Model experiments with several bow anti-pitching fins

REPORT No. 90

JULY 1962

SHIPBUILDING LABORATORY

TECHNOLOGICAL UNIVERSITY

-

DELFT

MODEL EXPERIMENTS WITH

SEVERAL BOW ANTIPITCHING FINS

by

REPORT No. 90

JULY 1962

9V/4(96J

T11

SHflPBU1LDING LABORATORY

TECHNOLOGICAL UNIVERSITY

- DELFT

MODEL EXPERIMENTS WITH

SEVERAL BOW ANTIPITCHING FINS

MODEL EXPERIMENTS VITH SEVERAL BOW ANTIPITCHING FINS. = = a = == = = = = = = = = = a = = = = = == = a = a = == =a = = =

Byl. Sonoda.

Deift Shipbuilding Laboratory.

Report No. 90.

July 1962.

Summary.

Seakeeping testa with a model of Todd

60

series (Cb m

0,65)

fixed with several bow antipitching fina were carried out on longl-tudinal regular waves and in still water. The purpose of the experi-mente was to inveatigaté the influence of relief alita or holes over

the fina on the motions, re8iatance and induced vibrations.

A model fixed with bow antipitching fins showed better seakee-ping characteristics at synchonous conditions than a model without

fine. The experiments confirm that the holes or slits over the fine are very much beneficial to reduce the induced vibrations andca any disadvantage to the motions and resistance in waves comparing

with the fine having no holes, or alita provided that their

Iztroduction.

Most of the intereat of naval architectß had been concentrated on the still water performance of ships in the early stages of the research. it was thought that a 8hip which Bhowed a good perforince in still water would also show a good performance at sea. It may be said that it was partly due to this opinion that ships with bulbous bows were designed to improve the propulsion charaöteristica.

Since, however, a ship spends moat of her life at sea, it seems quite natural that the ship performance at sea has recently been given footlights in the reserc.

Experimental and theoretical work and full scale investigations have.been carried out to improve the seaworthiness of a ship both

from the point of view of ship motions, and propulsion.

Nùmerouevaluabl& theorieahave been. established in the field of the rolling motion of a ship and we hayo succeeded to control the rolling motion of a ship by several means of antirolling stabilizers

The research work for the longitudinal motion of a ship has started at the same time and a theory concerning the increased resia. tance of a ship in waves has been developed. Many seaworthiness

tests have been carried out to get the most suitable form in waves. It ha. been realized that there is a very close relation between the longitudinal ship motion and the resistance in waves. Furthermore, it baa been confirmed that pitch is more dominant to both the sea-worthiness and the increased resistance than heave.

Thus, the idea arised of decreasing pitch by antipitching fina

and severa], experimental atudie have been carried out in this field including two full scale applications. One of the full scale

appli-cations which was on the Holland-America Lina vessel "Rijndam"

turned out to be quite unsucce8aful and discouraging. The 1J.S.S.

Compass Island was also equipped with antipitching fins and she brought us many valuable data.[1] . Both of these two cases showed

that the ships seriously suffered from transverse bow vibrations which were due to flow aeparat±oñ at the fins.

Many experimental reports, however, have indicated the

feasibi-lity of reducing pitch motion., although they aleo gave warnings about the transverse bow vibrations [2] [3) [k]

5I.

Nowadays it is

already an established opinion, that antipitching fins are effective

especially when a ship 18 in her synchronous condition. Ochi has recently attacked the study of the mechanism and the properties of

the induced bow vibrations ['6j

.

Several interesting facts have been revealed by his experiments.

The influence of the fin particulars to pitch, heave, resis-tance and induced vibratIons is treated in this paper. The experi-mente are especially concerned with the effect of relief holes or alita on the fins. Scale effects are ignored In, this

investiga-tion.

2. Experimental procedure0

2.1. Modal particulars.

A 1/5 fiber glass model of the Todd 60 serieS (DTI4B Model No. k211-W representing the 0,65 block coefficient) was, used for

the experiments. Particulars of the model are given in Table 1. A Bpecial fin fixing mount was inserted at the fore foot

of

the

mo-del so that fins could be eaaily exchanged. A photograph of the fore part of the model is shown in Fig. 1.

Table 1.

Model Particulars.

Masa moment of inertia for pitch (in air)

_1.85

kg.a.

Radius of inertia , 0,25 L.

Length between perpendiculars 2.258 m.

Breadth O311 m.

Draught _{0.125 1.}

Displacement 57.0 kg.

Blòok coefficient _0.650

Waterpiane coefficient 0.7k6.

2.2. Fin partioulare.

Several experiments have been carried out to study the effects

of fin particulars on pitch reduction, induced vibration- and rosis.-tance. The au-thor thinks it valuable to summarize the facts which ha been shown by -these experiments.

.). Fin area.

Th. fina of larger area produce larger pitch reductions, however,

the reduction is not proportional to the increase of area

-. Aàoording to [i.], the fins of which the total effective

area is 2 percent of the waterplane area were twice as effective as the 1 percent fina, the k percent fins, however, were only about three times as effective. There is leas probability for

vibration when the fin had a larger area-, but if cavities and/or

slamming do occur with the larger f n,h

_{ntnsiÏ of tfced}

vibration is proportional to fin area.-

-b). Aspect ratio.

-Small changes in fin dimeñsiona with constant fin area gave.

negligible effects on pitch reduction

_[3]

iTj. It is expected

that fins of greater span will be more effective to reduce the

induced vibrations.

o). Tjp fonce

Tip fences seem to have good effects on pitch reduction and

in-duced vibrations.

d)0 R.l-ióf mechanisms.

Relief' mechanisms are very useful to reduce the induced vibra-tion and moreover, they do not bring any remarkab]a changes on

pitch reduction compare4 to the fin which has no relief

mecha-nism.

-Considering these' facts-, the dimensione of the fins in our

ex-perimente were chosen as shown in Table 2 and Fig. 2. Aleo photo-graphe of these fins are shown in Fig. 3. Since the purpose of this experiment is to study the effect of the relief holes on the fins, -the main dimensions of the fins have been kept constant. The total

area of the holes in the ftns has aleo been kept constant. Fin B2,

-5-(

Location (aft of F.P)

(above base lin.)

