Increasing the sea speed of merchant ships

DAVIDSON.LABORATORY

STEVENS INSTITUTE OF TECHNOLOGY , HOBOKEN. NEW JERSEY

INCREASING THE SEA SPEED

OF MERCHANT SHIPS

by

Edward V. Lewis

FOR PRESENTATION AT A MEETING OF THE METROPOLITAN SECTION

SOCIETY OF NAVAL ARCHITECTS AND MARINE ENGINEERS

APRIL

ao.

1959

I

k:_ck aVaVjeklkak

it.

"

so.

MODEL READY FOR TESTING

(No.

2161 FOR PANEL 0-21)

MODEL UNDER TEST IN WAVES

DAV I DSON LABORATORY

SUMMARY

It is noted that the increase in attainable speed of modern merchant ships in rough seas is primarily a problem of reducing motions rather than increasing power. Possible measures for reducing motions are discussed

herein, including special devices for control of rolling and pitching, changes in hull form above as well as below the waterline, and modification of ship proportions to provide greater slenderness. The importance of the latter suggests that greater attention should be given to a ship's intended service when basic dimensions and characteristics are selected. Recommendations are made regarding research needed to permit quantitative predictions of ship speed and performance at sea from model tests.

TABLE OF CONTENTS

Page

Introduction ... .SOSSO ...

OS ... SS."

1

Logbook Analyses 2

Reasons for Speed Reduction 4

Means of Increasing Sea Speed 5

Anti-Rolling Devices 5

Anti-Pitching Devices 6

Hull Form 8

Ship Proportions 9

Design Implications of Increasing Sea Speed 11

Quantitative Predictions of Sea Speed 12'

Conclusions 13

Acknowledgements.... 15

References 16

Tables 17

INTRODUCTION

A serious problem for operators of ships in heavy weather services such as the North Atlantic and North Pacific is the loss of speed in rough storm seas. The problem of how to reduce this loss is becoming increasingly acute as the power and nominal speed of ships increase. Fortunately, recent pro-gress in research on the seagoing qualities of ships has produced a number of possible answers, and it is the object of this paper to discuss these and offer suggestions for practical steps which can be taken.

The Ship Technical Operations Committee of SNAME has shown great in-terest in the problem of sea speed, and for some time has sponsored a series of projects at the Davidson Laboratory, Stevens Institute of Technology. A new Panel 0-21 has recently been appointed under the chairmanship of Captain L.A. Renehan to provide advice and guidance for this work. Some of the mat-erial presented herein, as well as the movies to be shown later, has been obtained under the Committee's sponsorship. Portions of this paper simply review previously published material developed by the author under various projects sponsored by the Bureau of Ships and Mice of Naval Research.

R-744

-2-LOGBOOK

ANALYSES

An important step in the investigation of the sea speed problem at the Davidson Laboratory was the analysis of logbook data several years ago under the sponsorship of the Ship Technical Operations Committee. Through the cooperation of the Moore McCormack Lines, log data for.a number of voyages of both Victory and C-3 cargo ships in winter North Atlantic service were studied. The Moore -McCormack route to Baltic

ports is north of the British Isles -- with occasional stops at Iceland-and therefore is well-known as one of the roughest sea lanes in the

world. The author has also had the opportunity to examine logs of C-3's and the 'Schuyler Otis Bland' (C3-S-DX1) in North Pacific service between Japan and Puget Sound, which is another severe ocean route.

In all cases the relationship among daily average speed, heading, and sea condition was determined by graphical plotting. Average results for the Victory ships fully loaded are shown in Figure 1*. (Additional data are given in a report by Lewis'.) Only data for the days

in

which conditions remained reasonably constant were included. For all of the comparatively high-powered vessels mentioned, a consistent pattern has emerged, namely,

As sea conditions increase in severity, ship speed falls off slowly at first and then more rapidly. This falling off is much more pronounced than in the

case of lower powered ships such as Liberty vessels. The speed reduction is greatest in head seas and is

appreciably less as the direction of the sea swings around to bow, beam, and quarter. In fact, a moder-ate quartering or following sea may actually increase the speed of a ship.

