Pitch reduction with fixed bow fins on a model of the series Sixty, 0.60 block coefficient

RESEARCH AND DEVELOPMENT REPORT October 1956 Report 1061

alb. v.

Scheepsbouwkunde

Technische Hogeschooi

Delft

NAVY DEPARTMENT

THE DAVID W. TAYLOR MODEL BASIN

WASHINGTON 7, D.C.

PITCH REDUCTION WITH FIXED BOW FINS ON A MODEL OF

THE SERIES 60, 0.60 BLOCK COEFFICIENT

by

Ulysses A. Pournaras

PITCH REDUCTION WITH FIXED BOW FINS ON A MODEL OF THE SERIES 60, 0.60 BLOCK COEFFICIENT

by

Ulysses A. Pournaras

ABSTRACT

INTRODUCTION 1

DESCRIPTION OF MODEL 2

TEST PROGRAM 3

RESULTS AND DISCUSSION 3

Pitch 3

Heave 5

Phase Relationships 6

Location and Vertical Motion of Apparent Pitching Axis 6

General Observations 6 CONCLUSIONS 7 ACKNOWLEDGMENTS 7 REFERENCES TABLE OF CONTENTS Page 1 8

LIST OF ILLUSTRATIONS

LIST OF TABLES

Tablel

Principal Characteristics of Model 4607

-Table 2 - Model 4607 Schedule of Tests in Waves 4

Table 3 - Natural Periods in Pitch. and Heave, and Computed Critical Speeds

iii

Figure 1 - Model 4607, Series 60, Block Coefficient 0.60,

..

Figure .2 Planform and Profile Sections of Antipitching Fins ... ...

Figure 3 - Pitch Stabilization Fin PSF-1

age

9 9

Figure 4

Experimental Pitch Amplitudes, Waves of Various Lengths

and 2.5 Inches High 11

Figure 5- Experimental Pitch Amplitudes; Wavelength-Height Ratio ot 30 12

Figure 6 - Experimental Heave Amplitudes, Waves of Various Lengths

and 2.5 Inches High re 2' IN 13

Figure 7 - Experimental Heave Amplitudes, Wavelength-Height Ratio of 30i.--- 14

Figure 8 - Experimental Phase Angles of Heave After Pitch, Waves of

Various Lengths and 2.5 Inches High 15

'Figure 9 = Experimental Phase Angles of Heave After Pitch,

Wavelength-Wavelength-Height Ratio of 30 16

Figure 10 - Computed Location of the Point of Minimum Motion: Waves of

Various Lengths and 2.5 Inches High 17

Figure Ii- Computed Location of the Point of Minimum

Matibn,-Wavelength-Height Ratio. of 30 18

Figure 12 - Computed Vertical Minimum Motion, Four Waves of Various 41

Lengths and 2.5 Inches High ... u.-, 19

Figurer 13 - Computed Vertical Minimum Motion, Wavelength-Height

Ratio of 30 . 20 -10 -- 2 -- 4

NOTATION a Inertia coefficient Also with b _{Damping coefficient} Subscripts c Restoring coefficient F Exciting force F. Froude number L Length of model h Wave height, 2 r. r_in Wave amplitude

Z Dimensionless heave, em/T.

2 Heave amplitude

5 Phase, heave after pitch

A Wave length

qf Dimensionless pitch, tfrn,/t,

Om Pitch amplitude

t, Wave Slope

-ABSTRACT

The paper presents the results of model tests to determine the fea"s16i1ity

of reducing the pitching motion of ships by means of antipitching fins. A 10-ft self-propelled model representing the parent form of Series 60 and block

coeffi-cient 0.60 was tested in waves of various lengths and heights. Data are pre-sented nondimensionally for possible prediction of full-scale performance, and, where necessary, at model scale. It is shown that depending on speed and

wave conditions a pitch reduction of from 10 to 309ercent can be accomplished

with bow fins havin an area 2.5 percent of the water-plane area of the ship.