Fin

area (P & S)

Mean

span

(at one aide)

Chord length Max. thickness

Mean aspecto retio (at one Bide) Chord thickness ratio

Total area of the holeS

2.3. Instrumentation.

In the tests the model was free to pitch, heave and surge and restrained for roll, away and yawing motions. Reàiatance, pitch

and heave amplitude, wave height, bow vertical acoelerationand bow. transverse acceleratión were measured. Pesiatance measurements in still water and in regular waves were carried out with a gravity dynamometer fitted with a transmission of one to five (Inertia ef-fects resulting from the surging motion of the model and the appli-cated load were reduced in the same proportion. Heave and pitch

were measured by means of low friction potentiometers. 'Nave heights

were measured by meane f a conductance type wave height meter.

Statham linear accelerometers were used for the measurement of acce lerations. Model speeds were measured using a photo transistor and

electronic counter arrangement. (For further details, see and

[8J ).

the data except for resistancó, were recorded on two

five channel "Rapidgraph" penrecorders.

which is a variation from Fin B1, baa transverse alita of which the axis in the profile is declined backwards by 15 degrees. The crose section of the fins was streamlined to minimise the resistance.

All, the fins were made of plastica and fixed to the mounting block

by two screw bolts. (Fig. i).

Table 2. Fin ;articulara. 0,05 x L 0,20 x d 0,02k x 78 mm 80 mm 12 mm

0.98

6,67

8.8% of fin area

2.k. Test program.

The experiments were carried out in the Shipbuilding Laboratory of the University of Technology at Delft, using a gravity towing

sys-tem and a pnaumatic wavemaker for generating regular waves.. The tests were. carried out in regular head waves which have the dimensions as shown in Table 3. The teats for Fin A, Fin E and without fin were carried out for all six wave conditions. The wave conditions:

A/L

0,80, h/I = 1/ko and -/L = 1,20, hL = 1/ko were excluded

for the other f-Ins. Pesistance tests in still water were carried out for each fin.

Table 3.

h h/L

0,80 5,6 cm 1/ko

2,26 1,00 7,5 cm 1/39

2,71 1,20

3.

Analysis of test results.

3.1. Pitch and heave amplitudes.

Curves of pitch and heave amplitudes and the phase angle be-tween pitch and heave are plotted against Froude number in Fig. k

through 7. The various fins, are classified into three groups in

or-der to avoid oonfusion and to make it easy to compare. Both pitch and heave amplitudes are expressed in dimensionless units, that is,

amplitudes of pitch are divided by maxir'um wave slope and amplitudes

of heave by wavö amplitude. The variation of the wave amplitude in the various conditions was lese than 7 percent.

Fig. LI and Fig. 5 show that Fin E is as effective as Fin A and pitch reductions of about 25 percent to 30 percent were obtained at synchronous conditions with both Fin A and Fin E. FIg 6 and Fig. 7 also show that there is no remarkable.change in pitch and heave am-plitudes dependent on the shape and the distribution of holes or

slits in the fina.

-7-It should be noted,, however, that there _{is considerable}

diffe-rence betveen Fin B1 and Fin B2. Therefore it seems to be effective

to give an inclination backwards to the slits in the profiles.

It should be remembered that the total area of the holeB or slits is rather small (about 11 percent of the effective _{fin area).} Considering that the total area of slits _{was about 23 percent of the} effective area which was used in Ochi's experiment, _{it can be said} that the variation in shape or location of the holes or slits has

very little influence on pitch or heave amplitudes _{as far as the} total area of te holes or the slits is less than _{25 percent of the} effective area. The importance of the shape and location increases

with the total area of holes or slits.

It should be noted that "Antipitching Fins" are also

conside-rable effective as "Antiheaving Fins". The _{reason is that the}

coup-ling _{effect of pitch 'o heave is lagèr thai that of heave t}

pitch. According to Fig. 7a and 7b from

_tb]

, the coupling effect

of pitch to heave is significant at synchronous _{condition8 for the} Todd 60 series models (Cb z 0.6, 0.7). It is shown that the heave

amplitudes due to the coupled motion _{are almost twice as big as} that of uncoupled _{tio.'. This does not hold in generai because the}

coupling term depends much on hu].lparticulars.

. . o Resistance.

3.2.1. In stili water.

The still water resiBtance of the Model fixed with various

antipitching fins is given in Fig.

8.

_{Comparing Fin A with No Fin} in the range of practical speed, the resistance _{of the model with} Fin A is about 25-20 percent more than that without fins. It Bhould be noted that the resistance of Fin E is slightly _{less than that} of Fin Aat Fr. > 0.17. That means that the angle of attack of the

fins vttere not at an optimum with regard to the flow along the 'hull.

At higher speeds the flow is rectified by the slits, on the other hand, the flow is disturbed by them at lower speed. The _same argu-ment holds for the other fins except for _{Fin B1 which 13 always} worse than the others.

-8-a

-8-3.2.2e In waves..

Most of the experiments concerning antipitching fins have been carried out to 8tudy the effect of the fins on pitch reduction.

This ja also indicated by the naine "Antipitching Fins". The

reduc-tion of excess moreduc-tions of the ship itself is important for the com-fort of the passengers and the crew or for the prevention of cargo. damage. Since the pitching motion is the main cause for increased resistance in waves, the methods of reducing pick motion are also important te minimize the loas of speed at sea.

The dimensionless increase of resistance in wav3 with

vari-ous fins is shown in Fig. ₉ through li. The dimensionless transfer

funotion is expressed as: R

r

gr232/L

Some of the fins are considerably effective to reduce the increased resistance in waves especially at synchronous conditions. Fin A and

Fin E are noteworthy in this respect. Fin E is as effective as

Fin A but it is 'leBe effective at speeds higher than Fr 0,2.

The other fina aré effective in the following order; Fin C, Fin D,

Fin B2 and Fin B1.