The greatest reductions in speed in rough seas are found to result from voluntary reductions of power. Consequently, it appears that it is ship motions rather than lack of power which ordinarily limit rough weather speeds. Entries in logs suggest that the most frequent difficulty associated with ship motions is the shipping of heavy seas. This is particularly serious when deck cargo is carried, but even when it is not, there is danger of damage to hatch covers and deck outfit. However, when in ballast condition, bottom slamming is usually the immediate cause of speed reduction. High acceler-ations associated with pitch.ng and heaving do not ordinarily seem to be a reas,n for speed reduction for cargo ships.

4. Log entries indicate that large amplitudes of roll-ing, with large lateral accelerations often cause a change in course. A speed reduction usually follows!

1

such a course change.

The situation is clarified by other studies of sea performance, such as the work of Professor Aertssen2 in which shaft horsepower (SHP) as well as speed was considered. Figure 2 (taken from Aertssen2) shows typical results for a high-powered ship of 0.69 prismatic coefficient. It may be noted that the power requirement for a particular speed increases with the severity of the sea, but that there is an upper limit on both power and

speed. The situation is quite different for slow ships in which full pow-er can be utilized in all but the most extreme sea conditions. Speed re-duction is then due mainly to added resistance, reduced propulsive effici-ency, and inadequate reserve power.

R-744

4

-REASONS FOR SPEED REDUCTION

It is important to realize that the problem of sea speed for modern high-ppwered merchant ships has shifted from the question of power re-quirements to that of reduction of motions. What is the reason for this change? Aren't modern cargo ships designed for higher speeds?

Recent research provides the answer: they are designed for higher speed in calm water, with finer block coefficients, but with insufficient change in hull characteristics to permit higher rough-water speeds.

CompariNg. a Victory vessel with a Liberty ship, for example, one finds a reduction of block coefficient from .75 to .67, but very slight changes in the main dimensions or in bow freeboard. The forebody of the Victory ship is considerably finer and more U-shaped than the Liberty,

but this may not in itself make the ship more satisfactory regarding motions in waves. Yet the designed speed-length ratio, V/41.7, of the Victory ship is about 0.80 compared to only 0.55 for the Liberty vessel, while its installed power is 6000 or 8500 SHP compared to 2200. If the re-engined Liberty ship(Benjamin Chew was able to use only an average of 63 percent of its full power (6000 SHP) in service (See Newell and Chwirut3), it is not surprising that Victory ships cannot use full power

in rough weather either.

Similarly, a 460-foot ship designed for 1844 knot Isea speed' may perform very well in good weather but in a rough head sea be little or no better than a C-3 or a Victory ship with a rated 1644 knot 'sea

speed'. The important thing is that the hull form and proportions are usually selected with speed in calm water rather than in rough sea con-ditions in mind.

MEANS OF INCREASING SEA SPEED

The question which particularly concerns the 0-21 Panel is: What design changes can be made to permit higher rough-water sea speeds by modern cargo vessels? The following possible measures will be discussed in turn:

I. Anti-rolling devices Anti-pitching devices Changes in hull form

Changes in ship proportions Anti-Rolling Devices

One of the most important developments in recent years has been the extensive use of controllable fins to reduce rolling motions, particularly on passenger ships. Such fins have proved successful in increasing

pas-senger comfort, and it is believed they are also of importance for in-creasing attainable sea speed. The direct effect is to avoid changes in

speed and course otherwise necessitated by heavy rolling. This is an important consideration, particularly when deck cargoes are carried.

Another important effect of anti-rolling fins is their indirect influence on pitching. When a ship encounters heavy head seas and begins to pitch heavily, a change of course will usually reduce pitching. But if the course change results in heavy rolling, it is no solution. Speed reduction is then the only answer. However, if roll stabilization is provided, the course change can be made without loss of speed. Figure 3 shows the pitching amplitudes and accelerations of a roll-stabilized ship

in an irregular, short-crested sea as calculated for a Series 60, 0.60 block hull. (See Lewis4) This general trend of reduction in amplitude and acceleration with heading agrees with that observed on actual ships. It should be noted that even in a beam sea, the short-crestedness of the waves results in appreciable pitching motions. But it is clear from the

figure that a few degrees change of course may be very helpful in reducing pitching, if one need not be concerned about rolling.

