Seaworthiness of three designs for a United States coast guard 160-foot water patrol craft

o'

o

RESEARCH AND DEVELOPMENT REPORT

February 1956 Report 1009

Tcd'tìschc

=

NAVY DEPARTMENT

THE DAVID W. TAYLOR

MODEL BASIN

WASHINGTON 7. D.C.

SEAWORTHINESS OF THREE DESIGNS FOR A UNITED STATES COAST GUARD

160-FOOT WATER PATROL CRAFT

by

160-FOOT WATER PATROL CRAFT

by

George P. Stefun

TABLE OF CONTENTS Page ABSTRACT

i

INTRODUCTION

i

DESIGN CHARACTERISTICS

i

TEST RESULTS 5 Smooth-Water Resistance 5

Speed Reduction Due to Waves 5

Vertical Accelerations

Pitching and Heaving Motions 16

Phase Angles and Apparent Pitching Axis 19

Wetness 22

DISCUSSION 22

CONCLUSIONS 25

ACKNOWLEDGMENTS 26

LIST OF ILLUSTRATIONS

Page

Figure 1 - Body Plan for Design A 2

Figure 2 - Body Plan for Design B 3

Figure 3 - Body Plan for Design C 3

Figure 4 - Curves of Waterlines and Sectional Areas at Design Displacement 4

Figure 5 - Model Resistance and Running Trim 6

Figure 6 - Speed versus Wavelength at Constant Tow Force 7

Figure 7 - Cross-Fairing Method for Obtaining Speeds in Waves at Constant Power 8

Figure 8 - Speed versus Wavelength for Constant Effective Horsepower 10 Figure 9 - Average Amplitude of Bow Acceleration versus Wavelength

at Constant Tow Force 11

Figure 10 - Bow Acceleration versus Speed for Design A 12

Figure 11 - Bow Acceleration versus Speed for Design B 13

Figure 12 - Bow Acceleration versus Speed for Design C 14

Figure 13 - Amplitude of Average Acceleration at Amidships 15

Figure 14 - Amplitude of Pitch versus Speed 17

Figure 15 - Amplitude of Heave versus Speed 18

Figure 16 - Phase Lag of Heave Referred to Pitch 20

Figure 17 - Location of Apparent Pitching Axis 21

Figure 18 - Design B at Tow Force for Design Speed 23

Figure 19 - Design C at Tow Force for Design Speed 24

LIST OF TABLES

Table 1 - Ship Characteristics 2

Table 2 - Ship Speed in Waves, at Constant Tow Force 8

Table 3 - Ship Speed in Waves, at Constant Effective Horsepower 9

Table 4 - Average Acceleration at Station O in g's 16

Table 5 - Amplitude of Pitch in Degrees 19

The results of a seaworthiness investigation are presented for three designs of a proposed United States Coast Guard 160-ft Water Patrol Craft. The results were obtained from model tests in regular head seas with a

wave-length to waveheight ratio of 15:1.

A comparison of the three designs with respect to speed reduction in waves, magnitudes of vertical acceleration, and amplitudes of motions,

indi-cates that no one design is superior to the other two in all respects. The selection of the most suitable design, therefore, depends on which performance

characteristics are considered to be of major importance. The conclusion is reached that the design designated as B is the best with respect to amplitudes of accelerations and motions, and Design C is the best with respect to resist-ance in waves. The major difference in the two designs is that Design B has a transom stern and Design C a cruiser stern.

INTRODUCTION

A seaworthiness investigation of three designs for a proposed all-weather patrol craft was requested by the U.S. Coast Guard in order to determine the optimum of the three designs insofar as seaworthiness characteristics are concerned.1 The investigation was carried out with models built to a scale of 24:1.

Tests were conducted in the TMB 1404t tank which is equipped with a wavemaker to simulate regular head seas. Special emphasis was placed on severe sea states characterized by a wavelength to waveheight ratio of 15:1. The tests, therefore, represent the worst sea conditions which the vessels will probably encounter.

The results of the investigation are presented in terms of speed reduction due to waves, vertical acceleration at the bow and amidships, and amplitudes of pitch and heave. Qualita-tive estimates of wetness, bow emergence and submergence, etc., can be obtained from 16-mm motion pictures on file at the Model Basin.2 The comparative performance of the three designs is shown by tables and curves.

