The effect of design modifications on the natural course stability of full tanker models

November 197 1

LABORATORIUM VOOR

SCHEEPSBOUWKUNDE

TECHNISCHE HOGESCHOOL DELFT

THE EFFECT OF DESIGN MODIFICATIONS ON THE NATURAL COURSE STABILITY OF

FULL TANKER MODELS

by

Ir. C.C. Glansdorp

and

J.G.L. Pijfers

.

List of Fig4

Fig. Body plans of models.

Fig. 2A Arrangement of rudders of the original model.

Fig. 213 Arrangement of rudders of the modfied model-Free running propeller.

charteristic-Fig. 4 'Rudder forces and rudder sosonoc of ruder 1.

Budder. forces and rudder mument,...; of rudder IV.

increae of increment of sewa:ying derivative as a funtion

.of revulutions of the prs.peller.

amplitde-Yt-m', N' r r Y' N' V

'

v YI NI 6 ' 6 m' A Y etc. subscript s subscript R subscript p subscript s.T rudder area ship length ship speed

velocity of water near the rudder

revolutions per minute of ship propeller coefficient of advance

torque coefficient thrust coefficient

yaw damping derivatives sway damping derivatives

rudder derivatives dimensionless mass increment of derivatives refers to skeg refers to rudder refers to propeller refers to slinstream

swaying velocity amplitude

CL c)t

lift gradient

wake factor

6 _{rudder angle}

Centat.,:its.

4.

erìt..t.

tha* rett!ts of the to::;t-t.

'

L.;7 :Eesi:Cits nf -fr-esor

íT

ro,elier tets.

4.3 Pesuits tf rt:ider

General.

(:omvariscn of t-w-

'rare

Effet. of Hkec.

5.4 Effe.::tt hf soliese st.- in

slip

.. .treaTo.

5.5 Eff,*(2t

() Efft of'

t:t1,-*

Summary.

The course stability characteristics of two full tanker-afterbodies with different rudder-propeller-skeg configurations are experimentally

determined.

Natural course stability characteristics _{are improved by adding lateral} force generating devices in the afterbody.

Introduction.

The improvement of stability characteristics of full _{course unstable vessels} can be done principally in two ways. The first is to install _{an autopilot or} a similar electronic device aboard the ship in order to achieve directional stability of the newly created system, a course unstable vessel together with an electronic device. This solution is very attractive in an economic

sense.

The adjustment of the autopilot characteristics _{to make a course unstable} ship directionally stable is somewhat more difficult than for a course stable ship. Nevertheless current practice shows _{that large tankers, which} are very often course unstable, equiped with suitable autopilots _{have a} reasonable performance in unrestricted waters and these large tankers are certainly directionally stable.

Another way to improve the stability characteristics _{is to modify the} after-body and to improve the lateral force _{generation of the control devices} in the afterbody by well designed _{new configurations of the system:}

skeg-propeller- rudder. In this _{case the control-fixed stability or course} stability is improved and certainly _{facilitates the achievement of}

directional stability by either _{a helmsman or an autopilot.} Some work in this field, regarding

ship born manoeuvring devices has been done by English and Steel [1]. _{However some of their experimental}

set-ups are expected to be very expensive in practice _{due to the fact that they} need a more detailed development

before practical application. Therefore it is questionable whether _{these devices will be applied}

on large ships when relatively cheap electronic _{control devices are available which}

can

2

-Design modifications to improve the stability characteristics should be such that they do not effect the building costs of ships to _{a perceptible} extent including the costs of developing those modifications especially regarding problems of strength and vibrations.

In this paper relatively small design modifications of _{a parent model are} considered; the linear damping derivatives combined in _{a course stability} criterium are measured with a P.M.M. (Planar Motion Mechanism). _{It is shown} that these small modifications produce a relatively _{small improvement of}

course stability. None of the investigated configurations could make _{a very} unstable naked hull course stable. But one of the better configurations

together with a well designed electronical controlling device will _contribute, in the _{author's opinion, to a more secure and more economic directionally} stable system. It is known that the performance index _{as proposed by Motora} and Koyama [2] is a very good measure in comparing performances of different directionally stable systems exposed to random disturbances. Within

reasonable limits it is felt that the less _{course unstable a ship is, the} better the performance index will be if we assume that the ship has a suitably adjusted autopilot on board.

