Proceedings of the Symposium on Ship Handling, Publication 451 of the NSMB, Wageningen, The Netherlands

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SYMPOSIUM ON

"SHIP HANDLING"

P1973-5

Lab. v.

Scheepsbouwkunde

Technische Hogeschool

Delft

NOVEMBER 28-29-30, 1973 WAGENINGEN, THE NETHERLANDS

PUBLICATION NO. 451

NETHERLANDS SHIP MODEL BASIN

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Edited by.

M.W.C. Oosterveld

Publication No. 415

Netherlands Ship Model Basin Wageningen, The Netherlands

SYMPOSIUM ON "SHIP HANDLING"

November 28,29 and 30, 1973

Wageningen, The Netherlands

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Contents

Opening Remarks

SHIP HANDLING AT SEA

Lightering at sea - Ship handling aspects. by Capt. M.J. Laws, Shell International Marine Ltd., London, United Kingdom.

Tentative manual of ship handling in rough seas, II

by Prof. H. Tani, Mercantile Marine University, Tokyo, Japan.

Correlation between full scale and model measurements III

on ship manoeuvrability.

by H. Okamoto, Kawasaki Heavy Industries Ltd.,

Kobe, Japan.

Dynamic-stationing systems.

by H.J. Zunderdorp, Koninklijke/Shell Exploratie IV

en Produktie Laboratorium, Rijswijk, The Netherlands.

SHIP HANDLING IN CONFINED WATERWAYS

Operation of ship traffic in the port of Rotterdam. V

by F. Visee, Ministry of Transport, Hydraulics and Public Works, Rotterdam, The Netherlands.

Influences of the water depth on the manoeuvring VI

characteristics of ships.

by G. van Oortmerssen, Netherlands Ship Model Basin, Wageningen, The Netherlands.

Design of harbour configurations in view of navigational VII

and hydraulic aspects.

by R. Reinalda and J. Koster, Hydraulics Laboratory, Delft, The Netherlands.

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HUMAN ENGINEERING ASPECTS

Method of operation for the pilotage of large VIII

ships entering Hook of Holland.

by Capt. H.J. Maas,Ministry of Defence (Royal Netherlands Navy), The Hague, The Netherlands.

Analysis and training of ship handling capabilities. IX

by P.J. Paymans and K. Meurs, Netherlands Ship Model Basin, Wageningen, The Netherlands.

Mental load during the manoeuvring of a large ship. X

by C.L. Truijens, Institute for Perception R.V.0.-T.N.0.-I.Z.F., Soesterberg, The Netherlands.

Ship handling aspects in context with channel design. XI

by L.A. Koele, Ministry of Transport, Hydraulics and Public Works, Hook of Holland, The Netherlands.

MISCELLANEOUS

Statistical analysis of ship's manoeuvres. XII

by I. Oldenkamp, Netherlands Ship Model Basin, Wageningen, The Netherlands.

Docking and mooring of a VLCC inside a harbour. XIII

by J.W. Oosterbaan, Municipal Port Management of Rotterdam, The Netherlands and

J.U. Brolsma, Public Works Rotterdam, The Netherlands.

The approach and mooring of large tankers to an XIV

offshore buoy.

by Capt. J. Renton, Single Buoy Moorings Inc.,

Monaco.

Improvement of ship manoeuvrability by means of XV

automation.

by J. van Amerongen and N.D.L. Verstoep,

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OPENING REMARKS OF THE PRESIDENT OF THE NETHERLANDS SHIP MODEL BASIN

Ladies and Gentlemen:

On behalf of the management of the Netherlands Ship Model Basin I should like to welcome you very heartily at our third annual symposium. The goal of these N.S.M.B. symposia is to improve communications between industry, governmental authorities and the laboratory in order to contribute to highly qualified scientific industrial service. The central theme this year is "Ship Handling". The broad scope of the term "Ship Handling" has been illustrated by the titles of sessions and lectures as

mentioned in the symposium program.

Originally this symposium was planned in the International Agricultural Centre. However, the capacity of this Centre is restricted to 240 persons. Today we are here present with 400 participants, 250 of which are from abroad. From the United Kingdom 70, from Scandinavia, U.S.A. and Germany each about 30, France 15 and a small number of representatives of 16 other countries. Representatives of: Ministries of Transport, Boards of Navigation, Coast Guards, Port Management, Pilot Associations, Universities, Ship Model Basins, Hydraulic Laboratories, Nautical Institutes, Meteorology-, Radio-, Radar-, Decca Institutes,

Consultants, Ocean Engineering experts, Design Bureaux, Shipowners, Shipyards, Propeller factories and finally Classification

Societies.

We appreciate very much that so many experts from all over the world are present at this "Ship Handling" symposium to discuss problems of mutual interest. Since the operation of ships due to the increase in size of the vessels and the increase of traffic in important waterways has become a complicated matter, research has been started at many places to minimize hazards and losses caused by non-efficient ship handling. Besides the technical studies for the construction of offshore ship handling and berth facilities, also research is performed on the behaviour of

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helms-men and officers in command during the complicated actions in-volved with handling of large and fast ships. To study this be-haviour, the N.S.M.B. has built a ship manoeuvring simulator. The results obtained from investigations on this simulator over the past three years have shown to be of practical use for industrial service at the following items:

- Qualification of harbour designs with known ships and experienced pilots.

- Qualification of advanced ship designs from a viewpoint of ship handling and manoeuvring.

- Determination of the ship handling possibilities of non-existing large ships (500.000-1.000.000 t.d.w. tankers). - Design of traffic rules for waterways.

- Design of criteria to assist harbour authorities in their decision to give permission to ships to enter the harbour under known circumstances.

- The introduction of new nautical instruments.

- Training of pilots under circumstances not met before.

We have distributed the invited papers over four sessions: "SHIP HANDLING AT SEA" under chairmanship of Prof. Timman, University of Technology of Delft. "SHIP HANDLING IN CONFINED WATERWAYS" under chairmanship of Admiral Van der Graaf, Ministry of Defence, Royal Netherlands Navy. "HUMAN ENGINEERING ASPECTS" under chairmanship of J. van Dixhoorn, Ministry of Transport, Hydraulics and Public Works, and "MISCELLANEOUS" under chairman-ship of Dr. Hooft, Vice-President Netherlands Ship Model Basin.

All preparations and the coordination of this annual symposium are under guidance of Dr. Oosterveld, N.S.M.B., Vice-President, in charge of research and development.

I wish you all, chairmen, authors, discussors and audience excellent introductions, presentations and discussions.

NETHERLANDS SHIP MODEL BASIN

nen

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Thursday, November 29, 1973

morning session

SHIP HANDLING AT SEA

Chairman: Prof.Dr. R. Timman, University of Technology, Delft, The Netherlands.

Lightering at sea - Ship handling aspects,

by Capt. M.J. Laws, Shell International Marine Ltd.,London, United Kingdom.

Tentative manual of ship handling in rough seas, II

by Prof. H. Tani, Mercantile Marine University,

Tokyo, Japan.

Correlation between full scale and model measurement

on ship manoeuvrability, III

by H. Okamoto, Kawasaki Heavy Industries Ltd.,

Kobe, Japan.

