A review of reference material available on the subject of seaworthiness

1,RIME*4;

A 11.

/1..14.)

Experimental Towing Tank

Stevens Institute of Technology

Hoboken, New Jersey

Technical Memeeiltium No.92

January 1951

A REVIEW OF REFERENCE MATERIAL

AVAILABLE ON

THE SUBJECT OF SEAWORTHINESS

by

EXPERIMENTAL TOWING TANK STEVENSHNSTITUTE OF TECHNOLOGY

HOBOK.EN,

A REVIEW OF REFERENCE MATERIAL AVAILABLE ON THE

-SUBJECT OF SEAWORTHINESS

by

'Clifford L. Sayre, Jr.

(E;LT. PROJECT)

TABLE OF CONTENTS.

Page

Summary. ₁

Introduction 3

Classification of TechnicaP.Articles, Papers, and

Other Material Relating to Seaworthiness...i... 5

Abstracts and Reviews. .

KENT,, "EXPERIMENTS.ON MERCANTILE SHIP

MODELS !N WAVES"- 9

KENT,H.L.: "EXPERIMENTS ON MERCANTILE SHIP

MODELS IN WAVES" (SECOND SERIES).. ... .. . 11

KENT, _{"THE EFFECT OF WIND AND WAVES ON}

-THE PROPULSION OF SHIPS" .... .. .. . . 13

-KENT, J.L. "PROPULSION OF SHIPS UNDER

DIF-FERENT WEATHER CONDITIONS" .... . I'0..WPC.14' 17 KENT, J.L.: "AVERAGE SEA SPEEDS OF SHIPS

UNDER.W INTER WEATHER CONDITIONS" ... . .. 19 KENT, J.L.: "SHIP PROPULSION UNDER ADVERSE WEATHER

. . .. ... , .. . . 21

KEMPFy G. "THE BEHAVIOR OF DIFFERENT SHIP

FORMS 'IN A SEAWAY" ₂₃

KEMPF, G.: ."A STUDY OF SHIP PERFORMANCE IN

SMOOTH AND ROUGH WATER" .. ... . 24 KREITNER, J.: "CAN SEAWORTHINESS BE CALCULATED?" ₂₆ KREITNER, J.: "COMMENT ON'LURCHING"

2.7

HANCOCK C.Hfl: "SOME ASPECTS OF SEAWORTHINESS TESTS" ... 28

HASKIND, M.D.: "HYDRODYNAM:CAL THEORY OF THE

OSC/LLA-TIONS OF A SHIP IN WAVES" 30

Comments on Experimental Methods ... . . .... . . .. .... 31

Comments on Full-Scale Observations and Analysis 34

Concluding Remarks ₃₆

Appendix I: Weather: Wind and Waves ... 38

SUMMARY

This memorandum contains a collection of abstracts -or reviews of articles dealing with the subject of seaworthiness. The articles reviewed have been chosen as a .representative cross section of the types available for present study, and 'represent suitable starting points for more extended .investigations of the .subject. The .content of the articles includes research

on model 'behavior, observations of .actual .ship behavior, and .mathematical

:developments of theory: The following table gives a brief description of the .subject .matter contained in the various papers:

The .results of .model tests and .observations of ship -behavior .empha-size the importance of the ;influence of .form .on the motionl of .a ship in

waves. The .resistance in waves .increases with an increase in ;block

coef-ficient. This increased resistance due to waves .becomes such .a large

per-TM-92

-AUTHOR . TITLE ' : DESCRIPTION

. Kent, .

la.

.Kempf, G.

Hancock:, ! C11.

. 'Experiments on Mercantile: Ship

Models: in Waves : °

.'The .Beh,avior. of.Different Ship

, Forms: in: a: Seaway. ' .

!Some:Aspects. of. Seaworthiniss Tests.'

Towing-tank: tests of. ship. models of: different forms,: showing: the variation of. resistance, pitch

' angle,: and pitch period as: a . function, of' wave: height: and wave

length. . Kent,:..ILL. .. . Kent, :ILL. : Kent,:

34.

.

'The. Effect: ofLWind: and: Wavee . on: the Propulsion:.of: Ships:..

. .

'Propulsion. of. Ships% under:

Dif-: ferentDif-:WeatherDif-:ConditionsDif-: '

tAve.ragei Sea! Speeds oft Ships'

under Winter Weather

Condi-tions. '

Series. of: obseivationst during ocean:voyages, illustrating' the

effect. oftiorm: and; relative'l im-portance, of; wind.. and. waves. on

the t inereased. power' requirements.

i

, - / .. . , - 11 . . Kent, J.L. Kempf, . G. -' . 'Ship Propulsion under Adverse Weather Conditions:" .

'A' Study of Ship Performance

in Smooth: and'. Rough. Water:°

,

.

.

.

Correlation of. full-scale obser-vations, and. model tests of the' prototypes,: showing: the: effect 'of: fullness,: and:wave height, and

wave! length: oil, the, power: required

to maintain! Smooth-water, speeds.

.

Kreitner.

'

...

.

'Can. Seaworthinesst be,

Colon-t lated?`

.

General' information. on. the: motion

. of; aLsbipt in: waves.

. .

.

: Kreitner: J.

..

. .

!Comment. on. Lurching. '

A..

Analysis. of: the- motion. of: lurching.

.

.

. Flaskind,. BED.

.

'Hydrodynamical Theory. of: the Oscillations of. a, Ship: in : Waves.: ' . , .

.

Analysis of: the motion. of a: ship. in naves, with:six: degrees. of freedom, emphasizing: the importance. of. the damping, forces. and moments.

centage of the total resistance for block coefficients of about 0.75, that - it would appear inadvisable to exceed this figure in designing ships for

operating in areas where rough seas can be expected during most of a voyage. The increase in resistance for a given shape is about proportional to the increase in wave height. The maximum loss in speed (i.e., highest resis-tance) generally occurs when the wave length is. about equal to :the :ship

length. The bulb bow requires the least effective horsepower in waves, With the standard U-frame and V,frame next in that order.

One of the main advantages of the V-frame is apparently derived from the comparatively small accelerations produced aft which give better pro-peller performance. The standard.U-frame hull has much larger accelerations than either the bulb bow or V-frame. The use of flare in keeping decks-dry is only effective in the higher speed ranges and for short waves; but it should also be noted-that flare increases the resistance under these same conditions, .Full hulls tend to ship the most water.

Tests and experience show the desirability of keeping the pitch period .and period of encounter as different as possible. When the pitch period and.

period of encounter are the same, or nearly so, the largest. pitch angles end accelerations are encountered and the greatest increase in

resis-tance occurs. Since the only practicable way to prevent these conditions is to change speed, it is useful to keep in mind that ships having fine hulls -may often be able to increase speed.in moderate weather (at the expense of economy), while the fuller ships have no recourse except to reduce .speed.

INTRODUCTION

Presented herein is a survey of articles and papers treating the .aub-ject of seaworthiness. Although the term seaworthiness' is often

inter-preted .with a .legal connotation, it will .be considered in this .study only

the technical sense, and might be more .aptly .described as . seagoing qualities.' This project- was undertaken for scholastic .credit .in a course

entitled .'Special Problems in .Fluid Dynamics,' .which is part of the ,Fluid

Dynamics. Curriculum of the Graduate School of Stevens Institute of Tech-nology;* and represents the 'first of a .contemplated .series Of contributions

to the subject of seaWorthiness by successive students in the curriculum. As a primary .effort, therefore, it is intended to -give .a general review of

the subject .material .and to organize the various phases for more detailed

examination in .the--future, This paper is thus a compromise combining the

limitations of time .available for .reading,. writing, and securing .references

with the desire to outline _a field which is very .broad in scope. Perhaps it is best to examine this present effort las -a guidebook or program which gives

_an outline of the subject,, .the reference material _available, and .a .summary

of the problems, the 'solution of _which will increase the knowledge of the

subject and .eventually provide .useful information for direct .application

.to the design of ships.

The scope of the survey, for the purpose of this general outline of the information .available, includes the following components (originally formulated by J. Kreitner):

1. Reduction ,of .speed in rough water due to increased resistance

.caused :by :waves.

