derived from model testing
Fn = V
\/gL
Also used, but not dimensionless, is the Speed-to-Length Ratio:
Speed-to-Length Ratio = V
\/L
...Velocity is typically expressed in Knots (1 knot = 1.688ft/s)
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Typical Wave Patterns are made up ofTRANSVERSE and
DIVERGENT waves
Transverse wave
Stern divergent wave Bow divergent waveBow divergent wave
Coefficient of Wave Resistance, CW
Wave
Length
L
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Wave-Making Resistance
Transverse Wave System
- Travel at approximately the same speed as the ship
- At slow speeds, several crests exist along the ship length because the wave
lengths are smaller than the ship length
- As the ship increases speed, the length of the transverse wave increases
- As the wave length approaches the ship length, the wave making
resistance increases very rapidly
...This is the main reason for the dramatic increase in Total Resistance
as speed increases
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Wave Length
Wave
Length
SlowSpeed
High
Speed
Vs < Hull Speed
Vs Hull Speed
When the transverse wave length equals the ships length the vessel has
reached its HULL SPEED(Wave making resistance drastically increases
above hull speed)
Wave-Making Resistance
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Divergent Wave System
- Divergent waves consist ofBow and SternWaves
- Interaction of the bow and stern waves create the Hollow or Hump on the
resistance curve
Wave-Making Resistance
- Hump: The bow and stern waves are in phase, the crests are added up
creating a larger divergent wave system
- Hollow: The bow and stern waves are out of phase, the crests match
the troughs so that smaller divergent wave systems are generated
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Calculation of Wave-Making Resistance Coeff.
- Wave-making resistance is affected by:
- beam to length ratio - displacement
- hull shape - Froude number
- The calculation of the coefficient is far too difficult and inaccurate from
any theoretical or empirical equation
- Model test in the towing tank and Froude expansion are needed
to calculate the Cw of the real ship
Wave-Making Resistance
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It takes energy to produce waves, and as speed increases, the energy
required is a square function of velocity!
Lwave = 2V2
g
The limiting speed, or hull speed, can be found as:
V = 1.34\/Ls
Note: Remember at the hull speed, Lwave
and Lsare approximately equal!
Wave-Making Resistance
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Reducing Wave Making Resistance
1) Increasing ship length to increase the wave length
- Hull speed will increase- The hull speed will be greater for the longer ship (the wave-making
resistance of longer ship will be small until the ship reaches to the hull speed)
Wave-Making Resistance
2) Attaching Bulbous Bow to reduce the bow divergent wave- Bulbous bow generates the second bow waves
- The waves interact with the bow wave resulting in smaller bow divergent waves
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Bulbous Bow
Wave-Making Resistance
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Other Type of Resistances
Appendage Resistance
- Frictional resistance caused by the underwater appendages such as rudder,
propeller shaft, bilge keels and struts
- 2 24% of the total resistance in naval ship
Steering Resistance
- Resistance caused by the rudder motion (small in warships but a problem
in
sail boats)
Added Resistance
- Resistance due to sea waves which will cause the ship motions (pitching,
rolling, heaving, yawing)
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Increased Resistance in Shallow Water
Resistance caused by shallow water effect
- Water flow is restricted under the vessel,so water velocity under the hull increases
- The faster moving water decreases pressure causing the ship to squat
- Increases wetted surface
- Increases surface friction
- Waves tend to be larger compared to waves in deep water at the same speed
- Traveling through a canal can produce the same effect
Other Type of Resistances
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( )212
, Re,Fr TR VL V
C f fSV
Lg
= = =
When a model and its prototype are geometrically similar and
their two dimensionless coefficients (Re, Fr) are the same, theirresistance coefficients (CT) should be the same.
Dimensional analysis reduces the number of the related
parameters involved in model tests. However, it can take the
problem no further than the above conclusion.
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Model Tests of Ship Resistance
Model tests are widely used in the design and study of large
engineering constructions, such as harbor, breakwater, bridge
constructions, and ship buildings.
A ship model is geometrically similar to its prototype. Thesize of the model is usually much smaller than that of the ship.
Ship model tests are employed to predict the resistance, the
interaction between the hull and the propeller, seakeepingproperties of a ship, etc. Therefore, model tests are very
important in ship design and ship research. Here we focus on
model resistance tests.
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A typical resistance curve in a model test
V
gL
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A Towing Carriage and A Ship Model
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Towing tank
Resistance tests in calm waterResistance tests in calm water
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Resistance Test in Towing Tank
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Resistance Test in Towing Tank
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Seakeeping test in Laboratory
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Propulsion
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Sub Cavitating Propeller Fully Cavitated Propeller
Surface Piercing Propeller
(S.P.P.) Waterjet
Air Propeller
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Engine Reduction
Gear Bearing Seals
Propulsor
Strut
Shaft
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HUBROOT
BLADE TIP
TIP CIRCLE
ROTATION
LEADING
EDGETRAILINGEDGE
PRESSURE
FACE
SUCTION
BACK
Screw Propeller
PROPELLER
DISC
37
57
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Hub
pitch
dia
meter
The distance that the blade travels in one revolution, P
- measured in feet
Propeller Pitch57
P ll
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Propeller
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Propeller Geometry
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Propeller Coefficients
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Typical Chart
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B-Series Charts
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Blade Tip Cavitation
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p
Sheet Cavitation
Flow velocities at the
tip are fastest so thatpressure drop occurs
at the tip first.