Dynamic-stationing systems, IV

by H.J. Zunderdorp, Koninklijke/Shell ExploratLe en Produktie Laboratorium, Rijswijk,

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Ship Handling at Sea

LIGHTENING AT SEA - SHIP HANDLING ASPECTS

by

CAPT. M.J. LAWS

SHELL INTERNATIONAL MARINE LIMITED LONDON, UNITED KINGDOM

The concept of using small tankers to "top-up" or "lighten" car-go to or from larger tankers, when the loading or discharging port is draught restricted, has been part of tanker operations for many years, with the transfer normally taking place in shel-tered waters. The necessity for the ship-to-ship transfer arises from the tanker operator's requirement to optimise the freight advantage of using the largest suitable tanker for long ocean hauls, accepting the small double handling charge at one or both ends. In certain trades such operations have been a longstanding practice, but in general they have been of a stop-gap nature until such time as the facilities are developed to accommodate the larger ships on full draught.

In the mid-1960's, Shell placed orders around the world for a large number of VLCC's. Although it was appreciated that these vessels would be delivered before the tanker terminals in North West Europe were capable of receiving vessels of this size on full draught, there was ample justification for proceeding with the orders. It was only a question of time before facilities would be up-graded, and in the intervening period deadfreight, incurred by loading the vessels light in the Persian Gulf so that they would arrive at their discharge port on a suitable draught, would be acceptable.

Obviously there was considerable incentive to avoid this dead-freight, since its occurrence is always an anathema to the tanker operator. Accordingly, it was decided to look into the

possibil-ity of a lightening operation for these ships. A paper study

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indicated that the optimum sized ship for the lightening role would be a 70,000 ton deadweight vessel, and that the operation should not, as hitherto, be conducted on the threshold of the port to which the VLCC was programmed but rather should take place at sea. This would not only achieve the main objective of reducing the draught of the 220,000 ton deadweight vessel so that that vessel could enter port, but additionally could

facil-itate the smaller vessel distributing the lightened cargo to

smaller terminals, thereby taking fullest possible advantage of using the large ship for the major part of the voyage. This dis-tribution could be optimised by careful selection of locations for the lightening operation.

Before this ambitious scheme could be introduced, there were many problems to be overcome. Customs agreement, communications

(rendez-vous position being a key factor), local authorities concerned, scheduling and documentation arrangements, etc., all had to be considered. The major concern at this stage was, how-ever, over the equipment and manning that would be required for the lightening vessel. Being keenly aware of the consequences of any accidents which might occur during these cargo transfers,

it was very apparent that good ship handling would be fundamen-tal to thesuccessof the operation.

The resolution of this problem, and how the necessary high stan-dards were achieved initially, and have been maintained, by

personnel from within our Fleets, is now dealt with in further

detail.

The actual lightening operation, as envisaged, entailed the smaller 70,000 ton vessel being laid alongside the 220,000 ton vessel in open sea conditions. The climate of North West Europe does not, at first sight, suggest that suitable sea conditions will be sufficiently prevalent to permit this type of manoeuvre except under ideal circumstances. However, a study of the coastal waters off North West Europe led to the conlusion that under almost any circumstances it would be possible, in various locations, to obtain some degree of shelter from the weather, and that there were a sufficient number of such areas to permit

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convenient subsequent distribution of cargo.

From the start, it was strongly felt that much of the success of the operation would depend on the quality of the personnel cho-sen to command the lightening ships. The sheer size of the vessels involved obviously created a problem from the ship-hand-ling point of view, and the vagaries of local weather did not

lessen the difficulties. Whilst it could reasonably be expected that all Masters be proficient at handling their ships under most conditions, this particular operation did not fall into this category, and a more developed skill was going to be re-quired. It was therefore decided to select Masters who had

con-siderable experience of ship handling, albeit of smaller vessels. A small nucleus of such men were available from which selection

could be made, for various ships within the Fleet have been required, from time to time, to perform special services which have necessitated the Master undertaking his own pilotage and berthing. The experience was obviously not comparable to that required for this lightening operation, but such men were known to have gained the correct attitude to do this type of job.

How the manoeuvre would be executed had been planned and dis-cussed in detail, and once the experienced men had been selected, it was possible to decide the best means of achieving the safest operation. Consideration of the problem led to the conviction that the manoeuvre would be facilitated if both vessels were under way, so that both retained steerage way when berthing alongside. Constraint by the Authorities required that the lar-ger ship should be anchored first, and the lightening ship then brought up alongside. Initial trials indicated that this method created as many difficulties as it solved, and the original con-cept, of having both ships under way, was adopted and a suitable technique quickly evolved to the satisfaction of the Authorities.

To permit this operation to be executed safely, the lightening ship was equipped, on the portside, with four large whale fen-ders, housed in cradles on deck, which could be lowered to the waterline. These fenders were designed to absorb the impact of initial contact with the VLCC, and then to maintain a safe

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tance between the two vessels during the transfer operation. Smaller fenders were also provided at the stem and stern to avoidthe possibility of damage if the lightening vessel should become too angled when close to the other vessel.

To ensure that no confusion can arise, the master of the

lighten-ing vessel is placed in charge of the mooring and unmooring

operation, and this is clearly understood by both vessels.

The VLCC, on making the rendez-vous, is instructed to proceed on a set course at a given speed. The lightening vessel then

approaches from the stern, at a fairly wide approach and at a slightly greater speed, to produce an opening bearing. The speed and course of the lightening vessel is then adjusted so that

first contact is made with the forward whale fender of the

lightening vessel touching the forward hull of the VLCC. The

lightening vessel then maintains this position until the

moorings forward have been run, and she then drops back so that all whale fenders are landed. The actual technique varies

slightly between Masters, some preferring to manoeuvre so that all four fenders land simultaneously. All moorings are run away and secured as quickly as possible. The hose is then connected between the two ships as the vessels manoeuvre to a suitable anchorage. Once anchored, the cargo transfer takes place as quickly as possible. When the lightening ship is fully loaded she then lets go, and depending on the weather conditions, carries out a suitable manoeuvre which will take her quickly

clear of the now partly loaded and anchored VLCC. On rare

occa-sions, when weather/tide are exceptionally unfavourable, it has

been necessary for the VLCC to lift her anchor and both vessels

to get under way before separation can be achieved.

With the light freeboard of the lightening vessel and the loaded

freeboard of the VLCC during the mooring operation, and the

change in this aspect for the unmooring operation, it will be apparent that, when the wind is across the tide, this seemingly

straightforward operation becomes more difficult. With two vessels of this size secured closely together it will also be

apparent that wave induced motion affects both ships, causing

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unsynchronized rolling. Although on occasions, this may cause some disquiet to the uninitiated, experience has shown that there is no danger of contact between the two vessels. Notwith-standing either of these two facets of the operation, weather delays have been rare. This in no way reflects on the prevalence of good weather conditions, but indicates the viability of the system, and the capability of the Masters to cope with these conditions. In passing, it is of interest to note that when the manoeuvre was first discussed, it was decided that line throwing

rifles would be supplied to the lightening vessel, to permit the first mooring ropes between the two vessels to be run away when the two ships were still some distance apart. In practice these have only been used under exceptional conditions, and normally the initial close approach can be made in such a controlled fashion that a heaving line can easily be passed from one ship to the other for this purpose.

