Vessel traffic systems, ship handling and ship manoeuvring capabilities

MARINTEKNISKA INSTITUTET SSPA

SWEDISH MARITIME RESEARCH CENTRE SSPA

GoTEBORG

PUBLICATION NO 94 1982

VESSEL TRAFFIC SYSTEMS, SHIP HANDLING

AND SHIP MANOEUVRING CAPABILITIES

by

NILS H NORRBIN

Paper presented at

the Fourth International Symposium on Vessel Traffic Services

Distributed by: Liber Distribution S-162 89 STOCKHOLM Sweden

ISBN 91-38-07176-2 ISSN 0280-4255

CONTENTS

PAGE

SUMMARY 2

INTRODUCTION 3

SHIP HULL DESIGN AND POWERING PARAMETERS 5

Hull geometry 5

Resistance and powering parameters 12

FAIRWAY CONSTRAINTS AND DISTURBANCES - PERCEPTION AND IMPACT 14

MANOEUVRING CHARACTERISTICS 16

Stopping and slowing down 17

Manoeuvring in a wind 21

Steering and turning 26

MANOEUVRING IN FAIRWAYS 28

REFERENCES 30

3

SUMMARY

Man's awareness of the threat of technology to the environment but also the new technology in itself are promoting the implementation

and legislation of Vessel Traffic Systems of various degrees of

sophistication. In the strive towards shore-based control the limita-tions inherent in the dynamics of the ships are sometimes overlooked. The purpose of this paper will be to pinpoint some of these limitations and some of the ship handling problems built into new designs.

Following a brief look at ship design parameters an approximation of reference is formulated for the basic relation between hull form, size and specific resistance or power. Characteristic numbers are used to

describe operating hazard or control potentials as examplified by four

representative ships. A new method is advanced for the rational

de-scription and comparison of fairway alignments as well as for the any-lysis of ship manoeuvring history from prototype records or computer

simulations.

Three different operating situations are selected as being typical for

those facing a ship's master or pilot in an area, which may be subject

to surveillance and to traffic flow control: the slowing-down manoeuvre required to adjust the speed to the local regulations or to give way to

another ship, the change of course required at a way-point or in case

of meeting or passing another ship, and the passage through a coastal

waterway including straight reaches and bends. A simple formula is

de-rived for the distance to reduce speed by 40 per cent using a new engine

setting corresponding to steady propulsion at half the original speed.

The discussion of heading control during the slowing-down process is

illustrated by the comparison of helm activity exercised in alternative

simulated manoeuvres with a 60 000 tdw tanker in a bow wind. The

prob-lems of manoeuvring high-superstructure ships at low speeds in strong

winds are apparent from computer predictions of course-changing

manoeuvres performed for a passenger ferry. It is pointed out that the

common experience of the need for a weather helm may be violated in

cases where sideway drift is excessive. The final chapter includes

some results from a recent real-time simulator study of the behaviour

of a tanker in a windling approach with respect to the width of the manoeuvring lane and the requirements on the fairway alignment and

buoyage.

In view of the general experience drawn from the case studies cited

here a co-ordination of ship movements in high-traffic areas must be advoctated, and an up-dated information service will be useful.

Shore-based advice with regard to manoeuvring tactics may be misleading.

Any progress in the transmitting of own-ship position data from shore

surveillance stations to bridge-mounted manoeuvring predictors must

1. INTRODUCTION

The motion of a single ship over the ocean does not constitute a traffic problem, although the voyage may be endangered by improper ship handling or failures of hull, machinery or outfit, or by the

en-counter of extreme weather. Marine traffic, again, has been defined

as the combination of individual ship movements in a specified area, /1/. The safety of the traffic will depend on the topographic structure of the

traffic area and the fairway alignment, on the weather conditions

pre-vailing, on the number of ships in transit and on the engineering and human quality of the operation of these. This operation, again, depends on the standard of the individual ships and the navigational aids, of the operating systems and rules, and of the human elements behind. The existence of a vessel traffic system (VTS) implies some organisation of

the operation.

