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DOCUMENTATIE

POWERING CHARACTERISTICS OF A LOW-BLOCK DISPLACEMENT HULL FORM IN HEAD SEAS

by

Hugh Y.H. Yeh

Heinrich Schutz

Paul Plaia

APPROVED FOR PUBLIC RELEASE: DISTRIBUTION UNLIMITED

SHIP PERFORMANCE DEPARTMENT RESEARCH AND DEVELOPMENT REPORT

Tij,.hA H'sd"1

Th* Naval Ship Research and Development Center is a U S. Navy center for laboratory effort directed at achieving improved sea and air Vehicles. It was formed in March 1967 by merging the David Taylor Model Basin at Carderock, Maryland with the Marine Engìneenjig Laboratory at Annapolis, Maryland.

Naval Ship Research and Development Center Bethesdg, Md. 20034

* REPORT ORGNATOR

MAJOR NSRDC ORGANIZATIONAL COMPONENTS

OFFI CER-IN.CHARGE CAR DE ROCK 05 SYSTEMS DEVELOPMENT DEPARTMENT

*

SHIP PERFORMANCE DEPARTMENT SHIP ACOUSTICS DEPARTMENT 19 MATERIALS D E PAR TMEN T 28 N SR DC COMMANDE R 00 TECHNICAL DIRECTOR 01 OFFI CER-IN-CHARGE ANNAPOLIS 04 AVIATION AND SURFACE EFFECTS DEPARTMENT 16 COMPUTATION AND MATHEMATICS DEPARTMENT 18 PROPULSION AND AUXILIARY SYSTEMS DEPARTMENT 27 CENTRAL INSTRUMENTATION DEPARTMENT 29 NDW-NSRDC 3960/44 IREV. 8/71) OPO 917-872 STRUCTURES DEP AR TM EN T 17

i DEPARTMENT OF THE NAVY

NAVAL SHIP RESEARCH AND DEVELOPMENT CENTER

BETHESDA. MD. 20034

POWERING CHARACTERISTICS OF A LOW-BLOCK DISPLACEMENT HULL FORM IN HEAD SEAS

by

Hugh Y.H. Veh

Heinrich Schutz

Paul Plaia

APPROVED FOR PUBLIC RELEASE: DISTRIBUTION UNLIMITED

TABLE OF CONTENTS

Page

ABSTRACT i

ADMINISTRATIVE INFORMATION

INTRODUCTION I

MODEL AND PROPELLER PARTICULARS 2

TEST FACILITY 3

METHODS AND PROCEDURES ₃

RESULTS AND DISCUSSION ₄

RESISTANCE ₄

PROPULSION ₅

Regular-Wave Experiments Irregular-Wave Experiments

Prediction of Added Power in Random Seas ₅

CONCLUSIONS ₆

ACKNOWLEDGMENT 6

LIST OF FIGURES

II

Figure 1 - Hull Characteristics of Model 4360-I 7

Figure 2 - Open-Water Characteristics of Propeller 3448 8

Figure 3 - Added Resistance versus Wave Height for Constant Wave Length and

Froude Number 9

Figure 4 - Added Thrust versus Wave Height for Constant Wave Length and

Froude Number 10

Figure 5 - Added Torque versus Wave Height for Constant Wave Length and

Froude Number 11

Figure 6 - Added Shaft Speed versus Wave Height for Constant Wave Length and

Froude Number 12

Figure 7 - Added Effective Horsepower versus Wave Height for Constant Wave Length and

Figure 8 - Added Horsepower versus Wave Height for Constant Wave Length and Froude Number

Figure 9 - Exponent (n) for Added Resistance, Thrust, Torque, Shaft Speed, and Power versus Wave Height from Regular-Wave Tests

Figure 10 - Limits of Wave Height Used for Various Regular-Wave Experiments

Figure 11 - Nondimensional Transfer Function for Thrust versus Frequency of Encounter

Figure 12 - Nondimensional Transfer Function for Torque versus Frequency

of Encounter 18

Figure 13 - Nondimensional Transfer Function for Shaft Speed (RPM) versus Frequency of Encounter