3. Model particulars and test progam.

Two models of a 200.000 tons deadweight _{tanker were used for the experimental} investigations; they were made of reinforced _{fibre glass. The model of an} excisting ship is considered _{as the parent model; the afterbody of the} second model has been modified. The ship particulars are given in Table 1

including the particulars of the modified model in full scale dimensions. Table 1: _{Ship and model particulars.}

original model _{modified model} Length between perpendiculars _{310.00 m 310.00}

m Breadth _{47.16 m} 47.16

m

Draft 18.90 m 18.90

m

Displacement _{235,000 m3} 232,690 m3

Length center of boyancy _{+8.73 m}

+9.81 m

Propellerdiameter _{8.80 m}

8.80 m Pitch ratio

.700 _.700

Blade area ratio _.5143

.5143

Model scale _1:100

The body plans can be seen in fig. 1. The particulars of the used rudders are given in Table 2.

Table 2: Rudder dimensions.

With respect to control fixed stability the fixed _{portion of the rudder} can be considered as being an integrated part of the _{rudder with respect} to the calculation of the lift gradient. For turning _{circle calculations} another lower value has to be used regarding _{the effect of camber at a} certain rudder angle and the adverse _{effect of the fixed portion of the} rudder.

Rudders I and II are used in connection with the original ship; rudders III, IV and V in connection with _{the modified model. The lift gradient} mentioned in Table 2 is calculated _{according to a formula published}

by Whicker and Fehlner [3].

A survey of the configurations _{of the hull and the appendages and}

the

designation of these configurations _{for reference in section 4 where} the results are presented, is given in Table 3.

original ship _{modified ship}

I II III

*

*-IV

*-v.

balanced balanced Mariner fixed portion twin rudders fixed portion aspect ratio sweep angle taper ratio upper chord lower chord rudder height profile thick-ness ratio area L 2.87 0 1 7.25 m 7.25 m 10.40 m NACA0018 75.4 m2 3.063 2.30 0 1 10.00 m 10.00 m 11.50 m NACA0018 115 m2 2.683 3.36 100 .612 8.50 m 5.20 m 11.50 m NACA0018 78.8 m2 3.311 2.30 0 1 10.00 m 10.00 m 11.50 m NACA0018 115 m2 2.683 3.37 0 1 4.75 m 4.75 m 8.50 m NACA0018 2*38 m2 3.332 act

M(odified) ship, speed 15 knots.

_ 4 _

Table 3: Survey of testing conditions.

0(riginal) ship, speed 15 knots

When the propeller had been rotating _{the number of revolutions per minute} is indicated in the second or third column.

The measured configurations _{can be seen in fig. 2A and 2B.}

Swaying and yawing tests with the models were performed with _{the P.M.M.} available in the Delft Shipbuilding _{Laboratory. If the propeller}

ran during the P.M.M.-tests torque and thrust measurements were simultaneously _carried

out.

Designation propeller rudder

01

-

-02

N=85 I

03

- II 0 4 _N=67 II

05

N=85 II 0 6 N=102 II

Designation skeg propeller rudder

Ml

M2

-yes

-M3

- N= 85

-m 4

yes N= 85

-M5

- - III

M6

yes - III

Ml

-

_N=85

_III

M8

yes

_N=85

III

M9

yes - _IV M10 -

_N=85

_IV M11 yes N= 67 IV M12 _{yes N= 85 IV} M13 _{yes N=102 IV} M14 yes - _V M15 _{yes N= 85 V}

The free running characteristics of the model propeller have been measured in order to be able to estimate the wake in both cases.

Rudder tests have been carried out with different numbers of revolutions of the propeller and without propeller and the rudder force and rudder moment

exerted on the model were measured for all rudder types used in this investigation.

4.

Presentation of the results of the tests.

4.1

Results of P.M.M.-damping derivatives.