. Ease of steering ,in a rough sea.

Dryness of the deck, i.e. , reduction of the amount of water shipped aboard, and reduction of the damage to deck gear caused by the impact of water.

4. Seakindliness, i.e. , comfort of passengers and crew, and safety

of the .cargo, machinery, and deck gear -- essentially a ques-tion of inclinaques-tion

in

roll .and accelerations in roll, pitch, and heave.

Reprinted: by the Experimental Towing Tank: because, of'. its; interest to the Marine. Industry.

.TM-92

.

3-These components concentrate on the properties of hull shape rather than on the hull structure. Satisfactory structural design is, of course, a neceisary prerequisite to seaworthiness, but it has not been considered

in the present study. A section of references dealing with hull structure has been included in the bibliography.

The body of the survey is composed of abstracts or reviews of tech-nical papers available that give .information or that introduce concepts which aid in the understanding of seaworthiness. This in turn will give a better understanding of the motion of a vessel, so that ships may be con-structed to operate with more safety, economy, and comfort. This motion is produced by the combination of some .disturbing function and the .response

of the ship. The disturbing function, i.e. , wind or waves, is largely the don3ain of the oceanographer; the response of the ship is largely a problem

for the naval architect. It is .expected that these two.branches of .activity will eventually meet in .a solution to the study of motion. The .lessons

learned and concepts developed can then be applied to the modification of the hull shape and structure, The present hull shapes have probably reached an optimum form insofar .as smooth-water and frictional resistances are con-cerned. Future improvements

will

depend to a large extent on modifications of present designs to reduce the _resistance in .a seaway, and to make possible the maintenance of high speeds without .an unreasonably .large increase -in

powei..requirements or ,danger of .structural failure.

,.The articles .chosen for review are examples of,:. (1) general .papers dis-cussing _several aapects of .seagoing qualities and giving an over,all picture of the subject, and (2) articles on specific phases dealing .with .a particular isolated .component, The following papers are particularly recommended: G. Kempf,,

'A Study of Ship Performance in Bough",Water,' .SNAME, Vol., 44, 1936, p. 195; and

J.L. Kent, 'The Effect of ,Wind _and Waves on the Propulsion of Ships,' INA, Vol.

66, 1924, pp. 188-213. Two other papers of ,a fundamental ,nature which have not been .reviewed .because of their length and ,large content, but which are also very valuable because of .the wealth of information they contain are: J. Kreitner,

'Heave, Pitch, .and fiesistance of Ships ,in .a Seaway,' INA, Vol. 81, 1939, p. 203;

.and F.H. Todd, 'Fundamentals of Ship Form, IME, Vol. .57, No. 1, .February 1945, pp. 1-20.

CLASSIFICATION OF TECHNICAL ARTICLES, PAPERS, AND OTHER MATERIAL

RELATING TO SEAWORTHINESS

The following classifications .are suggested in the hope that they may lead to some standard method or system .by .which technical papers, abstracts,

and other data .relating to the study of 'seagoing qualities' of ships ,may be classified according to the effect studied, method of approach, and other parameters.

A general notation is established, so that common terms or expressions can le used in -describing the various phases of the subject. This particular

method of classification aims to set up a convenient code, or notation, by which technical articles can be classified briefly, but accurately. Thus,

when material on a particular phase of the subject is _required, the articles of interest to the researcher will be .apparent, obviating the .necessity of consulting material .which is not applicable to the subject under considera-.tion.

The code consists of a grouping of numbers .and letters of the follow-ing form:

The .meanings of the .Boman .numeral and first Arabic number are .given .in the following key

1. Ship Behavior.-- characteristics..usually considered in ,a .study .of

-seagoing- qualities, :i.e.., .seaworthiness:

1. .Resistance :studies .articles .dealing .with the increase in hull resistance ,caused :by .heavy seas.'

Course,keeping..studies . :material :covering :the:effects of .rough .water on .steering.

-.Dryness of .decks discussions of .hull .Shape,, :speeds, :etc.. with .respeet to :the :amount of .water :shipped :by the weather

.decks.

:4.- Seakindliness .,-- :articles :dealing with the .characteristics of

ship _behavior with .respect . to:

.a. Local .accelerations : .effects produced by :sudden

.move-ments or changes :in .direction.caused by :rough .water .af-.fecting passenger comfort and .cargo .safetY.

TM-792

5-:b. Special military considerations -- damage caused to flight decks, .gunnery -installations, etc.., by heavy

weather:

Structural Effects ,- articles dealing .with:

Local pressures -- studies of the effect of changes in forces .resulting from wave action.

.Bending moments discussions of the hog or sag of the ship :structure as a whole.

.111. Stability Control directional and dynamic stability as .influ-.enced :by:

Designer :control of :stability -- discussions of methods of

:stability control which can be built into the structure of the hull such :as bow shape, .bilge keels, hull shape, etc.

_Mechanical .control of stability papers on device's for

con-trolling _Stability such as tank systems, gyro stabilizers, activated fins, etc.

nr.

Topics Other Than Those Listed Above.

Admittedly, Group J covers all the phases usually .considered in.a ,study of seaworthiness. However, the topics :listed in Groups .II and .III are closely

connected with the subject, and material concerning them .is often found in the same article .discussing Items in _the first group. It was thought desirable

to have a means of ,including this additional material:so that the classifica-tion,of subjects might be as .accurate :as possible.

The _meaning of the second Arabic :number is given in the following :list.

These .items might be termed the .'initiating function or `.cause

. 1. -Rolling-- the .inclination in the transverse plane. 2. Pitching the _inclination in the' longitudinal plane.

.3. Yawing- .the .deviation from the .course in the horizontal direction .with .reference to .a vertical axis.

Heaving.-- the vertical rise or

Jail

of the entire vessel.

Combined motions .combinations of two or more of the .above elements)

The first capital letter describes the method or methods used by the author in obtaining his subject material or .discussing his :conclusions:

A. ,Analytical' a logical method ,developed from certain :basic :as

-.sumptions with a minimum of :mathematical .derivation.

E. _{Experimental -- results based primarily on performance of tests}

on models.

M. Mathematical .-- development of material .on the basis of

,deriva-tion and solu,deriva-tion of equa,deriva-tions.

Observed.-- conclusions reached on the .basis of tests or obser-vations of the behavior of full-sized vessels.

The second capital ,letter .refers to useful ,data or other materials

which might be included in the Particle such as charts, tables, illustrations, etc. .Note that in order to avoid confusion, these .letters .do not repeat those

found in the previous list.

C. _{Charts - - usually .bar graphs depicting comparisons of size or}

per-formance.

G. Graphs . - - line graphs plotted from tabular data _using two or more

variables.

. Illustrations -- pictures or line drawings explaining or simpli-fying descriptions given in the written text.

Photographs actual pictures of events or objects described in

the .article.

T. Tables -- tabular data .giving statistics or results .in .an organized .manner.

.For ease of reference, each of the .topics discussed in an article would have one coded dine, .found at the .beginning of the article. These lines .would

be arranged .in the order of importance, with the major topic .appearing on the

top line. For example, 1-2-3AT would designate an _article on ship behavior concerning the _effect of yawing on .steering. The text would ,be an analytical

discussion supplemented .by .tabulated data.

The .accompanying .abstracts have been classified in the _manner _described.

They serve to illustrate the .use of the .system and the meanings of various

terms.

TM-92

As time passes, .mozte and more material on the general subject of

seaworthiness becomes available. However, for persons interested .in.a particular phase, there has _been no means of determining pertinent ma-.terial except :by .wading through the .numerous references which can .be

found under .a general heading. Thus, .the' above method of classifications

was formulated .in order to .save :lost .motion and present .a more systematic

ABSTRACTS AND REVIEWS

KENT, J. L. _1-1-2-EG

"EXPERIMENTS ON MERCANTILE SHIP MODELS IN WAVES" 1-1-2-MG INA, VOL. 64, 1922, PP. 63-97.