DESIGN CHARACTERISTICS

The major particulars of the three proposed designs, designated as A, B, and C, are given in Table 1. The characteristics for both the design waterline and the load waterline are listed. The model tests were performed with all three designs ballasted to the load waterline, corresponding to the full-scale displacement of 380 tons.

TABLE i Ship Characteristics

Body plans for Designs A, B, and C are given in Figures 1, 2, and 3. Curves of sectional areas and of half-breadths at the design waterline are shown in Figure 4. As can be seen from the figures and from Table 1, differences in the three designs are minor, with

Q of Ship

L

1/2

Fig4lre i - Body Plan for Design A

Design A Design B Design C

Design Load Design Load Design Load

L.B.P. (ft) 160.0 160.0 160.0 160.0 160.0 160.0 B (ft) 25.11 25.19 24.54 24.56 25.08 25.13 I-! (fI) 6.58 6.83 7.0 7.08 7.0 7.35 A(tons) 361.0 380.0 373.0 380.0 351.0 380.0 CB 0.478 0.483 0.475 0.478 0.437 0.450 C, 0.733 0.652 0.737 0.655 0.779 0.610 0.780 0.613 0.690 0.634 0.701 0.642 12 14,f16 9 20

1/lili

119½

ffIII1I1UU

((11111111__

lII___o

3 18 -0 W.L.

Ii WA

Ii

11ViVA1VIII1i

F.;zV1V1V1IIIIL\

IAVIhIí

WIIIILt

16'- 0 "W. L. 1110 14'- 0"W.L. 12'-O"W.L, 10'

U'IVIIJJI

7'- 0"W.L 6

III

6'- 0"W. L

____

IHawAvMilg

81

5'. 0"W. L4' . 0" W. L 10 _{3' 0"W.L}

__

18 161 14 -

T-

i.II!Vjn i

2'- 0"W.L 12. 1'- 0"W.L I. oase Line D

b

b

411

b

b

4P?

Qof Ship

Figure 2 - Body Plan for Design B

Qof Ship

L0

Figure 3 - Body Plan for Design C

0"W.L. 0"W.L. Q"W.L. 0"W.L. 0"WL. 0"W.L. 0" D.W.L. 0"W.L. 0"W.L. 0"W.L. 0"W.L. 0"W.L. 0"W.L. ¡ne 13 16 199

iijtIIÏI%t

A.P.

i

18'- 0" 16'- 0" 14'- 0"

A.P. Fairing Station

IJIUW1I'

19 Fai ing Station F.P»

AV4VßßßI/

10'- 0» 8'- 0" 7'. 0" MI

-

1a

a

' 6 MIIii 6- 01 s' - o" 4' - 0" 3'- 0" 10 _{2' - 0"} 1'- 0" Base L D - 4"

-, 11

1WIII,..

iVNVDa

18'-16' 14'

tW

1LrtI1LiiIiArJi1r1rIIIIIi

UI1II

10'-ii!)

8' -'

IRAWJM%ßII

7'- 6'5' 4' -10 3'-12 2'- 1'-Base L -D _co ,, --= :, -,

i"

L. .L. .L. .L. .L. 0. L. V.L. t.L. 8. L. f.L. iLL. iLL. W.L. ¡ne

-N

A/di

N

A

_

-p_ii

Hall-breadth at D.W.L. A, D.W.L. = 6.58 ft B, D.W.L. = 7.0 ft _{C, D.W.L. = 7.0 ft} Design Design - Design A.P. ¡8 I6 '4 12 8 6 4 Stations

Figure 4 - Curves of Waterlines and Sectional Areas at Design Displacement

140 120 loo 80 60

o

40 20 o 14 12 Io

-t, C, o =

the major differences occurring at the stern. Designs A and B have transom sterns; C has a cruiser stern, which results in C having the smallest block and waterplane coefficients of the three designs.

The above-water portions of the three designs show that B has somewhat moreflare

and freeboard forward than either A or C. All three models are equipped with a guard rail at the deck level, not shown on the body plans of Figures 1 to 3. A bulwark above the guard rail was not included in the preli.minaryplans received from the Coast Guard, but may be used in the final design.