Large and stable region of
cavitation covering the
suction face of propeller.
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Consequencesof
Cavitation
1) Low propeller efficiency (Thrust reduction)
2) Propeller erosion (mechanical erosion as bubbles
collapse, up to 180 ton/in pressure)
3) Vibration due to uneven loading
4) Cavitation noise due to impulsion by the bubble
collapse
S P P
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S. P. P.
.
.
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S.P.P.
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S.P.P.
Waterjet
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Waterjet
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.
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Cavitation Tunnel
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58
Cavitation Tunnel
Applications: Assessment of Propeller and Duct Performance
Flow Visualization and Determination of Drag Characteristics forVarious Appendages
Cavitation Studies
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Propeller Test
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Geometrical similarity indicates the main characteristics of a
model & its prototype are in the same ratio.
or , for a model and its prototype
having the same Fr & Re, then we requir e
1, & ,
if both are run in water at the similar density &
temperature, .
Since 1
s
m
s s s m s
m m m s m
s m
Lm
L
V L V Lm
V L V L m
m
=
= = =
=
;
?
( ) ( ) ( ) ( )
, it is ,
and Re Rem s m s
Fr Fr= =
almost impossible to satisfy both
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1. In order to overcome this fundamental difficulty to satisfy
the similarity laws, a major (first) assumption was made
by Froude that the frictional and the wave-making
resistances are independent, and the frictional-resistancecoeff. depends only on the Reynolds #. The wave-making
orresidual resistance coeff. depends only on the Froude # .
1 2212
1212
2212
Frictional Resistance:
Wave-making Resistance:
T F R
FF
RR
R VL VC C C f f SV gL
R VLC f
V S
R VC f
V S gL
= = + = + = =
= =
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2. It is also assumed that the frictional resistance coeff. of a ship
(or a model) is the same as that of a smooth flat plate with
the same length and wetted surface area as the ship (or themodel). Therefore, CF orRF of a ship (or a model) can be
computed given the length according to the half-analytically &
half-empirically friction formulas.
3. Based on these two assumptions, we may determine the
resistance of a ship at a constant velocity given the results of
model resistance test. The steps are detailed below.
212
a. At , the total resistance of a model, , can be measured.
Thus ,
where is the model's wetted surface area.
m Tm
TmTm
m m
m
V R
RC
S V
S
=
d
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ndb. According to the 2 assumption, , can be computed given
the length of model according to a friction coefficient formula.
c. Computing the model's resistance coefficient
FmC
residual
2
.
d. If , namely, , then
,
the ship's residual resistance coefficient is computed.
e. Same as in Step b, can be comput
Rm Tm Fm
s m s s
m ms m
Rm RS
FS
C C C
V V V Lm
V LgL gL
VC C f
gL
C
=
= = =
= =
( )
ed given the ship's length.
f. The total resistance coeff. of a ship is given by,
.
TS FS RS
FS Rm FS Tm Fm Tm Fm FS
C C C
C C C C C C C C
= +
= + = + =
g The total resistance of a naked ship (excluding appendages)
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212
g. The total resistance of a naked ship (excluding appendages)
can be obtained, , at . When
two geometrically similar ships are running at speeds which
conform to the F
S TS S s S mR C S V V mV= =
2
2
roude Law, , they are said to be running
at . It is noticed that, .
rs rm
s s
m m
F F
S Lm
S L
=
= =
corresponding speeds
In most cases, the total resistance of a ship can be determined
accurately based on the model test results using the above method.
However, the method is based on the 2 major assumptions (a. CF
& CR are independent, b. CFS of a ship is equal to that of a flat platewith the same length). Sometimes the errors due to the
approximations may be significant. We will study the frictional,
wave-making and eddy-making resistances in detail, for
understanding the computation using the method & its validity.
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5.5 Frictional Resistance
Laminar and Turbulent Flow (review of CVEN 311)
Laminar flow: the fluid appears to move by the sliding of
laminations of the infinitesimal thickness relative to adjacent
layers.
Turbulent flow: is characterized by fluctuations in velocityat all points of the flow field and these fluctuations with no
definite frequency.
Whether a flow is laminar or turbulent flow depends mainly
on its Reynolds #. For a plate flow,6
8
6 8
when Re < 10 the flow is laminar,
Re > 10 the flow is turbulent,
10 < Re < 10 the flow is transitional
Friction form las for a flat plate
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Friction formulas for a flat plate
The following formulas are commonly used.
( )
15
5 1.5
212
10
1) Blasius formula. (Laminar flow)
1.32 / Re, Re 4.5 10 . Re , , thus, .
2) Prandtl and von Karman formula (turbu lent flow)
log Re , 0.074( ) , thus,
FF F F
F F N FF
RVLC C R V
SV
A
C M C R RC
= < = =
= + =
( )
1.8
8
10
.
3) Schoenherr formula (1947 ATTC line , derived based on 2))
0.242log Re , for Re 4.5 10 .
4) 1957 ITTC line formula (known as ship-model correlation line
not a friction coef
F
F
V
CC
=
( )
7
2
ficient for a flat plate, turbulent flow)
0.075, for Re 10 .
lFC =