Chp6-part1
Transcript of Chp6-part1
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Chapter VI. Propulsion of Ships
The propulsion system of a ship is to provide thethrustto the ship to overcome the resistance.
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6.1 Introduction Propulsive Devices (reading p205-209)
Paddle-Wheels: hile the draft varying !ith ship displacement"
the immersion of !heels also varies. The !heels may come out
of !ater !hen the ship is rolling" causing erratic course-#eeping"
$ they are li#ely to damage from rough seas.
Propellers: %ts first use !as in a steam-driven &oat at '.. in
*0+. ,dvantages over paddle-!heels are"
) not su&stantially affected &y normal changes in draft
2) not easily damaged) decreasing the !idth of the ship" $
+) good efficiency driven &y lighter engine.
/ince then" propellers have dominatedin use of marine
propulsion.
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Paddle Wheels Propulsion (Stern)
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Paddle Wheels Propulsion (idship)
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Propeller (!-"lade)
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Propeller (!-"lade)# $udder
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%et t&pe: ater is dra!n &y a pump $ delivered stern!ards as a
et at a high velocity. The reaction providing the thrust. %t1s use
has &een restricted to special types of ships.
'ther propulsion Devices:
1. oles (Duct) Propellers*main purpose is to increase thethrust at lo! ship speed (tug" large oil tan#er)
+. Vertical-,is Propellers: ,dvantage is to control the direction
of thrust. Therefore" the ship has good maneuvera&ility.
. Controlla"le-Pitch Propellers (CCP): The pitch of scre! can
&e changed so that it !ill satisfy all !or#ing conditions./. 0andem and Contra-rotatin Propellers: %t is used &ecause
the diameter of a propeller is restricted due to limit of the draft
or other reasons (torpedo). The efficiency of the propeller
usually decreases.
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%et Propulsion
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Vertical-,is Propellers
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Vertical-,is Propellers
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Controlla"le Pitch Propellers (CPP)
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Contra-rotatin Propellers
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0&pe of Ship achiner&
1. Steam 2nine(no longer used in common)
,dvantages: ) good controlla&ility at all loads" 2) to &ereversed easily" $ ) rpm (rotations per minute) matches
that of propellers
isadvantages*.) very heavy 2.) occupy more space
.) the output of po!er per cylinder is limited+.) fuel consumption is high
+. Steam 0ur"ine
,dvantages:
.) deliver a uniform turning tor3ue" good performance for large
unit po!er output" 2.) thermal efficiency is high.
isadvantages:
.) is nonreversi&le 2.) rpm is too high" need a gear &o4 to
reduce its rotating speed
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. Internal com"ustion enines (Diesel enine)
,dvantages: .) are &uilt in all sies" fitted in ships ranging from
small &oats to large super tan#ers" (less 00 hp 6 70"000 hp)
2.) 8igh thermal efficiency.isadvantages: .) 8eavy cf. gas tur&ines
+. 3as 0ur"ines (developed for aeronautical applications)
,dvantages: .) o not need &oiler" very light 2.) ffer continuous
smooth driving" $ need very short !arm; time.
isadvantages: .) e4pensive in cost and maintenance 2.) need a
gear unit to reduce rpm.
5. uclear reactors 4 tur"ine,dvantages .) do not need &oiler" fuel !eight is very small
2.) operate full load for very long time (su&marine)
isadvantages .) !eight of reactor and protection shield are heavy
2)
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Definition of Po5er
Indicated horsepo5er (PI)*is measured in the cylinders (/team
reciprocating engines) &y means of an instrument (an
indicator;) !hich continuously records the gas or steam
pressure throughout the length of the piston travel.
pm- mean effective pressure (psi)
L= >ength of piston stro#e (ft)n= num&er of !or#ing stro#es per second
A= effective piston area (in2)
n= num&er of cylinders
?550I mP p L A n=
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ra7e 8orsepo5er (PB)*is the po!er measured at the cran#shaft
coupling &y means of a mechanical hydraulic or electrical &ra#e.
!here Q= &ra#e tor3ue (l&-ft) $ n= revolutions per second.
Shaft horsepo5er (PS
)*is the po!er transmitted through the shaft
to the propeller. %t is usually measured a&oard ship as close to the
propeller as possi&le &y means of a torsion meter .