For all configurations the in-phase and quadrature _{components of the} forces on forward and after strut of the P.M.M. are measured in both modes, swaying and yawing. Due to the relatively high dimensionless

wL ,

frequency w' = _{during the tests ranging up from}

_.8

_{to 2.6, one may}

U

expect that frequency effects have an adverse effect _{on the measurements[4].} Nearly all damping forces and moments show _{non linearities in the}

measured range, so it is difficult to distinguish _{between linear and} non linear part of the forces and moments mainly due _{to scatter of the} few measuring points. It was decided to adopt _{one fixed non-linear term}

for the determination of the linear terms for all configurations belonging to one model. Although it is probable that the cross-flow _introducing

non-linear damping terms will not _{vary very much with the change of} configurations an error has been accepted in _{determining the linear} derivatives by this assumption.

In Table 4 the results of the measurements _{analyzed with the aforementioned} fixed non linear term are presented. For the sake of simplicity all

mass derivatives are omitted. The stability criterium used is:

N'

If

Yv-m

yl

then the model is course stable.

Reference is made to table 4 _{where the points of application of the}

-6

Table 4: Results of the P.M.M. tests with respect to damping derivatives.

It is to be noted that the dimensionless _{mass is for the original ship}

m'

* 105

= 820

and for the modified model m' 105 = 812

4.2 Results of free running _{propeller tests.}

A free running propeller test has been carried out at approximately _the same magnitude of the Reynolds number _{as was the case during the} P.M.M.-tests. The results of the tests _{can be seen in fig. 3.}

, ,

swaying yawing

Desig-nation Yv,.105

N'.105

Nv%Yv' (Y,'-m').105 Nr'.105 Nrt/(Yri-m')

0 1 -1562 -808 .517 _{-1232 -255 .207} 0 2 -1938 -668 .345 -1086 -314 .289 0 3 -1733 _-739 .426 -1109 -302 .272 0 4 -1836 -690 .376 -1092 -307 .281 0 5 -1821 -680 .373 -1079 -318 .285 0 6 -1878 -669 .356 -1053 -331 .314 M 1 -1438 -862 _.599 -1272 -232 .182 M 2 -1510 -831 .550 -1228 -256 .208 M 3 -1600 -811 .507 -1205 -250 .207 M 4 -1717 _-756

₄₄₀

-1149 -280 .244 M 5 -1559 -796 .511 -1209 -268 .222 M 6 -1619 -764 .472 -1124 -286 .254 M 7 -1716 _-735 .428 -1105 -303 .274 M 8 -1825 -700 .386 -1083 -322 .297 M 9 ..1663 -741 .446 -1104 -297 .269 M10 -1762 -696 .395 -1086 -316 .291 Mll -1768 -692 .391 -1068 -325 .304 M12 -1813 -684 .377 -1054 -332 .315 M13 -1829 -665 .364 -1049 -338 .322 M14 -1722 -748 .434 -1162 -294 .253 M15 -1832 -696 .380 _1105 -320 .289

For all rudders the rudderforces and ruddermoments exerted on the ship are measured as a function of the rudderangle. The models sailed along the center line of the towing tank without driftangle. In Table 5 the results of the linear derivatives are presented which are calculated from the experimental points using a least squares fit. In fig. 4 and

5 the results of the measurements for the rudders I and IV are given.

Table

5:

Results of rudder tests.

5. Discussion of results.

5.1 General

Qualitatively it is believed that _{the numerical reEults are accurate}

enough for drawing final conclusions. _{Quantitatively it appears as indicated} by the least squares analysis, that _{the average accuracy is as given}

in Table 6.

Differences between derivatives _{can not be considered significant when} they do not exceed the values mentioned _{in this Table.}

Y(5'105 _N6'.105

rudder-type propeller propeller

without N=67 N=85 N=102 without N=67 N=85 N=102 72 163 275 372 -34 - 91 -138 -176 II 124 250 305 377 -63 -104 -152 -191 III 94 217 277 330 -45 -101 -133 -166 IV 127 275 344 411 _{-59 -128 -162 -206} V 57 169 208 237 -29 - 76 - 98 -121

Table 6: _{Average accuracy of measured derivatives.}

IL

Yvl 105

25

IA

Nv1.105

15

IA

Yr1.105

20

-

8

-5.2

Comparison of the two bare hulls (01 and M1).

Both models are in bare hull condition very course unstable. The original model shows a point of application of the swaying force in front of the forward perpendicualr, thus indicating a negative damping in the afterbody, probably due to speed effects. The modified model shows an even more pronounced picture. The point of application of the swaying force _{is even a little bit further ahead of the forward perpendicular.} Looking at the stability criterium the ship with the modified _afterbody is not better than the original ship with respect to course stability; this fact was not anticipated.