The weight distribution of each model was arranged to give alongitudinal radius of gyration corresponding to 25 percent of the ship's length. Natural periods of heave and pitch were measured by manually oscillating the models in calm water. Results for Design A give

full-scale values of 3.5 sec for the natural heaving period, and 3.6 sec for the natural pitching period. Design B has periods of 3.7 and 3.6 sec respectively; C has a period of 3.5 sec for both heave and pitch.

rEST RESULTS

SMOOTH-WATER RESISTANCE

The 6-ft, 8-in, models were towed by a gravity system. Model speeds were measured electronically and were correlated to a known towing force which was corrected for the inter-nal friction of the system.

Smooth-water resistance tests were conducted to fix the towing forces to be used in the subsequent wave tests. The curves of model resistance and running trim for the three models at the load displacement (Figure 5) are provided, therefore, to indicate the relative magnitudes of the tow forces used to tow the models in waves. No attempt should bemade to extrapolate the model results to full-scale values since experience with small models at the Model Basin indicates that such results are not reliable, due to unstable flow conditions at low Reynoids

numbers.

SPEED REDUCTION DUE TO WAVES

The tests in waves were performed using three tow forces for each model, correspond-ing to still-water speeds of 80, 100, and 110 percent of the design speed. The predicted speed in waves for the three designs is given in Table 2 and Figure 6 for a range of wave-lengths from 75 to 180 percent of the ship's length. In general, the results at constant tow force indicate somewhat less speed loss for Design C than for A and B. All three designs, however, show considerable speed reduction in the severe sea conditions represented in the tests, the most serious speed reduction occurring for wavelengths of the order of theship's length. For a tow force corresponding to 80 percent of the design speed in smooth water, the

4.5 4.0 3.5 3.0 2.5 2.0 t .5 i .0 0.5 o

'5

t

1/

1/

//

1/

Design A (Modet Design B (Model Design C (Model 4592)

/ /

i

4593).

-4594)

/

1

/7

//

Design Speed 2.0

25

3.0 3.5 4.0 4.5

Model Speed in knots

Figure 5 - Model Resistance and Running Trim

2 _C)

G) C)

20 18 6 14 , 12 o .E IO -w, Q) 6 4 2

Figure 6 - Speed versus Wavelength at Constant Tow Force

I

Design Design Design

A

-iii-i

B

-:íi_ C

-0 16 32 48 64 96 28 160 92 224 256 288 Wavelength in feet Wavelength - Shiplength Ratio 0.2 0.4 0.6 0.8 1.0 1.2 1.6 1.8 '.4

R0

R V = R0 xV0 = Constant

TABLE 2

Ship Speed in Waves, at Constant Tow Force

speed in waves of length equal to the ship's length is about 25 percent of the smooth-water value.

lt is observed that the curves of speed at constant tow force (consistant resistance) tend to exaggerate the loss of speed due to waves. This is brought out by reference to

Figure 7. The resistance i? of the model is plotted against speed V in smooth water and in waves of lengthÀ1, À2, and À3. The speeds in waves at a constant value of resistance R0 are given by the intersections of the line i? ₌ with the curves of resistance for the various wavelengths. Thus, the speed in wave À1 is V1 and the speed reduction is given by V0- V1.

If, however, a curve for constant effective power given by the product Rx V (shown as a broken curve in the figure) is plotted instead of the curve for constant resistance, then the

Smooth Water

R = R0

Figure 7 - Cross-Fairing Method for Obtaining Speeds in Waves at Constant Power Wavelength

ft

Waveheight ft

Speed, knots Speed, knots Speed, knots

A B C A B C A B C 0 14.4 14.4 14.4 18.0 18.0 18.0 19.8 19.8 19.8 120 8.0 7.45 6.76 7.76* 16.02 16.55 16.82*

-

-

-168 11.2 3.14 3.28* 3.14 6.71 7.23* 7.01 8.21 8.88 9.28* 192 12.8 3.23 354* 3.43 6.32 6.78 699* 7.89* 7.88 7.87 240 14.6 4.47 4.67 5.10* 8,82 9.44 994* 10.58 10.57 10.79* 288 15.8 5.31 5.32 7.03* 12.15 12.37 13.39* 13.99 13.80 14.39*

*Ship with least speed loss.

speed in wave is T71 and the speed reduction is given by V1 - V1'. The figure shows that the speed in waves at a constant value of effective power is greater than that indicated by the speed at a constant value of resistance. Table 3 and Figure 8 were thus prepared from cross-faired values of speed and the product of speed and resistance to indicate the speed reduction at a constant value of effective horsepower corresponding to the smooth-water effective horsepower at 80 percent of the design speed.