Some of the factors involved in the ship-in-traffic complex _{are listed}

in Fig. 1, those above the ship and the waterway symbols being inherent in the designs and outfit while those below may be subject to varia-tions from one passage to another.

Fairway shall here denote a marked area of the waterway, or of that

part of the body of water, which is used for travel or transport. Two extremes of waterway are offered by the Mocambique Channel off the

East Coast of Africa and the Manchester Canal. More pertinent _{to the}

VTS problem, perhaps, are the Dover Straits, where ships are repeatedly

overtaking one another at close quarters, as well as a windling Baltic

approach, where ships may meet in hidden bends. In both these examples

the manoeuvring space available to the mariner-in-charge is _restricted.

If he feels reluctant to outside control, he may still take notice of

traffic information and advice from a shore-based VTS, which in lieu

of governing the movement of the vessel will govern the use of

re-stricted space" (Price, /2/). It may be required for a ship always to

ask for permission to enter a certain traffic area. _{It may clearly be}

unsafe, however, to require the ship to stop a passage without prior

notice, when such a manoeuvre may cause the loss of control of the

ship.

It _{is reasonable to expect that vessel traffic should involve a certain}

measure of safety, yet to be quantitatively described. _{If the same}

measure of safety is to be maintained in a more narrow area where ship

manoeuvres are restricted and traffic flow is congested, it will be

ne-cessary to improve the overall quality of the vessel traffic system.

This reasoning may be illustrated by the diagram in Fig. 2. The _level

of "acceptable safety standard" is indicated so as to reflect the

opinion that there are real "substandard" ships at sea as well as to

show that no ship can be safely operated to the extreme of its handling

capabilities in confined waters, _{whatever onboard equipment or traffic}

services available.

6

SHIP FAIRWAY

Dimensions Waterway restrictions

Proportions Shore topography

Prop.- & rudder arr. Fairway alignment

Type of engine Navigational aids

Outfit Charts

Surveillance

Load conditions Water level cit transit Manoeuvring characteristics Vind, tide, waves Functioning of outfit

Visibility

Crew training and experience

Traffic intensity

Pilot onboard Information Tugs available Advice

Control

FIG 1. FACTORS INVOLVED IN THE SHIP-IN-TRAFFIC COMPLEX

Factors above symbols are inherent in the design

and outfit of ship and fairway, those below are

subject to variations and judgement at transit.

This paper will deal mainly with the ship manoeuvring properties and the

way to use them in handling the ship in a given condition and a given

environment. Similar problems have been the subject of earlier papers

within the series of international VTS symposia; by Pafett mainly on

manoeuvring characteristics of ships in deep and shallow waters /3/. by

Hermans on a stochastic model of two-ship manoeuvres /4/ (where the

dy-namics of the ships are indirectly allowed for by the use of certain

laws of interaction derived from simulation), and by Abdelgalil on

colli-sions and groundings related to interactions between two ships or between

a ship and a side bank, /5/. It is hoped that the present paper may add

some new aspects of the engineering of ships and fairways, of the human

0

0

0

Measure of fairway constraint (ship-to-fairway dimension ratio)

FIG 2. A PERCEPTION OF SHIP-IN-TRANSIT SAFETY

2. SHIP HULL DESIGN AND POWERING PARAMETERS

Ship handling in open or confined waters may be a problem in case of

large or small vessels

alike,

and it is useful first to look for

non-dimensional figures to describe ship design and performance characte-ristics, and the constraints of the waterways.

Hull Geometry

The length is the fundamental dimension of a ship which largely deter-mines the cost of building, the economical speed at sea, and the minimum

turning basin diameter in a port to use, etc. The number of ship lengths

sailed is the natural measure of distance in the short range. Although

"length over all" may be more familiar to the mariner, L will here stand

for Lpp, the length between perpendiculars, the concern being focused on ships with L > 60 m or some 200', say.