Over six years of continuous service, the lightening fleet has been expanded and now contains an 18,000, two 70,000 and one

110,000 ton dwt. vessels, with a second of the latter class to be converted shortly. There has been an impressive record of success,and this has encouraged the extension of this concept to other areas outside N.W. Europe. Inevitably, minor incidents have occurred, as would be expected with the frequency of this type of operation. None of these incidents have involved more than superficial damage, and none have resulted in any interrup-tion to the service. In this time it has been necessary to ring the changes in the staff involved in this operation. Changing patterns in fleet operations made it difficult to obtain Masters with a lot of experience in ship handling, and suitable candi-dates for this job have to be selected on other bases. In all cases, Masters being introduced into this operation were, and are, required to serve in an observing capacity for a short period. Without exception, even new intakes with relatively little experience at close quarter ship manoeuvring, after ob-serving the operation once, express keenness to take over the reins themselves. Extreme caution is fostered in the early sta-ges, but as confidence builds up, this gives way to an expedi-tious, safe and efficient operation. It is abundantly apparent

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that those involved accept the task as an interesting challenge, and then gain considerable job satisfaction from these

appoint-ments.

Early concern regarding the adequacy of training methods, and the availability of personnel for this assignment was, in retro-spect, misplaced. This in no way implies that personnel for this job are no longer carefully selected, but rather that it is now appreciated that there are many more capable of doing this

exact-ing job than had been thought originally. The relevant training

that can be given is not very substantial, and it is now recog-nized that the learning process to change basic knowledge into practical skills can only be achieved by actually doing the job. This talent for ship handling lies dormant in most professional deep-sea mariners, since modern practice requires that the har-bour pilot performs most of these duties. However, it is abun-dantly clear that whenever this talent has been called upon it has not been found wanting.

The lightening ship Master, as will be realized, has an onerous

responsibility. In addition to his close quarter shiphandling duties in this operation, he is in overall charge of the cargo transfer operation; before arriving at, and on departure from, the rendez-vous he will have the normal problems associated with navigating vessels in these waters. Despite the heavy burden associated with this job, the assignment is much sought after by sea-going staff and those that have been appointed to this job are extremely reluctant to relinguish their commands for the more mundane demands of normal ocean trading.

Experience with this and other involved ship operations seems to

indicate that the opportunity to handle his ship is welcomed by

the Master; given his basic knowledge and training, confidence in his ability to do so is rapidly acquired by practice. The lightening operation has shown the adequacy of the practical yet necessarily limited training in ship handling that can be given to those selected for this assignment. However, it is equally apparent that there can be no substitute for this method of training, since it is eventually the ability of the man to

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cute these manoeuvres under the conditions that actually pre-vail, with a total awareness of the consequences of his errors, that will establish his confidence in his own skill.

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TENTATIVE MANUAL OF SHIP HANDLING IN ROUGH SEAS

by

PROF. H. TANI

MERCANTILE MARINE UNIVERSITY TOKYO

JAPAN

Introduction

In very rough weather in open sea,captains are always thinking of "how rough can the weather be without severely damaging the ship, and where is the point at which steps must be taken to protect the ship from the storm".

In the case of ships of usual size (understood to mean

10,000 to 20,000 dwt) there have been sufficient discussions on how to handle the ship to avoid the violent action of waves on the hull and to secure the safety both of life and ship.

Certainly ship handling in rough seas depends upon the types of ships, because each type reacts differently to the action of waves. Broadly speaking, however, some "Rules of Thumb" have been established among the mariners, based on their own long experiences, and have been used with success by almost all class-es of ships not larger than, say, 20,000 dwt.

The situation, however, might be somewhat different in vessels of larger size, such as the modern combined carriers and mammoth tankers. This is because most captains have had no time enough to gain sufficient experiences on account of the too rapid in-crease of the size of ships. In fact, these larger vessels usually have their navigation bridge so far aft that the longer distance from the bow causes the captains to underestimate the severity of wave impact on the bows. Since the larger the area offered to the seas the greater will be the shock of wave impact, these larger ships may be forced to reduce speed sooner than

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those of usual size.

In these circumstances the best solution of the problem is to give the captains some good indications which will inform them of when the ship is in danger and when is the suitable moment to start reducing the speed or changing the course. Strictly speaking, these indications can not be obtained without the exact knowledge of the strength of the ship hull in high waves. Unfortunately there still remain various unknown factors in the relationships between the operation, impact of waves and strength of ship. Nevertheless, to meet the needs of mariners at this moment, we are obliged to pass to some simplifying assumptions on the relation between ship motion and the ability of the ship structure to withstand the action of the waves.

The present paper deals with two possible indications with which the captain, driven into a violent seaway, would be able to make a good judgement in selecting the most suitable moment to start his storm ship handling. And our prime object is to present briefly the latest studies carried out in Japan, the results of which have been proposed in the form of a tentative manual. It is therefore beyond the scope of the present paper to give

rigor-ous mathematical treatments, but special effort has been made

to provide somewhat detailed information on the practices of ship handling which were made clear from the analysis of the investigations through questionnaire.

The first indication is the frequency of shipping water or the probability of deck wetness on the bow. The second is the in-crease in apparent slip ratio of propeller. Both of these have been analysed on the assumption that the ship was heading

directly into the waves. This is because such a situation can be considered not only as the severest condition, but also is much easier to handle mathematically.

Our manual is still only tentative. It should be revised with increase of our knowledge and experience. The captain should compare the methods given in the manual with reality,if the opportunity exists during a storm, and determine the best cources and speed for the ship.

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It should be specially noticed here that the present manual is for the time being confined to the handling of ships of

50,000 to 60,000 dwt. This is by reason that the maritime circles have been deeply concerned with these ships since we had

successive disasters of ore carriers of this size a few years ago. It would, therefore, be necessary to examine this problem carefully for larger ships with a view to being able to apply this manual.

This monograph consists of three sections. In the first section we will present the main section of the manual. The second and

third will outline the procedure through which the tentative limits of the two indications are determined.

I. Tentative Manual of ship handling in storm sea.

Usually the best way to minimize the pitching and heaving, as well as the effect of the impact of waves, is either to reduce speed or to change heading, or carry out both, when the ship is heading into rough seas in fully loaded condition.

Careful precautions should be made when the wind attains force 7 or so in Beaufort number. Good judgement is required in se-lecting the most suitable moment to start the operation. There are two good indications, the first of which is the number of occurrences of shipping water on the weather deck at the bow in a convenient definite time interval. The second is the increase in apparent slip ratio of propeller.

The following methods are preferable. 1.1 Method of shipping water.

Examine the relative frequencies of shipping water by counting the number of pitching occurred during two successive shippings

of water.

If the frequency exceeds the specific limits given in the following table, according as the ratio of actual bow freeboard to minimum bow freeboard, it is preferable to start reducing the speed or changing the course or both.

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In this table, the actual bow freeboard must always be measured at the location of the fore perpendicular, but the bulwark height should be excluded.

The minimum bow freeboard should be calculated by the following formula adopted by the 1966 International Load Line Convention.

For ships longer than 250 m,

Minimum bow height = 0.056 L (1

- --)x

1.36 , in meters

500 Cb + 0.68

where L and Cb denote respectively the length of ship and the block coefficient defined by the Convention.

1.2 Method of apparent slip ratio increment.

Note: The method is confined to the Diesel-engine driven

vessels.

Calculate the apparent slip ratio from log speed and revolutions of propeller by the following formula.

Apparent slip ratio, % = (1 - 30.9 x x 100,

NP

where N: revolutions per minute, P: pitch of propeller, in m,

V: average speed through the water, in knots. And then examine the excess of the slip over the calm-water-value during the voyage concerned. When the excess is larger than 25 %, it is preferable to start the operation.