VERTICAL ACCELERATIONS

Vertical accelerations were measured using two Giannini accelerometers with a range of ± 2g. The instruments were located at approximately Station 4 and Station 16. Values were extrapolated to the forward perpendicular and to amidships by drawing the envelope of the accelerations obtained at Stations 4 and 16; i.e., a line between corresponding values of

acceleration at 4 and 16 gives the acceleration at that instant for any point on the ship. The envelope of these lines represents the peak acceleration at any point.

The term "average acceleration" is used in this report torepresent the mean between peak upward acceleration (corresponding to downward velocity decreasing or upward velocity increasing), and peak downward acceleration (corresponding to downward velocity increasing or upward velocity decreasing). Except for the 120-ft wavelength, the upward acceleration is always somewhat larger than the downward acceleration.

Figure 9 gives the average acceleration amplitudes at the forward perpendicular for the three designs at constant tow force. The ship speeds at which these accelerations occur are those given in Figure 6 for the corresponding wavelength. In general, Figure 9 indicates that Design B has less bow acceleration than either A or C for the constant tow force conditions. In view of the severity of the sea conditions, the accelerations do not appear to be excessive

for any of the three designs. (Text continued on page 16.)

TABLE ;

Ship Speed iii Waves, at Constant Effective Horsepower Wavelength

fI

Waveheight ft

Speed, knots *percent Speed Loss

A B C A B c O 14.4 14.4 14.4 0 0 0 120 8.0 10.4 10.8 10.8 27.8 25.0 25.0 168 11.2 6.3 6.7 7.0 56.2 53.5 51.4 192 12.8 6.3 6.7 7.0 56.2 53.5 51.4 240 14.6 7.7 8.1 8.4 46.5 43.8 41.7 288 15.8 9.2 9.6 10.2 36.1 33.4 29.2

*percent Speed Loss Speed in Smooth Water - Speed in Waves <

o

20 8 6 14 8 6 4 2

Design A Design B Design C

Wavelength - Shiplength Ratio

0.2 0.4 0.6 0.8 1.0 Wavelength in feet

Figure 8 - Speed versus Wavelength for Constant Effective Horsepower

.2 .4 1.6 224 256 192 96 128 160 32 o 16 48 64

V) 0.8 = Q cv .2 o o 0.8 V, a = Q cv 0.4 G) L) L) o

Wavelength - Shiplength Ratio

0.8 1.0 1.2 1.4 1.6 1.8

Wavelength in feet

Figure 9 - Average Amplitude of Bow Acceleration versus Wavelength at Constant Tow Force

/

Tow Force for 19.8 knots in Smooth Water

/

Design A

Design B - -

-Design C

-

Tow Force for 18.0 knots in

_uï_ii

Smooth Water

rì

Zero S peed

Smooth Water

-Tow Force for 14.4 knots n

.6 _1.2 C C Ó

08

w o 1.6 Q 0.8 0.4 Wavelength = 168 ft Waveheight = 11.2 ft -Waveleng h 120 ft Waveheight = 8.0 ft

Upward Acceleration Average Acceleration Downward Acceleration

-Wavelength = 192 ft Waveheight

12.8 ft

-

-T

Figure 10 - Bow Acceleration versus Speed for Design A

0 2 4 6 8 IO 12 14 0 2 4 6 8 Io 12 Speed in knots Speed ¡n knots

I .6 U) 2 o 1.6 u) 12 a, C-, C-) 0,4 Wavelength 168 ft Waveheight = 11.2 ft Wavelength = 120 ft Waveheight 8.0 It I I I Wavelength = 240 ft Waveheight 14.6 ft

-Upward Acceleration

-Average Acceleration Downward Acceleration -Wavelength = 288 ft Waveheight = 15.8 ft Wavelength 192 Waveheight 12.8 ft ft 2 4 6 8 IO 12 14 0 2 4 6 8 IO 12 Speed in knots Speed in knots

u, L) 1.6 -Y' I 2 = o

08

G) L) 0.4 1.6 l.2 = = o

08

a, a) C) L) 0.4

-u..

