How to assess and evaluate the ships performance

Paper n. i

"HOW TO ASSESS AND EVALUATE THE SHIP'S PERFORMANCE"

B.J. Ostojic - TOTAL TRADING INTERNATIONAL S.A.

Lab. y. Scheesbouvktìdt

Ieckrische HoeschooI

Mt

NAV '88 - WEMT '88 SYMPOSIUM

ABSTRACT

Hall and propeller roughness have traditionally been "out of sight and out of the operator's mind." Once the ship was out of drydock, no more thought was given to what was happening to the hull below the waterline until the vessel was docked again and some more paint "slapped" on. Nowadays, this attitude can no

longer be accepted. Even an increase of 20 microns in average hull roughness can result in a 1 percent worsening of fuel consumption. Fuel costs are now frequently the largest element in total operating expenses.

Three main factors are responsible for the speed loss, or growth in power demand during the service of a ship:

- increasing ship's resistance

- decreasing water intake velocity due to increasing wake fraction - frictional losses due to increasing propeller blade roughness.

It is the objective of this paper to establish a calculation procedure

which provides detection of current and future trends in the ship's performance. This paper helps to minimize those cost components which are directly or in-directly exposed to and affected by the condition of the underwater hull and

propeller.

The study is based on careful analysis of numerous performance data which have been collected and evaluated during the past eleven years on board 30 ships. Thanks to the various number of service data received regularly from the ships and drydock paint suppliers, the author was in a position to analyse performance and to establish a vast number of coefficients and replicas. Therefore, if a

reliable estimate of the hull or propeller roughness is required, one has no al-ternative but to compute the various service figures by using the mathematical formulae which have been accurately developed with the aim of establishing a correlation between increased power and resistance. It is impOrtant to say that all analysed cases with roughened propeller have been carried Out Ofl ships with self-polishing paint applied or with an underwater hull surface free of foaling.

It has been estimated that approximately 350 million tons of fuel are burnt per year to propel the existing 500 million TDW of the world fleet. By saving only 6 percent of the world fleet's required fuel per annum, an estimated sum of 21 million tons of fuel could be left unearthed for the benefit of future

VESSEL' S REQUIRED PARTICULARS

Ship's Name Year Built

Dimensions:

Deadweight (DWT) in MT

Length overall (L) in metres

Machinery: Type Maximum rated BHP/RPM (NCR/RPM) Derated max. BHP/RPM (DNCR/DRPM) Continuous service BHP/RPM (CSR/CRPM) Propeller (F.P.): Diameter in metres

Pitch (WPITCH) mean in metres Nimber of blades

Propeller revolution at maximum rated BHP (RPM) Pitch selected for BRP/RPM (CSR/PRPM)

Blade finishing tolerances (R) in accordance with

Sea Trial Condition:

Speed laden/ballast (VST) in uiots

Sea trial power in BI-IP

Wetted area (S) in m2

Propulsive coefficient (fl)

Initial underwater hull roughness (KST) in microns

Sea water density (5)) in T/m3

Type of underwater paint applied

Asses

1.1

psessment 1. With Roughened Hull

Li Initial sea trial power increment computation due to initial hull roughness.

Ref. International Towing Tank Conference (ITTC 1978)

¿p

(%) =

0,0092 $:>.s.v3

[l05()

fL P -0,64

¿Vs=

10

-3

AVT = LV5

years (KM)

(KM/year)

Thus dropped service speed in relation to sea trial speed is:

Equation 1. Equation 2. (see Figure 1.) Equation 3. V ₌

_{VST - tVT}

(KM) or V =

VST - ¿Vs

years (KM) Equation 4.

where: = sea water density (T/m3)

V EVST

(KM)

P E

ST = initial sea trial power increment (%)

P E

ST = sea trial power (BHP)

K

E _KST initial hull roughness (microns)

S = wetted surface (m2)

Li = propulsive coefficient

1.2 After .. year(s) in service, speed (Ve) drop of ... KM is noticed.

Estimated speed loss per year could be calculated:

where: AVs could be indicated for:

- conventional paint as ¿VC0NV

A/B = constant as a function of DWT size

DWT (l0)

... MT

Figure 1.