Figure 14 - Nondimensional Transfer Function for Power versus Frequency

of Encounter 20

Figure 15 - Measured Sea Spectrum for State 5Sea 21

Figure 16 - Measured Sea Spectrum for State 6 Sea 22

Figure 17 - Typical Product of Sea Spectrum and Transfer Function 23

Figure 18 - Predicted Thrust, Torque, Shaft Speed, and Power Increase for Tests in Regular and

Irregular Waves, for States 5 and 6 Seas 24

19

Table 1 - Test Program ...3

Page

14

15

16

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ABSTRACT

Ship powering characteristics in a seaway were investigated for the model of a low-block codfieient. displacement-type hull in both regular and irregular head seas. The increase in power required to maintain speed was predicted by using both the regular-wave and the irregular-wave test results and these predictions were compared. Results indicate that when

used within certain limits, the assumption held that added drag is proportional to wave

height squared.

ADMINISTRATIVE INFORMATION

This work was authorized by the Naval Ship Systems Command under the General Hydromechanics Research Program of the Naval Ship Research and Development Center (NSRDC). Funding was provided under

Suhproject SR-023-Ol-Ol.

INTRODUCTION

One of the major problems facing a naval architect is to ensure that the new ship he is designing will maintain a certain speed. In order to do this, he must determine the horsepower needed to drive his design at the specified speed and, consequently, he must have methods for predicting the powering characteristics of related hull forms. In the past, these predictions have been based on data from specific model tests or on the results of methodical model-series tests in calm water. Obviously. ships will seldom, if ever, operate in such conditions. Usually, an arbitrary amount of horsepower is added to the calm-water requirement to compen-sate for the increased power needed to overcome added resistance due either to rough water or to bottom fouling. This procedure has been followed mainly because no reasonable technique has been available for

determining the effects of sea conditions on ship powering.

-In 1953, however, Pierson and St. Denis1 introduced the use of a statistical representation of sea con-ditions and a linear superposition technique. This prediction method gave a good approximation of ship-motion responses. More recently, seakeeping basins have been built in the United States and abroad and ship motion studies have advanced considerably. It is only natural that the same techniques are now being con-sidered for determining the added power required in a seaway.

Most investigators maintain that power increases are due mainly to increased resistance, i.e., that all associated propulsive coefficients remain the same. Therefore, if we know the added resistance of a ship in a

'Pierson, W.J.. Jr. and M. St. Denis, "On the Motion of Ships in Confused Seas." Trans. SNAME. Vol. 61. pp. 280-357

seaway, we can obtain the power required for a sea condition by applying calm-water propulsive coefficients to the added resistance. As a consequence, most theoretical papers in this area deal only with added

resistance; those most often referred to are by Maruo.24 The Maruo theory states that added_resistances

are essentially proportional to the square of wave height. Experiments have shown that this is generally true for merchant ships but that the theory is questionable when applied to low-block-coefficient ships.57

The purpose of the present study was to obtain more experimental data on the powering performance in waves of a low.block.coefficient hull form and to determine the applicability of the Maruo theory for

such forms, If, in fact, the "square law" does not hold, then the characteristics of the added-power demands were too defined and test procedures established for determining added power.

MODEL AND PROPELLER PARTICULARS

NSRDC Model 4360-1 was selected for the experiments. This single-screw, twin-rudder, low-block-model with bilge keels was fitted with NSRDC Propeller 3448. The 18.182-ft low-block-model was constructed of wood to a linear ratio of 16.94; ship and model data are given in Figure 1. Tests were conducted for a ship design displacement of 1890 tons at a draft of 11.91 ft, even keel. The propeller characterization and open-water curves shown in Figure 2 represent a 12.5-ft diameter, five-bladed propeller.

A 5-ft model of the same hull had been tested at the University of California by Sibul.57 It was

anticipated that testing a larger scale model of the same hull would provide additional information on blockage effects.

2Maruo, J., "The Excess Resistance of a Ship in Rough Seas," mt. Shipbuilding Prog., Vol. 4 (1957).

3Maruo, J., "Wave _{Resistance of a Ship in Regular Head Seas," Bulletin of the Faculty of Engineering, Yokohama}

National University, Vol. 9 (Mar 1960).