INTRODUCTION

Considerable attention has been given in recent years to the general problem of the motion of ships in both regular and irregular seas and to the operating limitations imposed on ships by such motions.1-5 The importance of reducing the pitching motion has been recog-nized and some attempts at solution have been initiated.

The problems associated with the reduction of pitch become evident upon inspection of the familiar equation of the uncoupled pitching motion of a ship:

A

d2O+Bc10+CO--Mei"

dt 2

dt

)

The three reactions of the left-hand side of E uation [1] are the inertia, damping, and restosIng reactions. Any one of these three terms can be altered, at least theoretically, to

produce de .e.d_cl._mt.nges in the motion. The inertia reaction, for example, can be altered by

redistributing the mass or sectional areas of the ship. The restoring reaction can also be altered by redistributing the ship displacement vertically or longitudinally. In practice, how-ever, desirable or imposed characteristics impose limitations on the naval architect's freedom to vary a design. The feasibility of reducing pitch thus becomes dependent on the feasibilit of increasing the damping coefficient of Equation [1] without introducing unacceptabledepar-tares from a desired form or from recognized shipbuilding practices.

It is, indeed, possible to design a hull form having relatively high damping character-istics; however, such form would deviate significantly from what is known as a normal form. On the other hand, it is generally recognized that only radical deviations from normal form are

sufficient to bring about a significant decrease in the motion. Qne possibility remains - that of increasing the damping characteristics of the ship by means other than modifications in the

basic ship form, for example by the use of fins, or wing appendages, located at selected points along the hull.

[11

'References are listed on page 8.

-Some experimental work has been carried out in this direction.° A noteworthy full-"scale application has been the installation of antipitching fins on the Holland-America Line

vessel RYNDAM. The RYNDAM experience has been somewhat unsatisfactory and

discour-aging. It has been reported that the ship suffered greatly by transverse vibrations attributed

X _{to the fins. These were installed at the bow of the ship not far below the static load}

water-line. After two days operation, they had to be removed before the vessel could continue her journey. Model test results, on the other hand, have given promise of large reductions in

pitching amplitudes through the influence of antipitching fins, and further investigations were thought advisable despite the RYNDAM experience.

The Taylor Model Basin has been active for some time in the study of antipitching fins. The present report describes a series of tests in waves with a 10-ft self-propelled model representing the 0.60 block coefficient, Series 60 parent form with and without antipitching

fins. It is emphasized that scaling up of the data pertaining to the model with fins may be

X

accompanied by serious errors resulting from possible scale effects which as yet have not been determined.

DESCRIPTION OF MODEL

TMB Model 4607 is a 10-ft plastic model representing the 0.60 block coefficient, Se-ries 60 parent form.7 A partial list of model characteristics is given in Table 1. The model was equipped with a TMB stock propeller No. 2071 driven by a 1.25-hp, de motor. The radius of gyration of the model was 25 percent of the waterline length for dynamic similarity with typical ships. The center of gravity was 0.15 ft aft of the midship section, the model being at even keel. A photograph of the model is presented in Figure 1.

TA13LE 1

Principal Characteristics of Model 4607

Agipitching fins, designated PSF-1, port and starboard, were essentially flat

X plates, 1/8 in. thick, shaped at the leading and trailing edges. The leading edge was swept

back. The span of each fin was 4 in., and the root and tip chords were 5 and 3.5 in., respec-tively. The planform area was 34 sq in. or 2.5 percent of the load waterplane area. The

4Ast rune =

Length, LBP, ft 10.00

Model Displacement, lb 265.70

Draft, ft 0.53

Maximum Beam, ft 1.33

Load Waterplane Area, sq ft 9.41

Load Waterplane Coefficient Cwp 0.706

Block Coefficient C 0.60

The

leading edge of the root chord was located 1 1/8 in. aft of the forward perpendicular. The fins were fitted with tip fences, 4 in. long and 3/4 in. deep by 1/16 in. thick. It might be noted that the sweat-back planform was arrived at from structural rather than hydrodynamic consid-erations. Reducing the load toward the tips results in a smaller bending moment at the root; this in turn lessens the structural difficulties of attaching the fins to the hull. Tip fences

_J

ere used to :..roach two dimensional flow.