1.3 What is the service power increment (P5) needed to maintain sea trial speed?

where:

CORRELATION BETWEEN DEADWEIGHT ANO SPEED LOSS

DUE TO HULL ROUGHENING ANO flECHAN/CAL DAllAGES

DEA 0 wEIGHr z jc (tir)

20 40 60 80 100 (20 /40 160 /50 200 ST i 'VS S

+P

ST

ST = power at sea trial (BHP) increased service power (BHP)

VST = sea trial speed

V5 = dropped service speed (KN)

x = exponent calculated from speed/power curve

Power increased in relation to sea trial is:

AP.

= P

i ST

or percentually expressed in relation to

ST is: A P.

=

100 ST

SEl.F-POLISHING PAIPIT (spi') (A V SP)

CoNvfNr.t SPP (REBLASTED HULL) (Av cosi') coNvEAfr,o,y4L

irnr

(AV CONy) (BRP) Equation 5. (BHF) Equation 6. 1 1 VST J

r

ed?

1.4 What is the estimated TJ/W hull roughness growth (AK.) causing the power

increment? i

From Equation 1., roughness may be expressed as:

where: Ks = (

l03,37flAP.

ST i + 0,6095 L /

K5 assumed service roughness (microns) .fl.. = propulsive coefficient

= power increased (%) ST = sea trial power (BHP) 5' = sea water density (T im3) S = wetted area (m2)

= dropped service speed (KN) L = length overall (m)

Equation 7.

Increased roughness AK. in comparison with sea trial roughness may be

expressed as: i

AK.

= K -

K

i S ST

FOST C

(PST

+

D)

(microns)

1.5 What is the fuel consumption increment (AFO1) due to roughened hull?

For basis sea trial fuel consumption corrected to actual calorific value,

the following equation may be used:

(MT/D) Equation 8.

where: CID = constant as a function of individual ship

ST = sea trial power (BI{P)

Increased fuel oil service consumption (FOs) would be:

FO5 = C

(5

+

D)

(MT/D)

where: C/D = constant as a function of individual ship

= increased service power

Thus increased AFO. consumption in relation to sea trial condition would

be expressed as:

AFO.

=

C. (s-ST)

i

or

FO.

FO (%) - i loo

FOST

1.6 Is the ship's roughened hull causing engine overload to attain designed

service RPM?

Propeller is designed to absorb ... Z power at CSR with ... % propeller RPM sea margin.

where:

The equation for the power absorption is:

RPM _CSR

P.A. (Z)

=

(

PRPM ) MCR

100

P.A. = power absorption (Z)

RPM = nominal revolution at MCR

PRPM = designed service revolution

CSR = continuous service rating (BHF)

MCR maximum power rating (BHF)

The equation for power absorption may be expressed for sea trial condition

as:

RPM

\3

ST

w

PRPM)

R

100

and for service condition as:

P.A. (%) = 7RPM

\3

_PS

\PRPM/

MCR

100

Thus increased service P.A. after sea trial would be:

( RPM \ PST) 100 P.A..

(Z)

=

PRPM)

i

Equation 10. Equation 11. Equation 12. Con Inc fol -

i:

-

i.

-e

s Pro: Ass 2.1 2.2

where: = increased service power

(BHF)

By plotting the figures in the propeller law curve, an individual ship

could be checked to determine whether the engine is mechanically overloaded 2.:

PM

onc1uding Reflections for Assessment 1.

Increased underwater hull roughness of approximately .... microns has caused the following performance deteriorations:

- increased engine load for about .... % to maintain sea trial speed

- involuntary speed loss of about .... KN at constant service power (load) - excess in fuel oil consumption for about .... MT/D ( .... %) to maintain

sea trial speed.

VT

- AVSP

years (KM)

where:

EVSP =

.... constant (KM/year)

ded 2.3 _After .... year(s) in service, what is the service power increment

(tP) to maintain trial speed?

Equation 14. Proposed action to be taken:

Alternative 1. drydocking maintenance

Alternative 2. underwater maintenance - hull cleaning o.