ISUI

Upward Acceleration Average Acceleration Downward Acceleration

-W w W w _{Wavelength = 240 ft Waveheight = 14.6 ft}

14

Figure 12 - Bow Acceleration versus Speed for Design

C Wavelength = 288 ft _{Waveheight = 15.8 ft} 0 2 4 6 8 lo 12 Speed in knots 8 IO 12 6 Speed in knots

= 14.6 ft

-=

8t

Wavelength = 168 Waveheight=1L2 ft Wavelength 120 ft Waveheight = 8.0 ft

-:

-2 4 6 8 Io 2 14 Speed n knots

Figure 13 - Amplitude of Average Acceleration at Amidships 0.5 0.4 0.3 0.2 0.. I 0.4 0.3 u, 0.2 = = 0.1 co 0.3 o o 0 = o. E < 0.1 0.3 0.2 0.1 o 0.2 0.1

PITCHING AND HEAVING MOTIONS

Pitching angles and the amplitudes of heave at the center of gravity were obtained from 35-mm motion picture records of the model tests. The camera was fixed in position 20 ft abreast of the model, giving a field of view corresponding to about 25 ft of model travel. A background grid, graduated in feet, provided a suitable scale for obtaining motion measure-ments.

The results for the pitching amplitudes of the three designs are given by Table 5 and Figure 14. In general, Design B appears to have less pitching motion than A or C, with C having the most. The heaving motion is given by Table 6 and Figure 15. Design B again seems to have a slight advantage over A and C, with A usually having the largest heaving

motion.

The trend of bow acceleration with speed for several wave conditions is given by Figures 10, 11, and 12. Accelerations for fixed speeds are listed in Table 4 to facilitate comparisons among the three designs. The table indicates that no design is superior to the other two in all of the test conditions, but B has the least acceleration in half of the 32 con-ditions listed.

The amplitude of the average acceleration amidships is plotted against speed in

Figure 13 for the three designs in several wave conditions. Here again, there is no consistent trend of superiority of one design over the others, but in general, B appears to have somewhat less acceleration than A or C.

TABLE 4

Average Acceleration at Station O in g's

Speed knots Wave Dimensions, ft 120 x8.0 168 x 11.2 192 x12.8 240x146 288x15.8 A B

CA

B C A B C A B C A B C 0 0.23* 0.24 0.26 0.37e 0.40 0,41 0.45 0.49 0.43* 0.65 0.53* 0.58 0.26* 0.32 0.31 2 045 0.34* 0.40 0.55 0.52* 0.62 0.59k 0.69 0.64 0.54* 0.65 0.59 0.41 0.40 0.35* 4 0.59 0.43* 0.52 0.75 59* 0.78 0.75* 0.87 0.83 0.55* 0.67 0.62 0.55 0.47 0.43* 6 0.63 0.51* 0.61 0.92 0.89* 0.92 0.92* 0.97 1.01 0.65* 0.71 0.69 0.67 0.56 0.54* 8 0.67 0.54* 0.68 1.07 1.01* 1.19 1.07 1.01* 1.12 0.83 0.80 0.79* 0.78 0.66* 0.66* 10 0.66 0.59* 0.70

-

-

-

-

-

-

1.02 0.93* 1.00 0.89 0.78* 0.81 12 0.64 0.59* 0.68

-

-

-

-

-

-

-

-

-

0.99 0.93' 0.98 15 0.56 0.48* 0.56

-

-

-

-

-

-

-

-

-

1.12' 1.25 1.20

Design Design Design A I Wavelength Waveheight = 288 = 15.8 ft ft

B

-__

- -

- - - -

-C

-l.