4Maruo, J., "The Theory of the Wave Resistance of a Ship in a Regular Seaway," Bulletin_{of the Faculty of Engineering,} Yokohama National University, Vol. 6 (Mar 1957).

5Sibul, O.J., "Increase of Ship Resistance in Waves," University of California, Berkeley, College of Engineering Report

NA-67-2 (Mar 1967).

6Sibul O.J., "An Experimental Study of Ship Resistance and Motions in WavesA Test for Linear Superposition," The University of California, Berkeley, College of Engineering Report NA-66-3 (Jan 1966).

7Sibul, O.J., "Ship Resistance and Motions in Uniform Waves as a Function of Block Coefficient," The University of California, Berkeley, College of Engineering Report Series 61, Issue 19 (Jun 1961).

TEST FACILITY

Experiments were conducted in the Harold E. Saunders Seakeeping Facility at NSRDC. This

rectang-idar concrete basin is 240 ft wide by 360 ft long and has a water depth of 20 ft. Pneumatic wavemakers arc located on adjacent walls of the basin, and both regular and irregular waves can be generated. Fixed

bar-type concrete wave absorbers are installed along the opposite basin walls. A steel bridge spans the

length of the basin and a model towing carriage operates along tracks hung on the underside of the bridge. Regular waves are generated by setting the dome pressure and by regulating blower rate of revolution (which governs the wave height) and valve-opening period (which governs the wave length). The dome pressure and valve period can be controlled by preprogrammed tapes to produce random sea conditions.

METHODS AND PROCEDURES

Resistance and propulsion experiments were conducted in calm water and in waves. The parameters

explored in the wave tests are outlined in Table 1.

TABLE I - TEST PROGRAM

lt was considered very important to obtain calm-water data in the same manner as rough-water data.

Therefore, the calm-water portions of the experiments differed from traditional calm-water powering

experiments in that an entirely free-running model was used. lt was assumed that the increase in power

due to waves did not differ greatly when the model was operated at ship propulsion point and model propulsion point; for simplicity, therefore, model propulsion points_{were used. Because the calm-water data}

were the basis of all rough-water experiments, runs in calm water were repeated several times (before, be-tween, and after rough-water runs) to ensure an accurate reference for comparing results. Although the differences between all calm-water runs were small (within test accuracies), only tests conducted on the same day as the rough-water tests were used in the analysis to ensure that the condition of the model and the water temperature were unchanged.

3 Propulsion Irregular Head Waves

F =

_n 0.10. 0.20, 0.25, Sea State = 0, 5, 6 0.30. 0.35 Experiments Regular Head

F =

_n 0.10, 0.15, 0.20. X/L = 0.4, 0.8. 1.0, 1.2, 0.25, 0.30, 1.6, 2.0 0.35 Waves 0. 1/130, 1/110, 1/90, 1/70, 1/50. 1/30 Resistance Regular Head F = 0.10, 0.15, 0.20,_n X/L =0.8, 1.0, 1.2, 1.6 0.25, 0.30, 0.35 Experiments Waves /X= 0,1/130,1/110, 1/90, 1/70, 1/50, 1/30

Resistance experiments were conducted with constant tow forces, and the carriage speed was set to keep pace with the model. Model speeds were then obtained by combining the carriage speed with the model surge record.

The model was free-running for propulsion experiments in waves and in calm water except for power supply wires and instrumentation leads. These were attached so as to impose a minimum amount of force

on the model. All experiments were carried out in head seas only. The actual wave heights were measured

by a sonic wave probe located one model length ahead of the model; they were recorded continuously during the experiment to provide instantaneous data on sea conditions. Heave accelerations were measured by an accelerometer located in the model, and data were integrated twice to obtain heave amplitude; positive heave was considered to be upward. Pitch angle was measured by a pitch gyroscope and the bow-down position was assumed as positive. In the pitch and heave calculations, the phase angles express the lead with respect to maximum wave elevation at midship.

