propeller designteacher.buet.ac.bd/mmkarim/propellerlecture.pdf · name 338: ship design project...
TRANSCRIPT
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NAME 338: Ship Design Project and Presentation
http://teacher.buet.ac.bd/mmkarim/
PROPELLER DESIGN
DR. MD. MASHUD KARIMPROFESSORDEPT. OF NAME, BUET
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http://teacher.buet.ac.bd/mmkarim/
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PROPELLER SPECIFICATIONShttp://teacher.buet.ac.bd/mmkarim/
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PROPELLER DIAMETER
Dmax = DB = aT
a < 0.65 for Bulk carriers and Tankers
a < 0.74 for Containers ships
a < 2/3 for single screw
X = 5~10% D
Y = 15~25% D
Z = Up to 5% D
Diameter shoud be as large as possible
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REVOLUTIONS PER MINUTE (RPM)
Torque Formula, Q = (5252 x HP)/RPM
Lower the shaft RPM and higher the HP, the greater the Torque
This is why slower turning propellers deliver more thrust—they are
recieiving more torque for the same HP
Low speed engine is heavy and bulky wasting valuable interior hull
space whereas high sped engine is light, compact and economical
A reduction gear is fitted between the crankshaft and the propeller shaft
to reduce shaft RPM so that a larger diameter and more efficient
propeller may be used.
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Where,
DB = Propeller dia at behind condition
D0 = Propeller dia at open water condition
PROPELLER DIAMETERhttp://teacher.buet.ac.bd/mmkarim/
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NUMBER OF BLADES
One blade: Ideal, it does not have other blades disturbing the water flow ahead of it.
Unfortunately, it will be unbalanced.
Two blades: It requires very large diameter to get the blade area required for effective
thrust
Three blades: it has been proven to be the best compromise between balance,
blade area and efficiency.
Four / Five blades: extra blades create more total blade area with the same or less
Diameter. Accordingly, more thrust is produced. However, efficiency decreases due
to additional turbulence
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PITCH
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EFFECT OF PITCH
Pitch converts the torque of the propeller shaft to thrust by deflecting or accelerating
water astern.
Large diameter, without pitch or angle of attack would not accelerate any water
astern
Similarly ordinary blades with too much pitch would attempt to force more water
astern more quickly than the engine can accommodate.
The fundamental task in selecting a propeller is to choose a pitch and diameter that
will generate the maximum thrust possible at normal operating speed without
overloading the engine
Increasing pitch increases thrust but increasing pitch too much reduces the
efficiency of the engine and propeller combination by slowing the engine
On the other hand, while too little pitch will not overload or slow the engine, it will
not accelerate as much water astern and thus will not generate maximum possible
thrust or speed
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RELATIONSHIP OF ENGINE POWER TO
PROPELLER POWER
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RAKE & SKEW
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HAND OF PROPELLER
If the propeller turn clockwise, it is said to be right-handed; if anti-clockwise, it is
said to be left-handed. In a twin screw ship, the starboard propeller is normally right
handed and the port propeller left-handed
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PROJECTED, DEVELOPED & EXPANDED OUTLINE
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BLADE NO. & BAR
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TORQUE & THRUST COEFFICIENTShttp://teacher.buet.ac.bd/mmkarim/
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COEFFICIENTS OF KT & KQhttp://teacher.buet.ac.bd/mmkarim/
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EFFECT OF REYNOLD’S NUMBER
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GEOMETRY OF B-SERIES PROPELLER
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GEOMETRY OF B-SERIES PROPELLERhttp://teacher.buet.ac.bd/mmkarim/
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VALUES OF V1
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VALUES OF V2http://teacher.buet.ac.bd/mmkarim/
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HUB AND SHAFT DIAMETER
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Assume Hub Diameter Ratio: 0.2
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B 3.35 BP-δ CHART
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B 3.50 BP-δ CHART
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B 3.65 BP-δ CHARThttp://teacher.buet.ac.bd/mmkarim/
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B 4.40 BP-δ CHARThttp://teacher.buet.ac.bd/mmkarim/
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B 4.55 BP-δ CHARThttp://teacher.buet.ac.bd/mmkarim/
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B 4.70 BP-δ CHARThttp://teacher.buet.ac.bd/mmkarim/
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B 5.45 BP-δ CHARThttp://teacher.buet.ac.bd/mmkarim/
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B 5.60 BP-δ CHARThttp://teacher.buet.ac.bd/mmkarim/
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HULL EFFICIENCY
R = Towrope resistance of the hull
T= Thrust of the propeller
Txt = Thrust deduction
t = thrust deduction fraction
1-t = thrust deduction fraction
T.VA = Thrust power (THP)
R.V = Effective power (EHP)
VA = Speed of Advance VS (1-wt)
V =VA/(1-wt)
Wt = Taylor wake fraction
VRtW
VT
t
A .)1.()1(
EHPW
tTHP
t
)1(
)1(
H
t
efficiencyHullTHP
EHP
W
t
)1(
)1(
VRtTV .)1(
RtT )1(
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SHAFT POWER, PS
QPC
VR
QPC
PP
QPC
SES
SD
HRoD
.
nD
VJ A
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The local Cavitation number at 0.7R is given by:
An approximate value for which is given by:
A mean thrust loading coefficient given by the following expression
is then obtained from Burril cavitation chart and AP is calculated. From
following expression, AD is calculated
And finally minimum expanded blade area ratio (EAR) required is calculated
assuming AD~AE
CAVITATION CHECK:
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BURRIL CAVITATION CHART
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EAR
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)DP229.0067.1(
AA P
E
Assuming AE=AD
4/D
A
A
A2
E
0
E
Expanded blade area ratio, EAR
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KELLER’S CRITERION FOR AVOIDING CAVITATION
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DIA, PITCH & RPM OF 3 BLADED PROPELLER
T = R/(1-t)
AREA OF BLADES AD= T/80
BAR =4.AD/(πD.D) D =?
ASSUME Pressure Loading of 80kN/m.m
VA = VS(1-w)
2222
2
422/
A
AT
VD
T
Dn
V
Dn
T
J
K
Use Chart and determine
P/D, J and η
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2
T
J
K
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nD
VJ A
Speed of
Advance,
nD
VJ A nD
VJ A
n =?
P/D=?
P=?
Number of blade = 3
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Obtain values of
NUMBER OF BLADES OTHER THAN THREE
Prepare a table as follows:
J
0.4 ?
0.5 ?
0.6 ?
0.7 ?
… ?
2
T
J
K
2
T
J
K 2J
Plot these data on a KT-KQ-Eta-J diagram and find the
intersection points with KT curve. Draw perpendicular at
these points and obtain eta for different j values and finally
plot it.
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The advance coefficient at which eta is maximum is the
optimum operating condition. From Jopt, find the
optimum rpm of propeller.