Fig 8 shows, on the other hand, that fins geuer..y increase the resistance in still water. Therefore the curves of the total resistance of the model fixed with Fin A, Fin E and without fina

are given in Fig. 12. and 13 in order to show the total effect of tk

fins on the resistance in waves. The figures show that a maximum reduction of 25 percent is obtained at aynchronous conditions. The slits on tc Fin E does not show any drawback with regard to

the it;ìnoe in waves. The figures also show that the resistance

of the model fixed with fins in waves is less than that without finE

in the range of practical speed at synchronous wave conditions.

3.3.

Induced vibration.

The magnitudes of the induced horizontal vibratïon were measu-red for the various fins in units of acceleration. The maximum double amplitudes of the induced vibrations are indicated in

Fig. 1k in units of the acceleration of gravity:

g.

-9-The measurement of the induced vibration was carried out by fixed towing. Two accelerometers were located on station No. 19, (0.05 L aft of the fore perpendicular), just above the center of the fin. One accelerometer was located 90 a/rn above the

baae line

and the otuer 2L15 rn/rn . The position of the center of rotation for torsional vibration is lLfO rn/rn above the base line using the assump.

tion from McGoldriok'e papar. The magnitudes of the vibrations in-dicated in Fig. 14 cannot be transferèd directly to full scale, as there is no dynamical similarity between thé model and the ship, which will be very difficult to be realized. However, the

qualita-tive vibration characteristics of the model are sufficient to jud.. ge the vibratory performance of the various fiflSD

Fin E is found to be the most favourable with regard to vibra-tion. Comparing Fin E with Fin C and Fin D, it can be realized that

if

is not iffective toredthó ibratiÓn.abythe hoIesuniformly

over the fins. It is aleo realized that distribution slits parallel to the fin span are not favourable with regard to vibrationa.

Oohi has found that the mechanism of the induced vibration je

of two types; one is the cavity type and the other is the siam-plt..

cavity type. He found that the impact force, due to collapse of the cavity and/or slamming, is applied to the bow side and to the fin. It is supposed to be favourable to locate the slits so far from the bow side that the collapse is induced to occur far away from the bow side to avoid that the impact force is applied to the bow. He has also found that the induced vibration is initially a torsio-nal rather than a pure horizontal flexural vibration which is also

confirmed by the experiments in this paper.

Various phase angels such as between pitch, heave, wave and the induced vibration are shown in Fig. 1k.

_CcPV

o means that

the induced vibration occurs at the instant in which the model is in horizontal position during ber downward stroke. The shift of the phase angle between pitch and the occurence of vibration should be noted. This is caused by the fact that the collapse of the cavity

is quickened by the slits.

-k.

Concluaions.

A considerable pitch reduction will be obtained by antipitching fins. Antipitching fine are also effective for heave reduction but this depends on the hull particulars of a ship. The magnitude of pitch reduction is not influenced by the location of the relief holes or by the shape of the holes as long as the total area of the holes is not large to the effective fin area.

The reduction of the resistance in waves due to the fins is

re-markable at synchronous conditions. Although the holes or slits on the fins are generally not favourable to reduce the resistance in waves, it is possible to have holed fins, such as Fin E, which reduce the resistance as much as Fin A.

Pitch reduction, even if it were not so magnificient, is

favoura--

lo

-bio to maintain the ship speed at sea because speed loss at sea is caused not only by the increased resistance but also by green

wa-ter on the fore deck, slamming and propeller emergence.

3.

Holes or slits on the fins are remarkable effective to avoid indu-ced vibrations. However, one must take care of the position and dimensions of the holes. Fin E was found to be the best, Fin C and

a

Acknowledgement.

The experiments have been carried out by the author under a

scholarship of the Ministry of Education in the Natherlands

The author wish to express his great thanks to the kind instruction of Prof.ir J. Gerritama0 Ria discussion and help during the test and

preparation of this paper are moat gratefully acknowledged.

Valuable advices were received from Messrs. Ir M,C. Meijer, Ir H.J. Zunderdorp and. Ir J.J. v.d. Bosch throughout the experimente

for which the author is deeply appreciative.

Thanke are also due to Messrs. E. Baaa, A. Rosat, W0 Beukelman and M0 Buitenhek for their active cooperationin the teats0

-R = resistance in still water

o

R = increase of resistance due to wave action V = model speed in meter per second

a = vertical acceleration of the bow

d = draught

g acceleration of gravity

h = wave height = wave amplitude

V1 = lower horizontal bow acceleration V2 .= upper horizontal bow acceleration

z0

= heave amplitude Fr = Proude number.. gL R g 2B2/L resistance. 12 -List of symbols.

OÇ maximum wave slop.

phase angle between pitch and heave

4.=

phase angle between wave and pitch

Era phase angle between wave and heave

¿= phase angle between wave and bow horizontal vibration.

(= phase angle between pitch and bow horizontal vibration

¿ phase angle between heave and bow horizontal vibration A. = wave length

Ç density

= pitch amplitude

B beam of ship

L length between perpendiculars R = resistance

13

-REFRENGES.

Becker, L.A. arid Duffy, D.J.

"Strength of antipitching fins and ship motions measured on U.S.A. Compass I8land

CEAG 153)".

D.T.M.B. Report 1282.

Pournaras, U.A.

"Pitch. redu'ction with fixed bow fins on a model of the series 60,

0.60 block cofficient". D.T.M.B. Report io6i. Pournaras, LA.

"A study of the sea behaviour of a mariner-class ship equipped

with antipitching bow fins". D.T.M.13. Report

108k,

k. Stefun, G.P.,

"Model experiments with fixod bow antipitching fins",

D.T.M.B. Re.porti1iS.

Abkowitz, M.A.

"The effect of antipitching fins on ship motióna".

SNAME 1959.

Ochi, K.

"Hydroelastic 8tudy of a ship equipped with an antipitohing fin"..

SNAME

1961.

7. Jaeger, H,E., do Does, J.Ch. and Gerritama, J.

"Description of the new laboratories of the department of naval architecture, University of Technology, Daift - Holland".

international Shipbuilding Progress, Vol. k, No. 30.

3. Gerritsma, J.

"An experimental analysis of ship motions in longitudinal regular

waves".

International Shipbuilding Progress

1958.

