Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines
Подождите немного. Документ загружается.
148 Performance
high reliability) allow the ship’s engineer to accurately assess the performance
of the engine at any time.
The values of bhp, mean indicated pressure and revolutions per minute are,
of course, capable of mutual variation within reasonable limits, the horsepower
developed per cylinder being the product of mean indicated (or effective) pres-
sure, the revolutions per minute and the cylinder constant (based on bore and
stroke). The actual maximum values for horsepower and revolutions to be used
in practice are those quoted by the enginebuilder for the given continuous serv-
ice rating.
ProPeLLer sLiP
The slip of the propeller is normally recorded aboard ship as a useful pointer to
overall results. While it may be correct to state that the amount of apparent slip
is no indication of propulsive efficiency in a new ship design, particularly as a
good design may have a relatively high propeller slip, the daily variation in slip
(based on ship distance travelled compared with the product of propeller pitch
and revolutions turned by the engine over a given period of time) can be symp-
tomatic of the changes in the relationship of propulsive power and ship speed;
and slip, therefore, as an entity, is a useful parameter. The effects on ship speed
‘over the ground’ by ocean currents is sometimes considerable.
For example, a following current may be as much as 2.5 per cent, and
heavy weather ahead may have an effect of more than twice this amount.
ProPeLLer Law
An enginebuilder is at liberty to make the engine mean pressure and revolu-
tions what he will, within the practical and experimental limits of the engine
design. It is only after the maximum horsepower and revolutions are decided
and the engine has been coupled to a propeller that the propeller law operates
in its effect upon horsepower, mean pressure and revolutions.
shp varies as V
3
shp varies as R
3
T varies as R
2
P varies as R
2
where
shp aggregate shaft horsepower of engine, metric or imperial
V speed of ship in knots
R revolutions per minute
T torque, in kg m or lb ft Pr
P brake mean pressure, in kg ft cm
2
or lb ft in
2
r radius of crank, in m or ft
If propeller slip is assumed to be constant:
shp KV
3
where K constant from shp and R for a set of conditions.
But R is proportional to V, for constant slip,
∴ shp K R
1
3
where K
1
constant from shp and R for a set of conditions.
But
shp
imperial
p A c r R
K R
2
33000
1
3
π
( )
When A aggregate area of pistons (cm
2
or in
2
), c 0.5 for two-stroke,
0.25 for four-stroke engines, or
T PAcr
R
K R K R
33000
2
1
3
2 2
π
( )imperial
or
T PAcr
R
K R K R
4500
2
1
3
2 2
π
( )metric
i.e.
2
T K R
2
where K
2
constant, determinable from T and R for a set of conditions.
PAcr T or
T
Acr
K
Acr
R
2
2
i.e.
3
P K R
2
where K
3
constant determinable from P and R for a set of conditions.
The propeller law index is not always 3, nor is it always constant over the
full range of speeds for a ship. It could be as much as 4 for short high-speed
vessels, but 3 is normally satisfactory for all ordinary calculations. The index
for R, when related to the mean pressure P, is one number less than that of the
index for V.
Propeller law is most useful for enginebuilders at the testbeds where engine
loads can be applied with the dynamometer according to the load and revolu-
tions calculated from the law, thus matching conditions to be found on board
the ship when actually driving a propeller.
Propeller law 149
150 Performance
FueL CoeFFiCient
An easy yardstick to apply when measuring machinery performance is the fuel
coefficient:
C
D V
F
2 3 3/
where
C fuel coefficient
D displacement of ship in tons
V speed in knots
F fuel burnt per 24 h in tons
This method of comparison is applicable only if ships are similar, are run
at approximately corresponding speeds, operate under the same conditions and
burn the same quality of fuel. The ship’s displacement in relation to draught is
obtained from a scale provided by the shipbuilders.
aDMiraLty Constant
C
D V
2 3 3/
shp
where C Admiralty constant, dependent on ship form, hull finish and other
factors.
If C is known for a ship, the approximate shp can be calculated for given
ship conditions of speed and displacement.
aPParent ProPeLLer sLiP
Apparent slip (%)
P R V
P R
101 33
100
.
where
P propeller pitch in ft
R speed of ship in knots
101.33 is one knot in ft/min
The true propeller slip is the slip relative to the wake stream, which is
something very different. The engineer, however, is normally interested in the
apparent slip.
ProPeLLer PerForManCe
Many variables affect the performance of a ship’s machinery at sea, so the only
practical basis for a contract to build to a specification and acceptance by the
owner is a sea trial where everything is under the builder’s control. The margin
between the trial trip power and sea service requirements of speed and loading
must ensure that the machinery is of ample capacity. One important variable on
the ship’s performance is that of the propeller efficiency.
Propellers are designed for the best combinations of blade area, diameter,
pitch, number of blades, etc., and are matched to a given horsepower and speed
of propulsion engine; and in fact each propeller is specifically designed for the
particular ship. It is important that the engine should be able to provide heavy
torque when required, which implies an ample number of cylinders with abil-
ity to carry high mean pressures. However, when a propeller reaches its limit of
thrust capacity under head winds, an increase in revolutions can be to no avail.