Conversely, the logbooks of cargo ships in North Atlantic service often record cases of heavy rolling developing at good speed in a beam or quartering sea. A change of course reduces the rolling, but heavy pitching results, and it thus becomes immediately necessary to reduce

speed. This situation would be avoided if rolling could be reduced by other means. In short, the control of roll gives the shipmaster much greater flexibility in the handling of his ship to minimize pitching.

R-744

-6-Hence, it is believed that anti-rolling fins should be installed on cargo as well as passenger ships. It is to be hoped that fin manufacturers will develop simpler, less expensive equipment specifically for cargo ship application. Depending on the speed, the fins may have to be comparatively larger thanthose installed on passenger vessels.

Another interesting development is the Navy's current interest in passive anti-rolling tank installations. Model tests and full-scale trials have shown the effectiveness of properly designed tank systems. Although such a passive anti-rolling device cannot be as effective as an active device, the reduction of roll is quite marked. The tanks also have the advantage of being effective even when a ship is at slow speed or hove-to. Hence, they should be considered for cargo ships -- particularly low speed

vessels.

,Anti-Pitching Devices

Assuming that rolling motions can be dealt with satisfactorily, the remaining motion of prime importance is pitching. The effects associated with pitching -- shipping of water and slamming -- appear to be the princi-pal reason for power and speed reduction in heavy weather. One obvious approach to the problem of pitch reduction is to develop and install devices to reduce the motion. However, it is well to point out here that the

prob-leth of pitch stabilization is somewhat different from that of roll stabili-zation. The wave forces and moments on a ship in the longitudinal direction are much greater than the lateral ones, and, consequently, it is out of the question to think of eliminating pitching entirely -- as has been virtually accomplished with rolling. Nevertheless, moderate reductions. in pitching amplitudes, with consequent reduction in vertical accelerations, appear to be within the range of feasibility. Even a few degrees of pitch reduction may have a great influence on the comfort of the crew and passengers or on the speed of a ship.

Fixed or oscillating fins should, of course, be located near the ends of a,ship, but the first question arises: which end? Figure 4 (from Spens5) shows the typical behavior of a ship in regular waves of

litt

at the speed which results in synchronous pitching. At synchronism not only are ampli-tudes high, but the bow plunges down into the crests and-tends to emerge at the hollows. The sterna on the other hand, tends to follow the waves. Taking into account the motion of the wave particles, the-forward motion of the ship, and its pitching and heaving (without fins), the resultant velocity vectors at bow and stern are found to be as shown in Figure 4.-

A

large vertical forces because of large angles of attack. These ara typical damping forces which-are in the correct phase to reduce pitching, and the

fins' action is analogous to bilge keels for reducing rolling.

However, the figure shows that fixed, fins at the stern develop small angles of attack and therefore generate small vertical forces. Hence, it -seems worthwhile to consider the use of controllable fins at the stern. Model tests have shown that, when operating in the correct phase relation-ship to regular waves, fins can be roughly of the same degree of.effective-ness as fixed bow fins. (See Spens5) The combination of fixed bow and controllable stern fins is almost twice as effective as either alone. .Ap-proximate comparative figures for-pitch amplitudes estimated on the basis of model tests are as follows:

No fins 90

Bo* fins (fixed) 7o Stern fins (controllable) 7.2° Bow and stern fins 5.60

If fins appear to be effective in reducing pitching, why can't ship-owners have all their vessels dry docked and fitted with them at once? The answer is that there are still some practical problems to be solved. Fixed bow fins have been in use for some time on the Compass Island, a Mariner ship converted to a naval auxiliary. Reports have indicated that the fins did reduce pitching, but they also caused some hull vibrations

under certain conditions.