5.3

Effect of skeg (M1 and

M2, M3

and

_{M4, M5}

and

M6, M7

and

M8, M10

and

_M12).

In Table 7 the contributions of the skeg were calculated.

Table

7:

Contributions to derivatives of skeg.

From Table 7 _{it can be concluded that the relation between the skeg}

derivatives, indicating the situation _{of the skeg with respect to}

the center of gravity, is almost perfect, _{regarding the accuracy. Comparing}

the

conditions M1 and M2 and M2 _{and 01 we may conclude that the ship with the}

modified afterbody together with _{a skeg has an equivalent course stability} as the original version. Increasing the _{skeg area improves course stability} as already was mentioned by _{many other authors and especially by}

Tsakonas

[5]

and Jacobs [6].

5.4

_{Effect of rudders not in the slipstream.}

(01 and 03, M2 and M6, M2 and M9, M2 and M1)).

comparing conditions

_(AYv')5.105

_(Al\y)s.105

_(AYr')s.105

_(AN1,1),.105

M 2

M 1 72 31

₄₄

24

M 4

M 3

117

_{55 56}

₃₀

M 6

M 5

60

₃₂

₈₅

₁₈

M 8

M 7

₁₀₉

₃₅

₂₂

_-19

M12 - M10

_{- 51}

_{12 32}

_-16

average of all conditions average of derivatives - 82

- 82

33

41 48 41

-21

21

In Table 8 the contributions of the rudders to the derivatives for the mentioned configurations are calculated and compared with the rudder derivatives Y6' and

N6'

in the same conditions.

Table

8:

Contribution of rudder to the derivatives.

In this case, the rudder is situated in the wake of the ship, the contribution of 1(AYv')R1 exceeds the magnitude of Ya'.

The explanation of this fact is given when the simple formulas (1) and (2) are compared:

(AYvt)R

= Y A

U,

_{n R DCL} U L2 *

9a

AR DC uR2 U2 L2 a UR

In this case

7

< 1 due to the wake effects, UR = U(1-4), therefore

IY6'

<

(AYv')R I°

In Table 4 it can be seen that the rudder, _{even if it is placed in the} wake of the ship has a very favourable _{effect on course stability.}

5.5

Effect of propeller

_{(M3 - M1).}

Comparing the conditions M3 _{and M1 the following contributions to the}

derivatives by the propeller can be calculated. Table

9:

_{Contributions of propeller.}

(2)

comparing

conditions

(AYv')R*105

(A,N ')105

v R* (AYr

')ii*

105 (LNr')R*105

Y6'.105

N(5'.1051

0 3 - 01

-171

+69

+123

-47

124

-63

M

6

- M2

-109

+67

+104

-30

94

-45

M

9 - M2

-153

+90

+124

-41

127

_-59

M14

- M2

-212

+83

+ 66

-38

i 57

-29

condition (AY

_{v p}

)

'.105

(ANv)p'.105

_{(AY )}

_'.105

r p (AN_{r p}) ' 105

M3 - ml

averaged all derivatives

-162

-118

+51

+59

+67

+59

-18

-29

If the propeller distance to the center of gravity is accounted for new average derivatives can be calculated.

It is to be noted that the fin effect of the propeller (no rudder present) is nearly of the same order of magnitude as the effect of the rudder

in the wake of the ship (no propeller present). The so-called fin effect of the propeller is in fact the generation of a stabilizing transverse force set up by an oblique flow on the propeller.

5.6

Effect of the slipstream.

(06, 05, 04

and

03, M13, M12, Mil

and M9) In Table 10 the effect of the slipstream on the derivatives has been calculated.

Table 10: _{Effect of slipstream on derivatives.}

If we assume a fixed relation between _{the derivatives then the averages} can be calculated. In fig. 6 the results of the average increase of AYv' in the slipstream _{are plotted as a function of}

_R.P.M.