A series of model experiments was conducted using three different bow shapes: (1) V-shaped.forward section and full bow waterline, (2) U-shaped

bow with fairly fine L.W.L., with considerable flare in the above-water sections, and (3) U-shaped bow similar to (2) but with vertical bow sec-tions. The following characteristics were investigated:

The increase of resistance of .a conventionally-shaped, low-speed cargo ship, in waves of different length and height, over the smooth-water resistance.

The effect of V- and Al-shaped bow' sections on resistance and sea qualities.

.3. The effect of forward flare in the above-water bow section on sea-keeping qualities.

4. _{The effect of ship speed.and wave length on pitching period and}

maximum pitch angle developed in .a uniform sea.

The effects of .different longitudinal distribution of load on

resistance and pitching.

The models were towed in a direction perpendicular ,to the wave crests, .no.attempt being made to determine resistance when running across the _sea.

The waves were generated by a wooden-flap wave-maker hinged and _oscillated by _a connecting rod from :a D-C mot:or, The wave length was -governed by the motor speed, and the height by adjusting the length of the crank arm. By .a

continuous variation of the terminal voltage of the motor in a regular .man-.ner, seas of a .variable character could be reproduced with accuracy by

care-,

ful attention to the timing-adjustment. The longitudinal load distribution was .varied to produce three different pitching periods for 'each -model. The models were tested in both head_and following _seas.

_Heavy pitching was encountered.and the :resistance fluctuations were .large with sudden reversals when the period of encounter was the .same'.as

the pitching period. The models experienced approximately the_{.same percent} of increase in resistance for the same wave height, and this percentage .was

TM-92 9

-approximately constant over the whole speed range. The loss in speed was approximately proportional to the .increase in wave height. Short waves produced a small angle of pitch; and .in long waves, the pitch angle was _approximately the same as the wave slope. Smooth,water resistance of the

models was essentially the same, varying only 3% at 9 knots. In waves of 400 ft., model (3) had the smallest power loss, wi,th model (2) showing

less than (1). The maximum pitch angles were about equal. In wave forms of varying lengths, model (2) performed best, but.models. (1) .and (3) did not

differ greatly. Model (2) was not noticeably more effective in keeping the bow dry (probably because of the low speeds used in the test.-- about'9 knots). In high waves of short length, models (1) and (2) had ..a slight

advantage in keeping the bow dry. In following waves, the fluctuations .in resistance for the same speed for waves of different lengths were small. The changes in load distribution point out the desirability of keeping the natural period of the vessel as different as possible from. the period of encounter.

A mathematical development of pitching in waves is given to correlate and explain the experimental results. An analytical .discussion is also

given of the effect of ship form on seaworthiness. From the .experimental observations arid mathematical results, it was found that the longitudinal shift of the center of buoyancy with change of pitch.(i.e..,, the longitudinal metacentric height) is a measure of the rough-weather qualities of the slow

cargo ships. This .inetacentric height .was affected more -by the fullness of

the waterlines than by the fullness of the transverse .sections. This full-ness .must represent compiornise, however, as too full a bow waterline would

increase.,smooth-water resistance.

KENT, J.L.

"EXPERIMENTS OF MERCANTiLE SHIP MODELS IN WAVES" (SECOND SERIES)

INA, VOL. 68, 1926, PP. 104-123.

This is a second series of model tests made to determine the effect of fullness of ship form on rough-water behavior. The results of the first

series are discussed on page 9 of this memorandum. Five models having the same lines were used, .the finer forms obtained from the full forms .by

di-minishing the amount of parallel middle body and lengthening the entrance and run. The models, representing ships of 400-ft. length, had prismatic ,coefficients of 0.80, 0.75, 0.70,,0.65, and 0.60; block coefficients of

0.79, 0.74, 0.69, 0.64, and 0.57, respectively; and equal mid,section.co-efficients. Wave heights of .4 .and 8 ft. were used, the lengths being varied

from 190 to 560 ft. The highest speed.used.for each model was based on the highest economical.speed in smooth_water (about 12 knots), the lowest :speed tested being four knotsbelow this figure.

In .a loaded condition, the comparative speed loss in 4-ft. waves for the three fine models was about the same (0.88 to 1.07 knots), .but .the _loss

in the two full models was cOnsiderably higher (1.41 to 1.74 knots). In waves of 8 ft., the full forms ,had the highest speed loss (0.55 to 1.26 knots), compared with 0.54 to 0.69 knots for the fine models. It.would.ap-pear inadvisable to use block coefficients greater than 0.75 if .a _ship will

regularly encounter waves higher than 8 ft. In a ballast condition, the loss in speed was even more pronounced, but the advantage was.with the full forms. The maximum pitch angle and pitch period apparently were not affected by form for the same speeds under the same sea condition's.

The fullest model shipped _water over the bow over a smaller range of conditions, but shipped it in .à muc.h greater quantity.than the fine hulls. For example, in 8-ft. waves , the finest model shipped water .over .a 380-:to

530-ft. srange.of wave length, while the full model shipped water in wave lengths _ranging from 420 to 520 ft. The _fullest model was swamped in 8-ft.x.

460-ft. waves, ,while the fine hulls shipped only a.moderate quantity.. The

pitch.angle or heave _amplitude was not a measure of dryness, but the period of encounter _was .a rough measure subject to fullness, speed, and _{wave height} factors. The maximum heave,of the full.ancl fine forms Ayes 7.5.to 9.2 ft., respectively.

An .analytical discussion .of .the following _elements of motion is given':

TM-92

11-(1) rolling or transverse oscillation, (2) pitching or longitudinal

oscil-lation, ( dipping or vertical oscillation, (4) yawing or horizontal

oscil-lation, (5) .surging or speed oscillations in the direction of .motion. The

.author discusses these elements in various _combinations in order to

.investi-gate the possibility of the occurrence of the maximum _effects .at the same

time, .and their effect on taking water over the .bow. The most undesirable

condition for shipping water occurs at the -lowest point of pitch (bow down), with the ship at the low point in heave, and with the wave crest .at the fore perpendicular.

KENT, J.

L.

_.I-1-5-0G

THE EFFECT. OF WIND AND WAVES ON THE PROPULSION OF. SHIPS".

.INA, VOL. 66, 1924, PP. 188-213.

The author presents a study of ship behavior during Atlantic cross,-ings of four different vessels. Three of the ,ships were powered by geared turbines: S.S. 'Montcalm', .a passenger .steamer; S.S. London Mariner,'

an express cargo steamer; and S.S. 'San ,Gerardo,' .an oil tanker. The

fourth vessel, S.S..`San Tirso,' was an Oil tanker powered .by quadruple-expansion reciprocating ,engines. The following data were .collected:

, (a) wind speed measured by a .specially -designed pitot tube, fitted with vanes and pivoted on .a 4-ft. steel tube.

(b) wind direction (relative) -- obtained by observing the .direc-tion in which the pitot tube pointed.

.(c) wave direction measured -by sighting along wave crests with

.a tube mounted on a fixed ,Auadrant..

wave height -- obtained .by comparing wave heights with known heights on the ship.

period of encounter -- obtained by taking the average of sev-.eral observations of the time between.meeting successive waves,'

measured by ,means of .a stop watch.

ship speed obtained.by.readings on a Walker Cherub .Log MK II, .measuring the time interval over the distance of. one mile.

.(g) .shafthorsepower. .calculated from:torsion.meterreading.on propeller shaft.

propeller RPM.- measured by timing flashes on the torsion meter

.scale.

pitch angle -- obtained by .a continuously recording,long-period pendulum.

pitch period -- measured from (i) by marking record sheet with an electrically operated pen controlled by a chronometer, with half-minute 'make and ,break' contacts.

.(k) rudder angle .--.observed by quartermaster

_as

_{the amount of} rud-der .necessary to, steer,a steady course.

TM-92

(1) course -- obtained from magnetic compass readings.

roll -- the angle measured by means of a , horizon apparatus' ;

the period by means of.,a stopwatch.. (The quantity oscillations per cycle' refers to the number of oscillations between _maximum rolls.)

notes -- observed special effects such as slamming, -vibrations,

etc.

) extracts from deck log ,- notes taken from ship' s log to compare

seaman' s opinion with observer's data.