If, in spite of the first trial of the above methods, the violent pitching and impact of waves still remain, the method of shipping water only should be tried again. The method of apparent slip is not applicable anymore, because once the speed is reduced, the slip will increase more than ever.

11,4

Actual bow freeboard _1.2

1.3 1.4 1.5 1.6

Minimum bow freeboard Relative frequencies of shipping water on bow deck

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Ship Handling at Sea 2. Critical probabilities of shipping water. 2.1 General procedure.

Relative frequency of shipping water forward is, at least for the present, a good specific criterion for detecting the approach of danger, when a loaded ship is heading into very rough seas. It is quite a simple measure which is easy to handle. As stated in the preceding section, however, it is quite a complicated

prob-lem to determine its critical value. Owing to this complexity

we are obliged to pass to a simplifying assumption, which will soon be described.

Our approach to the problem is based on the statistical predic-tion techniques which have been recently developed in the studies of seakeeping qualities. It is, therefore, not difficult to predict the probabilities of deck wetness for a particular ship

in a given state of sea. Difficulties really lie in finding what is the limiting wave height which determines the required

critical frequencies of shipping water.

In this connection an analysis has been made of the current practices of ship handling in storm weather, on the basis of

informations through questionnaire with shins of the usual size. Before proceeding to discussions of the results, it will be convenient to give a brief description of the general procedure of this paper, but in order to limit the size of the paper we

shall here confine ourselves to an illustration in the form of a block diagram.

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Block diagram of finding the critical probabilities of shipping water

(Questionnaire \ //Calculations

with 10,000 to\ with 10,000 to

20,000 dwt 20,000 dwt ships ) \ships Probabilities of deck wetness when started storm operation Standard deviations of relative bow motion and wave height R H Tentative limit of standard deviations Rm = 2.4 m Tentative limit of non-dimen-sional relative bow motion Z/L = 0.061 PP Check of theory and reality 11,6 (Calculations with 50,000 to 60,000 dwt \ships

Relative bow motion and wave height

R H

on-dimensional elative bow otion and wave eight Z/L H PP Assumed: Z/L = const. PP Limiting wave height for storm ship handling H = 7.1 m Probabilities of deck wet-ness and wave height

o H

--Critical frequencies of shipping water

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In the diagram the symbol Z denotes the greatest amplitude of relative bow motion expected on the average in 1,000 successive pitches, and is obtained from the standard deviation of relative

bow motion R by the well-known relationship:

Z = 3.8 R.

The success of the procedure depends upon the validity of the assumption that the non-dimensional relative bow motion Z/L

PP

should remain constant irrespective of the size of the ship. This assumption is based on the fact that in the present rules of the Classification Societies the required strength of the fore body is considered to be a function of ship length. It would, however, be necessary to examine this assumption more

deeply with a view to enable application to the problem considered.

2.2 Current practices of ship handling in rough _seas.

Information through questionnaire has been collected on 35 ships of the usual size, which range from 130 m to 175 m in length.

Table I shows the principal dimensions and _{speeds of the ships.}

All these ships are installed with Diesel _engines.

Probabilities of deck wetness have been determined for each ship at the moment when voluntary speed reduction or course change or both started. From them together with the effective bow freeboard, the standard deviations of relative bow motion of the ships were calculated by the formula:

R = fe/ 21og(1/q)i.

Effective bow freeboard, _{fe, is understood to mean the actual}

bow freeboard with additive correction for the so-called swell-up of the bow wave which was proposed by S. TASAKI (1).

The above analysis leads to the relationship between the

standard deviations of relative bow motion and corresponding observed wave heights as shown in Fig. 1.

Plots are considerably scattered. Because of the variety of

ship types, speeds, headings, wave periods, etc. of the _ships

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concerned, the discrepancies must be expected as an inevitable consequence of the complexity. We can notice that almost all plots fall in a triangular region bounded by three broken lines

©,

©, 0,

as shown in the figure. Outside of the above region,

there are several plots showing larger standard deviations. These are the data reported from ships which experienced for the most part spray over the weather deck at the bow (not shipping

of green water).

It will be of interest to note that the line° is nearly coinci-dent with the theoretical results, represented by two dotted

curves in the figure, which were calculated for two cargo ships

of 12,000 dwt and 13,000 dwt. These calculations were made on the basis of long-crested irregular head waves, Taking into

consi-deration the fact that the data were based on the reports of

different ships under different conditions, and furthermore some doubt might be thrown upon the accuracies of these data, the approximation of the theory to reality is felt to be quite satisfactory. These considerations will lead to the conclusion that the present theory can be used successfully for prediction of the behaviour of larger vessels.

Having got a general picture of ship handling practices, the

next step is to set up a tentative limit for the standard devia-tions of the relative bow motion which is to be acceptable as a criterion for storm ship handling.

Instead of arithmetic mean we have chosen the average of 1/3

lowest values as a tentative criterion, giving Rm = 2.4 m as

shown in the above block diagram. Such a choice might be

problem-atical , but it is our belief that at least for the present this

limiting value would be on the safe side and quite reasonable to be offered as a free trial.

Corresponding non-dimensional relative bow motion will be given

as follows:

Z/L = 0.061.

PP

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We shall use this auantity, in the next section, to estimate the

limiting wave

height

for 50,000 to 60,000 dwt combined carriers.

2.3 Criterion for modern combined carriers

The discussions so far have been limited to ships of 10,000 to

20,000 dwt. Similar calculations were made with two representa-tive bulk carriers of 53,000 dwt and 60,000 dwt. From the

calculated standard deviations of relative bow motion,

probabil-ities of deck wetness and non-dimensional relative bow motion

are derived. The results are shown in Fig. 2 as curves of

standard deviations against wave height. The corresponding non-dimensional relative bow motion can easily be read on the right hand side ordinate of this figure.

If we- assume that the non-dimensional relative bow motion

remains constant both with ships of the usual size and of larger-sized type, as discussed in the preceding section, we can find the limiting wave

height

Hc = 7.1 m , corresponding to

Z/L = 0.061.

PP

In addition to the above limiting wave

height,

the speed and

actual bow freeboard at the moment concerned are also required to determine the critical probabilities of shipping water. The speed would be lost to a considerable extent due to head waves by the time when some voluntary speed reduction is put in action, and the amount of loss differs with ships, but for our present purpose only a general picture is sufficient. In this connection, other investigations through logbook analysis were

made on modern combined carriers longer than 200 m , and the

results show that the speed loss due to head waves of about 7 m in height amounts to 37 percent of the normal speeds, on the average. If the normal speed be assumed 15.5 knots for these ships, then the reduced speed in rough seas will become about

10 knots.

Fig. 3 shows the calculated probabilities of deck wetness for

the two bulk carriers mentioned above, the ordinate being ship speed and the abscissa wave height. The curves have been drawn with constant probability of shipping water.

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It follows from the figure that the criterion for probabilities of shipping water is about 1/16 for a 53,000 dwt bulk carrier

with 9.17 m _{of bow freeboard, and 1/48 for a 62,000 dwt bulk}

carrier with 10.20 m of bow freeboard. This difference, of

course, indicates the effect of bow freeboard height.

With a particular ship, if we know the probabilities of shipping water for a particular bow freeboard, then for any other free-board height we can find the corresponding probability of

shipping water by using the diagram of Fig. 4. Any bow freeboard in this figure is represented in terms of the ratio to the first

one.