The fins were attached to the hull by fitting them to a cylinder 7 1/2 in. long and 1 1/4 in. in diameter. The cylinder was cut and shaped into the hull so that it was tangent to the keel, with the root chords of the port and starboard fins parallel and 1 1/4 in. apart. The nose of the cylinder was faired to the forefoot on the lower side, and its upper side was faired to a spherical surface of a radius equal to that of the cylinder. No attempts were made to fair the attachment with fillets. Schematic plans and photographs of the fins and the installation are shown in Figures 2 and 3.

TEST PROGRAM

The model was self-propelled in waves according to the schedule of Table 2. All tests were conducted in head seas. Wave size and motion data were obtained by means of motion-picture records and also by electronic pickup and recording systems. The model was guided by an arrangement which allowed freedom in heave, pitch, surge, and sway but restricted the model in roll and yaw.

RESULTS AND DISCUSSION

Prior to testing in waves, the natural periods of heave and_pitch had been obtained for Model 4607 with and without fins. These are shown in Table 3 along with the computed critical speeds at which the amplitudes of the motions are maxima.

PITCH

The pitch amplitudes of the model heading into waves of various height h and with lengths A from 0.75 to 1.50 times the length of the model L, are presented in Figures 4 and 5; these graphs show the pitch amplitude vs. Froude number and speed for the model with and without fins. The experimental points indicated have been corrected linearly for wave height e.g., by multiplying the experimental values by the ratio of the nominal to the actual wave height. The variations of the measured wave heights were less than 7.5 percent.

Pertinent points to be observed are the shifting of the critical speed, i.e., the speed of maximum motion, and the magnitude of the resulting amplitude reductions as a function of speed, waVe length, and wave height. The shift in the critical speed to lower values in the case of the model with fins can be explained by the increase of the natural period in pitch caused by the fins with the consequence that a lower speed of advance is required for

3

-Model 4607 - Schedule of Tests in Waves,

4

' TABLE 2

TABLE 3

- Nat-La-al Periods in Pitch and tleave and Computed Critical Speeds

7 =_ Ar-ea- r o' ele-.1.4"

-7i Fri ;2 z-jf v =--er

-,

letr-' 3eve*. 7-c % v.< OW, 4IKP _ - 2,4P ₌ v A ft AIL h in* V h

-

-Speed Range.. Without ;Fiats ; knots With Fins, knots . - fl---7.5 0.75 2.5 36 0 to 3.2 0 to 3.2 2.5. 48. 0 to 3.2 0 to 3.2

10.01.00

4.0 30 0 to 2.7 I 0 to 2.7 12.5 1.25 2.5 60 0 to 3.2 0 to 3.2 5.0 30 0 to 3.7 , 0 to 2.7 15,0 .50 2.5 72 0 to 3.2

/

G to 3.2 _ 6,0 30 0 to_2.7_ 0 to 2.7' 1 ;Natural Periods sec

Critical Speeds, knots

A=7.5 ft A.---ao.o ft A=12.5 ft A=15.0 ft

.1

\=.r.,,

-Heave , ,Without Fins.

With Fins 1.045 YX 1.065 0.59 1.42 0.50 1.33 2.35 2.22 3.32 3.16, _

Pitch Without Fins With Fins , _ , 1.011k 1 0.73 1 4

(ii''' '

0.09.g9 1 . 2.59 1.53 3.61 1 2,35 =

synchronism. The effects of the forward speed, wave height, and wave length will be consid-ered simultaneously in the explanation of the effect of the fins in reducing the amplitude of the motion.