The length-over-beam ratio is typically around 7, but when special ships

are included we must allow for variations in the range of 4 < 1117.- < 10

(cf. Fig. 3). The higher figure relates to destroyers, container ships and some inland craft, the beams of which are limited by narrow lock

8 6 B/T 8 2 4 6 8 10

1/B

12 0 LiTr 20

®

30

®

°

FIG 3. SHIP UNDERWATER-HULL PROPORTIONS L/B, BIT AND L/T

(Cf. Fig 5.)

gates. In general, a high length-over-beam ratio contributes to a good

"dynamic stability" on a straight course, although relative draught and

trim as well as shape of end hull sections are also of importance.

Shallow draught and V-shaped sections endanger stability, a trim by

stern will improve it. (Cf. below.)

One would imagine that the combatant naval craft would rather benefit from a high manoeuvrability, whereas the typical long voyage tanker

should be designed for stability on course. It is well known, however,

that the modern full form large tanker is relatively short, i.e. it has

a low length-displacement ratio uvin or a high ratio of

"mean-section-area" to "length-area : L2. It is also well known that such tankers

often exhibit the characteristics associated with dynamic instability. During the two last decades tanker mariners have learned to live with

the instability, much because of the rate control included in modern

autopilots on the one side, and because of training and new knowledge of ship manoeuvring on the other. New designs of stern and propeller/

rudder arrangement have come to improve handling capabilities. One particular ULCC with special shallow draught features and a twin-skeg/twin-rudder arrangement sails successfully with a L/B equal to 4.4. Other special ships are built with even lower length-to-beam

ratios.

0

/

2

10 DI TA IF 9 501 100 150 200 250

Lm

300

FAG Li. MINIMUM SHIP DRAUGHT (AFT AND FORWD) FOR SAFE HANDLENG

AS RECOMMENDED BY THE PANAMA, CANAL PFLOT ASSOCIATION

(Acc to tables in ref. /7/..)

Note, that the Tength-displacement ratio does not indicate the degree of block fullness, unless combined with the length-over-beam and

length-over-draught ratios. In practical low speed hydromechanics the

free surface may often be treated as a plane of symmetry, i.e. the flow

around the underwater hurl is assumed to be equal to the flow around a

completely submerged double-hull of displacement 2V. The quotient 2T/IL appears as the proper aspect ratio AB in the hull/low-aspect-ratio wing analogy - cf. Fig. 3 again. The forces and moments due to a forward

motion (with speed component u) combined with a lateral drift (with yaw

velocity r.--tdItildt=4)) are therefore al] essentially proportional to the

product of lateral area and aspect ratio, or to the square of the

draught. The drift force centre-of-pressure is well forward on the

null,

Ship mean draught T = 11(TA+TF) and trim TA-TF vary with load condition,

of course, as does the volume displacement 7. Recalling the wing ana-logy it will be seen that a trim by the stern transforms the lateral

area from a rectangular planform into a slender delta wing. Even a small

trim will be accompanied by a large aftward shift of the

centre-of-pressure of the "lift" due to lateral drift, and so by an increase of

the dynamic stability lever. The Marpol convention on segregated bal-last tanks, stipulated minimum mean and forward draughts with a view to retain the seakindliness of tankers of a length of more than 1501 mi Stern draught was to be adjusted to full submergence of the propeller, /6/. Based on the experience from handling a wide variety of ships the Panama Canal Pilot Association has recommended minimum draughts for all ships in ballast or partial loads, /7/. The minimum values at stern

exceed those at the bow by 2 feet throughout Fig- 4 = but bow draught

values are higher than the 1MC0i requirements.

-1 0

1"--1

FIG 5. FOUR REPRESENTATIVE SHIPS A, B, C, D

Passenger and car ferry 60 000 tdw tanker 50 000 tdw container ship 500 000 tdw ULCC (Cf. Table I.) B: C: D:

The full-load even-keel beam-over-draught ratio is, typically again,

around 2.7, with normal variations in the range 2 < BIT < 6. (Cf.