-_-

-Wavelength = Waveheight = 240 ft 14.6 ft

- -

-N= 8t

Wavelength = 168 ft Waveheight=11.2ft

---

=2Oft o 2 4 6 8 Io 12 14 Speed in knots

Figure 14 - Amplitude of Pitch versus Speed Io 9 8 C) C) C-C C) Io L) Q-9 C) = 8 E lo 9 8 7 6 3 2

6 5 4 4 3 2 2 Design Design Design Wavelength = 289 ft \Waveheight = 15,8 ft A

B

-C

7

Waveheight Wavelength=240 ft 14.6 ft

-/

Wavelength = 192 ft Waveheight = 12.8 ft

-

---Wavelength = 168 ft Waveheight= 11.2 ft

-Wavelength = 120 ft Waveheight = 8.0 ft

==

- - -

--0 2 4 6 8 Jo 12 14 Speed in knots

Figure 15 - Amplitude of Heave versus Speed

9 8

7 6

TABLE 5

Amplitude of Pitch in Degrees

TABLE 6

Amplitude of Heave in Feet

PHASE ANGLES AND APPARENr PITCHING AXIS

The phase lag of heave with respect to pitch is plotted against speed in Figure 16. These results, together with the curves for the location of the apparent pitching axis given by Figure 17, are presented merely to complete the motion analysis. The results are of little

help in choosing the best design, since the error in phase angle measurements is about± 5 percent. Actually, a single curve for all three models might well be a realistic representation for the data of Figure 16. Similarly for the data of Figure 17, following the methods of

Speed knots Wave Dimensions. ft 120X 8.0 168X 11.2 192X 12.8 240 X14.6 288X15.8 A B C A B C A B C A B C A B C 0 2.50 2.40* 2.88 5.20* 5.58 6.25 7.31 7.30* 7.60 8.42 8.25 8.10* 799 773* 7.87 2 2.62 2.53* 2.99 6.00v 6.10 7.36 9.41 933* 9.71 9.03 8.66* 8.83 8.52 8.11* 8.27 4 2.62 2.53* 2.98 6.70 6.48* 7.71 10.18 973* 11.03 9.48 9.02* 9.40 8.94 8.47* 8.68 6 2.50 2.43* 2.83 7.10 6.63* 7.53 9.65 9.30* 10.60 937* 9.81 9.23 8.77* 9.02 8 2.31 2.23* 2.64 7.20 6.53* 6.86 8.47 8.22* 9.13 9.98 9.63* 10.14 9.48 9.06* 9.38 10 2.07 2.00* 2.39

-

-

-

-

-

-

10.11 9.84* 10.42 9.66 9.31* 9.70 12 1.18 1.70* 2.09

-

-

-

-

-

-

-

-

-

9.80 9.52* 997 5_ 1.31 1.19* 1.61

-

-

-

-

-

-

-

-

-

9.98 977* 10.26

*Shjp with least pitching motion.

Speed knots Wave Dimensions, ft 120 x 8.0 168 x 11.2 192 x 12.8 240 x14.6 288x15.8 A B C A B C A B C A B C A B C 0 0.55* Q55* 0.72 1.90 1.83 1.67* 3.03 2.44* 2.85 4.86 4.20* 4.48 5.75 5.36* 5.50 2 1.02 0.90* 1.10 2.28 2.08 2.04* 2.85 1.90* 2.58 4.30 4.30 397* 5.70 5.23* 5.56 4 1.27 1.08* 1.31 2.73 2.39* 2.60 3.12 2.09* 2.86 4.25 4.43 4.03* 5.82 5.31* 5.75 6 1.35 1.13* 1.38 3.22 2.79* 3.22 3.71 2.93* 3.69 4.65 4.69 4.61* 6.04 554* 6.04 8 1.29 1.09* 1.35 3.78 3.32* 3.81 4.52* 4.58 4.98 5.35 5.09* 5.60 6.37 5.90* 6.39 10 1.15 Q93* 1.25

-

-

-

-

-

-

6.27 5.72* 6.89 6.73 ß35* 6.80 12 0.93 0.81* 1.10

-

-

-

-

-

-

-

-

-

7.19 6.93* 7.25 15 0.53 0.48* 0.80

-

-

-

-

-

-

-

-

-

7.88 777* 7.98

o, 60 C, -c .E _so 70 Design DesignB A

-

o

X

-N

Design

_-

Wavelength Waveheight = =

-288 ft 15.8 ft

-4

-C

I.

o

"

I Wavelength Waveheight = = 240 ft 14.6 ft a Wavelength Waveheight = = 192 ft 12.8 ft