!here dS= shaft diameter (in)" G= shear modulus of elasticity of
shaft material (psi)" = measured angle of t!ist (degree)"
LS= length of shaft over !hich @ is measured $ n= revolution per
second
2 ? 550BP nQ=
( )+
"0
S
S
S
d G nP
bL
=
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Delivered horsepo5er (PD)*the po!er delivered to the propeller.
0hrust horsepo5er (PT)*
T= Thrust delivered &y propeller (l&)
VA= advance velocity of propeller (ft?s)
2ffective horsepo5er (PE 9 or 28P)*
RT= total resistance (l&)
Vs= advance velocity of ship (ft?s)
?550T AP T V=
?550E T sP R V=
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Propulsion 2fficienc&
0otal propulsion efficienc&
can also &e replaced &y or
, more meaningful measure of hydrodynamic performance
of a propeller is: a 3uasi-propulsive coefficient"
"
" !here is the shaft
ET S B I
S
D
ED
D
DS S
S
PP P P
P
P
P
P
P
=
=
= transmission efficiency
and thus" .
- 9*A for ships !ith main engine aft
- 9BA for ships !ith main engine amidship
- smaller if a gear &o4 is used.
T D S
S
=
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6.+ Propeller 3eometr& and 0erminolo&
Coss
Cac#
8u&cap
Dace
um"er of lades: 2" 9 /9 !"Eoss
8u"cap
Shaft
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0he face surface of a "lade is a portion of a holicoidal surface
0he helicoidal surface: Fonsidering a lineABperpendicular to
a lineAAand supposing thatABrotates !ith uniform velocitya&outAAand at the same time moves alongAA!ith uniform
velocity" the surface s!ept out &yABis a helicoidal surface.
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Pitch*P!hen the lineABma#es one complete revolution
and arrives atAB. %t traveled an a4ial distanceAA"
!hich represents the pitchof the surface. The propeller&lade is part of that surface and the pitch is also called the
pitch of the &lade.
Pitch anle tan or tan2 2
Gitch ratio: tan
P P
r rP PR
PRD
= =
= =
P
o
,
2 r
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p*0
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Developed ,reaAD
2pended ,reaAE
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oss*(a#a" 8u")
oss diameter= The &lades at their lo!er ends or roots are
attached to a &oss !hich in turn is attached to the propellershaft. 0he maimum diameter of this "oss is called the
"oss diameter . The &oss diameter is usually made as small
as possi"leand should &e no larger than the siesufficient
to accommodate the "lades and satisf&in the
re:uirement of strenth. %t is usually e4pressed as a
fractionof the propeller diameter.
,t one time propeller &lades !ere manufactured separately
from the &oss" &ut modern fi4ed pitch propellers have the&oss and &lades cast together. 8o!ever" in controlla"le
pitch propellersit is of course necessary for &lades and
&oss to &e manufactured separately.
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lade outline*it is decided &y propeller series diarams.
;2panded "lade outlineeading edge
Trailing edge
G* figure 0.5
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$a7e (a &lade isperpendicular or titled !.r.t the &oss )
S7e5 (the s#e!ness of a &lade !.r.t. the center line)
Pitch ratio
%n case that the pitch"P,is not constant" then the pitch is
defined asP = Ptip (the pitch at the tip of a propeller).
Clade area ratio HAD/A0
AD - Total (developed) &lade area clear of that of the &oss
PPR
D=
2
0 ? +A D=
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6. 0heor& of Propeller ,ction
,ssumptions*
) replacing the propeller !ith a stationary actuating dis# across
!hich the pressure is made to rise
2) neglecting the rotational effect of propeller
) neglecting vortices shed from the &lade tip" $ frictional loss.
DVA
VA(1+b)VA(1+a)
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omentum Conservation=orce > net momentum flu
(horiontal)
( )( ) ( )
( )
0
2
0
H H (mass conservation)
A A
A A f A a
A A
T Q V b V
Q a V A V A b V A
T QV b A V a b
= + = + +
= = +
2ner& 2:uation
( )
( )( )
222
2
2
0 0
0
2 2
2 " " 2
2 2
or
2 2
AA
A
A
V bV P
g g
b b VTA P T T A b b V
A g
b ba a
++ =
+ = = = +
+ = + =
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( )
( )
0
0
0
2
0
2 02 2
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2tension of momentum theor&
Fonsider the rotation of the flo! passing through the propeller
disc." the reduced ideal efficiency &ecomes"
2
2
2
I $ I 0.