Damping generallydepends on the vertical component of the velocity, which in turn,,

_Q

is dependent on the frequency and amplitude of the oscillation. Other things being equal, aq _{_ver../4,4/t60} increase in the vertical velocity results in greater damping. The same argument holds for the

case of increasing wave lengths at a given speed since longer waves imply lower frequencies and, therefore, lower damping. Past the critical speed, also, the amplitude of the motion de-creases and so must the damping. It will be observed that at speeds above the critical, the

model with fins experiences a milder decrease of amplitude than the model without fins. The (trend is for equal amplitudes) at some higher speed outside the range of the tests when the

amplitudes are small and the effect of added damping is only minor.

The preceding remarks must be viewed as general. Considerations as to the effects of hase relationshie between heavin and pitching, which govern the amplitude of the verti-cal motion of the fins and possibly their contribution to damping, have been omitted.

HEAVE

In Figures 6 and 7, the heave amplitude is presented in the same fashion and for the same conditions as in the case of pitch. Again, the experimental points indicated have been corrected linearly for the wave height. The shift of the critical speed to a lower value for the

model with fins is again present but is not so obvious. The damping of the fins, as reflected by the increase of the natural period, results in a shift of the critical speed. This shift alone, however, cannot explain the reductions effected in the amplitude of the motion since the

change in natural period is hardly significant. One must also consider the effect of the fre-quency of encounter, the amplitude of the motion, a,nd above all, the two mosLimportant, factors: coupling effects, and the phase between the motiona_in henve nrid

The importance of coupling effects is generally recognized but little is known concern-ing their magnitude. The magnitude is assumed to be _proportional to the amplitu_de of the motion. The phase angles evidently assume some importance also and, therefore, any dis-cussion that follows must remain within a general and qualitative nature. The equation of the heaving motion, for example, including pitch coupling terms is

d2,1, iwt Ai dt2d2z +B,dz_dt + C,z + a +b dill+ccif= Fe a 2 2 _2& dt

We observe from Equation [2] that the effect of pitch on heave depends on the angle of pitch. Pitch angle, in turn, depends partly on the coupling of heave and pitch and, therefore, on the

displacement in heave. It is noted that for all practical purposes, the natural periods in heave

and pitch are identical for the model without fins. Consequently, the critical speeds in heave and pitch are also identical. ate effect of the fins is toincrease the natural period in pi teh

[2]

much more than the natural period in heave. The natural periods are thus separated as are the critical speeds. With fins, the critical speed for pitch is reached first. Upon reaching the critical speed in heave, the amplitude of pitch has already declined, and the coupling effects are apt to be smaller for the model with fins. The algebraic sign of the coupling effect will be determined by the phase relationship between heave and pitch. The significance of the phase

relationship between the two motions thus becomes apparent. PHASE RELATIONSHIPS

The phase relationship between heave and pitch are presented in Figures 8 and 9. The experimental values are plotted directly without any correction for wave heiaht variations and must, therefore, be treated with the proper caution.

LOCATION AND VERTICAL MOTION OF APPARENT PITCHING AXIS

The location of the apparent pitching axiss and the amplitude of its vertical motion are presented in Figures 10 through 13. By definition, the apparent pitching axis is the point of minimum vertical motion along the ship. The longitudinal location of the apparent pitching axis and the amplitude of its vertical motion can be used together to asseis further effects of the antipitching fins. As the curves of Fi ures 10 through 13 have been computed on the basis of phase angles, they should be viewed:211y as indicative of a trend.

GENERAL OBSERVATIONS

It was found that the model without fins shipped quantities of green water over the bow,

especially in the longer and steeper waves. The shipping of green water was less severein

the 10-ft waves (A/L = 1.00) of 30:1 length-height ratio, and increased in severitywith speed. The most severe condition was at speeds approaching 2 knots

(F0.20) in the 15-ft waves

(A/L =1.50) of 30:1 length-height ratio. Large quantities shipped over the bow and flowed over the deck to near amidships. Forefoot and forebody emergence were frequently observed to occur in the 30:1 waves, particularly in the upper half of the speed range.