- ---.--

' fr-

a X Wave length = ].68 ft Waveheight = 11.2 ft

-

I- - . -

___

a o 4 6 8 Jo 12 14 Speed in knots

Figure 16 - Phase Lag of Heave Referred to Pitch

90 80 70 60 80 70 60 50 40

V, c- 24 V, E < 6 o C) u-C 'C CC C 24 cv C. C-C o C', 32 24 16 8 o Design Design Design A

B - - - -

_{Wave length} Waveheight = 288 = 15.8 ft ft C

=

-Wavelength Waveheight = 240 = 14.6 ft ft ______C._._.____.-.__________...i:=: Wavelength Wave height 192 ft 12.8 ft

-- ___--

Wavelength Waveheight = 168 ft 11.2 ft -4---4 o 2 6 8 Io 12 4 Speed ¡n knots

Figure 17 - Location of Apparent Pitching Axis

32

24

16

Reference 3, calculations for the location of the apparent pitching axis are based on phase lag measurements.

WETNESS

The severity of the sea conditions represented in the model tests makes _any determina-tion of the relative wetness or dryness of the three designs extremely thfficult. All three models were very wet for most of the test conditions, with C perhaps the worst ìn this respect because of its larger pitching motion and somewhat greater speed in waves at constant tow force. Decks of the three models were dry in the 120-ft wavelength at all _{speeds tested. They} were also dry at low speeds in waves of length equal to 1.80 shiplengths. For intermediate wavelengths at low speeds (tow force for 14.4 knots in smooth water) the degree of wetness varied between slightly wet and wet. At higher speeds, corresponding to smooth-water _tow forces of 100 and 110 percent of design speed, all three models were very wet in waves longer than 0.90 shiplengths.

Figures 18 and 19 show a few examples of the large amount of green water shipped in various sea conditions. The photos were taken at the approximate peak of the pitching cycle. The figures also give a good indication of the large amount of forefoot emergence to be expected in severe storm seas. The problem of bow submergence does not appear to be as serious as that of forefoot emergence. The film of Reference 2 shows that only a small portion of the bow is submerged when large amounts of green water are shipped. This might be largely eliminated by providing a bulwark above the guard rail. The guard rail itself is effective in preventing wet decks only in the short wavelengths of the order of 0.90 shiplength or less.

The problem of stern emergence is not as serious as bow emergence but is present to some extent in all three designs in the more severe sea conditions. Design C has the least stern emergence, therefore this design is less apt to lose speed from propeller emergence.

DISCUSSION

The results of this investigation should be viewed in the light of the very severe sea conditions under which comparison of the seaworthiness characteristics of the three designs was attempted. Heavy seas, especially when they result in large forefoot emergence and

serious shipping of green water, tend to obscure somewhat, differences in behavior resulting from form characteristics. As was intended, the tests probably represent the poorest operat-ing conditions in which the vessel may find herself, but these conditions will prevail for a relatively minor portion of a ship's seagoing experience.

The above remarks are not intended to imply that the results obtained for the higher waveheights are not indicative of the actual performance in smaller waves. Rather, it is believed that more moderate conditions may better bring out differences in sea behavior due

sOU)j g'9 :pds

X ;1 6I :saA

g

N P21-61727

Waves: 144 Feet X 9.6 Feet;

Speed: 13.5 Knots

Waves: 240 Feet x 14.6 Feet;

Speed: 9.9 Knots Figure 19 - Design C at Tow Force for Design Speed

Waves: 192 Feet X 12.8 Feet;

to variations in design in the three models. For example, Reference 4 indicates thatfor a Destroyer Escort operating under milder conditions, the advantage of a cruiser stern over a transom stern in maintaining higher speeds in waves is greater than that indicated by the present investigation. Also, it is quite evident that the somewhat greater freeboard and flare of Design B will show to more advantage in moderate seas insofar as wet decks are concerned. It can also be assumed that differences in the three designs with respect to themagnitudes of the pitching and heaving motions will be more definite in milder sea conditions. In this respect, the slightly greater "V" form of the forward sections of Design B will probably show an even greater advantage of less motion, as compared to Designs A and C.5'6

On the basis of the results of the present investigation, no clear-cutconclusion can be reached with respect to the design with the best overall performance. The choice appears to be between B and C, and the final selection shoulddepend on which performance charac-teristics are considered to be of major importance. Thus, if resistance characcharac-teristics are of primary interest, Design C has a definite advantage. If, on the other hand, magnitudes of motion and accelerations, and dry decks are the main interest, Design B is superior.

