Speed Power Pred

1Institutt for marin teknikk

Making speed-power predictionsfrom model tests

Sverre Steen


ITTC57 Correlation Line

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10

Reynolds number Rn

I

T

T

C

'

5

7

F

r

i

c

t

i

o

n

l

i

n

e

C

F


Friction lines (formulas to calculate the frictional coefficient)Turbulent flow


Scaling of Resistance

Measured resistance of model

Viscous resistance, model

Air resistance, model

Residuary resistance, model =

=

-

-Correlation allowance

Viscous resistance, ship

Air resistance, ship

Residuary resistance, ship

=

+

+

Total Resistance, ship

+

= Calculated from empirical formulas


Ship Resistance ScalingTransom stern dragAir resistance

Model scale

resistance components

RsBDmAAmFmoTmRm CCCCkCC =+= )1(

Residual resistance model Residual resistance ship=

BDsAAsAoFFsRmTs CCCkCCCC ++++++= )1()(

Measured model resistance

Viscous resistance

Full scale resistance Viscous resistance Air resistance

Transom stern dragCorrelation coef. Full scale resistance com

ponents


Calculated resistance components

mmm

TmTm

SV

RC

=2

2 Total resistance coef., model

Air resistance coefficient

Transom stern resistance

Appendage resistance

SACAA T001.0 = CD0.8

2/1

2/3

)()/(029.0

F

BBD C

SSC =


Viscous Resistance

Frictional Resistance

Form factor

Roughness allowance

2)2(log075.0= nF R

C

[ ] 221.0 33.403)(31.110 FssF CVHC =

(ITTC57)


Determining the form factor

When wave resistance, air resistance, and base drag is subtracted from total resistance, you are left with viscous resistance

How can the form factor (1+k) be determined? By running at low speed so that CR0 (typically Fn=0.1) By using Prohaskas method By using an empirical method

(1 )Tm o Fm AAm R BDmC k C C C C= + + + +

10

Institutt for marin teknikk

Prohaskas metode for finne formfaktor

y = 62.981x + 1.251

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005

Fn4/CF

C

T

/

C

F

(1+k)=1.251

11


Prohaskasmethod

The exponent for Fnis chosen so that thedata points fall on a line that is as straight as possible

The exponent shouldbe in the order of 2-9

12


MARINTEK Form Factor Based on a regression, instead

of measurements on each model

Intentionally excludes viscous pressure resistance, since pressure resistance should be scaled as wave resistance

Form factor30.6 75ok = +BTT

LC

FPAPWL

B += )(where

13


Correlation Coefficient CA

Accounts for systematic errors in the scaling method Derived from analysis of full scale speed trials -0.15E-03 CA -0.3E-03 for conventional ships. Value depend

on stern shape and appendix arrangement It is important to get access to full scale trial results of high

quality to maintain a good correlation!

14


Propulsion Test

Dynamometer

Tow rope FDMeasurement of:Torque QThrust TRate of revolutions n

42 DnTKT =

52 DnQKQ =

Thrust coefficient

Torque Coefficient

15


Open Water Test

42 DnTKT = thrust coefficient

52 DnQKQ = torque coefficient

2=

Q

TO K

JK propeller efficiency in open water

V

Measurement of:Torque QThrust TRate of revolutions nSpeed V

10*KQ

Efficiency

KTKT

,

1

0

*

K

Q

DnV

J A=Advance number

16


Analysis of Propulsion Test

Wake fraction:

DnVJw O

=1

Relative rotative efficiency:Q

QOR K

K=

Hull efficiency:wt

H =

11

Quasi-propulsive coefficient: RHOD =

Thrust deduction fraction:TFRt DT =1

Results:

KQ0

Advance numberto find J

0

Enter with KT from propulsion test

10*KQ

Efficiency 0

KTKT

,

1

0

*

K

Q

DnV

J A=

Open water diagram:

17


Performance Prediction

R and thrust deduction t are assumed free of scale effects Wake of single-screw vessels is scaled according to:

The full scale propulsion point J* is found from solving the equation:

Fm

FFsomos C

CCwwww ++= )( two += 04.0where

2222 )1()1( ssTsT

wVDtR

JK

= From towing test

From open water test

From propulsion test

18


Performance Prediction (cont.)

*)1(60JV

DwRPM ss =

R

QD

KRPMDkWP = 35 )

60(

10002)(

Rate of revolutions

Delivered power

Brake power

This KQ is found from the full scale open water diagram for J

M

DB

PkWP =)(

A procedure for powering prediction is given in Annex E in the lecture note

19


Load-varied propulsion tests British method

Thrust T, torque Q and propeller speed n in model scale is known as functions of the tow rope force FD

Interpolate (or extrapolate linearly) to find the model resistance:RTM=Thrust when FD=0

Calculate correct FD for each speed and find actual values of Thrust T, Torque Q, and propeller speed n. From here on the procedure is the same as for the continental

method

20


Multiple-screw propulsion

If the propulsors are equal: use average values of thrust and torque when calculating propulsive factors and determining the propulsion point

If the propulsors arent equal: do a separate analysis of each propulsor, finding its full scale RPM and power Problem: special tests are generally required to determine the part

of the resistance carried by each propellerPossible solution: make an assumption about how the thrust deduction is distributed between the propulsors

Example: A double-ended ferry using both forward and aft propulsors during transit.The forward propulsor will have much higher thrust deduction that the aft propulors. Tests running each propulsor separately can be used to determine the thrust deduction of each unit

Making speed-power predictions from model testsITTC57 Correlation LineFriction lines (formulas to calculate the frictional coefficient)Scaling of ResistanceShip Resistance ScalingCalculated resistance componentsViscous ResistanceDetermining the form factorProhaskas metode for finne formfaktorProhaskasmethodMARINTEK Form FactorCorrelation Coefficient CAPropulsion TestOpen Water TestAnalysis of Propulsion TestPerformance PredictionPerformance Prediction (cont.)Load-varied propulsion tests British methodMultiple-screw propulsion

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Speed Power Pred