TILLustaLfirualittALtaits2jni

i_ppngofireen water in termgth,

When shipping of green water did occur, less water was shipped by the model with fins at the same speeds and wave conditions.

In no case did the forefoof ret2pdy Qf the Diode'

with fins emerge. During the tests in the 15-ft by 6-in. waves (A/L= 1.50, A/h= 30) and for the full speed range, the fins were observed to come near the surface of the water but never to break clean and skim. The fins also approached the surface in the 10-ft by 4-in. waves (AlL= 1.00, Alh=30) at zero speed and at very low speeds of advance.

A flow phenomenon was observed which may be serious and which should be further investigated. On their downward stroke, the fins forced water from the lower to the upper side around the leading and trailinedges. The flow appeared in two distinctsheets. The leading

edge sheet, probably under the influence of the forward speed, closed in overthe upper sur-face of the fin faster than did the trailing edge sheet. The two met near the surface of the water and produced a whirl. As the fins came near the surface in the more severeconditions,

the two sheets partially escaped into the air and produced an audible slap at the sides of the bow. Removal of the tip fences durinIspot check runs did not influence the amplitudes of the motion but produced a third sheet of water around the tip. This sheet xe.c:Lon er than the other two and slapped the sides of the bow with greater intensity. It is believed that such flow will result in large unsteady forces on the hull in the vicinity of the fins. Scale effects will probably be introduced because the atmospheric pressure is the same for the model as for the ship.

CONCLUSIONS

The test results indicate that reduction of the pitching motion of ships can be effected by means of fixed 1, ntipitching fins. Further theoretical and experimental investigations are considered necessary before the useful range and limitations of antipitching fins can be

ade-quately established. The,pitst_ireduction attributable to the fins considerably improves dray:

ness of the model in head seas. The practical speed range, as restricted by motions' of undesirable magnitude, is also giatmlecl. Forefoot and forebody mergence occurring during the tests without the fins were notobserved when the fins were installed.

ACKNOWLEDGMENTS

The author is greatly indebted to Dr. V.G. Szebehely foi s valuable suggestions and encouragement during the course of this study. The cooperation of the members of the Ship Dynamics Branch in obtaining and analyzing the data is gratefully acknowledged. The author is particularly indebted to Mr. G.P. Stefun who analyzed the motion records.

REFERENCES

Weinblum, G. and St. Denis, M., "On the Motion of Ships at Sea," Transactions Society of Naval Architects and Marine Engineers, Vol. 58(1950).

St. Denis, M. and Pierson, W.J., Jr., "On the Motion of Ships in Confused Seas," Transactions Society of Naval Architects and Marine Engineers, Vol. 61 (1953).

St. Denis., M., "On Sustained Sea Speed," Transactions Society of Naval Architects and Marine Engineers, Vol. 59 (1951).

Lewis, E.V., "Ship Speeds in Irregular Seas," Transactions Society of Naval Architects and Marine Engineers, Vol. 63 (1955).

Szebehely, V.G. and Todd, M.A., "Ship Slamming in Head Seas," David Taylor Model Basin Report 913 (Feb 1955).

V6.

Edstam, U.B. and Vytlacil, N., Jr., "Hydrodynamic Effect of a Bow Hydrofoil on Three Vessel Types," Master of Science Thesis, Massachusetts Institute of Technology (1955).

7. Todd, "Some Further Experiments of Single-Screw Merchant Ship Forms

-Series 60," Transactions Society of Naval Architects and Marine Engineers, Vol. 61 (1953). V 8. Szebehely, V.G., "Apparent Pitching Axis," Forshungsheft fiir Shifftechnik (Jan 1956).

2.

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Figure 1 - Model 4607, Series 60, Block Coefficient a.60

i 7 7 3 _{Eye"-rrfr-vz?T-7.--'}

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Figure 2- Planform and Profile Sections,. Anti pitching Fins

0.R.