L

01d

20-031-04 FIG. 2 )

Ir

'o 'o O' FIN B FIM E J

PLAN VIEV0F ANTIPITCHING FINS

SCM.E. Il MATIAL WOOD NOP. No.: 1121 I4OLNO.: 31

TODO IORtES ShO

'o

1.5 OES S r. 1.0 0.5 o 120 lOT 60 60 40 PITCH .1,20 0 ° 1.0 1.0 0.5 120 100 3. .' 60 60 Lo - 23 HEAVE FIG. h PITCH 0.5 0 S.S 120 ISO 60 60 LO 20 01 F 02 03 S NOFIN ® FIN A o FIN E HEAVE PITCH 01 02 03 01 02 03 PHASE LAO 02 0.3 0.5 05 o 120 100 ¡ 60 00 40 20 NOFIN ' FIN A FINE HEAVE PHASE LAO PITCH A/L .1.20 0 O I0 0.5 05 HEAVE 0 01 02 03

J3. 100P::'LAO0

I0 to 0$ 120 3. 100 60 60 40 20 HEAVE l'o 1,5 al 02 03 .03 01 Fr PHASE LAO 5.3 CI 02 PHASE LAO DI 0.2 02 01 02 PITCH FIG. 5 A/L .1,00 h/ .'/oo PITCH 2VL .0.60 h Fr 52 03 0:1 02 0.3 o _0I 02 03 01 03 01 02 PHASE LAO 01 02 Fr 03 Si 0.2 03 0 Fr 0I 02 Fr 03

H 0 1.0 o DI 02 03 N I-0 0.5

I

FIG. 6 0.1 0.2 03 Fr 03 HEAVE Fr -HEAVE 1.0 N 1.0 f 0.5 lo 0.5 1.0 0.5 FIN C o FIN D PITCH .1.20 0.1 Fr 0.3 io f 0.5 0 DI 0.2 03 0 BI 02 03 f 0.5 Fr 02 NO FIN 03 01 .0.2 03 0.1 Fr 03 03

¡0

0.5 FIG. 7 PITCH 3, L .tOO 5/L 1/30 1.0 a

045

PITCH h, .0.80 .1/30 _Fr 03 Fr 02 00 HEAVE HEAVE FIN FIN B, B. PITCH À/ /. 1/33 PITCH A, L .1.00 /L .Yoo PITCH A/L I, L 0.00 i 130

mo 0mo loe mo mo mo t'io mo t-mo mo loo too * NO FOI FIN * OtO 015 FIG. 13 a at to 0.0 0.1 0.0 - 03 01 0.3 03 t.N ti 10 ti 14

too toi o.oe

at 0,2 0.1 04 DI 05 03 :. o ti ti ti LS

t-010 .._F 015

---Io F10. 16 _Fr 515 53F __ .:: -Vo. ¿0 10 .10 .- VSI C 00 .10 e. FIN E am F' 015

MODEL EXPERIMENTS WITH SEVERAL BOW ANTIPITCHING FINS.

3y T Sorioda,

Deift Shpbuildin _Laboratory.

Repqt No, 9O

Juj 1962.

Soakesping teste with a model of Todd 60 series (Cb

o,6)

fixed with several bow antipitching fina were carried out on longi-tudinal regular waves and in still water. The purpose of the experi-mente was to investigate the influence of relief alita or holes over

the fine on the motions, resistance and induced _vibrations.

A model fixed with bow antipitobing fins showed better

seakee-ping _{characteristics at synchonous conditions than a model without} fine. The experimente confirm that the hole. or slits ov.r tbø Lins are very auch beneficial to reduc. the induced vibrations

any disadvantage to the aotiona and resistance in waves comparing with the fine having no holes or alita provided that their dimen-sions and position should be very carefully decided.

Itrodu9 tion.

Jiost of the interest of naval

_{architecte bad been concentrated}

on the still water performance of ships in the early stages of the

research. It was thought that a ship which showed a good perfrtice

in still water would aleo aho

_{a good performance at sea. It may be}

said that it was prtly due to this opinion that ehip with bulbous bows were designed to improve the propulsion chavaöteriac

Since, however, a ship spende _{moat of lier life at sea, it seems} quite natural that the ship pQrformanQe at sea has recently beon given

footlights in the research.

xperionta1 and tbeoretjoa3. work and fuli Scale investigations

have been carried out to improv, _{the seaworthiness of a ship both} from the point of view of ship motions and propulaio.

Numerous valuable theories have been established in the field

of the rolling motion of

_{a ehip and we bave succeeded to control}

_the

rolling motion of a ship byeeveral means of antirofling stabilizer5

The research work for the longitudinal

motion of a sbtp haa started at the same time and a theory concerning the ±ncreaeed reei.

tance of a ship in waves has been

_developed.

_Many

_{seaworthiness}

tests have been carried out to get

_{the most suitable form in}

_waves.

It has been realized that there

_{is a very close relatton between}

_the

longitudinal

ship motion and the resistance _{in waves. Furthermore,}

it has been confirmed that pitch je

_{more dominant to both}

_{the sea-e}

worthiness and the increased

resistance

than heave.

the idea arised

_{of decreasing pitch by antipitohing}

_fina

and aevor]. e3cperimental studieu _{have been carried out in this}

field

including two Lull scale applications. _{One of the full scale appli..} catione which was on the _{Holland..Amertca Line vessel "Rijnda&'} turned out to be guite _{unsuccessful and discouraging. The U.S S.} Compass Island WaS aleo equipped with antipitobirig _{fine and she}

brought us many valuable

data.f i]

_{Both of these two caSes showed} that the ships seriously suffered from transverse bow Vibrations

which were duø to flow

_{separation at the fins.}

Many experimental reporta,

_{however, have indioated the feasibi-.}

lity of reducing pitch

_{motion, although they also gave}

_warnings

about the transverse bow

_{vibrations f2) [3] [41 [5]. Nowadays it is}

already an established opinion that antipitching fina are effective oepeciafly when a ship is in her synchronous _{condition. Och lias} recently attacked the study of the mechanism and the properties of th. induced bow vibrations _[6J

. Several

_{interesting facts have been}

revealed by his

experiniente.

The influence of the fin

_{particulars to}

pitch, heave, resis-tance end induced vibrations is treated in thu paper. The experi-mente are especially concerned with the effect of relief holes

_or

alita on the fine.