Two major items influence the lift _{generation of the rudder in the}

slipstream. The propeller accelerates _{fluid particles thus giving a higher} incoming velocity of the water onto the rudder, and secondly the

transverse velocity of the obligue flow behind _{the propeller is decreased} due to the propeller action, both _{phenomena introducing the straightening} effect of the propeller, making the effectiveness of the rudder less. comparing

condition (AYv's.T.l' 0

(ANvs

)'.T-5

-,

in)s.0

(Ay)' T.105 s. (AN '

NT*1

0 4 - 03

_103

+49

+17

_{- 5}

averaged

_ 64

+32

+32

-16

0 5 - 03

- 88

₊₅₉

+30

-16

averaged - _{82 +41 +41}

_-20

0 6 - 03

_145

+70

+56

_-29

averaged

_128

+64 +64

-32

M11 - M9

-105

+49

+36

-28

averaged - ₉₇

₊₄₉

₊₄₉

_{-2 4}

M12 - M9

-150

+57

+50

_-35

averaged

_-126

₊₆₃

₊₆₃

_-31

M13 - M9

_166

+76

₊₅₅

_-41

averaged

_148

+74

₊₇₄

_-37

Also the straightening effect of the hull plays a certain role in the nature of lift generation of the control devices in the afterbody.

The real nature of this effect and the way to measure it, is still a bit obscure [7].

5.7 Comparison of the rudders in the slipstream (02, 05, M8, M12, M15). This comparison is in fact the most important because the conditions

mentioned are more or less like the situation for the ship sailing in fully loaded condition under maximum power. In Table 11 the results are recalled.

Table 11: Comparison of derivatives under normal sailing.

The following conclusions can be drawn from the results in this table. None of the investigated configurations is course stable. The old version with the original rudder seems to be the best configuration, but with respect to the value of significant

difference of arm the differences in course stability _{are of minor} importance. Compared to the actual performance on full scale of this configuration, reference r8] gives some evidence that this configuration was indeed course unstable.

Comparing 02 and 05 and M8 and M12 show that increasing the

rudder area not necessarily gives a better course stability. Considering an area of 115 m2 as _{a limit for a single rudder}

configuration then it seems justified for achieving _{course stability} to use twin rudders. They ought to have considerably _{more area in} total than applied here during this investigation.

1(1-ea-st,l(g)--n Y-,; .105 N .105 _Nv%Yv' (Y; -m')105 N; .105 t Nr AR Yr'-m' 2 0 2 -1938 -668 .345 -1086 -314 .289 75.4 m 0 5 -1821 -680 .373 -1079 -318 .285 115 m2 M 8 -1825 -700 .386 -1083 -322 .297 78.8 m2 M12 -1813 -684 .377 -1054 -332 .315 115 m2 M15 -1832 -696 .380 -1105 -320 .289 2*38 m2 signifi-cant dif-ference in arm .035 .020

-

12

-In Table 12 thecontributions of rudder and propeller are calculated for the 5 different rudders and compared with the results of the rudder tests.

Table

12:

Contributions of rudders and propellers and comparison with rudder derivatives.

It can be seen in this table that the propeller contribution to the

derivatives is about 70% of the rudder contribution. Compared to the rudder derivatives Y6' the contribution of the rudder to the swaying derivatives

is about 60% of the rudder derivatives. The total measured value of the influence of rudder and propeller is of the same order of magnitude of the measured value for the rudder derivative during the rudder _tests with the exception of the twin rudders.

5.8

Effect of wake.

The thrust and torque measurements during the trials _{and the free running} characteristics of the propeller could be used _{for calculating the wake.} designation

(AYv').105

_(ANIr')105

(AYr').105 _(ANr')105

Y6'.105

N6'.105

0 2 - 01

-376

+140

+146

_-59

-

_ averaged

-296

+148

+148

-74

275

-138

o 5 - 01

-259

+128

+153

-63

-

-averaged

-268

+134

+134

-67

305

-152

M 8 - M2

-315

+131

+145

-66

-

-averaged

-283

+141 +141

-71

277

-133

M12 - M2

-303

+147

+174

-76

-

-averaged

-312

+156

+156

-78

344

-162

M15 - M2

-322

+135

+123

-64

-

-averaged

-274

₊₁₃₇

+137

-68

208

- 98

propeller averaged

-118

+ 59

+ 59

-29

-

-rudder I

-178

+ 89

+ 89

-45

275

-138

rudder II

-150

+ 74

+ 74

_-37

₃₀₅

_-152

rudder III

_-165

+ 82

+ 82

-42

₂₇₇

_-133

rudder IV

-194

_{+ 97}

_{+ 97}

-48

₃₄₄

_-162

rudder V

-156

+ 78

+ 78

_-39

₂₀₈

- 98

This is done for all types of rudders; it was shown that the

wake-factor doesnot vary with yawing or swaying amplitude and the wakewake-factor was nearly 0.52 for the original version and nearly 0.55 for the modified version as can be seen in fig.