5

of

9

Figure 3 - Pitch Stabilization Fin PSF-1 The photographs show plan view, profile (fences removed), and the fins as installed aboard Model

4607.

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---4

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Model Speed , knots

Figure 5 - Experimental Pitch Amplitudes, Wavelength-Height Ratio of 30

0.8 0.6 0.4 0.2 0 1.0 0.8 0.6 0.4 0.2 0 1.4 12 1.0 08 0.6 0.4 0.2 --

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I 1 I 4 I I , 0,

o 0 0 0 Modil Speed knots 025 ID 0.8 06 0.4 0 0 0 0.2 04 06 08 ,10 12 1.4 I6 18 2.0 2.2 2.4 26 2.8

Model Speed ,,knots

Figure 6,- Dcperimental Heave Amplitudes; Waves of Various 'Lengths and 2.5 Inches High

1.0, 0.8 0.6 0.4 1;6 114 1.2 1.0 0.8 0.6 04 b -1 I

21 -NE

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MIPMMIN

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Model. Speed ,, knots

Figure 7 - Experimental Heave Amplitudes, Wavelength-Height Ratio of 30 14 0.25. 0.8 0.7 0.6i O5 0.4 0,3 0.2

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Model Speed , knots

Figure 8 - Experimental Phase Angles of Heave After Pitch, Waves of Various Lengths and 2.5 Inches High

15 1 1 Froude Number 0.10 0.15 0.05 0.20 0.25 80 o_ 60 40 20

-02 04 0.6 0.8 110 t 2 14 16 18 2.0 2.2 2.4

Model Speed , knots

Figure 9 - Experimental Phase Angles of Heave After Pitch, Wavelength-Height Ratio of 30

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Figure 11 - Computed Location of the Point of Minimum

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---Froude Number 0.05 0.10 0.15 0.20 -L

---0

0 0.8 0.6 0.4 0.2 1.4 1.2 1.0 0.8 0.6 0.4 c- 0.2 1,6 g 1.4 1.2 0.8 0.6 0.4 1.6 1.4 1.2 1.0 0.8 0.6 19 With Fins Legend Without Fins ---Wavelength =15 ft, A'/L .1.5

Figure 12 - Computed Vertical Minimum Motion, Four Waves of Various Lengths and 2.5 Inches High

0.32 g 1.28 1.12 096 0.80 0.64 0.48 0.32 Froude Number 0.05 0.10 0.15 0.20 0.25 0.64 0.48 0.32 0.16 0.96 0.80 0 0.64 0.48 7 0.32 0.16 1.28 ₀ 1.12 0.96 0.80 0.64 048 1 Wavelength=7.5 ft, A/L.=0.75 Wavelength =10 ft,AA_ =1 ,/

Wav leng h=I2.5 ft, A/L =1.25

2.8

0.4

0 02 0.4 06 08 1.0 1.2 14 1.6 1.8 2.0 2.2 2.4 26

Model Speed, knots

-a

1

I

1.6 1.2 0.8 0.4 2.8 1.6 1.2 3 g 0.8 0.4 0 2.8 *1 2.4 2.0 1.6 1.2 0.8 0.4 0.05 Froude Number 0,10 0.15 0.20 0.25 02 04 06 0.8 10 1.2 14 16 18 20 2.2 2.4 26 2.8

Model Speed in knots

Figure 13 - Computed Vertical Minimum Motion,

Wavelength-Height Ratio of 30 0.60 0.40 0.20 0.96 a, -o 0.80 0.64 ,g 0.48 0 0.32 0 0.40 0.266 0.133 0.16 0.80 75. -0 0.666f,

_a

0.533 a I I 1 1 --1 ---- ----...---- -- ----_...,...-_,--- _{Wove ength .10 ft,} .--- ...---Wavelength .12.5 ft, AiL .1.25 Wavelength . 15 It, X/L .1.5 With Fins Without Fins Legend

-1 I A/L 0

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