_{Scale effects are ignored la}

_{this investigai.}

tion.

2. _{Exerimeijta3. procedure.}

2.1. Model partj.cu13r5.

A 1/5k fiber glass model of the Todd 60

_{series (DTMB Model}

No. k211-W

representing the 065

block

_{coefficient) was used for}

the experimente. Particulars of the model

ar. given in Tabu

_1.

A secial tin fixing mount was inserted at the fore foot

_{of th.}

mo-del 80 that fins could be easily

_{exchanged. A photograph of the}

fore part of the model is shown in _{¡ig. 1.}

Table 1.

Mdel Particulars.

Length between perpendioulare _{2.258 s.} Breadth 0.311 s. Draught 0,125 ni. Displacement _{57.0 kg.} Blàok coefficient _0.650 Waterplene coefficient

_0.7k6

Waterplane

area

o.ak ¿

M&se moment of inertia fo pitch (in air) _{1.85 kg.a. eec2.}

Radius of inertia

0,25 L.

/

22. Pin partioularsd

Several experiments have been carried out to study the effects of fin partiou1a. _{on pitch reduction, induced vibration}

and. resis-tance. _{he author thinks it valuable}

to summarize the tacts which have.'

been shown by these _experimente.

Fin are.

Thøfinof larger area produce _{larger pitch reductions, however}

the reduction is not proportional to the increase of _{area [3]}

According to [4), the fine of _{which the total effective}

area is 2 percent of the watarplane area were twice as effective as the 1 percent fins, the 4 peroent fina, however, were only about three times _{as effective, There ie ].eaa probability for}

'vibration when the fin had _{a larger area, but if oavitiea and/or}

slamming do occur with the larger fin, the intensity of_t1sifloed vibration ta proportional to fin area.

Áspeot ratio.

Small changea in fin dimensions with Constant fin area gave

negligibl, affecte on pitch reduction

[J

_1. I is expected that fi.ue of greater _{span will he more effective to reduce the} induced vibrations,

o). iD snOe.

Tip fences Seem to have good effcta on pitch reduotio _and'in

duced vibrations. d), Pf _{èfmeohanipma.}

Relief meohaniain _{are very useful to reduce the}

'inducid

vibra-tion and moreover, they do

_{not bring sne.rka}

changea on pitch reduction oomparedi _{to the tin which has no relief meoha..} flea,

Considering these facte, the _{dimensione of the fins in our} ex-perimente ware chosen as ahown in Table 2 and Fig. 2. Also photo graphs of these fine _{are shown in Fig.. 3. Since the}

purpose of this experiment is to study the effect of the relief holes on the fins,

the, main dimensions of th. fins have been kept constant. The total area et the boles in the fine lias _also

_{been kept Cofl8tant.}

Pin

-5-which is a variation from Fin H1, bas tranaversé elite of -5-which the

axis in. the profile le decilnett backwards by 15

degroeß,

Th

Croes

section of the fins was streamlined to

_{minimise the resistance.}

A]] the fina were made of plastics and fixed to the

_{mounting blòok}

by two screw

boita. (Fig. i).

Table 2.

particulars.

Looatio (aft of F.P.)

_{005 z L}

(abov, base line) _{0,20 z d}

Fin area (P & S) _{0,024 x}

Mean span (at oria side) _{78 mm}

Chord length _{80 mm}

Nax, tbickrieee _{12 mm}

Mean aspecto ratio (at one aide)

_0,98

Chord thickneos ratio _6,67

Total area of the holes _{8.8 of fin area}

23. Xnstrumentatjot.

Ib the teats the model was free to pitch, heave

_{and surge}

and restrained for roll,

sway and yawing

motions. Resistance, pitch

and heave amplitude, wave. height, bow vertical acceleration end bow

transverse acceleration were measured. Resistance measurements in still water and in regular waves were carried out with a gravity dynamometer fitted with a tr4nemisaiou of one to five (Inertia

et-feats resulting from the surging

motion

of the model and the appli-catad load were reduced in the same proportion. Heave and pitch were measured by means of low friction _{potentiometers. Yave heights} were measured by nekns of a codUet*.s type _{wave height meter.}

8tatham linear acoe].arometa _{were used for the measurement of}

ace..

leratious. Model _{peeds were mea5ured ueing a photo transistor and}

electronic counter arrangement. (Por

_{further details, see}

_IJ

_and

[8] ).

A] the data except for resistance, were recorded

on two

2.4. ;e.t

The Øxperjments were carried out in the Shipbuilding Laboratory

of the 1Yniversitp of Technology atDelft, _{using a gravity}

_towing

ay.-tern and a pnaumatic wavemaker for _{generating regular waves. The tests}

were carried out in regular iead waves which _{have the dimensions as}

shown in Table 3.

The tests for Tin A, Fin E and without fin

_were

carried out for aU

8ix

_{wave condition.. The wave conditions:}

0,80, h/] 1/40 and .-/L 1,20, h/I, - 1/40 were excluded

tor the other fine. Resistance testa

in still water were carried out

for each fin.

Tbl. 3.

h

hL

0,80 _{5,6 cm 1/40}

2,26 1,00 _7, cui _1/30

2,71 1,20

3.

Analsispf teat resulte.

3.1. Pitch and )eave amplittd.s

Curves of pitch and heave amplitudes and _{th. plia.. angle} be-tween pitch and heave are plotted against Froude _{nusber in Fig 4} through . The various fine are

classified into three groups in

or-der to avoid confusion and to uiake it

easy to øouipaDI. Both pitch

and heave auiplitud.a are expressed in dimensionless units, that

ia,

amplitudes of pitch are divided by

maximum wave slope and amplitudes of heave by wave amplitud.. The variation _{of the wave amplitude in} the various condition. was leas than _{? percent.}

Fig. k and Fig. Y show that Fin E _{is as effeotive as Tizi A and} pitch reduction, of about 25 percent to _{30 percent were obtained at} synchronous conditions with both Fin

A and Tin E. Fig.

_{6 and Fig. 7} aleo show that there is no remarkabl, _{change in pitch and heave} am-plitudee dependent on the shape and the distribution o1 hole. or slits in th. fins.