The author was .accompanied by an assistant on each voyage. The period of time occupied in taking .data for each observation was . from 1 to PA hours.

The tabular forms .used in recording data are shown in Figures 1 through 4.

:/q4ALTSIS.OF DATA

_WIND:AND,WAVES:ENCOUNTERED

. Wave -lengths were calculated from the _observations

of ship speed,

period of encounter, and relative direction of the wave progression. Wave

slope .was calculated for :a trochoidal shape . from the observed :lengths .ad heights. :It was found that the :calculated :wave 'lengths and :heights, ,using

the .formula involving the wind -speed, were inaccurate on these occasions

:because of the :absence of :a :steady ,wind :speed :and :direction.'

SPEED AND PONER OF SHIP ON VOYAGE

The power losses due to 'rough water ,were :,caused by:

1., retarding forces which caused ship to slow for a given RPM,

thus increasing true slip and decreasing efficiency;

. loss of propeller efficiency from rudder action needed to main-tain a -steady course (on twin-screw vessel, maximum torque

varia-tion occurred on screw toward which rudder was turned);

3. the torque variation Joss in roll or pitch which was approximately

proportional to the maximum angle of roll or pitch (the times of maximum torque appeared to occur at the frequency of the roll. or

pitch periods; maximum torque in rolling did not always occur at lowest point of roll except when rolling was regular

TM-92

-15

PITCHING AND ROLLING

The main types of pitching motion were:

small oscillations, irregular in period And angle;

larger.oscillations regular in period, amplitude being periodic, Varying from maximum to minimum angle_in.approximately regular intervals;

.3. _{:larger oscillationSirregulat..in periOd.and.of.a periodic}

charac-ter with.regard.to pitch.angle, but.cycle.of change.occurring.at

...irregular time intervals;

4. approximately regular oscillations:containing.occasional.damped oscillations.

. EFFECT.. OF WEATHER ON STEERING

The effect of the wind was dependent upon the longitudinal distribution of the superstructure. Vessels.with.unbroken deck,line.and.centralupperworks:

were.less.affected.than.veSsels having separate islands.ofAeck.erections. The:effect.of.waves was dependent upon.the.relative direction:ana.wave.height.:

PR I NC I PAL. CAUSES. PROD UC 1 NG LOSS OF SPEED IN A SEAWAY

The most significant effects which resulted in increased power.require-.ments or:loss.of.speed.were:

1. increased .resistanee due to waves;

.2. increased wind resistance.due,to.superstructure of ship;

.3. decrease.in propeller_efficiency;

WIND AND WAVE DATA

PONER, AND SPEED DATA PITCH AND ROLL DATA

FORCES AFFECT (On single 13crew- vessels, SHP

column: is omitted.. ) , , . DATE "''' ,

z

-. PARTICULARS OF WIND SHIP'S

OWE

c '

PARTICULARS OF: WAVES

FROM.. FORMULA cn 41 ° = ''.4 P-t.:

-(../ _ 0 E-r 1 a 0

z

rA TRUE MEAN SPEED, MAXIMUM FLUCTUATION ..BEWA . OF: SPEED, , . . 11FDORT°TALE .

g8

W E,, ' ,.A..i (74 cz ' DIRECTION ' ! LENGTH,

:

-. MAXIMUM HEIGHT, . :CALCULATED ' 'MAX: SLOPE, ' . M'nZI _.1-!.-, C.7. la Z. !-4 W ' II

a

e...,.. 5:, 4 0" 1.4 .--. '

r '

' WI IL ED ...1

'

-OBSERVED

FROM DECK LOG

,

z

0 (4 Z W -m

c

I . , , , :SHP - '-. SPEED, I . -' SHAFT' HO ' ' -. .mni . ' , , CALCULATED ....! `41 1:6 c..;" . 1 i4 CV , . WAKE! FRACTION . . SCREW: EFFICIENCY PORT ! STBD. 'PORT STBD. . , , PORT STBD... 1 1 ' ' -PORT r : STBD.

8

a

0 CZ Z 4.1

2

0

PITCHING i PERIOD OF ENCOUNTER. I SLC: , ROLLING .

PERCENT' VARIATION ON TORQUE

PITCHING ROLLING PE RIOD SEC. ' MAX. ANGLE DEG. OSCILLATIONS PER CYCLE PERIOD, SEC MAX. ANGLE, DEG. OSCILLATIONS PER CYCLE PORT STBD. PORT s STBD.

0

r

a

0

CC Z. 41 Cn SHAFT HP RELATIVE WINFX WAVES ' RUDDER

ANGLE AND _DIRECTION,

DEG. . PORT STBD. DIFFERENCE SPEEDSEED,. .

/7

, DIRECTION, DEG. LENGTH, FT. HEIGHT. FT. DIRECTION, DEG.

KENT, J.L.

"PROPULSION OF SHIPS ..UNDER D?FFERENT 'WEATHER CONblTIONS"

iNA,

VOL. 69, 1927, PP. 144-163.

This paper is a resume of a second series of sea voyages taken during the winter months on the Atlantic. The type' of data collected and methods of observation are .described in the .author' s earlier paper on the first

series (INA, Vol. 66, 1924, pp. 188-213), which was .a _similar study of the ,behavior of ships having block coefficients ranging._ from 0.73 to 0.83. The

.vessels studied here are twin-screw, .geared-turbine passenger ships, the S,S. 'Oroya' and S.S. 'Oropesa, which are of a finer form than the earlier vessels, having block coefficients of 0.68 and 0.73, .respectively.

.A comparison of smooth-water performance for the .'Oroya' was obtained by using data .collected in waves "less than 4 ft. , and in the case of .the

'Oropesa, by extrapolating steam trial data furnished by the builder. Graphs are given showing the required increase in thrust for varying wind .and sea conditions. The 'Oroya' showed a necessary increase in thrust of

52% to 460% over smooth-water requirements in weather which .varied from good to 'extremely bad. The requirements of the 'Oropesa' varied from 87% to 155% .under .similar but somewhat less severe maximum conditions (7-ft.

waves, 30-knot wind). Variation in torque on the propeller shafts was ob-served to be periodic, the maximum torques occurring .with .a .frequency

roughly equal to the pitching period of the ship. The torsional fluctua-tions of the ,shaft on the weather side were always greater than those of the propellers to .leeward.

Slowing the ship in .a head .sea lessened the maximum pitching .angle,

.lengthened the period of encounter, and reduced the .number of oscillations

,**

from maximum pitch to maximum pitch* but did not alter the pitch period appreciably. Rolling motions differed .between the two ships, but could .be

accounted for by the difference in .metacentric height.

460% represents the required thrust if the ship had been able to maintain speed. Actually, it was necessary to slow from 13.4 to 8.2 knots.

- !". For each- new, roll or pitch, the amplitude of the complete roll- or pitching, cycle, usually varies slightly. The. amplitudes. often, increase as the: ship. tends to. synchronize, iith, the wave: motion

until, some maximum, amplitude. is reached,. at, which point: the Period, to. recover. has!. beei:E so

lengthened: that: the: ship falls out, of synchronism with: the wavea, and, the: new. pitch, or: roll is. damped by the oncoming, waves. The author. uses( the term: ' oscillations'. toi describe: complete cycles of, motion.' Thus,- the, number, of oscillations, at. which. a. maximum, condition occurs, is some integer, times the. average. pitching, period.

TM-92 17

-The author describes two types of pounding.! The first is the effect produced :by forcing a vessel, through wave: swells at such a speed that the

ship .cleaves its .way . through wave . crests on . a more . or . lest . even .keel, and

gives .rise to panting (occurring on high-speed vessels suchHas destroyers). The second effect is that

of

slamming, which . consists of_ a single . severe'

.blow on the .underwater body,. occurring at irregular intervals, .and .observed

-Most. .often in cross -seas.Because the slamming effect was_ observed at

ir-regular :intervals in the pitching .cycle , it is believed that slamming and

pitching .are not related directly.

-Waves .caused .both ships to yaw :in .the: same direction ( AproadsWe on

to the .waves ). The yawing tendency .increased with the increase in :wave

height .and .wave 'length, when it lexceeded the ship 'length, . and was

:especially

KENT, L

"AVERAGE SEA SPEEDS OF SHIPS UNDER W HITER WEATHER CONDITIONS" INA, VOL. 69, 1927, PP. 291-313.