3. Increase in apparent slip of propeller.

By the time when the captain decided for himself to start voluntary speed reduction in rough head seas, both revolutions of propeller and ship speed are decreased as compared with those in calm water. Since the rate of speed loss is normally larger than that of decrease in revolutions, apparent slip ratio will be increased. Hence the amount of slip increment can be adopted as a possible criterion for the storm shin handling.

Investigations have been made by collecting logbook data on combined carriers longer than 200 m.

3.1 Speed loss in head or bow waves.

Ship speeds, at the moment immediately before the first volun-tary speed reduction or course change, have been plotted against Beaufort number as shown in Fig. 5. Additional scale of wave height on the abscissa is based on Roll's well-known work (2). It is seen that plots are considerably scattered along the

ordinates. This might have been caused mainly by the facts that

the same Beaufort number does not always correspond to the same

sea state, and further the collected information contains

different wind directions and wave directions within the range of about 60 degrees relative to the ship's heading.

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Shin Handling at Sea

general trends of some remarkable speed loss with increase in Beaufort number. The speeds have been represented in terms of the ratio to the calm-water values in the voyage concerned. It follows from this figure that the speed in rough head seas will decrease on the average to 83 percent of the calm water speed with Beaufort number 6, and to 67 percent with Beaufort number 9. With stronger wind, still more speed reduction might be ex-pected, but we have had too little information to estimate the

amount.

It should be noted here that no observed wave heights had been given in the logbooks, and hence we can not confirm whether Roll's average wave heights for North Atlantic Ocean are valid also for North Pacific Ocean where our investigations were made.

In the information through questionnaire with ships of

10,000 to 20,000 dwt are contained the observed wave heights,

and they are generally higher than Roll's average by 1 2 m.

3.2 Apparent slip increments.

Increases in apparent slip ratio have been nlotted against the

reduced speeds as shown in Fig. 6, using the relation given in

Fig. 5, and a regression line has been drawn to _{approximate the}

general trend of apparent slip in rough seas. The regression

line is marked with Beaufort number.

The average value of slip increments is 18.6 % and the

corre-sponding average of reduced speeds is 76.2 %. This point seems

to be between Beaufort number 7 and 8 on the regression line. By the word "apparent slip increment" we will mean the

differ-ence _{between the slip ratio in rough seas and that in calm}

water at the voyage concerned.

It must be emphasized that the values of apparent slip ratio in log records are the average taken over four hours, and hence they might be lower than those at the moment when the ships were actually in critical conditions. Taking this into consideration, the average slip increment mentioned above might be too low to

be acceptable as a criterion. In other words _{it might be too}

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We have eventually chosen the average of 1/3 highest values as a tentative limit. This becomes 25.5 % and is between Beaufort number 8 and 9. Corresponding reduced speed will be found about

69 %.

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Conclusion

Our tentative limit of deck wetness corresponds to the sea state

expressed by wave height of 7.1 m , while the limit of apparent

slip increment corresponds to the state between 8 and 9 of Beaufort number.

If "wave height" be understood to mean the significant wave height and Roll's average relationship be assumed, Beaufort

number 8 _{9 will correspond to the wave height of about 6 m}

and hence there would be some discrepancies between the two criteria in the corresponding wave heights. However, it appears to the writer that, in the absence of such pertinent data as experiments furnish, the degree of agreement between the two criteria is felt satisfactory from a viewpoint of their

tenta-tive role.

It must be noted here that the program outlined in this paper was undertaken by the JAPAN ASSOCIATION FOR PREVENTING MARINE

ACCIDENTS during 1970 1971. The writer would like to express

his thanks to the Association for their permission to reproduce the contents of their investigations.

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References

R. TASAKI: On Shipping Water; Report of Shin Research Institute, Vol. 11, No. 8, 1961.

H.U. ROLL: Height, Length and Steepness of Seawaves in the North Atlantic, and Dimensions of Sea-waves as Functions of Wind Force; S.N.A.M.E.,

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Table I Principal dimensions and speeds

Notation: fe : Effective bow freeboard, in m

Vo : Calm water speed, in knots; No : Calm water r.p.m.

V : Reduced speed due to waves, in knots;

N Reduced r.p.m. due to waves.

SHIP L No. PP B CI, fe Vo No V N V/Vo N/No 1 153.09 _21.0 .791 10.54 13.4 101.5 11.0 96.5 .821 .951 2 130.00 20.0 .754 6.85 11.5 140.0 7.5 125.0 .652 .893 3 129.00 20.0 .742 6.20 12.8 120.0 8.0 110.0 .627 .917 4 145.00 19.6 .673 11.00 18.5 107.0 17.0 104.0 .919 .972 5 142.50 20.0 .640 8.45 17.0 104.3 14.3 102.9 .841 .987 6 140.46 19.0 .702 8.15 15.0 103.0 10.0 95.0 .667 .922 7 145.00 19.6 .486 9.60 17.4 107.0 14.0 96.0 .805 .897 8 140.10 19.4 .658 8.60 16.0 117.5 14.0 98.0 .875 .834 10 146.00 19.6 .693 10.60 16.7 108.0 16.1 105.0 .962 .972 12 145.00 19.5 .666 8.56 18.0 111.0 9.5 70.0 .528 .631 13 151.25 19.4 .675 8.20 15.0 104.0 12.0 95.0 .800 .913 15 140.00 19.0 .686 8.00 15.5 96.0 15.0 95.0 .968 .990 16 145.00 19.5 .636 11.94 18.5 110.0 16.0 102.0 .865 .927 17 147.00 22.4 .548 11.40 18.0 112.0 11.0 94.0 .611 .839 19 139.00 18.3 .742 9.60 13.5 115.0 12.0 109.0 .889 .948 20 140.00 21.0 .595 11.75 18.5 125.0 16.0 122.0 .865 .976 21 142.50 22.0 .614 12.25 18.5 110.0 16.0 99.0 .865 .900 22 145.00 19.4 .672 10.20 16.5 105.0 12.5 99.5 .758 .948 23 145.20 19.6 .676 9.20 17.2 103.5 16.5 101.0 .957 .976 27 132.00 23.0 .699 7.75 13.8 113.0 7.0 100.0 .505 .885 28 175.00 25.2 .592 11.55 22.5 103.0 18.0 90.0 .800 .874 29 145.00 19.5 .664 6.20 18.5 114.0 16.8 105.0 .905 .921 30 140.00 19.0 .680 8.61 13.5 85.0 10.5 83.0 .778 .965 34 160.00 23.0 .560 11.70 21.0 115.0 17.0 90.0 .810 .783 35 140.00 21.0 .560 8.95 17.5 127.5 14.0 115.4 .800 .905 37 156.00 22.6 .563 8.20 19.5 115.0 17.0 110.0 .872 .957 38 130.22 20.8 .649 10.80 15.5 125.0 9.0 100.0 .581 .800 40 142.60 19.2 .521 10.88 16.5 106.5 15.5 104.7 .939 .983 41 145.00 21.8 .575 12.00 19.5 109.0 14.0 95.0 .718 .872 43 175.00 26.0 .540 13.01 22.0 108.0 21.5 105.0 .977 .972 45 175.00 25.7 .569 11.00 22.5 110.0 20.0 107.1 .889 .974 46 131.78 19.0 .640 8.38 16.0 128.0 13.5 123.0 .844 .961 47 145.00 19.6 .672 10.20 16.8 101.0 14.8 94.0 .881 .931 48 145.38 19.5 .670 7.99 17.0 108.0 16.0 103.0 .941 .954 53 156.00 23.2 .563 12.06 21.0 108.0 20.7 107.0 .986 .991

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80

0 0

2

60

3 0

40

0 (f) R

_{< 20}

5 LJ 0 0 0 SPRAY 0

©

0

a

Z ©

08

0

o

/

THEORY'4

00 /0/0

rz

0

n 0

/

000 --, 7/ / ₀

/0

I I

5

OBSERVED WAVE HEIGHT, H (n)

Fig. 1 Standard deviation of relative bow motion

and observed wave height.