-7-It should be noted, however, that there _{is considerable dif} te-rence between Fin B1 and Fin 22. Therefore it esame _{to be effective} to give an inclination backwards to the _{elite in the profiles.}

It should be remembered

that the

_{total area of t) bolee or}

slits is rather amai]

(about 11

percent of the effective fin area). Considering that the tota). _{area of elite was about 23}

_{percent of the}

sff.otiv. area which was used in Ochi's experiment, _{it can b. said} that th, variation in shap. or location of th. bolee or slits hae very little influence on pitch or heave amplitudes _{as far as the} total area of the bolee or the alite is lees than 25 percent of the effective area. The importance of the shape and location increases

with the total area of holes or slits.

It should be noted that "Antipitohing _{Fine" ara aleo} coneide-rable eff.tiv aa "Antiheavftig Fine". _{The reason is that the} coup-ling _{effect of pitch to heav, je larger than that of heave to} pitch. According to Fig. 7a and 7b from [ioj th. coupling effect

of pitch to heave is significant at synchronous conditions for the

Todd 60 series modele

_{(Cb * 0.6, 0.?). It is ShOwn}

_{that th. heave}

amplitudes due to the coupled motion are almost twic. as big as th*t of uncoupl. _{mntioi. This does not hold in general becaus. the}

coupling term depends much on hu)3.partjouiars.

3. .

Rejtance.

3.2.1. lu stil.] watet.

The stil]. water resistance of the mode3 fixed with various

antipitohing tine is given in

Fig. B.

_{Comparing Fin A with No Fin}

in the

range of practical speed _{th. resistance of the model with}

Fin A is about 25.20 percent mor. than

_{that without fine. It should}

be noted that th. resistance of Fin E i. slightly lese tba that

of Fin A at Fr.> 0.17. That _{asan. that th. angle of attack of the} fine were not at anoptjmu _{with regard to the flow along the}

hull.

At higher speeds the flow is _{rectified by the alite, on thea other} hand, th. flow is disturbed by them at lower speed. The _same

argu-ment held. for the other tiria _{except for lin which is always}

worse than th. others.

8--8..

3.2.2. Inwavee.

Moat of the experime _{concerning antipitching fins have}

_been

r'riea out to study the effect of UlLS fins on pitch reduction.

This te aleo indicated by the _{name "Antipitobing Fine's. The} reduc-tion of excess moreduc-tione of the ship itself is important for the

corn-tort of the passengers and the _{crew or for the prevention of cargo}

damage. Since the pitching _{niotion is the main cause for increased}

resistance in wavee, the method. of reducing pich motion are aiea important t. minimize the lose of speed at sea.

The dimeneion1es increas, of resistance in waves with

vari-oua fine is shown in Fig. 9 through lip The

_{dimensionless transfer}

function Le expressed as:

g

r23/L

Some of the tins are _{oonaid.rab3.yt effective to reduce the increased} resistance in waves especially at synchronous conditions. Fin Aand

Fin E are noteworthy in 'this respect. Fin I ta as effective as Fin A but it is lees effective at speeds higher than

The other tin, are effective in the _{following order; Fin C,}

_{Tin D}

Fin B2 and

_Fin

Fig. 8 _{ehowa, ot the other band, that fins geuer.1].y increase}

th. resistance in still water. Therefore _{the curves of the total} resistance of the model fixed with Fin A, Fin E and withoiat tte

ars given in Fig. 12 and 13 in order to ehow the total effect of the

fins on the resistance in waves. The figures show that a maximum reduction of 25 percent is obtained _{at synchronous condition..}

The al,tn on t3 _{Tin E do*a not show any drawbac1 with}

regard to

the ritanoe in wavss. The figures _{aleo show that the resistance}

of th. model tixd with fine in _{waves is loss than that without fini} in the rang. of practical speed at synchronous wave oondition

3e3. Indqçed ibration.

The magnitudès of the induced horizontal vibration were me&sz

red

_{for the various fine in unite}

_{of acceleration. The maximum}

doublò amplitudes of the induced vibratjo

at,e indicated

in

Fig. 14 in units of the acceleration _{of gravity: g.}

qn - n _s

r

The measurement of the induced vibration was carried out by

fixed towing. Two aøcelez,ometera Were located on station No. 19, ($.O _{L aft of the fore perpendicular), just above the conter of}

the tina. One aoc.l.rometar was located

_{90 m/m abov. the baee lin.}

and the other 25 rn/rn _{. The position of the center ot rotation for}

toraiena]. vibration is i0

_mIa

above the base line

using the asaump.

tian from Z1cOeldrick'a

_{paper. The magnitud.. of the vibrations}

in-dicated in Fig. 14 cannot be transfired

_{directly to tul]. scale, as}

there ie no dynamical similarity between the model and the ship,

which will, be very djtfinult to be

_{realized. Rowever,}

the

qualita-tive vibration characteristics of the model

_{are sufficient to jud}

ge the vibratory performance of the various fins.

Fin E is found to be the moat favourabl, with

rer4 to

vibra-tion. Comparing Fin E with Pin C and Pin

_{D, it can be realized that}

it is not effectiv, to reduce the vibrations

_{by the holes uniformly}

over the fine. Xt is also realized that distribution slits

_parallel

to the fin span are not favourable with regard to vibratiens

Oohi bas found that the meohantem of _{the induced vibration is} of two typee one is the cavity

type and the other is the

elum-plue-cavity type. _{e found that the impact fore., due to collapse}

of the

cavity and/or slamming, la applied to th. bow side and to the tin.

ft is supposed to be favourable to locate the slits so far from the bow aide that the collapse is induced _{to occur far away from} the bow side to avoid that the impact _{toro. is applied to the bow.}

e has aleo found that the iduoed vibration is _{initially a}

torsio-nal rather than

a pure horizontal flexural vibration which is aleo

confirmad by the experiments in this papar.

Various

phase angels

_{auch as between pitch,}

_{heave, wave and} the induced vibration ere shown in Pig. 14. = O means that

th. induoed vibration _{ocours at the instant in which the mode]. is} in horizontal position during ber downward _{stroke. The shift of the,} phase angle between pitch and the 000urence of vibration should be

noted. This is osueed by

the fact that the collapse of the cavity

lo

-k. Conclu4ene.