The results of two previous papers concerning ohservations undertaken by the author on a series of voyages (INA,

Vol. 6011924, pp. 188-213; INA,

Vol. 69, 1927, pp. 144-163) are consolidated in the paper. Although the

.number of voyages was not sufficient to draw any .sweeping general deductions, the results do give qualitative information on the. factors governing the powering of ships for winter weather .conditions. The types of weather are classified .as conditions of wind ,speed and wave height, which are the 'factors

primarily responsible for causing an increase .in power requirements. Winds

are given four categories '; (,I)- Beaufort number 0, to 4 .= light airs to

mod-erate breeze,

(2)

.Beaufort number 4 to 7 = moderate .breeze to moderate gale, Beaufort number 7 to 9 moderate gale to strong gale, (4) Beaufort number 9 to 12 strong gale to hurricane. The .conditions of the sea are classed -as: (1) smooth sea = wave ,height less than 3 ft:, (2) moderate sea = wave height from 3 to 10 'ft. ,

(3)

'rough sea =.wave height from 10 to

30

ft. ,

very rough sea = wave height greater than

.30

ft. Although .various wave heights and lengths occurred, there appeared to .be a maximum ratio, 1:11, which was _never exceeded.

It is .interesting :to ,note. that the percentage of observations _during

which .winds of :a particular .c lass .occurred was .approximately. .equal to that

:during .which :seas .of the same class prevailed, .although lin:actual practice

they di:d no occur :simultaneously. This _approximation of :frequency of ocL

.currence was used to classify the 'total .weather .under .four :headings.: (1)

'fine, .(2 ) _moderate, (3) rough, and (4 ) .very.rough, .which .correspond to :a

_combination of ,classes .of :wind: and .waves :as previously :described. On this

:series of voyages, which took place from January :to .March, _moderate .or

:rough weather .was :experienced .about 70% of the .total :time :at .sea..

Values.of the power _coefficient (ci ) .are _given for each

ship

in .each

'type .of .weather, as well as an .average. valne for .a given. voyage. A table

is .also .constructed giving 'the .margins of,. power ,used over or under the :average power in use throughout the voyage for .each type of .weather. For

SNP' x 427.1

2/ 3 v3 where V = speed; in knots and'6..= displacement. in to,14..

'TM-92

.I-1-5-0T

the fuller vessels, these .requirements varied from 10% more in fine weather (when the ship was making her best speed) to 7% less than average power in rough weather (with a corresponding reduction in speed to avoid structural damage) . The finer vessels were actually able to .increase 'their power to

maintain speed (at the ,expense of economy) in moderate or,

rough weather,

A correlation of

model

experimental data andt.ctUal observations taken at sea serves to unify J.L. Kent's extensive investigations, some of which are discussed on pages 8 through 20 of this memorandum. Voy-ages on the S.S. '43.erengaria,) S.S. 'Londdh Mariner,' and S.S. 'San Alberto' provided 'additional material to that already collected for the

five, vessels discussed in previous Papers. 'Average rough weather' 'coni

ditions were determined from plots of Wind speed and wave height vs. ship speed, and represent conditions which, if met continuously through-out the voyage, would have.allowed each:ship'tocomplete her voyage in

the same lehgth.of.time as was taken.under the Varying conditions.ac-tually encountered. These .conditions are described as being winds of 20 knots' accompanied by waves.apprxitatelYtix high'.

On the assumption that.average performance under these. average rough. weather' conditions represented a constant loss.in-speed, the margin of power that would have been necessary to maintain smooth-water performance was calculated. This percentage increased with the block . coefficient, becoming excessive when the fullness exceeded 0.74. By' using. model test values for EHP, DHP, and wind resistance at various speeds, the loss in power.ind the increased resistance due to rough water were determined. A graph showing the relative-importance of the effect of.wind.and. waves on the'total.resistance shows'that wind'and wave resistances were about equally divided until the block coefficient -became large.

Consideration is also given to the relative efficiencies of various, sized screws under adverse.conditions...The author concludes.that the

change in torque due to.increased slip should be as small as possible even though smooth-water efficiency may have to be Somewhat reduced. It .is also shown' that fOr rough-water performance, a large-didmeter propeller'

working at low revolutions gives better results than -a smaller high-speed

screw. .

A table shows the effect of weather on large- and small-diameter' propellers. Graphs are plotted showing the loss in speed.due to the in-crease in wave height, and the relative wind and rough-water losses (Cs vs. ship speed).

TM-92 .

KENT, J.L. -1 -5 -OGT

"SHIP PROPULSION UNDER ADVERSE WEATHER CONDITIONS" -1 -5 -EG NEC, VOL. 53, 1938-37, PP. 55-64.

Some typical speed and power losses for'ships with various block co-efficients are listed in the following table:

BLOCK COEFFICIENT 0.60 0.70 0,71 0.73 0.742 0.745 0.785 0.83 LOSS OF SPEED, KNOTS 1.0 1.1 1.1 1.05 1.7 1.35 2.15 3.7 MARGIN OF POWER TO MAINTAIN

SMOOTH-WATER SPEED, % 13 29 25 24 50 33 91 224

Loss OF POWER DUE TO WIND, 3 11 10 9 13 10 7 16

KEMPF, GUNTHER

"THE BEHAVIORJW DIFFERENT SHIP FORMS IN A SEAWAY"

1-42EG

SHIP SLDR. & ME BOR., VOL. 40, 1933, PP. 229-231,

.(THIS ARTICLE :APPEARED ORIGINALLY IN: WERFt-REEDEREI-HAFEN, 1932, P. 176)

Dr. Kempf describes the results ofa series of tests made on'three -models having approximately the same Principal dimensions, but varying

in form -- a normal hull (U-frame), a Maierform (V-frame),.and a bulbous :bow shape. The models were towed free,to-trim:but were

restrained.longi-tudinally.by the resistance.dynamometer..A.motion-picture.camera.was_used

-to .record the instruthent readings-so.that.instantaneous.values might be

.used.

The.normal:hull:fOrm.was found-to:have:a:reasonably:steadystern with considerable movement.a:the.bow, the .model Oivoting.on.a point. 2/3 L aft of the:F.P. The-bulbous:bow had approximately.uniform.amplitude:fore ,and:aft,:and pivoted .amidships. The Maierform,whichshippedrthesmallest

amount.of:waterlorward, had its greatest.motion-forward, pivoting about .a point .3/4 L:from the.F.P. Acceleration.curves.were obtained.by.differ-.entiating.heave:and pitch.results:twice: The.accelerationsim the normal hull.were:about:twice-thelmagnitude.of those .of.each of:the other.models. ,The.bulbous:bow produced.equal,accelerations...fore.and:aft,:while the

.Maierform produced,comparatively:largeracCelerationaJore.andsmaller:aft. The resistance In waves .showed the bulb..to be superior .up to about .27

knots, the Maierformphaving the highest .resistance. The author comments that.despite:thedarger.resisiance.of the Maierform, the.steadiness.of.the ,stern might give higher propeller.efficiencies_under:similar.conditions.

Plans.and:a table.giVe:bodysections.and principal dimensions. Pitch- .ing.diagrams:giving.taximum.emersion:and:immersion,.and.maximumlaccelera-:tion.at:thelow.and stern are also included.

TM-792

KEMPF, GUNTHER

"A STUDY OF SHIP PERFORMANCE IN SMOOTH AND ROUGH WATER"

SNAME, VOL, 44, 1936,

PP. 195-211.

,I-1-2-EGT

-I-1-2-0GT

_Observations taken on .board the M.S. 'San Francisco' are _compared

with data obtained from .a series of model tests in an effort to determine if towing tank procedures can be used to predict full-scale ,behavior. in

rough water.. The limit of _accuracy .of the model data is .considered to .be

1.0%, while the ,accuracy of the full-scale observations is estimated _as

±3.0%. The method of .analysis used involves three main .steps: (1)

propul-sion tests

in

smooth water, (2) propulsion tests in sinooth water with ditional tow4rope resistance (EHP.artificially increased), (3) propulsio'n

tests under various _weather and wave .conditions. Observers recorded the

following .data -for the ship: (a) speed, .(b) propeller RPM, (c) propeller

thrust, (d) propeller torque, .(e) resistance of the .towed .bodies (by

dy-.namometers) (f) velocity _and direction of apparent .wind, (g) ,rudder ,angle, (h) pitch, roll, .and heave motions, (i) length .and height of waves.