0

I I I

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Bulk-carriers of 53,000 dwt and 62,000 dwt 0 0 2 3.0 0 en ce 2.0 IL 0 0 5 IL 10

SIGNIFICANT WAVE HEIGHTS H (m)

2 3 4 5 6 7 8 a a 0.06 0 0 2 0.05 3 0 0.04 i=

0.03 <

0 LT)

0.02 2

0 001

Fig. 2 Standard deviation of relative bow motion at F.P. 4.0

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10

53,000 dwt bulk-carrier with bow freeboard 9.17 m.

62,000 62.000 dwt

\

---e

11/20 15 10

dwt bulk-carrier with bow freeboard

53.000

dwt \\\

\\

1/10 1/10

\\

\I/20

10.20 m.

\

5 Fig. V50 1/100 Ii 5 3 6 7

SIGNIFICANT WAVE HEIGHT , H (m)

Probabilities of deck wetness. 8

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1000 500 200 100 50 1/q 20 10 2 f/f0 20

1.6 1.4 1.2 1.0 0.8 07 06 0.5 I

III

I I

III

I 2 5 10 20 50 100 200 500 1000 1/q0

Fig. 4 Probabilities of shipping water and

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Combined carriers of 50,000 60,000 dwt Ship Handling at Sea

11,20 0 REGRESSION LINE I I 6 7 8 9 10 BEAUFORT NUMBER I I I I I I 3 4 5 6 7 8

SIGNIFICANT WAVE HEIGHT, (m)

Fig. 5 Reduced speed in rough seas.

0 100 90 80

0

70 60 50 0 40 I I. I

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Ship Handling at Sea 50,000 % 60,000 dwt combined carriers GO REGRESSION LINE 10 0

Fig. 6 Apparent slip increment and reduced speed.

50 0 9 IN BEAUFORT NUMBER 0 I 1 I I I 40 50 GO 70 BO REDUCED SPEED , IN ../. 0

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CORRELATION BETWEEN FULL SCALE AND MODEL MEASUREMENTS ON SHIP MANEUVERABILITY

by

H. OKAMOTO

KAWASAKI HEAVY INDUSTRIES, LTD., KOBE, JAPAN

I. Introduction

The maneuverability as an important theme for the design of a hull form of large tankers seems to have originated from a need to establish a reasonable compromise between an economic ship design and a good maneuverability, because the design requirement to get a reduced length/breadth ratio and increased block

coefficient to get a so-called blunt hull form for economical reasons is considered a cause of reduced course stability. For a successful solution of this problem, it is necessary to estimate the maneuverability of the design hull form. In an effort to solve

it, a series of approaches have so far been exercised by many

naval architects to pave the way towards establishment of an authentic method of model testing and finding out of a correlation between the actual ship and its geosim model. The problem was dealt with also in the Recommendation of the International Towing Tank Conference (ITTC), and it was handled as a task work of the ITTC in its 11th Convention and has thereafter been continuously the main theme for the study. But it does not seem that a

satisfactory solution has been reached for the problem. Especially in a blunt hull form of a large tanker, the effect of the flow separation appears remarkably in the model performance because of the full hull form, and this is the very reason why the problem

is too complicated and difficult to be solved. (1), (2)

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it was reported by Professor Nomoto that the course stability increased in a range of a gentle turning motion, which phenomenon is said to be an unusual phenomenon and characterizes the model ship. Presence of this phenomenon associated with the model alone is a troublesome problem in the estimation of the performance of an actual ship from the model test results, and it is desirable to establish a method of model testing involving no unusual phenomenon described above. For this reason, the scale effect of full ships should be discussed starting from general cargo ships, for which there are many experimental data and reports on the scale effect available, though for these latter ships the scale effect is not so large as for full tankers. In the present report, the scale effect in the maneuverability of vLCC's is dealt with. The research on the scale effect in model maneuvering tests for VLCC's at present is not satisfactory and much is left to be solved by

theoretical and experimental research in future. The report presents the nowaday's situation of the research on the scale effect on the maneuverability of VLCC's, together with several examples of tests with a large scale model which may be an approach

to the mentioned problem.

2. Maneuvering test data of model and full size 2.1. Method of test and unusual phenomenon

The scale effect in the maneuverability may be divided into the following two. These two are essentially related with Reynolds' number, but they differ in mechanism of occurrence. One is the well-known, usual scale effect. This effect varies continuously

with Reynolds number and is seen in cargo ships or very fine

tankers. The occurrence is mainly attributed to the larger velocity of the propeller race in the scale model compared with that on full size, which results in an increase of the effective-ness of the rudder and a better course stability of the model. The fact that the propeller race of the model has a larger velocity than that of the actual ship is derived easily from the difference

of Reynolds' numbers between model and ship. To avoid this scale

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area to meet the effectiveness of the actual ship's rudder, or the model propeller load to meet that of the actual ship by an air propeller for friction correction.

The second effect is an unusual scale effect, so-called unusual phenomenon, which is mainly dealt with in this paper. This phenomenon is supposed to occur by the loss of the similarity in the flow field at the stern due to flow separation in the case of a blunt ship model. The main cause of this phenomenon is the smaller Reynold's number in the model, but it is subtly affected by the shape of the frame and there are some cases where no unusual phenomenon occurs in spite of a blunt body. First, a brief explanation is made of the aspects of the unusual phenomenon in the maneuvering experiment with a full tanker model. Following

the progressive trend towards full hull forms of tankers,

parametric studies have been performed by Kawasaki and (1), (2),

utilizing various model ships differing in LIE, Cb, lcb and AR/Ld, etc., in an effort to reveal the effect of the change in the hull

form parameters on the maneuverability. Results of these studies have shown interesting phenomena, beyond the conventional recog-nition, associated with the hull form characterized by L/B < 6.0 and Cb > 0.8. As shown in Fig. 1, the L/R curve representing the obtained results of the spiral test has a reduced slope in the vicinity of the origin for the fuller ships compared with that of

ships with conventional hull form, and, with a rise of (5 the slope

first increases and subsequently declines. This implies a larger turning resistance in the region of straight going, in other words,

a better course stability.

It

is usual in an ordinary fuller ship

that the ratio of the turning resistance to the turning rate is reduced and.the course stability and the response to steering are also poor. As described before, the course stability of the model is increased by the usual scale effect, but its degree is not so large and a careful examination will make it possible to estimate the actual ship's performance in spite of the scale effect. However, the unusual phenomenon entirely changes the results, and

an accurate estimation of the actual ship's performance is hardly

possible. In this section, the spotlight is directed to the

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running ship model, with a view to study the scale effect by comparing the results of model tests with those of tests with actual ships. Unless otherwise mentioned, the test procedures hereinafter described concern the model tests conducted with ship models similar to the actual ships at the Froude numbers just corresponding to those of the actual ships, with no special regard being given to the propeller load.