In tank tests with models for powering experiments the following particu-
lars are given.
Quasi-Propulsive Coecient (Q )
Q
model resistance speed
2 torque rev/min
work got out per
π
min
work put in per min
total shaft horsepower at Propeller (ehP)
EHP
ehp
p
Q
where
EHP effective horsepower for model as determined by tank testing;
p increase for appendages and air resistance equal to 10–12 per cent of
the naked model EHP, for smooth water conditions.
The shp at the propeller for smooth sea trials is about 10 per cent more
than that in tank tests. The additional power, compared with sea trials, for sea
service is about 11–12 per cent more for the South Atlantic and 20–25 per cent
more for the North Atlantic. This is due to the normal weather conditions in
these areas. The size of the ship affects these allowances: a small ship needs
a greater margin. By way of example, 15 per cent margin over trial conditions
equals 26.5 per cent over tank tests.
The shp measured by torsionmeter abaft the thrust block exceeds the EHP
by the power lost in friction at the sterntube and plummer blocks and can be as
much as 5–6 per cent.
The bhp exceeds the torsionmeter measured shp by the frictional power lost
at the thrust block. Bhp can only be calculated onboard ship by multiplying
the recorded indicated horsepower by the mechanical efficiency stated by the
enginebuilder.
Required bhp for engine EHP sea margin hp lost at sterntube and
plummer blocks hp lost at thrust block.
Propeller performance 151
152 Performance
Typical values for the quasi-propulsive coefficient (Q) are as follows:
tanker 0.67–0.72, slow cargo vessel 0.72–0.75, fast cargo liner 0.70–0.73, ferry
0.58–0.62 and passenger ship 0.65–0.70.
Power buiLD-uP
Figures 5.3 and 5.4 are typical diagrams showing the propulsion power data for a
twin-screw vessel. In Figure 5.3 curve A is the EHP at the trial draught; B is the
EHP corrected to the contract draught; C is the shp at trial draught on the Firth
of Clyde; D is the shp corrected to the contract draught; E is the power service
curve from voyage results; F shows the relation between speed of the ship and
engine revolutions on trials and G is the service result. The full-rated power of
the propelling engines is 18 000 bhp; the continuous service rating is 15 000 bhp.
In Figure 5.3 curves A to D show shp as ordinates and the speed of the ship as
abscissae. In Figure 5.4 powers are shown as ordinates, revolutions as abscissae.
Speed in knots
16 17 18 19 20 21
80
0.60
0.70
rev/min ehp/shp
shp Service
E
A
B
shp
10000
rev/min Service
G
Contract draft
F
D
C
Contract draft
T
o
r
s
i
o
n
m
e
t
e
r
s
h
p
t
r
i
a
l
d
r
a
f
t
E
q
u
i
v
.
s
h
p
t
r
i
a
l
c
o
n
t
r
a
c
t
e
h
p trial
d
r
a
f
t
e
hp
c
o
n
t
r
a
c
t
d
r
a
f
t
rev/min Trial
110
100
90
8000
12000
14000
16000
18000
Trial draft
Figure 5.3 Propulsion data
Figure 5.5 shows the relationship between revolutions, power and brake
mean pressure for the conditions summarized in Figures 5.3 and 5.4. A fair line
drawn through the observed points for the whole range shows the shp to increase
approximately as the cube of the revolutions and the square of the bmep. For
the range 95–109 rev/min, the index increases to 3.5 for the power and 2.5 for
the bmep. Between 120 rev/min and 109 rev/min, a more closely drawn curve
shows the index rises to 3.8 and the bmep to 2.8. In Figure 5.5 ordinates and
abscissae are plotted to a logarithmic base, thus reducing the power/revolution
and the pressure/revolution curves to straight lines, for simplicity.
traiLing anD LoCking oF ProPeLLer
In Figure 5.6, the normal speed/power curves for a twin-screw motor vessel on
the measured mile and in service are shown.
The effect upon the speed and power of the ship when one of the propellers
is trailed, by ‘free-wheeling’, is indicated in the diagram. The effect of one of
the propellers being locked is also shown.
Figure 5.7 shows the speed/power curves for a four-screw motorship under
the following conditions:
1. When all propellers are working
2. When the vessel is propelled only by the two centre screws, the outer
screws being locked
3. When the ship is being propelled only by the two wing screws, the two
inner screws being locked.
trailing and locking of propeller 153
4000
80 90 100 110
rev/min
Torsionmeter shp
trial draft
shp Endurance
trial at 21′–10
1
/2′′
draft
C
D
E
shp service
Equiv. shp trial
contract draft
Trial draft = 21′–0′′F
22′–9′′A
= 21′–10
1
/2′′ mean
contract draft = 26′–9′′ mean
18 000
6000
8000
10 000
12 000
14 000
16 000
shp
Figure 5.4 Propulsion data
154 Performance
astern running
Figure 5.8 summarizes a series of tests made on the trials of a twin-screw pas-
senger vessel, 716 ft long, 83 ft 6 in beam, trial draught 21 ft forward, 26 ft aft,
26 000 tons displacement.