Yaw, sway, and roll were measured, but the results were not analyzed since the measured magnitude is small in head-sea conditions. Model surge was detected by a sonic probe and was combined with carriage velocity to yield the velocity of the unsteady model. The model surge signal was also fed back to the propeller drive system to keep model speed constant; therefore, surge was generally very small and could be neglected. Yaw and sway output signals were used to control the rudder servo. This arrangement

kept the model on course with minimum rudder angles (usually less than a couple of degrees). Relative

bow motion was measured by a sonic probe at Station I. Propeller thrust and torque were measured by reluctance magnetic gages, and carriage speed and propeller shaft speed were monitored with slotted-disk,

magnetic-pickup devices.

All test data were recorded simultaneously on analog tape and later converted to digital form for

analysis. Some information was also recorded on strip chart recorders and RMS voltmeters for monitoring during the experiments.

RESULTS AND DISCUSSION

RESISTANCE

The results of resistance experiments conducted at constant tow forces were compared with those of

other experiments8'9 in which the model was restrained in surge motion and even forced to surge. No

significant differences were found in the results of the two types of experiments. Therefore, since the restrained-model experiment is easier to conduct, the technique in which the model is restrained in surge motion should be pursued more fully in resistance experiments.

8Wahab. R. and L.W. Moss, "On the Added Drag of Destroyers in Regular Head Waves," NSRDC Report 3704 (Aug

1971).

No results arc reported for Xfl. = 0.4 and XII. 2.0 because the differences between results of

calm-water and wave tests were sufficiently small to he within instrumentation accuracy.

PROPULSION

Regular-Wave Experiments

As stated in the introduction, one of the main objectives of this study was to investigate the appli-cability of the "square law" for predicting the added resistance in waves of low-block-coefficient hulls.

Therefore, several wave heights were investigated for each wave length and speed (see Table 1). Figures

3-8 show how the various parameters were affected by changes in wave height. The straight lines on these

figures indicate the slope of 2. If the square law holds, then a line drawn through all data points should

forni a line which coincides with or is parallel to this line.

lt can be seen that this was not the case, If we curve-fit the data by the least-squares method, the

exponents of the lines are between 1.5 and 3 (Figure 9). However, if we eliminate the extreme conditions as designated in Figure 10 (that is. omit the waves which are too small or too large), then the majority of the data points fit the square law fairly well, at least for engineering purposes. This procedure can be justified by the argument that because the power required in calm water differs only slightly from that

required in very small waves, the accuracy of such differences is therefore questionable. On the other hand, very large waves cause shipping of green water and the emergence of forefoot or propeller and will

ob-viously lead to a complicated nonlinear domain. Accordingly, all the transfer functions were determined without the extreme data and using the slope-2 lines which appeared to best fit the data points. These are

presented in Figures 11-14 as functions of the frequency of encounter for each Froude number. Irregular-Wave Experiments

Model experiments were conducted at five speeds for each of two sea conditions (States S and 6, with

significant wave heights of 10 and 16.9 ft. respectively). For each condition. 30 min of (equivalent

full-scale) data were obtained in order to have sufficient sampling for a statistical analysis. Because of the

limited length of the tank, this entailed several crossings for many of the conditions. The measured sea

spectra are given in Figures 15 and 16.

Prediction of Added Power in Random Seas

The power required in random waves was obtained from regular-wave data by applying the theory of

linear superposition and integrating the products of the transfer functions (Figures 11-14) and the sea spectra

(Figures 15 and 1 6). Figure 17 shows a typical example of these products. The results of the integration

are shown as lines in Figure 18 (open circles indicate the data obtained from irregular-wave tests). The details of this method are given by Gerritsma et al.10 For this particular case, the agreement was very good.

CONCLUSIONS

The "square law" can be applied to low-block-coefficient hull forms within certain limitations of wave

height.

Care should be exercised in using the results of model experiments to obtain the transfer functions for increase of power or drag in waves. The waves should be large enough to generate significant differences of power or drag over calm-water results, but should not be large enough to create violent hull

motions. This is in contrast to motion study experiments which require only that waves should be small and stay in the linear range.

Additional data on powering requirements in waves are needed to define the applicable limits of the "square law."

ACKNOWLEDGMENTS

The authors are grateful to Messrs. R. Long and D. Huminik for instrumentation support and to Mr. G. Rossignol for computer analysis of the data.