Assessment 2. With Roughened Propeller

2.1 The service power increment (P) computation due to increased propeller

roughness may be expressed as:

/ ¿fr

APsp

= 0,071 R5

(l-0,00352 R)

Equation 13.

.on

where:

SP (%) service power increment

R5p = service propeller roughness (microns PVA)

2.2 After ... year(s) in service, speed drop of ... KN is noticed.

Assuming that SP paint has been applied, estimated speed loss per year would be constant in correlation with deadweight (see Figure 1.):

tVSP

(KN/year)

Total estimated speed loss after ... years in service would be expressed

AP.

(%) = 0,26 . (year) + 0,03

iP

3 4 5 6 7

AGE OF SHIP (YEARS .SINCE LAUNCH)

Figure 2.

2.4 What is estimated blade roughness growth (

AR) causing power penalties?

= 15,43

AP.

(oo67

.

AP.

+ i)

ip iP iP SP = ST iP iP 0,071 .

AR.

(l-0,00352 .

AR. )

+ 100 Equation 15. (see Figure 2.) Equation 17. Equation 16.

(see Figure 3.)

where: = increased power since launch (70)

POWER PENALTIES AGAINST TI/-lE FOR TYPICAL RATES OF

PROPELLER BLADE SURFACE DETERIORATION

where: = increased power (%)

AR.

increased propeller roughness (microns PVA)

r

n 17.

= increased service power (BHF)

where: FOsp = service overconsumption (MT/D) = increased service power (BHF)

FOST sea trial consumption (MT/D)

CID = constant coefficient as a function of

individual ship PSP

increased propeller roughness (microns PVA)

5.

2.) 'ST

sea trial power (BHF)

POWER PENALTIES AGAINST PROPELLER 8LADE

SURFACE ROUGHNESS

G, O .0.

'-40

'u t t I 3,o. I 'u I I 'li o I

i4

I I I I-. 1,o '4, ¡ i I- -Lu t- l4J

lL

20 40 60 80 100 120 140 f60 les?

PFOPELLER BLADE ROUGHNESS R0('PVA)

Figure 3.

2.5 What is the fuel overcorisumption (L FO1) due to power increased? 16.

3)

Based on Eqiition 8., we may

express the following:

FOsp = C (Pep + D)

FOST = 's

+ D)

or

or

ST

2.6 Is the roughened propeller causing engine overload to attain designed service RPM?

Increased service power absorption (P.A.5) could be expressed as follows: Thus increased ¿FOconsumption in relation to sea trial condition would be

expressed as:

EFO.

= FO5p _- FO51

iF = C

(s

-

_PST) iP ¿ F0.

AFOP

(70) = F0 iP (MIlD) Equation 18. IRPM \ P.A.sp ( = PRPM) R 100

Thus increased P.A. after sea trial would be:

(RPM \ 3 (

_s1)

loo

P.A..

(%) = \PRPM)

P

Equation 19. Equation 20.

Concluding Reflections for Assessment 2. 3.

Increased propeller blade surface roughness of approximately .... microns has

caused the following performance deteriorations:

- increased engine load for about .... % to maintain sea trial speed

- involuntary speed loss of about .... KM at constant service power (load)

- excess in fuel oil consumption for about .... MT/D to maintain sea trial speed.

Proposed action to be taken:

- underwater brushing and polishing of propeller.

Assessment 3. With Permanently Roughened Hull

i 18.

lows:

n 19.

n 20.

speed.

Speed drop could be calculated as follows:

or permanent speed drop in relation with sea trial condition is:

or

VS cos

=

+ F . 10

-3

AVT

=

AVCOSP

. years

V _{VST - VT} (Ku) (KN) = VST -

tVC0SP

years

tRPM \3

_PER P.A. (%) = PER \PRPM1 MCR (KNiyear) Equation 21. (see Figure 2.) Equation 22. (KN) Equation 23. Equation 24.

ud be

and SPC-ed. It is important to say that the ship will never reach the

initial hull smoothness as it had on the sea trial. Therefore speed drop or power increased should be considered as a permanent deterioration in

comparison with the sea trial condition.

where: VC0SP = speed loss for combined conventional and SP

paint application and reblasted hull

ElF = constant as a function of DWT size

Thus total speed loss over .... years would be:

3.2 Does the service propeller with fixed pitch have to be modified due to

has

permanent performance deteriorations?