17 000
16 000
15 000
14 000
13 000
12 000
11 000
10 000
shp →
90 95 100 105 110 115
rev/min →
5.5
5.0
Slope = 3.8
shp
Slope = 3.5
Slope = 2.5
+
+
+
bmp
Slope = 2.8
+
bmp (kg/cm
2
)
3.5
6.0
4.5
4.0
Figure 5.5 engine trials: power, revolutions and mean pressure
Service results
One engine in use
(One shaft locked)
One engine in use
(One shaft freewheeling)
Measured mile results
10 000
B.H.P.
8 10 12 14 16 18
Speed in knots
0
8000
6000
2000
4000
Figure 5.6 speed/power curves of a twin-screw vessel
As plotted in Figure 5.8, tests I to VI show distances and times, the speed
of approach being as stated at column 2 in Table 5.1. The dotted curves show
reductions of speed and times.
astern running 155
16 000
14 000
12 000
10 000
8000
6000
4000
2000
0
6 8 10 12 14 16 18 20
Speed in knots
Two wing screws only
B.H.P.
Four screws working
Two centre screws only
Figure 5.7 speed/power curves of a quadruple-screw ship
0 2 4 6 8 10 12 14
Time in minutes
0
I
II
VI
III
A
B
F
C
D
E
V
Stop
Nautical miles
16
16
18
18
20 20
10
10
8
0.8
6
0.6
4
0.4
2
0.2
12
12
14
14
IV
Speed in knots
Figure 5.8 ship stopping trials
156 Performance
The dotted curves A to F respectively correspond to curves I to VI. In test
I, after the ship had travelled, over the ground, a distance of two nautical miles
(1 nm 6080 ft), the test was terminated and the next test begun.
In Table 5.2 a typical assortment of observed facts related to engine stop-
ping and astern running is given. Where two or three sets of readings are given,
these are for different vessels and/or different engine sizes.
Trials made with a cargo liner showed that the ship was brought to rest
from 20 knots in 65 s. Another cargo liner, travelling at 16 knots, was brought
to a stop in a similar period. The engine, running full power ahead, was
brought to 80 rev/min astern in 32 s and had settled down steadily at full astern
revolutions in 50 s.
A shipbuilder will think of ship speed in terms of the trial performance in
fair weather, but to the ship owner ship speed is inevitably related to scheduled
performance on a particular trade route. Sea trials are invariably run with the
deadweight limited to fuel, freshwater and ballast. Because of the difference
between loaded and trials draught, the hull resistance may be 25–30 per cent
greater for the same speed. This has a consequential effect upon the relation
between engine torque and power, and in the reaction on propeller efficiency.
Adverse weather, marine growth and machinery deterioration necessitate a
further power allowance, if the service speed is to be maintained. The mean
wear and tear of the engine may result in a reduction of output by 10–15 per
cent, or a loss of speed by up to one knot may be experienced.
table 5.1 ship stopping trials
1 2 3 4 5 6
test no. ahead speed
of approach
knots
(rev/min)
Propellers Propellers
stopped
(min)
ship
stopped
(min)
Distance
travelled
(nautical
Miles)
P.
s.
i 23.0 (119) trailing; unlocked 16.4 14.9 — 2.0
ii 14.5 (75) trailing; unlocked 12.5 12.1 17.0 1.4
iii 13.5 (75) trailing; locked 1.5 1.5 13.0 0.8
iv 15.0 (75) ahead running
checked; no
additional astern
power
1.3 1.3 5.2 0.5
v 14.8 (75) engine stopped;
astern as quickly
as possible
0.7 0.8 3.1 0.4
vi 22.4 (116) trailing; unlocked 1.5 1.6 15.0 1.1
When selecting a propulsion engine for a given ship, a suitable power
allowance for all factors such as weather, fouling, wear and tear, as well as the
need to maintain the service speed at around 85 per cent of the maximum con-
tinuous rating, should all be taken into consideration. (C.T.W.)
astern running 157
table 5.2 engine reversing and ship stopping
ship engine
type
ahead
(rev/
min)
time for
engine
stopping
(s)
engine
Moving
astern
(s)
astern
running
ship
stopped
rev/
min
s min s
Large
passenger
D.a.
2C.
(twin)
65 — 30 — — 4 2
small fast
passenger
s.a.
2C. tr.
(twin)
217 53 63.5 160 72.5 2 15
195 35 45 160 60 1 54
220 45 59 170 81 2 5
Passenger Diesel
electric
(twin)
92 110 — 80 225 5 0
Cargo
s.a. 2C.
(twin)
112 127.5 136 100 141 —
Cargo
D.a.
2C.
(single)
90 24 26 90 115 2 26
88 12 14 82 35 3 25
116 35 40 95 50 —
Cargo
s.a. 2C.
(single)
95 31 33 75 45 —
110 12 21 110 53 3 21
116 10 55 110 110 4 6