The .results.of the .ship and _model 'tests are presented in graphical

and tabular form. The method used in expanding the frictional .resistance of the model to full :size is .explained in detail. Data for the open-water model propeller tests _are .given in tabular and graphical form, with .additional plots correlating the torque and thrust characteristics .of the model and fulLscale propeller.

Revolutions of the ship and model screw for .corresponding speeds .agree with .a 1.0% scale correction -for the wake. The power 'requirement is.about

3.0%_ higher for the ship, which-.may be due to the greater relative .roughness

of the ship screw, or to some inaccuracy in estimating the surface .roughness of the hull of the ship. The torque and thrust coefficients of .the model _and

ship propeller show .good :agreement, ,especially at the higher .values of

:Rey-nolds number. The comparison of predicted .and _measured power and thrust .is

not satisfactory, the author .recommending that further _investigation .is

.nec-essary to determine the causeof the difference (see comment at .end of

..ar-.ticle).

The :model was run -through various regular wave trains, and the

creased resistance corrected for the effect of the .wind _which would have

produced :such.a.- train. The model predictions for loss of .speed .agree .very

well with the .observed conditions if the 'following wave proportions are used:

:,;

The ship observations and model predictions of pitching angles are

in good .agreement. The author concludes

that

_{quantitative results can be} obtained :in the towing _basin using regular _{waves to predict the behavior} of _a vessel in an irregular seaway.

(Reviewer.' s.. Comment : The disagreement. of: the. model.. and. Ship- thrust predictions: is. difficult to reconcile, with the good agreement of the! thrust and torque..coefficients.:.As stated- in the

ar-ticte; the author.. does: not! attempt. to evaluate:the:cause. The mailman): differeneetin thriat, between

predicted: and measured' is: about 16%, the: average: being- 5%, to. 8%.': This appears to: be excessive when: considering, the:accuracy of, smooth,-water. EHP predictions, but, when: the: more t complicated

motion, involved:is:considered,. the discrepancy, does, not: appear: as: an,..error". of great, magnitude in-view. of the,. lack. of', more accurate, means- of, testing:ancl:measurement available, at present.)

TM-792

.25.

.lengths of 492 :ft. A plot _of the .losses .in speed vs.. .Beaufort Scale, !on

:semilog, paper, .gives :a .smooth:curve: The following table.shows :some typi

-.cal points .corresponding .to :data . as .given. on:additional. plots _of :Beaufort

.number .vs. .wave height-, and .Beaufort number v6.!_wave length.

:BEAUFORT No. WIND SPEED, WAVE HEIGHT, WAVE 1ENGTH, LOSS IN SPEED-,

KNOTS .FT. - FT.

3.8 12 5.5 125 10

KREATNER ,

J.

'!CAN SEAWORTHINESS BE CA LCU.LATED?1

MARINE NEWS"; VOL. :26, No. 3 AuOusr 1939, P. 39-42.

The author presents a qualitative discussion of the interaction of

ship and waves. Heave and pitch are analyzed as resonance problems to be evaluated as mechanical oscillations encountering damping effects. The difference in damping energy between the bow and stern sectiong is taken

as a resultant hydraulic pressure, which, in ,effect, increases the hull resistance. Illustrations and graphs show (1) the ,linear variation in

wave height for.an average ship,- (2) an. illustration of the production

of waves by the ship, .and(3) a curve of calculated pitch alleles, with several points given by observations made _under .actual conditions. .An

ex-planation on the basis of the author's theory is given for the performance of the Maierform (V-frame) forebody.

The mathematical development of the discussion in this _article is presented in a paper: _aHeave, Pitch, and Resistance of Ships in a Seaway,' given by the author at the INA spring meeting in 1939.

INCLINED WATER SURFACE

NORMAL WATER SURFACE

N

//r/..,///,,

TM7-92

27.

KREiTNER, J.

J 4 5 AI

"COMMENT_ON LURCHING"

SNAME, VOL. 48,.1940,.PP. 89-791.

Lurching is defined as an isolated, long,and deep roll. The action is characterized by.a ship.'hanging at.an angle of.roll.which is greater than the angle of the wave slope for periods.of.time.which.are excessive in comparison with the normal frequency and amplitude.of.rolling. The .author treats,iurching.asa problem in simple static.forces. He.analyzes

' the problem.with the ship.and.water.surface.inclined.with.respect.to.the'

horizontal, .and mutually inclined .with respect.to.each:ethef. This.condi-.tion.causes the.metacenter (V) to move off the.vertical.centerline.of the .

cross section to a position vertically.abdve the .center.of gravity.(G) .where.the ship_will.maintain.the.same angle:of.heel_as long.as,the water

surface maintains .the same relative position.with.respect.to the ship. .Thus, if.a ship_were.moving at.or.near the wave propogation velocity, this

static equilibrium.condition.would.be:Valid: The.author.suggeats.using.the ratio of the .metacentric.radius to.the.metacentric height U1V/GO.aa.an .additional index..to.stability,.since.this ratio.determinas:the:magnitude

of the_angle.at.which the ship_will,'ihang.' The.diagram.given

indi-cates graphically.the.authors.analysis.of the action. The .geometry of the diagram defines the.angle.at which the.ship.Will.hang.

Angle at .which the ship will hang. = where:

=.angle.betWeen waterplsne and horizontal ,AM = metacentric radius'

C. H. HANCOCK

-oOME ASPECTS OF SEAWORTHINESS TESTS"

RAPER PRESENTED AT 1948 MEETING OF AMERICAN TOWING TANK CONFERENCE, REPRINTED 1N BULLETIN 1 OF THE SNAME.

I-1-2EG

III-7-2EG

Ii I-8-1EGP

1-4-2EGF

The author emphasizes the study of hull form above the Waterline as important in its effect on seagoing qualities.; He recommends the gravity tow deviee as being the closest approach to. prototype action, since the model testsare carried out at constant thrust, rather than at constant

. speed, which. is the predominant carriage technique. He suggests modifying

constant-speed carriages by means of a local weight and pulley arrangement so that thrust, rather than speed; would be constant. By using the constant-thrust technique in a disturbed tank,. the loss

of

speed in A head seacan be Measured directly. He points out that loss of speed is the subject of most

tests, with measurements of pitch angle, erticat accelerations, and heave following in that order of importance.'

A study is given Of the effect of wave length on reduction of speed.._ From graphs of Wave, length vs- speed, it is shown that, in general, the re-duction in speed is about 10% for waves.of about 75% of theship length. The reduction

in

speed increases until at wave lengths equal to the ship length, A maximum reduction of '50% to 60% is produced. Recovery

is

more gradual as .

wave length increases, until ,at wave lengths of two ship lengths, speed re-duction is 15% to. 20% of the corresponding calm-water speed:

The effect of flare is also studied in relation to: (I) speed

reduc-tion, (2) pitching-angles, and (3) vertical acceleration at the bow, with

these three quantities plotted against wave length. Four different speed/ length ratios are chosen with three different amounts of flate., Theresults show that in waves up to one-half of-the ship length, the most flare causes the most reduction in speed, the least flare, the least amount of speed re-. duction, Above the wave length equal to one-half of the ship length, no

def-inite superiority can be noted for any of the three amounts of flare. The. - author points out that most designers experiment with variations in bow

Shape,

and that it would be-desirable to Undertake systematic investigations of the effect of the stern and after .section as well.

The results.of testing Maierform bow and stern sections are compared with the results obtained from testing

a

standard hull under similar

condi-tions,. The normal model shows less speed reduction

in

waves hp to one ship length, at _which point the normal and Maitrform sections exhibit an equal

re-- TM-92

29

4:1 duction in speed; but the normal model has .a higher pitch _angle (about 30%)

_In the longer .waves, the Maierform has a higher pitching.angle .but the speed reduction is Jess, .and the decks are drier.