2 I
2
!here is the rotation velocity of flo! after the propeller"$ is the rotation velocity of the propeller.
I
aa
a
a
= >
+ = =
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lade 2lement 0heor&%n the momentum conservation of a propeller" no detailed
information can &e o&tained !ith regard to the effects of the
&lade section shape on propeller thrust and efficiency.
( )
( )
The total velo. at radius " " 2 .
Thrust: cos sin
Jesistance: sin cos
Koment: $ and
is a function depending on section shape (!in
r A T T
T L D
F L D
F L D
r V V V V rN
d d d
d d d
q d r d d f
f
= + =
=
= +
=
r r r
( )
( )
g
section theory). Dor a propeller" the relative advance velocity
of the fluid at the disc" is $ the rotation velocity is
I .
AV a
r a
+
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aVA
LL1
( ) Ir a Ia r
rV r=
r
AV
a $ aare determined &y e4periments
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6./ Similarit& ?a5 for Propellers
,lthough theoretical studies and C=Don propellers are very
important and provides valua&le guideline for designing propeller"
a great deal of #no!ledge concerning the performance of propellers
has &een o&tained from propeller model tests. 8ence" it is
necessary to e4amine the relation "et5een model and full-scale
resultsas the case of resistance.%n open 5ater(not &ehind a ship)"
( )" " " " " "
- rotational speed" - diameter of propeller- pressure in !ater" - dynamic viscosity
- speed of advancing" - Thrust
A
A
T f D V g n p
n Dp
V T
=
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2 + 2
2
22
2 +
Msing .," the non-dimensinal formula is given &y"
" " "
Droude N: "
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%n open !ater" the propeller efficiency coeff.:
.2 2 2
hen all the dimensionless parameters are the same for the
t!o propellers" the t!o propellers
!ill &
A T A T!
Q Q
TV V J
nQ nD
= = =
1eometricall& similar
2
e .
/cale ratio:
Dor the same Droude N:
Dor the same advance ratio (most important)
indicating the model rotating faster.
s
m
As s
Am m
s As m
m Am s
DD
V D
V D
n V D
n V D
=
= =
= = =
d&namicall& similar
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2 2 2 2
Dor the same < = + a
vacuum (cavitation) tunnel.
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Dor the same Je: "
!hich is contradict to the similarity of Dr. Therefore" it is almost
impossi&le to satisfy the Dr $ Je similarity la!s simutanously.
/imilar to the assumption made
As m s
Am s m
V D $
V D $ = =
in model resisrtance tests" !e
assume viscous force is independent of other dynamic forces.
8ence" it may &e computed separately. %n reality" viscous force
is usually a small portion of the total force. The smilarity of
Je is neglected in propeller model tests.
Therefore" propeller model tests follo!s $ (advance
ratio) similarity la!s. %f the cavitation is relevant" then the
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6.! Propeller odel 0est, test on a model propeller is run either in a to!ing tan# or a
running flo! in a !ater tunnel (cavitation tunnel) !ithout a model
hull in front of it" !hich is called ;open 5ater< tests.
) VA= velo.of flo!
2.) n- rotation ofmotor
.)p!- pressure can
&e controlled
KeasureVA% Q% T%
andn&
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Development
of cavitations
of a propeller
in a
cavitation
tunnel
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Trust coeff Toe3ue coeffT Q
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Q
T
Testing results
0
AV
JnD
=
/lip ratio " Gitch ratio " section types $ N of &lades.AV P
nP D
= =
2 + 2 5Trust coeff. " Toe3ue coeff. "
.2 2
T Q
A T!
Q
Q
n D n D
TV J
nQ
= =
= ='pen- 5ater efficient
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Purpose of open-5ater tests
%t is usually to carry out open !ater tests on standard series of
propellers. Their features (such as N of &lades" &lade outline
shape" &lade area ratio" &lade section shape" &lade thic#ness
fraction" &oss diameter $ pitch-diameter ratio) are
systematically varied. The result data are summaried in a set of
particular diagrams" !hich can &e used for design purposes. e!ill study ho! to use these diagrams later for designing a
propeller.
/tudying the efficiency of a propeller and find a propeller !ith&etter efficiency
/tudying the e4tent and development of cavitations over a
propeller.