A considerable pitch reduction will be obtained by antipitching fine. Antipitehing fine are aleo effective for heave reduction but thu depend. on the hull particulars of a ship. The magnitud.

of pitch zeductien i. not influ.noed by the location of the relief

holes or by th. shape of the helee _{ai long ai the total area of}

the holes is not large to the effective fin area.

The reduction of the resistance in waves due to the fina is

re-markable at synchronous canditione. Although the holes or slits on the fins are generally not favourable to reduce the resistanc. in waves, it is possibl. to have holed fine, auch as Tin , which

reduce the resistance ai much a. Fin A.

Pitch reduction, even if it were xwt so magnificient, la favours..

bis to maintain the ship spe.d at esa because apeed loas at sea is

caueCd not only by the inorea.ed reeietancø but aleo by green w&

tar on the fore deck, slamaing and propeller emergence,

Holee or elite on the fine are remarkable effective to avoid indu

ced vibrations. However, one muet take care of th. position

and

dimensione of the holes. Fin E was found to be the beat, Tin C and

=, 11

-k31.dem.rt.

The experiments have been carried out by the author under a

scholarship of the Ministry of Education in the _Netherlands.

The author wish to express his great thanke to the kind instruction of Prof.ir J. Gex'ritaaa. Hie discussion and help during the test and

preparation of this paper ara moat gratefully acknowledged.

Valuable adviciea were received from Messrs. Ir M.C. Meijer,

Ir H.J. Zunderdorp and Zr J.J

v.d.. Bosch throughout the experiments

for which the author is deeply appreciative.

Thanks are aleo

due to Mesare. E. Baas, A. Noest, W,

Beuke].Ína'n

-1s

of gbo*.

maximum wave elope

a

phase angle betwø.n pitçh and heave

(,,.a phase angle betwe.n _{ways and pitch} ¿. phase angle between wave and heave

e phai. angle betwesn _{wave and bow horizontal vibration.}

¿, _{phase angi. btws,n pitch and bow horizontal, vibration}

a

_{»ha.. angi. between heav, and bow horizontal}

_vibration

7.. -

wave length

a

density

a

pitch amplitud.

B z bean of ship

L

a

length between perpendiculars

R

a

_re.ietan.

= resistance in still water

R increase of resistance due to wave

action

V _{model speed in meter per second}

a

vertical acceleration of the bow

d draught

g

a

acceleration of gravity

h - wave height

a wave amplitude

a

lower horizontal bow aoee].eratjon

V2 _{upper horizontal bow aooeeration}

z0

a heave amplitude

r

a

_{. Proud. number,}

gL

o

R

- _{dimensionless transfer function for increase} of

g

13

-RE(RENCES.

Becker, L.A. and Dut fy, D..?.

"Strength of antipitohing tins and ohip _{otione measured on U.S.A.}

Compase Is].and (EAG 153)". D.T.}T.3. Report 1282.

Pournaras, LA.

"Pitch rsduetion with fixed bow tine OU a model of the series 60,

0.60 block oofficent". D.T.M.B. Report 1061.

Pournaras, Uk.

"A study of the oes behaviour of a mariner-daBs ship equipped

with antipitching bow fins". D.T.M.13. Report 1084.

k.

Stefun, a..

"Model experiments with tixod bow antipitching fins", D.T..M.B. Report

iii8

5. Abkowitz, LA.

"The effect of

antipitching fins on

ship motions".

3NA}E 1959.

6 Ochi, K.

"Hydroslastic study of a ship equipped

with an

_{antipitohing fin".}

SNAME 1961.

Jaag.r, H.E., de Do.e J.CIi.

and

Gerritema, J.

"Deacriptien of the new laboratoiiee of the

_{department of naval}

architecture, University of Technology, Deif t - Holland".

International Shipbuilding Progrese, Vol. 4, No. 30. Gerritetna, J.

"An experimental analysie of 8hip motions in

longitudinal

regular waves".

International Shipbuilding Progress 1958.

O

FIG.2

190

FIN

B2 A 1 j D a

e

-4

FiNO

4-15{

PLAN VIEWS OF ANTIPITCHING FINS

SCALE: 11 _MATERIAl.: WOOD

EXP-NO.: 6128 MODELNO.: 31

TODD 60 RIES S 65

/

/

/

/

/

,

34

_I.

e

,

0.5

-Ñ

_'LO U.S

0

pTCH

/L

i.20

i

_/L

_1/4Q rn. 0.2

Fr

e.

O

ol

Fr

120

N 100

3.

.

e

u) ₈₀ .0.2

f

0.3

k

60 40

\

₂₀

.0.3

0.2

N

10 o 120

N 100

a

w ₈₀ 60 40 20 HEAVE 0.1

/

,

.-/1

_/

05

_/

/

,

-

-,

F/

-.

0.2 Fr 0.3 0.1 Fr

02

0.3 o 0.1 Fr 0.2 0.3

i.o

N 1.0

î

N :3. W 1.5 0.5 0.5 O 120 100 80 60 40 20 o 0.1' HEAVE G

PHASE LAG

af

___-0.1 PITCH 0.2 0.2 o o e 0:3 0.3. 3 ® NO FIN

FIN A

FIN E

XI

.f.L

hi

/L

0.80

i

=1/4.0 0:1 .0.2 0.3

i

0.5 o

i

0.5 o 120 100 80 60 40 20

-NOFIN1

®

FINA

e PITCH 0.1

,

Fr HE AV E 2

IL =1.20

h/

/

/L

=1/30

0.3.

3

0 01 0.2 0.3

-Fr

0.2

_ Fr

PHASE LAG

- -=---.

0.3

t

0.5 8.0. 60 40 20 HEAVE N 120 Fr

? ioo

PHASE LAG

0.1 PITCH Fr 0.2

A/L

_=1.00

h/

ii

'L =730

3

o 0.1 0.2 0.3 0.2 0.3 0 _0.1 1.0

Î

0.5.

OES 0.5 O

N 120

w 80 60 40 20 HEAVE

-- - - --4

- -

2

-PHASE LAG

al

0.1 PI:TC H - 0.2 __ Fr 0.2

Ai

_/L

OE«80

h,

!/30 0.3 0.3 0.3 O 01 0.2

.