-There is also the possibility that even though fins reduce pitching amplitudes they may not reduce the wetness of decks forward. This depends on the phase relationships between bow motion and the waves. In tests for the 0-21 Panel at the Davidson Laboratory the performance of a model with and without fins was compared, and,

ih

this case; the wetness forward was only slightly decreased when fins were used. This will be demonstrated in

the film to be presented later.

Undoubtedly these problems can and will be solved. Professor Abkowitz has been working on the bow fin problem for some time at Massachusetts In-stitute of Technology, and is scheduled to present a paper at the next fall meeting of the SNAME. Oscillating stern fins involve not only mechanical

and structural problems for such large movable surfaces, but also 'a serious problem to devise a control system which will cause the fins to move in the desired manner in relation to the irregular motions of the ship. Work at

the Davidson Laboratory indicates that these problems, too, can be solved, but there remains some doubt as to whether the results will be worth the cost.

R-744

-8-Hull Form

It has been a dream of naval architects and research men to find some hull form modification which will significantly improve the sea behavior of a ship without penalizing its good weather performance or increasing its initial cost. The most promising variation is the use of V-shaped forebody sections, the most extreme version being the so-called Maier form. Experi-ments described by the author6 indicate that smaller motion amplitudes are obtained with V-sections, and theory (See Korvin-Kroukovsky. and Jacobs7) explains this as resulting from a reduction in the natural pitching period and an increase in pitch and heave damping. For high-speed ships, however, a V-form is not usually suitable for minimum resistance, and some compro-mise is therefore necessary. Although the author does not expect

practica-ble changes in underwater shape alone to result in drastic improvement in performance over good, conventional forms, underwater shape does require careful consideration by the designer. Model investigations of some changes in hull form are contemplated in Panel 0-21's research program.

For ships operating frequently at ballast draft, an important aspect of hull form is the effect of bottom shape forward on the frequency and severity of slamming. Form modifications which reduce the area of flat bot-tom can be expected to permit higher speeds before slamming occurs in bal-last condition. Research is needed on possible changes in lines which are practically feasible for merchant ships, and the Hull Structure Committee of the Society (Panel S-14) is now considering a project along this line. Another important factor affecting slamming incidence is the forward draft. The provision of adequate liquid ballast capacity is of great im-portance for ships which must frequently make rough weather passages with-out full cargoes. Of course, if all of this ballast is carried in low deep tanks, it has an unfavorable effect on metacentric height and rolling. This has led to suggestions that 'tween deck ballast tanks be employed.

Even though possible underwater hull form modifications do not offer much hope for drastic improvement in attainable rough-water speed, there can be no doubt that above-water shape is of great importance. Bow free-board and flare particularly do not have an appreciable effect on the mo-tions, but they obviously do have a large effect on reducing the frequency of shipping water. Unless measures are adopted to reduce pitching motions, there should be a trend of increasing bow freeboard/length ratio as the nominal sea speed increases. Usual values are given in Figure 5, which shows an upward trend of freeboard with increasing speed. However, the in-crease shown may not be great enough and consideration should be given to

a more rapid increase in freeboard as nominal speed is increased. The ef-fect of a large increment in freeboard at the bow was included in the pro-gram for the 0-21 Panel and will be shown in the films.

Adequate flare is also a necessary feature of good hull lines. Its

effect on ship motions is difficult to detect in model tests, but it assists greatly in keeping solid water off the foredeck. Bence, the flare should be sufficient to deflect water outward as the bow pitches downward into a

wave, but not so great as to cause local impact loads on the structure or whipping of the hull girder. (For a discussion of observations on an

air-craft carrier rounding Cape Horn, see Jasper8.)

Another important design feature affecting attainable sea speed is a clear, uncluttered weather deck, especially forward. The object is to make it possible for a reasonable amount of water to be safely taken aboard and quickly put overside. To this end more attention should be paid to placing

equipment such as the anchor windlass below deck, using rugged steel water-tight hatch covers, adding a good margin of strength to the forecastle deck, and providing really effective breakwaters. Examples of good bow design are the old Normandie and the new Canadian destroyer escorts of the St. Laurent class. Several breakwaters on the forecastle deck have been tried in the 0-21 model project, and the following appear to be desirable features: sharply pointed plan view, concave sections, and generous height (six to eight feet).