A specific test of the effect of free _water :on the .rolling of .an LST

is .also discussed. Graphs of rolling angle vs. wave length indicate that the size of the openings in longitudinal .bulkheads in the _bottom tanks has

little effect:on the .amplitude of roll. The graph shows that the presence of about 2 ft. of water in the bottom hold produces .a reduction of about 22% in the amplitude of .roll compared with the roll produced with the .dry

bottom.

Another specific test is discussed in which the effect of pitching on the swimming pool of the S.S. 'Americd is studied. Graphs and photographs show how .a slight reduction in the length of the pool offset resonant os-cillations which were .normally _encountered during .moderate pitching.

HASKIND, M.D.

I-4-5M

"HYDRODYNAMICAL THEORY OF THE OSCILLAT,ONS OF A SHIP iN WAVES"

PRIKLADNAYA MATEMATIKA MEHANIKA (APPLIED MATHEMATICS AND MECHANICS) VOL. 10, 1946, PP. 33-66.

THE FOLLOWING IS A COPY OF THE ABSTRACT PREPARED BY DR. J.V. WEHAUSEN FOR THE MATHEMATICAL REVIEWS, VOL. XI, No 3, MARCH 1950, P. 228,

In this_paper, the author gives a systematic exposition of the hydro-dynamic theory of the motion of a ship among waves. Certain simplifying as-sumptions are made from the beginning. The ,linearized boundary conditions for the free surface and small oscillatory motions of the.ship_are assumed; the free surface is assumed to have a train of sinusoidal waves coming .in from infinity in some direction; the water is taken infinitely deep; and, although all six degrees of freedom of motion are allowed, they are assumed to be periodic functions of time. Determination of the velocity potential of the motion leads to .a Fredholm integral equation (not discussed here),

and depends upon the expression for :thevelocity potential of .a source of

pulsating strength situated beneath the free surface and either stationary or moving with constant velocity. The methods follow those developed by

Kochin in his various papers on the surface waves caused.by submerged bodies. The first part of the paper deals with the case when the average posi-tion of the ship is fixed; the second part with the case of constant average velocity. In general, the work.is pointed toward finding the forces and mo-ments to which the .ship is subjected. .Equations for these are derived, and it is shown that they may be divided into inertial forces (and moments), damping forces, hydrostatic forces, forces caused by the striking and re-flection of the oncoming waves, and if_the.ship has a constant average velo-city, the usual wave resistance. Emphasis, however, is on the damping forces and moments for pitching and heaving motion. In order to estimate the effect of hull form and wave length of oncoming waves on these damping coefficients, Michell-type ships (an analogue of thin symmetrical wings in supersonic flow) are considered. Results of numerical calculations are presented for a special-ly chosen famispecial-ly of water-plane and transverse sections. For this famispecial-ly, it is shown, for example, that transverse sections with flare have higher damping co-efficients.than full sections, and that damping coefficients decrease as the ratio of ship speed.to wave speed increases. It is

of

additional interest to note that whereas there is no coupling between heaving_and pitching of .a ship which is symmetrical fore and aft when the ship is not underway, there is

COMMENTS ON ,EXPERIMENTAL METHODS

Several.methods_have.been nsed4in-conducting.towing:tank tests of .model,behavior- These.methods.are: (1):a:conStant4thrust:technique,with.the'

model free.to.Move longitudinally, (2).a-constant,speed procedure with the model.restrained.longitudinaily.by a dynamometer, and (3) the self-propelled test, .

The self-propelled.method.gives.the.closest.approximationtoactual ,conditions; however, the..dynamometer.apparatus.should:not.be such that the

longitudinal surging bf.the.modelj.srestrictedunless provisions can

be

.thade.for:taking.a Series of instantaneous .readings.

. The.constant,speed procedure is, of course, the:usual.method=of testing

model hulls. .in_waves,.howeverthe.load.onthe.dynamometer.iariesin.some .cyclic.manner. Itis.therefore.necessary.to.obtain.either_aseries.of

instan-tanedus_readings for.various.attitndes.ofthe,model or.to.compute:sote.sort .of:average.resistance.over.an entire:run,under.a.given set.ofinitial.condi-Itions.'Jt.might.be possible to .use sufficient damping in the-resistance

dyna-mometer to produce :a fairly constant reading for.a particular run. ,A.useful

Study. Might be made to determine.whether such.average conditions.cOuld,be -used to Correlate or.represent the cycle.experienced.by.the instantaneous

.re-suits. To be of value, the test resnIts:shouldlae.unique fora given set of initial conditions, and should be reproducible withinclaselimits. .J..L. Kent's investigations ('1Xperiments.with MerCantile

Ship Models:in.

Waves-,' INA,

.Vol, '61, 1922, pp. 63-97, and INA, Vol-. 68, 1926, pip, 104-123) were performed

at constant speed, .using average values of resistance. .17

, The constant-thrust technique offers some useful applications since the,

f

model.actually loses.speedin the manner of.a real ship. This type.oUtest can be accomplished by towing with .a falling.weight,:or by towing with a ' carriage connected tothe.model by.some pulley.and.weight.or spring linkage

which does not restrain the longitudinal.movement

of

the-.model.

CAL.Hancocks

experiments ('Some Aspects of_SeaworthinesS Tests,' Paper presented at 1948' Meeting of the American Towing Tank Conference, Reprinted in Bulletin I of the SNAME) were conducted:using:the.falling_weight tow;.anditis.believed AG. Kempf ('A Study.ofHShip Performance in Smooth:andliough_Water,' SNAME,

Vol. 44, 1936, pp. 195,211):used.a cartiage:mtidifiedto produce constant thrnst,.although:a.:good,description.of.the.apparatusis_not.givenin the.ref-.erences'.cited, Perhaps itcan,be shown'thatthe constant=speed.method.produces-:results:which ate:as-accurate.or.as.applicable.asthose.pf.the.constantthrust

procedure.

TM-92

31-It is possible to examine the results of the, various methods . and the

criticisms .of technique .which have . been offered, but little .has . been done to .evaluate the methods themselves- It is significant that . the .results of

these. _different procedures have been . compared with . full-scale observations with :varying _degrees of :accuracy; but it is .desirable to .see _whether chance or empiricism has _distorted the :actual .events and :their : true significance.

-A Jew .other points.are .worth _noting . in. connection .with . model testing'.

-- The . first . is . that the self --propelled . teSt .

in

.smooth _water .might .be :extended :to include towing some .submerged . object . or .adding .dynamometer resistance

(artificially increasing EHP). .G. Kempf .used 'the :results of . such ,a :test as

:an aid . in . ana lyz ing .the .rOugh,water tes ts , since .the added :resistancesitu-;fated yto :some .degree the :increased .resistance caused by .waves. .A :second :consideration-concerns the .choice .of suitable :wave length/wave height ratios'

for .experiments, C.H. Hancock ,speaks . of .wave length. in terms . of . a .speed/ ..length .ratio; J.L. Kent :describes .conditions .at two .standard .heights but of

.varying length; :and G.Kempf- finds..S :good correlation'. of .results .by

length/height ratio graduated to increase with increasing wave length, A useful .study .could :be made to determine . the .most desirable 'parameter . or

parameters to . be used in . describing the ;waves .and .in comparing . the :effects

. on objects of.varying lengths :and/or beams. 'Many-ships are built . for .a

particular .trade route- .or .with. a particular

.geographical .area under

consid-.eration. .would .be .desirable .to :test .such models . under the average rough :sea .conditions to :be .encountere& as :well as .under the maximum rough- sea

conditions expected. Indicated modifications in hull shape :or :structure

mightwell increase. .the over-all safety and :economy, :even _though they might

_slightly impair the .smooth-water ,Characteristics . This last 'thought .is not

original, :since _developments such :as the _Maier form :system :represent .attempt to , accomplish . these . improvements, but the absence .

of

.material .(compared.with

.what has been :written on .smooth-water . characteristics ) .indicates .a field

.which is still fertile for : creative .achievement.