'0Gerritsma, J. et al., "Propulsion in Regular and Irregular Waves," International Shipbuilding Progress, Vol. 8, No. 82 (Jun 1961).

APPENDAGES:

RADIUS OF GYRATION = O.25L

VCG = 12.94 FT

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e

Figure 13 - Nondimensional Transfer Function for Shaft Speed (RPM) versus Frequency of Encounter

600 500 (\J 4) )400 -1 o o 300 200 100 o

1.0

2.0

Encounter FreQuency we

1/sec

Figure 14 - Nondimensional Transfer Function for Power versus Frequency of Encounter

Encounter Frequency we in 1/sec

Figure 15 - Measured Sea Spectrum for State 5 Seas

14 12 10 8 6 4) o a) _U) 4 2 o o 1.0 2.0

3.0

50 140 30 20 10 O o a)

9

('J H a) 3 ti) o 1.0 2.0 Encounter Frequency e in 1/sec Figure 16

Measured Sea Spectrum for State 6 Sea

1 2

Encounter Frequency We in 1/sec.

Figure 17 - Typical Product of Sea Spectrum and Transfer Function

23 50 ,000 a) Cn CID 140,000 o 4-, C)

30,000

20 000

.4-) C) cl) Cn cl)

T

10 000

o

ri) -6 - l-1 5 4 Q -I ,< 3 1) o _H -o -o o 3 -4 li o : 20 10 O r A

p

-r r A

o

i A

__

Thrust r A

o

i

Regular Waves Irregular Waves

à

Sea States 5 6 L o ¡ A I A Powe r A

o

A

-

I I

o

I o I Torque I L o I I I L A L I 0 I RPM i I-0 I-5 20 25 .30 .35 .10 l-5 20 25 .30 .35 Froude Number F

Figure 18 - Predicted Thrust, Torque, Shaft Speed, and Powei Increase for Tests in

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UNCLASSIFIED

SPCUTItV Clos si ficat tori

D D

FORM 1473

NOV 65 I (PAGE 1) UNCLASSIFIED

DOCUMENT CONTROL DATA - R & D

S dir it) clos s, fit ation of title, brrdy uf abs trac t id ,ndcs ing anno Uit/cm inn-I Oc e tIered when (Ire overa!! report s dtnssi(ied) ONILINA TINO ACTIVITY (Corporale author)

Naval Ship Research & Development Center Bethesda, Maryland 20034

2a. REPORT SECURITY CLASSIFICATION

UNCLASSIFIED

2b. GROUP

N/A

3 REPORT TITLE

POWERING CHARACTERISTICS OF A LOW-BLOCK DISPLACEMENT HULL FORM IN HEAD SEAS

4 DESCRIPTIVE NOTES(Typeof reportancl inclusive dates)

Research & Development Report

AU THORISI (First name, middle initie!, tas! name)

Hugh Y.H. Yeh, Heinrich Schutz, Paul Plaia

6 REPORT DATE

June 1973

7. TOTAL NO. OF PAGES

29

7h. NO OF REPS

lO

ea. CONTRACTOR GRANT NO

1m, PROJECT NO

SR-023-0l 01

mc.

d.

98. ORIGINATOR'S REPORT NLJMBER)SI

4059

9h. O TISER REPORT NOISI (Any other numbers that may be assigned this report)

IO. DISTRIBUTION STATEMENT

Approved for public release: distribution unlimited.

ti. SUPPLEMENTARY NOTES f2, SPONSORING MILtTARY ACTIVITY

Naval Ship Systems Command 03412

Washington, D.C. 20360

13 ABSTRACT

Ship powering characteristics in a seaway were investigated for the model of a low-block coefficient, displacement-type hull in both regular and irregular head seas. The increase in power required to maintain speed was predicted by using both the regular-wave and the irregular-wave test results and these predictions were compared. Results indicate that when used within certain limits, the assumption held that added drag is proportional to wave height squared.

I. JN('LASSI FI EI) Security Classification FORM

I

UNCLASSIFIED 4 KEY W080S LINI' A LINK O LINK C

ROLE WT ROLE WT POLE WT

Added Drag in Waves Powering in Waves Speed Loss in Waves