Another important consideration is adequate forecastle length. Even in the straight-crested head seas of the model tank, solid water is often shipped abaft the forecastle if its length is not sufficient. It is de-sirable whenever possible to extend the forecastle far enough aft to reach the full beam of the ship.

Ship Proportions

Finally, one of the most effective means of improving the attainable sea speed of merchant ships is believed to be the increasing of length in relation to other dimensions. The reason that improvement is to be ex-pected from such a change in proportions has been dealt with at length elsewhere by the author8. Briefly the argument may be stated as follows: Generally the most violent pitching and heaving motions and greatest speed

reductions occur in head seas, as previously noted. (See Figure 3) In

typical storm seas a ship responds most violently to the wave components which the ship encounters at its natural pitching (or heaving) period. However, if the speed is reduced until synchronism occurs only with wave

R-744

10

-components shorter than the ship, the motion amplitudes will be greatly

re-duced. The longer the ship the less the reduction of speed required to ameliorate the violence of the motions.

This idea is illustrated by Figure 6 which shows the general trend of speed for severe pitching and wet decks withA/(L/100)3. It shows how the

trend ofA/(L/100)3 for modern ships has been parallel to the limiting curve. A trend toward lower values ofA/(L/100)3 labeled 'proposed trend' would

permit higher sea speeds.

Good confirmation of the tendencies shown in this chart has been ob-tained by Aertssen2. On the basis of ship observations he has obtained a' series of curves which run generally parallel to the limit curve of Figure

6. (See Figure 7) Service performance data on lengthened Liberty ships also show a clear advantage in extra length. (See Newell and Chwirut3)

In order to obtain further confirmation of these ideas in connection with the work of the 0-21 Panel, two models were built to the same scale. The first basic model selected by the Panel for comparative testing

--was of the new Moore McCormack freighters. The lines of this class of ships are essentially the same as those on the 'Schuyler Otis Bland' (C3-S DX1) except for their expansion in beam. Comparative particulars are given

in Table I. The second model was obtained by increasing the length of the other by ten percent, with corresponding reductions in beam and draft so

that the displacement remained the same. Midship freeboard is the same, as is the shear line throughout most of the length. The shear line for-ward is continued upfor-ward in such a manner that freeboard/length at the bow is the same for both models. (See Figure 8)

Comparative model tests and moving picture records taken in an irre-gular sea corresponding to 7 on the Beaufort Scale (significant wave height 20 feet) showed the following:

At equal speeds a considerable reduction in pitching amplitude, in vertical acceleration at the bow, and -- most important -- in shippage of water forward

for the longer hull.

For comparable wetness in this particular sea, the basic model was limited to 8 to 10 knots, while the other model could contipue to a much higher

speed.

It is believed that these results and the moving pictures will show clearly the advantage

of

increasing length. It should be noted that the reduction in beam is not essential for good performance in head seas, but was intro-duced only to keep the displacement unchanged.'

DESIGN IMPLICATIONS OF INCREASING SEA SPEED

The suggestion of increasing length to improve seagoing qualities will undoubtedly lead to the objection that it is too expensive a solution. The answer is that, as in all other aspects of naval architecture, compromise is necessary. _{This compromise should be made on the basis of overall operating} costs, any additional initial cost being weighed against the savings in sea

speed. An attempt at such an economic evaluation was made by the author9. This means that the optimum proportions for a ship must depend on its

ser-vice, that is, on the proportion of time that rough _{seas are expected. Hence,} it is not enough to select length, beam, depth, draft, _{etc. merely by} compar-ison with other similar ships. Nor for rough-water services does it suffice to select form and proportions on the basis of calm-water resistance consid-erations and then to add a rough water 'power margin'. As speeds increase, a point is reached beyond which additional power becomes unfeasible. _The hull must be of such a design that it can be driven to the speed desired.

