4. BASIC OUTLINES OF PROPELLER DESIGN Propellers are designed to absorb minimum power and to give maximum efficiency, minimum cavitation and minimum hull vibration characteristics. The above objectives can be achieved in the following stages: 1. Basic design 2. Wake adaptation 3. Design analysis
a) Propeller Design Basis The term propeller design basis refers to: i. ii. iii.
Power Rotational speed Ship speed.
that are chosen to act as the basis for the design of the principal propeller geometric features. Resistance and Power Estimation
A ship owner usually requires that the ship will achieve an average speed in service condition (fouled hull in full displacement and rough weather), V SERVICE, at a certain engine power. Initial acceptance will be the basis of demonstration of a higher speed on trial condition (clean ship usually in light displacement), V TRIAL, at some power, i.e. V TRIAL = V SERVICE + δ V
where δV is the speed increase due to the fouled hull, rough weather and other effects, and is usually taken as 1 knot.
P ESERVICE = (1 + x) P ETRIAL
where (1+x) is the sea margin that the ship resistance is increased usually by 10 to 20% in average service conditions.
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b) The Use of Standard Series Data in Design Propeller design diagrams of Standard Series, such as Wageningen-B, can be used in five design options as: Known variables 1. Known power (P D), rotation rate (N) and advanced velocity (V A)
Required Dopt
In this case the unknown variable optimum diameter, D opt, can be eliminated from the 5 5 diagrams by plotting KQ /J versus J instead of K Q vs J. KQ /J can be written as: K Q J
5
3
Q
ND 5 QN = ( ) = ρ N 2 D 5 V A ρ V A5
since P D = 2π QN K Q J
5
=
P D N 2
2πρ V A5
This parameter Bp is plotted against
1 J
≈
NP D1 / 2
= δ =
2.5
V A
ND V A
= B p
and these diagrams are called Bp-δ Bp- δ
diagrams. On these diagrams optimum η0 and δ are read off at the intersection of known Bp value on the optimum efficiency line. D opt is then calculated. 2. Known power (P D), diameter (D) and advanced velocity (V A)
Nopt (required) 3
If the delivered power PD and the diameter D are known a diagram of K Q /J can be obtained. K Q J 3
=
P D ND 3 QN ( ) = = ρ N 2 D 5 V A ρ D 2V A3 2πρ D 2V A3 Q
From these diagrams N opt is calculated on the optimum efficiency line. 3. Known thrust (T), diameter (D) and advanced velocity (V A)
Nopt(required)
If the resistance values of the ship are available, thrust T can be calculated using the thrust deduction factor t: T= RT /(1-t) 2
T and D are known optimum rate of rotation Nopt can be eliminated using KT /J . K T J 2
=
T
ND 2 T ( ) = ρ N 2 D 4 V A ρ V A2 D 2
2
4. Known thrust (T), rotation rate (N) and advanced velocity (V A)
Dopt(required)
T and N are known and optimum diameter D opt can be eliminated similarly using KT /J4. ND 4 TN 2 = ( ) = J 4 ρ N 2 D 4 V A ρ V A4 K T
T
5. Determination of Optimum RPM and Propeller Size (Diameter) (General Case)
To determine the propeller diameter D and rate of rotation N for a propeller when absorbing certain delivered power P D in association with the ship speed V S. i) ii) iii) iv) v) vi)
Propeller type is chosen depending on the ship type, initial installation coast, running costs, maintenance requirements. Number of blades is determined by the need to avoid harmful resonant frequencies of the ship structure and the machinery. BAR is initially determined First it is necessary to determine a mean design Taylor wake fraction (w T) from experience, published data or model test results. r esults. Advance propeller speed V A can be determined as V A=(1-wT)VS Diameters of behind hull and open water are calculated as D max is assumed to be usually % of the draught Dmax = D B = aT
a<0.65 for bulk carriers and tankers a<0.74 for container ships where DB and T are the behind hull diameter and draught of the ship, respectively.
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When the diameter is determined the diameter should be as large as the stern of hull can accommodate, to obtain the maximum propeller efficiency. The typical figures of the clearances of propeller-hull, propeller-rudder and propeller-baseline should be: X Y Z
5% to 10% of D 15% to 25% of D up to 5% of D vii)
Open water diameter D0 is then calculated by increasing the D B by 5% and 3% for single and twin screws respectively. D0 =
D B
1 − 0.05
for single screw propellers
viii)
This diameter D0 should absorb the delivered power for trial condition at the optimum RPM which would correspond to the maximum propeller efficiency.
ix)
From the Power-Speed diagram (PE vs. VS) PETRIAL is read off at the VSTRIAL
12000
10000
8000
) W k ( 6000 E P
Trial Condition Service Condition
4000
2000
0 12
13
14
15
16
17
18
19
20
VS (Knots)
x)
Propulsive efficiency ηD is obtained by iteration for the optimum RPM, N. Initially ηD is assumed (i.e. ηD(assumed)=0.7) and the delivered power P D is calculated as: P D (TRIAL) =
xi)
P E (TRIAL)
η D ( assumed )
Bp and δ values are calculated for a range of N (e.g. N=80 to 120)
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xii)
From the Bp-δ diagram open water propeller efficiency, η0 is read off at the corresponding Bp-δ (D0)
xiii)
N (RPM) vs. η0 diagram is plotted and η0max and N values are read off from the diagram
0.624 0.622 0.62 0.618 0.616 0
0.614 0.612 0.61 0.608 0.606 0.604 80
90
100
110
120
130
N (RPM)
xiv)
ηD is calculated η D ( calculated ) = η H η Rη 0 =
xv)
1 − t 1− w
η Rη 0
The difference between the ηD(calculated) and ηD(assumed) is calculated as: ε = η D ( calculated ) − η D ( assumed )
iterate if required if ε if ε value > εthreshold, go back to step (x) and assume ηD(assumed)=ηD(calculated) if ε if ε value ≤ εthreshold, ηD(calculated) is converged and ηD is the latest calculated ηD(calculated) xvi)
Based upon the latest value of η of ηD break power in trial condition P B(TRIAL) is calculated as: P B (TRIAL) =
xvii)
P E (TRIAL)
η Dη S
=
P D
η S
Installed Maximum Continuous Power is taken as =
5
P B (TRIAL)
0.85
xviii) PD = PBηS NP D1 / 2
B p = 1.158
xx)
PB /DB is read off at the calculated (B p, δB) from the Bp-δ diagram and the mean face pitch is calculated.
2.5
V A
and δ B = 3.2808
ND B
xix)
V A
are calculated
3. Engine Selection
xxi)
We have optimum RPM (latest), brake power in trial condition P BTRIAL and installed maximum continuous power. For engine selection refer t o engine layout diagrams provided by the manufacturers.
4. Prediction of Performance in Service
Prediction of the ship speed and propeller rate of rotation in service condition with the engine developing 85% of MCR
5. Determination of Blade Surface Area and BAR (Cavitation Control)
Cavitation control is carried out for trial condition. This is due to fact that the ship will have the maximum speed in trial condition If the calculated BAR is less than the selected BAR, the design stage is completed. If the calculated BAR is greater than the selected BAR, a new BAR greater than the calculated BAR is chosen. All the calculations are performed for the new BAR.
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