CONCLUSIONS

Model tests in regular waves representing head seas with length to height ratios of 15:1 indicate that Design C, in general, loses less speed in waves than either A or B for a given smooth-water tow force or smooth-water effective horsepower. Design A suffers the greatest speed reduction in waves.

In view of the very severe sea conditions represented by the model tests, vertical bow accelerations are not excessive for any of the three designs. The largest vertical accelera-tions at the forward perpendicular are of the order of ± 1.1 g. Differences in the three designs are about 0.1 g or less. No one design has consistently less vertical acceleration but, in general, B has the least acceleration both at the bow and amidships.

Design B has consistently smaller pitching amplitudes than either A or C. Design C had, in most instances, the largest pitching amplitudes.

Design B has the smallest heaving amplitudes, and A the largest heaving amplitudes. The decks of the three designs are extremely wet in waves of length greater than the ship's length, except at very slow speeds. Design C is the worst in this respect because of its somewhat greater motion and speed in waves, atconstant tow force.

The selection of the design with the best overall performance depends on which per-formance characteristics are considered to be of major importance. Design C appears to have the best resistance characteristics in waves. Design B, however, shows the bestperformance with respect to dryness and magnitudes of motions and accelerations.

ACKNOWLEDGMENTS

The author is indebted to Mr. U.A. Pournaras for his valuable aid and suggestions throughout the investigation, and to Mr. J. Bonilla-Norat, Mrs. F. Poole, and Miss _D.

Randolph for their help in obtaining and analyzing the test data.

REFERENCES

United States Coast Guard letter ENE-2 of 16 June 1955 to David Taylor Model Basin. David Taylor Model Basin 16-mm Motion Picture NP21-M1178.

Szebehely, V.G., "Apparent Pitching Axis," Forschungsheft fur Schifftechnik (Jan

1956).

Gover, S.C., "Model and Full-Scale Studies of Ship Motions," paper presented at the American Towing Tank Conference (Apr 1953).

Weinblum, G. and St. Denis, M.1 "On the Motions of Ships at Sea," Transactions of the Society of Naval Architects and Marine Engineers, Vol. 58 (1950).

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

Copies

14 Chief, BuShips, Library (Code 312)

6 Tech Library

1 Tech Asst to Chief (Code 106)

i Asst, Chief for Ship Des & Res (Code 300)

i Ship Des (Code 410)

1 Hull Des (Code 440)

12 COMDT, USCG

2 USCG Academy, New London, Coon.

2 USCG Yard, Curtis Bay, Md.

1 DIR, ETT, SIT, Hoboken, N.J.

1 DIR, Experimental Nay Tank, Univ of Michigan,

Ann Arbor, Mich.

1 Head, Dept of NAME, MIT, Cambridge, Mass.

1 Capt. R. Brard, Directeur, Bassis d'Essais des

Carènes, Paris, France

i Dr. L. Malavard, Office, National d'Etudes et de

Recherches Aéronautiques, Paris, France

i Gen. Ing. U. Pugliese, Presidente, Instituto

Nazionale per Studi ed Esperienze di Architettura Navale, Rome, Italy

1 Sr. M. Acevedo y Campoanior, Director, Canal de

Experienceas Hidrodinaniicas, Madrid, Spain

1 Dr. J. D ieudonné, Directeur, Institut de

Recherches de la Construction Navale, Paris,

France

i Supt, Nederlandsh Scheepsbouwkundig

Proefstation, Haagsteeg, Wageningen, The

Netherlands

INITIAL DISTRIBUTION

1 Prof. J.K. Lunde, Skipsmodelltanken, Tyholt

Trondheim, Norway

1 Dr. Hans Edstrand, Director, Statens

Skeppsprovningsanstalt, Göteborg, Sweden

1 Dr. S.L. Smith, Director, British Shipbldg Res

Assn, London, England

1 Dr. J.F. Allan, Supt, Ship Div, National Physical

Lab, Middlesex, England

I Dr. H.W.E. Lerbs, Director, Hamburg Model

Basin, Hamburg, Germany

WAVY OPPO PH?« WASH DC

Copies

S ALUSNA, London, EngIan

3 CJS