2.2. Description of tested ships

The particulars of the actual ships and the ship models tested by us, Kawasaki, are as follows:

For the testing three types of ships were used:

Actual ships A and B were built in Kobe works, while ship C was

built in Sakaide works of Kawasaki. The approximate hull forms

of the three ship types are shown in Figs. 2, 3 and 4. Principal dimensions of the tested ships are listed in Table 1. Ships A and B are the tankers having the hull form represented by Cb = 0.81

and L/B = 6.125 and 6.19, while ship C is characterized by L/B =

5.75 and Cb = 0.82. All of them are equipped with ordinary balanced rudder. The rudder of the A and C type ships have a horizontal fin installed at the bottom for prevention of circula-tion loss at the lower end of the rudder. The actual ships of types A, B and C have not experienced the particular problems or difficulties in the maneuverability and have been operated satisfactorily.

2.3. Actual ship tests

Ordinary spiral (or reversed spiral for ship C) and Z steering tests were carried out for all three types of the actual ships. For the test contents and the test conditions, etc. at the

testing, reference is made to Table 2. The tests were carried out at their sea trials before delivery. The measured items were the heading angle (or turning rate), rudder angle, ship speed and RPM

A. (100,000 DT tanker, 3 ships of this series were built),

B. (130,000 DWT tanker, 3 ships of this series were built),

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of the main engine. The measuring was carried out with digital detection apparatus (3) developed for the experiment with the actual ships at the Technical Research Laboratory of Kawasaki. The devices were designed to print numerical figures, after measuring simultaneously the above-mentioned four items, at intervals of 1 or 10 seconds. The detecting method of each measurement was as follows:

Turning rate and ship speed were detected using a digital "Shaft-encoder" connected to repeaters in the apparatus which were operated by signals from the main signal transmitter of the gyro-compass and the pressure-log of the ship, respectively. To measure the rudder angle and RPM of the main engine, the digital

"Shaft-encoder" fitted directly to the steering gear and the pulse generator were used, respectively. The turning rate in the ordinary spiral test was determined according to the differences of the heading angle between two adjacent timings at intervals of I second. The analysis of the result of the Z steering test was performed with approximation of the first order equation of

motion, according to the method of the least squares (4).

2.4. UcadQl_tQ.ata

The model tests were carried out, in principle, in the model maneuverability testing pond of Kawasaki (5). This has a free water surface area exceeding 200 m x 200 m and is capable of testing the free running ship model. Outfit and method of the test are the same as the ones usually used. The model was powered by generators driven by a gasoline engine and controlled by radio from the station on the bank. The data of heading angle, turning rate, rudder angle, ship's speed and revolutions of propeller, etc. were measured by a free gyro, rate gyro, potentiometer, and current meter, all of which were mounted, and the results of the measurements were continously recorded on an electromagnetic oscillograph. The position of the center of gravity of the running ship model was determined by cross bearings from two points on the bank to obtain the tactical diameter. The spiral tests for a 2 m Lpp model of ship A and a 4.5 m Lpp model of ship type C were performed in an 80 m x 80 m test tank in Mitaka Facilities of the

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Ship Research Institute of Japanese Government (SRI) by the Tokyo University and SRI respectively.

2.5. 14.5 in LE2 model and ex2eriments

The outline of the model experiments carried out by us for a large model (L = 14.5 m) of ship A is as follows:

2.5.1. Ship model

The ship model used for the experiment has a wooden shell reinforced by light gauge angles, and has a bulwark of minimum requirement and a longitudinal radius of gyration corresponding

to that of the actual ship. (cf. Fig. 5) The draft was adjusted

by water ballast, while the trim was controlled by small ballast weights, which enabled reduction of the static longitudinal bending moment to a negligible level. In conjunction with the transverse stability, no longitudinal bulkhead was provided but the necessary reserve buoyancy was ensured and the free water effect was prevented by using blocks of expanded styrene properly distributed inside the hull, in order to assure the said stability. This ship model was regarded as a vessel requiring registration under the maritime law and regulation and had to be manned by competent officers (captain and chief engineer). For the testing, 2 or 3 experimenters were on board the ship to conduct the

measurements.

2.5.2. Instruments and testing procedures

The block diagram of instruments is shown in Fig. 6. The ship model was equipped with an automobile engine of about 45 HP as a main engine. Besides this engine, two 1 kVA generators powered by a gasoline engine were installed aboard the ship. For maneuvering the ship, a means was provided to enable automatic

Z steering by an electric motor with use of the Z testing mechanism usually employed for wireless control of the model as well as manual steering with a suitable link mechanism. As instruments were installed a free gyro, rate gyro, current meter and some

other instruments specially designed for the model tests. To determine the tactical diameter, the bearings of the running

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ship were observed from stations on land. The ship was run in waters where there was no influence of wind and waves within

the port of Kobe.

2.6. Results of experiments

The results of the spiral tests and the Z steering tests conducted

for both model and actual ships A, B and C are discussed below.

Ship A:

From the results of the spiral tests shown in Fig. 7, it is understood that, especially in the starboard turning, the curves are obviously divided into two groups; one for the actual ship and the 14.5 m model and the other for the 6 m and 2 m models, and the models having an Lpp of 6 m or less show smaller L/R ratios in the region of small rudder angle (+ 100 or less) and a reduced turning ability. These findings concern the unusual

phenomenon already mentioned in this report. Fig. 7 illustrates

no appreciable difference in the intensity or magnitude of the unusual phenomenon between the two models with Lpp 2 m and 6 m. In the case of turning at a large rudder angle, it is noted that the value of L/R shows considerably good agreement between 6 m Lpp model and the actual ship, as has been said (6). The ship model of 14.5 m Lpp hardly involves the unusual phenomenon as experienced by the small ship models and shows good agreement with the actual ship for turning at a small rudder agle. In the

range of larger helm angle, r of the 14.5 m Lpp model is larger

than that of the actual ship. This is considered to be caused by the difference between the main engine characteristics of the actual ship and that of the model ship. Namely, the main engine of the 14.5 m Lpp model ship had a relatively too large horse power as compared with that of the actual ship, so that even if

the resistance was increased by turning, the RPM of the main engine did not decrease due to residual power and the rudder was placed in the relatively strong propeller race, and the better turning ability was obtained as a result. The same can be said of

the results of the Z steering test plotted in Figs. 8 and 9. It

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seen in the spiral test results of 14.5 m Lpp ship model, was experienced in the 5-degree Z tests. In Figs. 10 and 11 L/R, 1/T' and K' are plotted against the approximate Reynolds number for both actual ships and ship models tested. Inasmuch as the turning at a large rudder angle of the 14.5 m Lpp ship model is incompati-ble with a successful comparison for reason of the characteristics of the main engine as precedingly mentioned, the comparison was carried out only in conjunction with the turning at a small rudder angle, with the resultant finding that the ship model of 14.5 m Lpp provides results much closer to those of the actual ship than

the other ship models of 6 m Lpp or less, thus suggesting an

appreciable usefulness of a large ship model for the

maneuverabil-ity test. Ship B:

Also in conjuction with the 6 m Lpp model of ship B, the unusual phenomenon was seen in both spiral test (Fig. 12) and Z steering test results (Fig. 13), said phenomenon being not so conspicuous as in the 6 m Lpp model of ship A. The test of ship 43 has provided

results implying the general trend that correlation is better in the turning at a larger rudder angle.

Ship C:

In the case of ship C, the reversed spiral test method was adopted for both model and ship. The results shown in Figs. 14

and 15 for the model and ship show no unusual phenomenon at all,

and the agreement between model and ship is good.