_It is .also important 'that .the _mode l . tests .inc lude acce lerati on .mea -sureMents. The.inagnitudes .of .movement _are largely-important in resistance;

.but .the .accelerations :are .necessary

:in

.considering structural :safety,

:com-fort of passengers :and , the dryness .of .the decks.. .C.H. Hancock and G. j(empf

'have sincluded -.data

on

:accelerations .in their papers Perring ( 'Some

_Factors Affecting .Resistance . of .Ships in Waves,' .Engineer, Vol. 40, :Dec: 1925, pp, 686-688) :has used :Kent's sdata . on :model _experiments :to .discuss _some

,of the :effects.of .acceleration _onlresistance. .Hancock :and .Kent present :some investigations .

of

sthe question . of :the :dryness . of :the _deck.,

Another aspect of model testing is the determination of wind resis-tance. Kent's investigations indicate that the added effect of wind may be significant, particularly on fine vessels -(which .are often passenger liners

with extensive superstructure). The present methods of calculation are highly empirical and susceptible to wide variation in interpretation. Here again, an evaluation of testing methods and .methods of analysis of data offers a wide range of possibilities for original .work. In addition to the added resistance of the wind, the effect of asymmetrical foreward .and after grouping of superstructure on .a .vessel .might also be considered. It is

realized that the _models presently used in towing tank tests are .unsuitable

for such investigations, but the field does offer problems for the solution in the wind tunnel by an .aerodynamic approach.

TM-92 33 7

COMMENTS ON FULL-SCALE OBSERVATIONS AND ANALYSIS

Kent, Kempf, .and, their co-workers have .contributed the greatest :.bulk

of .material _concerning .the seagoing qualities

_of

.ships. The parameters_ of :observation. are largely:the . same, _but the .method .of analysis-and

presents-. .

tion .of results .are .very different.. Kent reduces most . of .his . observations

to .sets . of .average _conditions, producing:a .statistical analysis. .Kempf 'presents his conclusions in .a :form . which is .best :described .as .a .modifica

-tion.of .the .usual _method .of _analysis .of smooth;water: test results.

The power . constant or power coefficient (Cs) ( see Page 19) used by

.Kent :appears to be ,of .value in describing . the comparative power

:require-.Ments . of . the same :ship ( or :similar :hulls ) .under ,varying .sea .conditions

However, _since Cs is a._modified .Admiralty _coefficient and is not

dimen-sionless, there is probably. a

Jack

.of .applicability .under .a wide :range of f

:circumstances .for .various types .of .ships. An improved form, .which . also

in-.cludes the :block :coefficient is suggested . below

Slip

'Power .coefficient = K

A V.8

where

SHP =- shaft horsepower

V = ship speed in .knots

A = displacement in tons

8 = block coefficient

K = a . suitable constant to make numerator . and .denominator

dimen-sionless .

IA preliminary ,survey :of the.applicability.

of

.this modified.coefficient iu .di ca tes.that some additional .modificatiOn .would . be . necessary : for . _ccimpari=';

son .of :single and twin-screw vessels.

,1.

The :suggestion .has often been .made that most full-scale tests . are

superfluous and that data, are :readily .available from . the deck and :engineer

-.ing. logs .of _vessels

in

.service,-.

.In

.fact, this _method .has .been. used .by

J.L. Taylor . ('.Statistical :Analysis . of Voyage Abstracts., .INA, Vol. 70, 1928) ...The results are purely :statistical in _nature:, . and while information useful :in :modifying :design .might ,be obtained, .nothing .is . contributed . to

the, understanding of the :fundamental problem . of the -motion of .a 'ship .in

.de-'scribed and analyzed,. any such method would be of limited value. It might

also be commented that the observations of wave length and height are

.usu-ally ,inaccurate or contradictory, even .when made _by experienced mariners.

Trained observers usin_g various types of instruments often .disagree.

Per-haps the :Use of .a :stereoscopic camera technique will lead

_Ao

more

_consis-tent .results. .As mentioned in the discuSsion.of Model .testing, is :de

sirable .to know the :average :sea :conditions ,and :the probable.roughest

:con-.ditions expected for .a given .area. :Investigations

of

this kind :might :be, or .may already have :been, :assumed by :the :weather .ships of .several :nations .which :are .stationed .at _various points in the .Atlantic and :which are :staffed

with .trained _meteorologists.

TM-792

CONCLUDING REMARKS

Thus far, some of the present methods of testing, observation, and. analysis of _results have been examined, and some of the more pressing problems, those which deal mainly with increased resistance, have been outlined. Probably the most important point for attack is the question

i\

of the motion of the ship. When this motion can be described.analytically and translated in terms of increased resistance to the model or vessel, a large part of the problem will have _been solved. Much work

has

.been

done on the components of motion and some exists for combinations of the various components. Analytical methods are hampered by the presence of a _large number of variables; and the ,simplifying assumptions which are

usu-,

_ally made restrict the solution to such.a degree that the results .are un-real or artificial and hence cannot be applied to un-real problems. For

in-stance, an analysis which treats each motion separately and combines the _motions in ..a final solution, but which omits the ,coupling forces, may

pre-sent a picture which is far removed from the events that are actually taking place. Perhaps some variation of- the matrix method used in the treatment of flutter in.aircraft_will allow the retention of a sufficient number of variables to give.a realistic solution. J. Kreitner's paper ('Heave, Pitch.and Resistance of Ships in .a Seaway,' INA, Vol. 81, 1939, p. 203) is .a valuable contribution since he considers the .effects of

.several degrees of freedom rather than single isolated components.

Until such time as .suitable equations of motion are derived 'to:assist in the prediction of results, model tests probably offer the best.approach. It must be .remembered, however, thai, model testing is limited, in general,

to straight-dine.motion into.a uniform wave train. These conditions assist in simplifying the conduct and _analysis of the tests, but they also restrict the applicability of the conclusions. It has already been suggested that it is desirable to determine.which kind of a model test.technique will pro-duce the _most _useful _results. ,In order to establish some manner of

correla-tion for model prediccorrela-tions, full-scale observacorrela-tions or tests of similar _models in varying sizes .are necessary. The methods .used.and observations

_made.in full ,scale tests appear to be adequate . (provided _acceleration .mea -surements are .included), with the possible .exception of the description of the wave dimensions. The determination of wave dimensions also leads to the question of _describing the ,nonuniform seaway in terms of the regular train. ,waves used .in tank testing. A possible means of .comparison.might be a

TM-92 37

and presentation of results might be investigated with profit.

These remarks are not meant to disparage any of the past achievements. An attempt has been made to point out some -of the most useful paths leading to more satisfactory results. Much of the present work, correlation, and predictions are empirical and thus offer room for a more fundamental and realistic evaluation. It is within the real!'" of possibility that the

opti-mum form and structure cannot be attained at present because empirical methods will only indicate modifications to existing shapes. An analogy to illustrate this Might .be drawn from aircraft development: it was only after airfoil theory was well advanced and understood that the laminar-flow' ' airfoil could be envisioned,.much less constructed. It is also realized

that some of the investigations which are suggested are very broad in scope and represent a large investment in time and money. Most research activities receive their support from commercial or government contracts and can devote only a small part of their program to projects, which do not offer _more or

less immediate results. It is here that.the.student can make a significant

' ontribution in time and thought. e student _needs guidance; but at the

1.c

The

same time, a fresh approach, uninhibited by 'accepted' methods or techniques, often produces .results.

The suggestions rfor the .evaluation of present methods of testing and

_analysis of results. and the ,introduction of a uniform notation for the classifications of _articles sound .like a proposal for standardization. The

author realizes that the term 'standardization' is _a rather trite and over= worked expression often offered as panacea for all kinds of ills. However,

' comparison of re,sults, ideas, and conclusions is the only means of

increas-ing the understandincreas-ing of any subject, and an intelligent discussion should le based on iprrie common footing of ideas or terminology. A new'.approach or

change in method needs some foundation as a basis for departure. Most of the present material.available on seaworthiness has little in common for establishing an over-all picture, and it is this situation which prompts a suggestion for some semblance of uniformity.