Sooner or later, however, the power requirements in both smooth and rough water must be determined. In this connection it is significant that the longer model showed a distinct advantage in calm water.

In a discussion of the economics of ship speed, the _{problem of cargo} handling inevitably looms up as a factor that cannot be ignored. So long as ships spend half their time in port, any saving in sea speed is only partially effective. For example, doubling the sea speed would provide no more than a 50 percent increase in the quantity of cargo carried per year. But when the port time is reduced to _{a few days by use of vans or containers,} or by using bulk handling methods, any time saved at sea will immediately be

reflected in increased effectiveness of the ship as a means of transport. Hence, one can afford to spend more for increasing the speed of ships that have quick port turnaround.

R-744

-12-QUANTITATIVE PREDICTIONS OF SEA SPEED

So far only the comparative performance of different hulls has been considered. Another important goal for research is the prediction of ship performance and speed by means of model and/or theoretical techniques. This

involves the following important steps:

More complete and accurate observations of ocean wave patterns, with results presented in the form of spectra showing the relative importance of different wave-length components.

Full-scale observations to establish quantitatively the upper limits of factors governing attainable sea speeds of different ship types. That is: How much water must be shipped before speed must be reduced? How great must be a slamming impact? What values of vertical or lateral acceleration are excessive? Recent full-scale ship tri-als with elaborate instrumentation, though of great value, have failed to give us answers to these questions.

Correlations between ship performance and behavior of models in irregular tank seas. A step in this direc-tion has been taken at the Davidson Laboratory, again under the sponsorship of the Ship Technical Operations Committee. The basis for the correlation has been

sim-ply the visual observation of water shippage over the bow, as shown in the film.

It is hoped that research will be pursued along all of these lines, in order to permit better predictions of attainable sea speed in addition to qualitative comparisons of models.

CONCLUSIONS

Although much remains to be learned about ship behavior in waves and means of improving the attainable rough-water speeds of modern merchant ships, it is believed that certain conclusions can be drawn at this time, namely,

Slender ship forms, as indicated by low values of

hi(14/100)3 , appear to be advantageous for

obtain-ing high rough-water speeds.

Accordingly, choice of hull proportions and basic characteristics at the beginning of a design should take into account the intended service and the de-sired sea speed.

Practicable variations in underwater hull form of-fer some possibilities for modest improvements in sea performance (including reduction of slamming) and should be continually investigated. Meanwhile, modifications in the abovewater hull are known to

be of great importance for minimizing water ship-page and eliminating water on deck quickly and without damage. Hence, important factors are ad-equate forward freeboard, well-designed flare, clear weather decks, effective breakwaters, struc-turally adequate decks, watertight closure of openings, etc.

Dynamic devices such as fins and tanks for control of rolling appear to be of great value for cargo ships as well as for passenger vessels. Fixed or controllable fins for reduction, but not elimina-tion,of pitching appear promising, but should not be considered a substitute for other more basic

steps.

In order

to

make quantitative as well as qualita-tive predictions of ship performance at sea, re-search is needed particularly on the following:

Recorded characteristics of ocean wave patterns, in the form of spectra

indicat-ing the relative importance of different wave components.

Shipboard observations of conditions which cause speed reduction, such as amount of water shipped, severity of slamming, and magnitudes of lateral or vertical acceler-ations.

Correlations between model and full-scale performances.

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-14-When these three steps are sufficiently advanced, it should be possible to make use of model tests to pre-dict the attainable sea speed of a specific design in any service.

'able II summarizes the data analyzed to date in the comparative mop del test program being carried out at the Davidson Laboratory for Panel

0-21. This work is continuing, and further results will be made available in due course.