Permanent P.A. increment could be expressed (as per Equation 10. or 11.)

as follows:

where: P.A.PER = permanent increase of P.A.

PPER = permanent increase of service power

Permanent increase of service power could be calculated by using Equation 5. as follows:

3.3

Thus actual propeller pitch modification could be written:

NWPITCH = WPITCH

1+0,5

(P.A.)

100 Equation 27. (BHP) Equa t ion 25. =

. [

f

vp\x1

ST ST

VST )

j

where: =

permanent speed drop (KM)

3.4

VST =

sea trial speed (KM)

ST = sea trial power (BHP)

In the case where P.A. level is continuously on the high side, which can be checked against propeller law curve, it is perfectly clear that the working

propeller has become too heavy. Therefore propeller pitch has to be reduced, Propeller reconditioning.

Reconditioning of a propeller with fixed pitch is advisable if, in the broadest sense, there exists a discrepancy between the current point of operation of the propeller and the point for which it has been designed. This may occur for a variety of reasons:

- if the propeller is too light, it will be impossible to develop full power of the engine because of overspeed. If the propeller is too heavy,

the engine will be overloaded at high power.

- during the lifetime of a ship, the need for sailing at a lower speed may arise either with the aim to save fuel or after having entered a different

type of service. For a fixed pitch propeller it can be, in some cases, economically advantageous to alter the power absorption.

The cure for improper power absorption can be achieved as follows: - The establishment of proper power absorption to a level close to the

designed figures:

¿P.A.

= .:t

%

Changing the pitch equation may be written:

APITQ

(7e) = 0,5 .

(P.A.)

Equation 26.

F ir

where: positive sign means altered pitch be

and negative sign means reduced pitch

is

n 25. 3.4 where: Effect tions. Thus where: Percentual

NWPITCH = new modified pitch (m)

WPITCH = working propeller pitch

+AP.A. = predicted reduction/alteration

power absorption

of propeller pitch modification on the change

change of propeller revolution may be expressed (m) of of propeller revolu-as: Equation 28. can be orking reduced. e of ed. i heavy, d may f fe rent ses, e RPM (%) = 0,7142

(+PIT)

actual propeller revolution change could be written:

NRPM =

SRPM - PRPM

0,7142

(APIT)

_{Equation 29.} 100 NRPM * _SRPM

PPM

..ttPITCH

*SPpM

=

PRPM+

= (+) increased (-) decreased = actual service = designed RPM pitch was = percentual (t _{RPM PRPM)} selected change propeller RPM for which of pitch speed working propeller (%) Equation 30. 100 SRPM = PRPM 1 +

(RPM)

100

where:

ARPM =

% (see Equation 28.)

Finally, it should be mentioned that a computer-aided system of this study will be available in the near future. The software is currently in production and

is designed to be loaded on any machine using the MS-DOS or PC-DOS operating

Mr. Branko J. OSTOJIC received an education in Naval Architecture from the Department of Shipbuilding Engineering at the University of Split, Yugoslavia.

He worked eight years for CARGILL GESTION S .A., Geneva (formerly TRADAX)

in the Ocean Transportation Division. During that period Mr. Ostojic's primary ac-tivities included the following areas:

- Monitoring and analysing the vessel performance of CARGILL's fleet,

particularly the main engines, speed, and power absorption.

- Studying the use of various types of paints for the coating of the vessels' hulls and their effect on extending maintenance of underwater

parts. This study also encompassed possible modification of

the

pro-peller in order to optimize the main engine performance.

Mr. Ostojic is presently employed by TOTAL TRADING INTERNATIONAL S.A. in Geneva,

Switzerland.

Professional Address Residence

TOTAL TRADING INTERNATIONAL S . A. Route de Sauverny i

Case Postale 509 CH-1290 Versoix-GE

CH-1211 GENEVA 24 SWITZERLAND

SWITZERLAND