Fig. 3.) Often the higher figures are found for large passenger fer-ries which enter shallow port terminals and for which wide beams are required for reason of deck space and metacentric height. As such ferries are mostly designed with high superstructures over their full length, the ratio of wind area to underwater lateral area may be high,

or too high - the effects of the trend are exaggerated due to the

pre-sent restrictions on design power brought forth by the rapid increase

in fuel costs.

It appears that the ships in traffic display a wide variation of hull form proportions, exemplified by the data for four representative ships

(A-D) compiled in Tables I and II and even more evident from the sil-houettes given in Fig. 5.

The general relations between hull form and dynamic stability or manoeuvring capabilities have been briefly indicated only. Some

cha-racteristics for the ships in Tables I and II will be Further detailed

in Chapter 4, however.

The overall design of the stern arrangement is important, and in

par-ticular rudders should always be in a behind-screw position. The diagram

in Fig. 15 will illustrate the benefit of the screw race to the rudder-on-ship effectiveness; see further discussion in Chapter 4.

The modern twin-skeg screw + rudder arrangement is highly efficient from a propulsive point of view as well as for normal helm control, but RPM or pitch control of the screws is here less effective for low-speed

steering than in the conventional twin-screw/twin-rudder arrangement, where a difference in port and starboard thrust generates a considerable

suction to one side of the afterbody, /8/.

Rudder size is made to yard practice or chosen to satisfy the minimum requirements formulated by Det Norske Veritas, /9/, and where the sta-bility inherent in the design is poor further enlargements are often suggested. An increase in rudder size alone makes little to improve the dynamic stability, however, due to the straightening of the flow in-duced by the propeller. The working propeller + rudder should be treated

as one effective fin element, in wich the propeller may well dominate.

Again, an increase of rudder area may still improve manoeuvrability and

yaw checking characteristics which contribute to the directional

stabi-lity of the steered ship.

Due to their relatively small power demand and the engineering difficul-ties associated with low RPM, large tanker propeller diameters are

limited and so the fin effect contributed. A stopped propeller may spoil the effect all together, and also cause a destabilizing separation of

afterbody flow.

Table 1

- tesign data for ship

---B 'C _ D ' Type of ship Pass a Car Ferry 60 000 tdw Tanker

Large Cont Ship

500 000 tdw ULCC Spec Dim Symb Ratio, Dim fig Ratio Dim fig Ratio Dim fig Ratio Dim fig Length o a in L o a = 128.0 -2-11.,2 = 289.6 =. 364.1 = Length b pp IL i/B 115:7 5.26 201.2 6.25 274.3 8,52 350.0 4.43 Beam in In a D BIT D/B 22.0

12.03.73

0.55 r 32.2 17.7 I 2.64 0.55 32.2 24.6 2.64,

076

79.0 30.7 3.29 I 0.39 Draught (nom) m T L/T 5.9 19,5 12.2 16.5 12.2 22.5 24.0 14,6 Volume displ M3I V L/V" 8480 5,69 63600 .5.06 6300 6.92 540090 4.32 , weight displ ,tonnes A ''''' 8670 65190 -.. 64580

-553500 e Deadweight tonnes OW ' DW/A 21001 0.24 54000 0.83' 38000 0.'59 499000 0,92 1

Side wind area

m2 AL" AL/T2 2090 59.6 1940 12.0 2350 15.8 3370 .5:85 Prop thrust/area = .,.-KW0/V0 -0.56

aI

0.81 -1(80 -1.40 AL Rudder area , Propeller diam m2 im I At. 0 Ar/LT

1x 16.8

CP 3.95 0.025 1 x 42 6.80 0.017 , = t x 63 6,20 0.019

-2 x 125 8.60' 0,030 Engine type

-Diesel ' Diesel . ,---' Turbine Turbine 1 1 Shaft power KW KW° 2 * 6600

-1 x 13700 -2 * 30500

-2 x 194.00 . [

Power per tonnes Astern power ratio

KW/tonnes -KW0/A = -KWA/KW0 1.52

.

