Factors currently used at NPL to predict the performance of single screw ships on measured mile trials

(1)

FACTORS CIIRRTLY US

AT NPL TO PREDICT THE PERYO RMANCE OF SINC-LE-SCRiiW SH]PS ON i1EASURED-MILE TRIALS

by

B.S. Bowden

In this note the present practice at NPL

for estimating the performance

of single-screw ships on measured-mile trials is briefly described and. the

values of the prediction factors currently used are given.

In March 1965, the British Towing Tank Panel agreed on

a standard procedure

for the prediction of ship performance from the results of resistance and

pro-pulsion experiments with ship models;

this procedure is published in reference

(i).

The results derived from the analysis of the model experiments may be

used to estimate the performance of the ship under specified

weather and sea

conditions;

estimates are made of the shaft power F5, or delivered power

and propeller rate of rotation N3 at ship speeds IT.

The ship powers are defined

by

(1+x)PE S

and

PS = 11S where

(1+x)

is the power prediction factor,

is the standard effective power for the ship and

is determined, using the moulded ship displacement and the ship resistance coefficient at the standard temperature of 15°C

_(59°F).

The latter is the naked resistance coefficient for the ship as derived from the measured model value plus an allowance, where

appropriate, for appendages.

is the quasi-propulsive coefficient for the ship.

S

the shafting efficiency, is normally taken as 0.97 for ships with

machinery amidships and 0.98 for ships with machinery aft.

Power Prediction Factors, (1x)

In reference ('1) the B.T.T.P.

presented

power prediction factors, based on

an analysis of extensive ship-model correlation data, for single-screw ships

(2)

extended to include factors for ship lengths down to 100 ft. and in so doing the factors for ships with lengths less than about 510 ft. were

amended..

(Reference 2).

The NPL factors for ships up to 510 ft. in length are based on information

available for vessels such as trawlers and coasters and, for the best hull and.

best trial conditions as defined by the BTTP, they are in close agreement with

the values used very satisfactorily at NFL since 1952.

There is a need. for

first-class ship-model correlation data for these shorter vessels and.,

eventually, some modification to the PPF values may be necessary.

However,

present indications are that any modification is likely to be small.

Present practice at NFL for model experiments in No.1 tank

is to use the

factors for average hull and. best trial conditions.

The latter are defined,

in accordance with the BTTP 1965 procedure, as follows:

Average hull condition.

Nominally all-welded, with plate bilge keels of orthodox

proportions and. position, and. solid. bronze propeller.

Surface roughness 0.007 inch for 2 inch wavelength, 20 days out of dock.

Best trial conditions. No wind.

No waves or swell.

Sea temperature, 15°C (59°F);

the average value on U.K.

measured. miles for the warmest month.

Depth of water 75 fathoms.

At NFL the standard effective power for the ship,

_E'

is normally

derived

from the model results using both the Froude method and the ITTC 1957

model-ship correlation line and the corresponding values of (1+x) FROUDE and

(1x)ITTC currently used for average hull and best trial conditions are given

in Table IA and Figure 1

Values of the incremental

resistance coefficient

CA for the ITTC 1957 line have also been derived and these are given in

Table lB and. Figure 2.

Power prediction factors for use with the Schoemherr line, based on NFL data, are given in Table IA and Figure

3.

At present the same value of prediction factor is used for both loaded and ballast conditions, However, there

are indications

that the factors for light ballast conditions may be somewhat higher than for the same ships at loaded

draughts,

in some instances

by

as much as 10 per cent. As more data

are obtained. for

light ballast trials it may be possible to introduce a correction for the effect of draught,

(3)

-3-A brief note on the power prediction factors for best hull conditions is given in the Appendix.

Quasi-Propulsive

Coefficient

To enable the effect of scale to

be taken

into account the BTTP assumed in reference (1) that for ship performance estimates the propulsive coefficients for ship and model

could

be related by

-

klrD

S M

Furthermore, the BTTP recommended that until more information became available

k1 should be

taken as unity

and, at the present time, this is the value used at

NPL.

When estimating ship performance on trials, rD and the model propeller

M

rate of revolutions NM, are determined from the model resistance and propulsion

results at the loading corresponding to the appropriate power prediction factor

for the ship.

Propeller Rate of Revolutions

The ship propeller rate of revolutions is given by N8 =

k2N

where k2, the prediction factor for propeller revolutions, is determined

from

the empirical equations

k2

= 1.265 -

0.1

(1x)FROUDE _-

0.2%

(1)

= 1.265 -

C1 (1x)ITTC - 0.2CB

(2)

=

1265 - C2

(1+x)SCH -

0.2%

...

₍₃₎

= _1.165 +

_{C3 CA ITTC -}

_O.2CB (ii)

The constants C1, C2, C3 in

equations (2) to (4)

are functions of the ship

length and are shown in Figures L and

5.

For ships of lengths greater than about 510 ft. the propeller revolutions factors used at NFL, and. given by the above equations, agree with the values

published in the BTTF 1965 Standard Procedure.

References

1. "B.T.T.P.

1965 Standard Procedure for the Prediction of Ship Performance from Model Experiments", British Towing Tank

Panel.

NFL Ship Report

No8O, October 1966.

(4)

2. "Perfonnance Prediction Factors for Single-Screw Ships in use at Ship

Division, NTPL.' Dawson, J. Appendix XIII. Report of Performance

Committee.

11th International Towing Tank Conference.

Best hull conditions are defined as:

Nominally aU-welded with plate bilge keels of orthodox proportions

and

position,

and clean

new solid bronze propeller.

BSRA. surface roughness 0.003 inch for 2 inch wavelength. Clean (less than one day) out of dock.

Figure

6,

shows values of (1-i-x)PROUDE for best hull

and

best trial conditions together with the corresponding values of (1+x)ITTC.