Fr

1.0

N

t

o

-0.1 0.2 0.3 01 0.2

3

-

Fr

to,

N

05

PITCH HEAVE Fr

A-IL

1.00

h/

ii

/L

Y30 o,.1 _{0.2 0.3}

Fr

0.1 0.2 0.3

1.0

N

i. OE

05

o PITCH HEAVE

AI = 0.80

h

ii

L = ¡30

0.1 0.2 0.3 Fr 0.1 0.2 0.3

PITCH

FIN C

FIN D O 0.1: _{0.2 0.3} Fr 0.1

- Fr

0.3 1,.0 0.5 1.0 0.5

1.0 3 0.5 HEAVE 1.0 PITCH 01 0.2 _0.3

XI

'L ..tOO

hj

IL

h/30 ai. 0.2 0;3

1.0 3 0.5 1.0 0.5 HEAVE Fr P ITCH

Al

IL =0.80

hi

'L

=1/30

- - - - -

- -.

o 0.1 0.2 0.3 o 0.1 0.2 0.3

900 800 700 600 L-D) 500 o 400 300 200 100 NO FIN

FIN A

FINE

0.6 015 0.7 0.20 0.25 I I I 0.30 0.8 0.9 1.0 1.1 1.2 1.3 1.1.

Vm/sec

-0.6 0.7 0.8 0.9 1.0 -

Vm/sec

1.1 1.2 1.3 1.4

006.007

-._

NO FIN

-®-

FIN A

__

FIN E

0.05 o i o

F1G19

BYL 015

Rr

0.20 0.25 A/L _0.80 h1 0.30

,

,

,

,_#_

-/

,

,

0.20

Fr

X/L 1.00 h1 1/40

o 0..05 o .io 0.15

Fr

006.008

L 10

FIG. lo

Q.25 _0.30 o 0.05 010 0.15 0.20 Fr

D D

006.009

lo

o

-i--

FIN B1

--

FIN B2

- FIN C

-_

FIN D

-- ---

--- ---

_{- - -}

-- --

-0.05 010 ç

_grzB,

Rr

0.15 NO FIN

FIG. 11

0.20 _0.25

X/

'L

0.80

hi

'L = /30

0.30

0.05 010 0.15

Fr

10

io oo 900 800 700 600 I-t: ₅₀₀ 400 300 200

loo

o

-.--

NO FIN

e--

FINA

FIN E

/

J

/

/

, -

,--."

/.-.-/_-_

---010 0.15

Fr

0.20

II

I I I

r

i I I I i 1.00

h,

i/

'L

'30

0.25 - 0.30 I I 0.1 0.2 0.3 0.4 0.5 0.6 - 0.7 0.8 0.9 1.0 1.1 - 1.2 1.3

14

rn/sec _H

XI

'L

i.00

h,

_i

¡ L = 40 1000 900 800 700 600 L O)

j

500 400 300 200

-e-

--7

-100

7

7

- -.-- -

-O i 0.1 i Ó.2 i 0.3 i b.4 Fr 0.10 0.15 _{0.20 0.25 0.30}

I--

--J I I

-l-

-- i 1 i

i-

! i 0.5 0.6 0.7

08

0.9 1.0 1.1 1.2 13 1.4

--,

V. rn/sec

1100 1000 900 800 700 I-600 500 400 300 200 100 o _0.1

+

NO FIN .®- FIN A I I 0.2 0.3 0.4 0.10 0.5 0.6 0.15 .1' 0.7 0.2 0.8

09

1.0

y rn/sec

1.1 0.25 1.2 1.3 0.30

1100 iood 900 800 700 t-c ₆₀₀ ¿00 300 200 100 O

=

0.10 0.15 0.20

-,

i r i i j. 0.1 0.2 0.3 0.L 0.5 0.6 0.7 0.8 0.9 1.0 1.1

-

y rn/sec

XI

'L

1.20 h/L

/o

0.25

030

I -- J 1.2 1.3 1.4

1200 1100 1000 900 800 700 C) Q 600 500 400 300 200 100

f

- -

.

--

.._

.

-I - I I ---I--0.1 0.2 0.3 0.4 0.25

X/

'L

0.80

kh/30

-

Fr

0.10 0.15 0.20 I

J----

- I - 1 1- -- - I- -I - i - -I ----I 0.5 0.6 0.7 0.8 0.9

iii

1.1 1.2 1.3 1.4

V rn/sec

0.30

-e---

NO FIN

FINA

FIN -E

1300 1200 1100 - 1000 900 800 700 L l ₆₀₀ 500 400 300 200 100

.- /

_,

_/

.-

-.-

..--.

_-.

-

,

-

-.-- -.--

- -

-- -- -- -- --

-- --

_0.10

'2

0.15 _0.25 -1 -J - t

-I

I J J I - - J. I j

----

_{J -.} I I I 0.1 0.2 0.3 0.4 0.5 Ó.6

07

0.8 0.9 1.0 1.1 1.2 1.3

t4

rn/sec

-

Fr

0.20 _0.30 - I

010

Fr

À'

IL1.20

hi i-

_{lL = "30}

0.20 0.10 o

,Fr

70 60 50 N ₄₀ -(kf ₃₀ 20 10 o 40 > 30 (43 20 > 20 .30 _40 60 50 L; '43

FIN 4--

₃₀

--FIN

20

FIN E

o _1 O N 20

-.0

40 40 3o (4i 20 f O

FIG. 15

0.10 Fr 0.15

't,

/L

1.00

h,

i,

'L

/30

lo

o _1 O 0.10 Fr 0.15 0.10 0.15

0.3

02

0.1 o 0.6 0.5 0 4

---0.80'

'yL=14o

005

0!05 010 010 015 0.15

Fr

Fr

0.20

20

0.25

025

0.30 0.30

0)0.3

0.2 0.1

0.3 0.2 0.1 0 0.5 4 0.3 ..p, 0.2 t 0.1 O

-0.05 0.05 010 010 0.15 0.15 0.20

e-Fr

0.20

Fr

0.25 0.25 0.30 0.30