.1'.. 1 0.21 * ... 0,85 .0.94

-0.40 0.07

A'

0.40 I Bow thruster KW KWT T._ 1 x 735

-2 x 735 -A Ratio m Depth, -1

-Table

II

- Performance data for ship

A B _ C D Type of ship

Pass & Car Ferry

60 000 tdw Tanker

Large Cont Ship

500 000 tdw ULCC Spec Dim Symb Ratio Dim fig Ratio Dim fig Ratio Dim fig Ratio Dim fig Ratio Speed knots V ,o FnLo 22 0.34 17 0.20 28 0.26 16 0.14

Prop rpm Shaft power min-' KW RPM() KW° - -190 13200 - -114 13700 _ 125 61000 - -80 38800 - -Spec resistance R/A R/A -0.0095 -0.0018 -0.0055 _ 0.0007

Crash stop track reach

m ST ST/L 440 3.8 2670 13.2 2300 8.4 4300 12.3

Turn circle advance (average)

m AD AD/L 415 3.6 660 3.3 1125 4.1 1150 3.3

Tactical diameter (average)

m TO TO/L 500 4.3 600 3.0 1225 4.5 1045 3.0

Linear steering gain (at V V)

-

sI

K K -0.085 -0.87 -0.057 -1.70 -0.027 -1.78 -0.053 -2.25 Time constant s T T. 13.9 1.36 116 3.96 152 3.51 147 3.47

Nom "rudder efficiency"

--1<7T' -0.64 0.43 0.34 0.65

-I I I

0..008 0.006 '0.004 i0.002 14 Fr,

FIG . DIAGRAM OF RESISTANCE PER UNIT

DISPLACEMENT AS A FUNkTION

OF FROUDE NUMBER FOR SHIPS OF DIFFERENT LENGTH AND LENGTH

DISPLACEMENT RATIO (Cf. Eq. (2_1).)

Resistance and Powering Parameters

!It would certainly be unfair to the non-technical reader to lead him to bellive that the resistance of a ship can be derived from a simple

for-mula with.any degree of accuracy. This difficulty is even more obvious

fn case of the propulsive power, which is sensitive to the detailed

shaping of bow and afterbody, and to the design. of the propeller.

Re-gression formulas including a great number of form parameters have

fai-led to predict a ten per cent difference of performance between two almost similar

models-11116. LIU

_jim

II

I/

11%6_

100

NI*

_Virehhi

/?,4

'

1:MAIMIll

IlirAir

/AIM

VI

k*Ii

4M11

1594

I

kb

a

I

Lid/3= 7 ___

_.-ri,

0.7 02 03 01, V knots 10 20 30 40 0

Nevertheless, except for the length-displacement ratio, the main

di-mension ratios are of no major importance to the "specific" resistance

of the ship, i.e. to the resistance per unit of weight displacement

A = pgV. A discussion of basic relations will be helpful to a further understanding of speed change manoeuvring.

In crude terms the total resistance of a ship is the sum of a

fric-tional, RE, and a residual resistance, RR, mainly due to wavemaking.

The frictional resistance is proportional to the wetted area S = k.VC7

and to speed V squared, the friction coefficient falling off with

in-creasing Reynolds number (or product VL). To the first order the

resi-dual resistance varies with the forth power of the Froude number,

Frit_ = V/VegT. The wave system interference is responsible for an

ad-ditional waviness with "humps" around Fni_ = 0.20 and 0.29 and with an

amplitude of a few per cent of the total residual resistance).