ACKNOWLEDGEMENTS

The author wishes to acknowledge the assistance of Mk. Clayton Oden-brett in carrying out the model tests in the 0.21 Panel's project and to Mr. Herant Deroian for the color moving picture photography. The members of the 0-21 Panel on Feasibility of Higher Sea Speed provided helpful sug-gestions in the planning of the project. The members of this Panel are:

Captain L.A. Renehan, Chairman Captain E.G. Barrett

Mi. Frank Grafton Mr. Wilbur Marks Captain S.J. Swanson Prof. E.V. Lewis

Mr. Harrison R. Glennon, Jr., Assistant Vice President of Moore Mc-Cormack Lines (Chairman of the Ship Technical Operations Committee), and Captain Swanson of the American Mail Line have been of great assistance in connection with both log analyses and comparative model testing. Captain Wentworth of the American Mail Line kindly furnished film of his ship in heavy weather in the North Pacific.

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-16-REFERENCES

1. Lewis, E.V.: "Sea Speed of Victory Ships in North Atlantic Service", Stevens ETT Note.438, prepared for Ship Technical Operations Com-mittee, SNAME, August 1957.

2: Aertssen, G.: "Sea Trials on a Passenger - Cargo Liner with Block Coef-ficient 0.672 and on a Large Tanker with Block CoefCoef-ficient 0.770", Dimi, January 1959.

3. Newell, R.Y., and Chwirut, T.J.: "Service Experience of Liberty Ship Conversions Involving Four Different Propulsion Systems", Chesapeake Section, SNAME, April 1959.

4.. Lewis, E.V.: "Developments at Stevens on Behavior in Irregular Seas", Symposium on the Behavior of Ships in a Seaway, Wageningen, Holland, September 1957.

. Spens, P.: "Pitch Reduction by Oscillating Stern Fins", article in

notes of Second Summer Seminar on "Ship Behavior at Sea", Stevens 'ETT Report 708, November 1958.

Lewis, E.V.: "Ship Speeds in Irregular Seas", Trans. SNAME, Vol. 63.

1955.

Korvin-Kroukovsky, B.V., and Jacobs, W.R.: "Pitching and Heaving Motions of a Ship in Regular Waves", Trans. SNAME, Vol. 65, 1957.

Jasper, N.H., and Birmingham, J.T.: "Strains and Motions of USS Essex (CVA9) During Storms Near Cape Horn", DTMB Report 1216, August 1958. Lewis, E.V.: "The Sea Speed of Cargo Ships in Rough Weather Services",

International Shipbuilding Progress, June 1956. Additional Reference

Odenbrett, C.: "Log Analysis of C3-S-A2 Cargo Ships and C3-S-DK1 Schuyler Otis Bland", Stevens ETT Note 467, February 1958.

TABLE I

COMPARATIVE SHIP CHARACTERISTICS

°Determined by o

1 oseill

f 'models in a.largo essk.

n3.0.

Bland" Moore McCormack (Model 1963) Modified design (Model 2161) Length B.P. 450'-0" 4581-0" 503'-10" Breadth 66'-0" 68 -0" 64'-10" Depth 41'-6" 41'-9" 40'-5" Draft 28'-6" 28'-6" 271-2" Freeboard midships 13'-0" 13°-3" 13'-3" at bow 32'-3" 32'.-5" 35'-7" Bow freeboard/Length .072 0.071 0.071 Displacement, tons, S.W. 15,910 _16,147 16,147 ARL/100)3 174 168 126 T * seconds P ' 7.83 7.54

T//1,

_P 0.362 0.336

R=744.

TABLE

COMPARATIVE TEST RESULTS IN IRREGULAR -HEAD SEAS

CORRESPONDING TO BEAUFORT 7

(Significant Height 20 feet) FOR PANEL 0-21

0Data not analysed; ahould be the same as Model 1963 with normal forecastle.

,. MM Detign, '

Model 1963 .

MM Design,

.Raisedjecsl.

MM DeaUgh,

Bow Fins

10$ longer Model '2181 _

-Tim. freeboard _{32'-5" 40'_S" 32+-5"} : ": 35.+-7" Packing amplitude, Degrees, at 10 knots , . . : , . Average . Average, 1/3 bigbeSt. 6.4 10.2 .. 8,*01 4.8 , . 7.8 5.4 . 9:.1

' Cases of Shipping Solid

Mater Over Bow

, . 8

knot!