3. Proposed remedy for prevention of unusual phenomenon-Flow-smoothing fin

From the observation of the flow field at the stern of the ship

model, it is reasonably conceivable that the installation of a

suitable flow-smoothing fin on the stern of the ship is highly

effective for prevention of the unusual phenomenon. Two types of

flow-smoothing fins were tested: one vertical and the other

horizontal, each type then being changed with respect to their

size and position. The basic conception for the flow-smoothing

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normal to the hull surface and acts to provide a fairing effect by being placed to cross the vortex, while the vertical type serves to accelerate the stream along the hull surface so as to avoid separation and smoothen the flow, availing the suction of the propeller, realized by installation of a plate having ogival sections approximately parallel to the hull surface. The

effectiveness of these fins has successfully been confirmed by experiments.

3.1. Effect of flow-smoothing fins on free running ship model Z-tests with the 6 m Lpp model of ship A were carried out to see the effect of each of the nine flow-smoothing fins (vertical type

x 3 and horizontal type x 6) installed from station 1 1/2 towards

the stern, and the obtained results were compared with the

corresponding results of the actual ship test. Fig. 16 illustrates the arrangement of the flow-smoothing fins and Fig. 17 shows the test results. As shown in Fig. 17, the values of (TL' x XL)

(means, overshoot angle/helm angle) generally tend to be enlarged by installation of the flow-smoothing fin. Horizontal fins HM ©,

® and and vertical fin VMa have provided test results in which no occurrence of unusual phenomenon can be discerned because of

the absence of a noted drooping or plateau in the course of the curve probably representing the unusual phenomenon in the turning at a small rudder angle. It seems that the larger the size of the vertical fin the larger the value of (TL' x XL), but, for

suppressing the unusual phenomenon, the installation of the fin should preferably be more astern. As to the effect of the horizon-tal fin, the HM®or HM.0) fin give preferable results, but the

cannot eliminate the unusual phenomenon, which means that the position of the fin has a very delicate effect on the results. In conjunction with the position of the HM g fin, a delicate change in the effect of the size of the fin was noted, confirmed with the finding that a large (TL' x XL') was obtained at the Z tests over 100, while a drooping, probably representing the unusual

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of vertical type and the HM @fin of horizontal type provided nearly identical results. Fig. 18 illustrates the comparison between the 1/T' and K' values of the tested model as affected

by the HM

0

fin and VMa fin and those of the actual ship and

seems to suggest a possibility of not only a prevention of the unusual phenomenon but also of an apparent reduction of the scale effect usually experienced between the actual and model ships.

3.2. Flow observation and measurement of lateral force 4.nd turning moment

The four variations (Fig. 19) of the horizontal type fin, including the A fin corresponding to HMO and the B2 fin corresponding to HM

0,,

were installed on the side of 2 m Lpp

models of ship A and were tested for a range of drift angles

(8) of 00 to 10° in the circulation tank of the Tokyo University.

Flow observation by tuft-grid method has revealed that, at the

drift angle of 00, there are weak vortices on both sides of the

ship's hull, and that, if the drift angle grows, the vortex of

the face side decreases and becomes substantially zero at a drift

angle larger than 20, with a gradual growth of the vortex on the

back side. So far as the visual observation is concerned, the

vortices hardly differ from each other in spite of installation

of the fins, which seem to have no effect at all on the occurrence

of the vortices of the hull. This is quite contrary to the

presupposition that the flow-smoothing fin will eliminate vortices

and smooth the flow along the hull, resulting in a reduction of separation possibility and removal of the unusual phenomenon. Nonetheless, the flow-smoothing fin will doubtlessly provide an effect of smoothing upon the stream just passing along the surface

of the fin. We found a great need for confirmation of the change

of the flow pattern along thehullsurface by close observation,

which, to our regret, could not be done in aseriesof model tests

herein reported. This is the very reason why the conclusions of the author are now inevitably limited to an assumptive expression that the effect of the flow-smoothing fin upon the stream seems to prevent the local separation by smoothing the flow around it without affecting the whole vortex flow. Fig. 20 shows the force

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2

Y and the turning moment N in the form of Cy = Y/P/2L.d V and

2

CN = N/0/2i, .dV2 obtained from the force measurement tests in

the circulating tank and Fig. 21 shows the acting point (0 of the resultant side force. In the following, the result of each type of fin is described, compared with that of the original

(without fin). In conjunction with the fin A, CN and C are less

than those of the original and the non-linearity for drift angle conspicuous. If B approaches 00, the acting point of the resultant force is shifted appreciably forward, even ahead of FP at about 5.5o or less. The forward shift of the acting point of force caused by the change of (3 is unique which can never be seen with

the other fins. Unlike with fin A, Cy and CN with the fins B1 or B2 are increased relative to those of the original, but the acting point is shifted forwards by about o.1 Lpp, substantially

independent of B. Also in conjunction with the fin C, the acting

point of the resultant force is shifted forwards as well as C and

CN rise, and with 13 in the left direction, the acting point shows

a forward shift, where 13 is small. The course stability is

expressed by the following parameter:

CMw

)

CYB (mx - CYw)

(mx - Cyw Y(3

CNI3/Cy13 _{means the acting point of the side force cause by the} drift, and the more forward this point is located, the less the value of A and the course stability become. The term Cmw/(Mx-Cvw) representing the damping by turning motion has not been determined, because of the infeasibility of the experiment in the circulating tank, but it is highly conceivable, from the confirmed forward shift of the acting point, that the installation of the flow-smoothing fin tends to decrease the course stability. Concerning the fin A, corresponding to the HMO fin found the most effective, for the 6 m ship model, it has been confirmed that where the value

of (3 is less, or where the motion is gentle and the unusual

phenomenon is more remarkable, the forward shift of the acting point is larger. This proves the results of the previous experiment with the Lpp 6 m ship model are true.

(47)

HM @fin, the remarkable shift of the acting point, as obtained with the fin A, was not seen and also results with the 6 m Lpp

model were not preferable. These facts seem to suggest that the probability of a shift of the acting point obtained in the

circulating tank tests and a drop of course stability in the free running model tests are correlated with each other to a consider-ably high degree of undeniability. However, it is still left questionable whether the effect of the flow-smoothing fin is really the one providing the suppression of occurrence of the unusual phenomenon in a hydrodynamic manner or whether the compensation is derived from a hydrodynamic force produced

according to a mechanism entirely not associated with the unusual phenomenon, namely, what is a real picture of the effect offered by the flow-smoothing fin.

As may have been understood from the above, it is probable that the flow-smoothing fin can be utilized as a means for avoiding the unusual phenomenon sometimes found in the results of ship model experiments. At this time when the mechanism of occurrence of the unusual phenomenon itself has not yet been revealed, there is no measure to control the effects of the flow-smoothing fin, the said effects being subjected to a fairly large variation, depending on the position of the fin and its size. For this reason, the author is of the opinion that further detailed investigations should be made to obtain more and more knowledge on the not-yet-solved problems of the usefulness of the flow-smoothing fin and extensive research from a hydrodynamic view-point, such as observation of local stream or measurement of

pressure.

4. Consideration

The maneuverability of the blunt ships as correlated with the spiral test and the Z test have been described in this report, and the author showed that the maneuverability is appreciably

affected by the unstable flow and separation occurring in the stream field at the stern of the hull. The pattern of the stream field at the stern differs considerably between the ship model and the actual ship and represents the most influential factor