7.

These values are rather high and some scale effect in the wake must be assumed, thus influencing the incoming velocities and making the pertinent derivatives larger.

6.

Conclusions.

The following general conclusions can be derived.

Course stability characteristics are improved by adding lateral force devices in the afterbody.

The modified afterbody did not give better course stability characteris-tics. In fact a skeg was needed to have nearly the same performance as

the original version.

The effect on course stability of a rudder in the wake of a ship is of the same order of magnitude as the effect of a rotating propeller without rudder.

The effect of propeller slipstream is important for the improvement of course stability.

None of the investigated configurations proved to be course stable. Increasing rudder area doesnot always imply a better course stability. For this type of tankers one could hardly expect that _{course stability} can be achieved by relatively small changes in the design, apart from the question to what extent course instability can be accepted.

The total improvement in course stability of rudder-propeller configu-rations is for the greater part due to the rudder action. _{However the} propeller seems to have a substantial effect in both, the generation of stabilizing transverse forces and reducing the stabilizing effect of the rudder by producing a straightening slipstream, _{and secondly} making the incoming water velocities in the rudder higher.

7.

References:

[1] _{English, J.W. and Steele, B.N.:}

"Ship-borne Manoeuvring Devices"

Paper read at the 1st International Tug Conference,

_1969.

[21

Motora, S. and Koyama, T.:

"Some Aspects of Automatic Steering of Ships"

Japan Shipbuilding and Manen Engineering, July

_1968.

13]

Whicker, L.F. and Fehlner, L.F.:

"Free Stream Characteristics of a Family of Low Aspect Ratio, All Movable Control Surfaces for Application to Ship Design"

DTMB Report

_933,

May

1958.

[41

Glansdorp, C.C. and Pijfers, J.G.L.:

"Systematic Horizontal Oscillation Tests with Models of Series 60, Blockcoefficient

.70

with Varying Length-Breadth Ratio's"

Report

322

of the Shipbuilding Laboratory of Delft. University of Technology, August

_1971.

Tsakonas, S.:

"Effect of Appendage and Hull Form _{on Hydrodynamic Coefficients of} Surface Ships"

Davidson Laboratory, Report No.

740,

July

1959.

Jacobs, W.R.:

"Estimation

of

_{Stability Derivatives and Indices of}

Various Shipforms and Comparison with Experimental Results."

Journal of Ship Research, September

_1966.

f7]

Mandel, P.:

"Some Hydrodynamic Aspects of Appendage Design" Transactions SNAME

1953

[8]

Glansdorp, C.C. and Buitenhek, M.:

"Manoeuvring Trials with a

200.000

_{Tons Tanker"}

Report

248

of the Shipbuilding Laboratory _{of the Delft} University of Technology, August

_1969.

Fig. 1

modified version

2A links

APP

///zez,/, 7,40

APP

100

2E rechts

50

APP

3

10

KQ

KT

1

0.6

0.4

0.2

Wageningen

B-series

-Experiments

---

KQ)

KT

..

....,

..

...

K

....,%0

I

<1...Kr

_".,

'

0.2

04

0.6

j

08

1.0

RUDDER I

nks

x

O

A

without propeller

200

n

=

67 R.P.M.

_{viS .105}

n

=

85

s.

n

=

102

..

1

100

.

_3Ö0

_.45

'''

1

°

,...

s

300

if

O

O.

100

A

200

1

N

-105

50

100

1°

s

Nzt

4 rechts

o

5 links

RUDDER TE

incl. skeg

200

105

100

Nsb.105

50

1

6

Yvi '1

-5

5

-10

-150

o

R .P. M.

40

80

120

)

Rudder 1E

O

Rudder ISE

7

Original, with propeller

_{and rudder I}

o

_Modified

_,

_{with propeller, rudder Er}

and skeg

.05

-0