The

ratio

(1+x)ITTC/(1x)PROUDE

_{= ©}

FROUDE/®ITTC is derived from NFL data and is,

of course,

unity

at the model length. The actual values of both (1x)FROTJDE

and (1x)ITTC are unity for the model length and ideally, with perfect model

and. ship conditions and a perfect extrapolator, (1x) should be unity for all

ship lengths.

Present practice at NFL for model experiments in No1 Tank is to use factors for average hull and best trial conditions for both model propulsion

analysis and

ship power estimates. Compared with the values plotted in

Figure 6 for best hull conditions these are 0.05 higher for (1x)FROUDE and. about 0.06 higher for (1+x)ITTC. Similarly for (1x)SCHOENHERR and CAITTC the values foi

average hull

and

best trial conditions are, respectively, about 0.057 and 0.00015 higher than for best hull and best trial conditions for a ship of the same

(5)

-5-Table IA

Power Prediction Factors for Single-Screw Ships Average Hull and Best Trial Conditions

(1+x) ITTC _{(1 x)PR.OUDE} _{1 +x)SCHOENHERR}

100 to 110 1 11 100 to 120 1 .05 100 to 156 1 .09 111 to 132 1.12 121 to 160 1 .04 157 to

₂₄₀

1.10 133 to

158 1,13

161 to 200 1 .03 241 to

₄₀₃

1.11 159 to 191 I ,14 201 to

₂₄₀

1 .02

404

to

₄₅₅

1.10

192

to

₂₄₀

1,15

241 to 280 1 .01

₄₅₆

_to

485

_{1 .09} 241 to 430

1.16

281 to 320 1 .00

486

to 516 1 .08 431 to

485 1,15

321 to

₃₆₀

_0.99 _{517 to}

₅₄₀

_{1 .07}

486

to 515

_1,14

361 to

₄₀₀

_0.98 _{541 to}

₅₆₅

_{1 .06}

516 to

540

1,13 401 to 440 _0.97 566 to 586 1 .05 541 to

560

1 .12 ₄₄₁ to 24-80 _0.96 587 to 610 ₁ .024-561 to

₅₇₉

1 .11

481 to 520

_{0 95}

611 to

₆₃₆

_{1 .03}

580

to

₅₉₇

I 10 521 to

₅₄₀

_0.94 _{637 to 672}

_{1 .02}

598 to

625 _{1 .09}

541 to 560 0.93

₆₇₃

to 716 _{1 .01}

626

to

652 1 ,08

561 to 582 0.92 717 to 770

_{1 .00}

653

to

685

1 .07

583

to 609 0.91 771 to 870 _0.99 686 to 720 1.06 610 to

642

0.90 871 to 1000 _0,98 721 to

₇₆₅

1 .05

643

to 680 _0.89 766 to 825 1 .04 681 to 725 0.88

826

to ₉₅₀ 1 .03 726 to 780 0.87 951 to 1000 1 .02 781 to 860 0.86 861 to 1000 0.85

(6)

Table

i:

Incremental esistance Coefficient for use with ITTC 1957 Lines For Single-Screw Ships, Average Hull and Best Trial Conditions

C AITTC x 10-100 to 185 0.50 186 to 325 _0.1+5

326

to ₄₀₉ _0.1+0 to 176

0.35

177

to 540 0030 51+1 to

₅₉₁

0.25 595 to 651 0 20 652 to 726 0.15 727 to 870 0.10 871 to 1000 _0.05

(7)

13 1'2

1. 1

10

09

100 AVERAGE

HULL

FINISH

AND BEST

TRIAL

CONDITIONS

1000

C)

SINGLE - SCREW SHIPS

(1

i- c) I.T.T.C.

-

© FR.OUDE

I

FROM N.PL.

I

DATA

(1 -I- x) FROUDE ©

I.T.TC,

N

6.Trp

(j

)FoUDE

200

300

400 POWER PREDICTION

500

600

700

800

900 Lp

ft.

(8)

04

03 iO3 x CA

_rrTC.

oa

01

0

100 SINGLE -SCR.EW SHIPS

200

300 4-00

500

600

700

800

900 1000

Lpp f+.

INCREMENTAL RESISTANCE COEFFICIENT FOR USE WITH

I.T.T.C. 1957 LINE

(9)

t 3

1. 1

10

09

08

100 1000

'I

200

300 +00

500

600

I I I I I

SINGL.E - SCREW

SHIPS

_{(ii-x) 5CR}

_©FROUDE

FROM N.RL. DATA

700

800

900 Lpp f+.

POWER PREDICTION FACTORS

FOR

SCHOENHERR

AVERAGE

HULL

FINISH

AND

BEST

TRIAL

CONDITIONS

11

(10)

0.05 0090

0085

0080

100

200

300

400

500

600 Lpp ft.

700

800 CONSTANTS

C1 AND C2

_{FOR DETERMINING PROPELLER REVOLUTIONS FACTOR}

1000

N

= 1265

-C1

-C2

(i.

(ix) SCH

i.nc.

- oa

-ca CB

C/

Ca

(11)

250

200

150

100

50

I

0

50 CONSTANT C3 FOR DETERMINING PROPELLER REVOLUTIONS FACTOR

2 WHEN

USING

_CA

_LTTC.

0 C3 CA

_,.T.1c.

oa

a

1165

CB

1

300

400

500 Lppf+.

00

700

800

900

10

100 ZOO

'1

C) c-n

(12)

I2

I.0 O9

0'S

PERFORMANCE PREDICTION FACTORS FOR BEST HULL FINISH

& BEST TRIAL CONDITIONS

(

x) ITC.

(t + x') F0UDE

® FROUDE

©

rrtc.

-n