, ,.. _{10 knots} ,-, _-I .

12 knots

-14 knots

.16

knots

18 knots

1 bad

1 small

1 very bad 1 moderate 1 very bad 1 moderate. 2 small .., --, ,4., , . . 1 moderate 1 moderate . -'

none

1 small

none

none

1 moderate. 1 small

1 bad

1 moderate 1 small 1 very bad 1 small -,-__ 1. moderate 1 bad . .. none-. . none c-Lmoderate

none-FIGURE

I

TREND OF SPEED WITH SEA CONDITIONS

FOR VICTORY CARGO SHIPS

BEAUFORT SCALE

1 18 16 14 12 10 POWER 1 i I REDUCED I e 1

I

QUARTERING

FOLLOWING SEA

AND

SMOOTH

SEA

BEAM SEA

MWRATE

BOW SEA

I ROUGH. HEAD

VERY

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I

9

10

FIGURE 2

TYPICAL SPEED-POWER CURVES FOR DIFFERENT SEA CONDITIONS AND

HEADINGS

Plate I

"Sea The& on a Passenger-Cargo Liner with Block Coefficient 0.672 and

on a Large ranker with Block Coefficient 0.770."

Paper by PROFESSOR G. AERTSSEN,

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FIGURE 3'

180

- 0-AVERAGE ACCELERATION DUE TO PiTCHING RAGE PITCH 120 105 0.6 0.5 4CCELERATION 0.4

9

0.3 0.2 O./ 90 10

CALCULATED PITCHING

_{MOTION FOR}

SERIES 60 HULL (0. 60 BLOCK) AT VARIOUS

_HEADINGS

_BOWAN IDEAL IRREGULAR STORM

_{SEA (NEUMANN)}

R-744

-22-S /TRW

ant

L

...

MA*. PITO L.. MAx. PITCH _

---FIGURE 4

DIAGRAMS SHOWING DIRECTION OF FLOW AT BOW AND STERN

SERIES 60(Cba0.60) MODEL IN WAVES I-1/2x MODEL LENGTH HEIGHTE1/40 MODEL LENGTH AT SYNCHRONOUS SPEED (NO FINS)

.08

.07

FBD

.06

.05

+L

.04

FIGURE 5

TREND OF FREEBOARD RATIO

WITH SPEED-LENGTH RATIO, VAX

OTIS

HI

SCHUYLER

BLAND+

+

1

SEAFARER

C 2

+

MARINER

+

C3

±

+VICTORY

IBERTY

+HOG

1

ISLAND

-ItIB

.55

.60

.65

.70

.75

.80

.85

V

R-744

-24-

_{FIGURE 6}

SEA SPEED OF CARGO SHIPS IN RELATION

TO ZONES OF SEVERE AND MODERATE PITCHING

IN IRREGULAR HEAD SEAS

Key: + Nominal Sea Speed

Typical Heavy Weather Speed

1.2

1.Modern

Ship

Trend

"Marittrt

der. Draft (5)

Prop+210

Trend

C3

I

C4

ZONE OF MODERATE

PITCHING AND DRY DECKS

I I I

ZONE OF SEVERE

PITCHING AND WET DECKS

"Mariner-Design L.L. I

"Seafarer"

n

GIB Hog Is.A

1-lbert y

i!san Tim; (2)

of s

Older

Ships

Length

100

150

200

250

3

Displacement-Length Ratio, .0./(L/100)

0.3

1.2 .2. 4/ 4*). 04.

-

re

OBSERVED LIMITS OF SEA SPEED

OF CARGO SHIPS

I BY

AERTSSEN2

474, oc-4"4.2o 0

ZONE OF MODERATE

PITCHING AND DRY DECKS

7

FIGURE 7

ZONE OF

PITCHING AND

4I/7. A

eivo r

SEVERE

WET. DECKS

Pee

-0.3

447.15 ry6:4Ql

100

150

200

250

LW1.

FIGURE 8

COMPARATIVE PROFILES AND BODY PLANS