A family of curves of ITTC 1957 friction coefficients drawn for constant

values of length to a base of Froude numbers may be approximated by

hyperbolas, for which the product constants are proportional to the

in-verse of the square root of L. The form factor to be applied to the

friction coefficient may be ruled out together with the variation of the wetted area coefficient k. As for the residual resistance an analy-sis of SSPA model test data compiled in /10/ indicates that the

co-efficient

CR = R./2 SV2_{' 2} is roughly proportional to (L/V13)-2.5. From above the total specific resistance may be approximated by the

function L ) F + ( L ) nL F + c ( ) nL

F4

(2.1)

vio6

nL

vo

vo

A limited analysis of a set of randomly selected SSPA data gives the tentative values a = 2.4-10-7m, b = 0.0026 and c = 3.0, all up to the

second hump.

The diagram in Fig. 6 has been designed on base of equation (2.1) to show the dominating effects of length and length-displacement ratio. Where displacement weight payload is of concern relative speed and

length-displacement figures must both be kept low. If a high speed is

essential relative length must be high, but the resistance is then more or less proportional to the displacement, and volume loads may still

pay their way. Please note again that the detailed design of hull and propulsive arrangements may modify this simplified picture. Here the

curves of Fig. 6 will be used in support of the discussion in Chapter 4.

Within the approximation offered the thrust of the propeller may be estimated from the same diagram using a thrust deduction equal to

t = 0.20 throughout.

15 (

16 0 4.1 W eec ke, C _ c-<Ne L ee 9 0 de ,t.,.,o9 ck.e°' 0 \ c'' cee's oc rninishing

_f

or

indreos,ngirrvoy

_width IV width constrict,on

e/w

FIG 7. NON-DIMENSIONAL PARAMETERS OF WATERWAY CONSTRAINTS Describing increasing deviation from straight reach

in deep unrestricted water

3. FAIRWAY CONSTRAINTS AND DISTURBANCES - PERCEPTION AND IMPACT

As seen from the bridge, the traffic area or fairway may be perceived

from the appearance of islands and buoys and other ships, and from the

distances and bearings to objects ahead. Sounding figures and curves

in the chart give a more detailed information on the manoeuvring space available to the own ship clear of the effects from shallows or banks. So is especially the case where large ships are to enter natural port

approaches also used for smaller vessels. In a few cases special edge

a

range beacons or light sectors define the safe channel for these ships.

More seldom still can they be sure of a "clear cut" entrance, which

would require an active traffic control. Bridge-to-bridge

communica-tion by VHF, however, should be equally natural in any sea traffic area

as is the car-to-car contact-by-eye accepted in street traffic.

Fairway dimensions are naturally related to the dimensions of the ship. Distances along the track(s) are measured in terms of the number of ship

lengths sailed - s/L - as is head reach and advance in stopping or turn-ing manoeuvres. Civil engineers design dredged channel bends with

con-stant radius R, and the fairway line indicated in a chart is generally

made up of circular arcs and straight reaches. With respect to ship hand-ling capabilities the relative curvature L/R is a better measure of the deviation from a straight course, which is closely connected with the

turning rate dWdt = TV. In a similar way the appearance of a width

con-straint becomes ever more evident with the increase of the quotient B/W. Let Wp (negative) and W, denote the offsets of port and starboard sibanks from a fairway centre datum line; the sideway deviation y then

de-fines the port and starboard bank distance parameters np = B/(Wp-y) and

ns = B/(W5-y),

which have proved to model the growth of the "bank suction force" and "bow-away-from-wall" moment constituting the classical bank effect, /8/. Now, as the ship moves from deep to shallow water of depth h the turning resistance and the bank effect both increase in proportion

to the increase of the under-keel clearance parameter = T/(h-T).

The relative impact of the fairway constraints may be defined in the

re-ference frame of Fig. 7, equally applicable to illustrate fairway design

criteria or ship manoeuvring capabilities.

The chart fairway centreline shown on top of Fig. 8 may be typical for

port approaches in Scandinavian waters. In the second diagram the same

curve is illustrated by the local radii of bend as a function of the

dis-tance along the original curve. In the third diagram the radii ordinates

are replaced by the correspodning curvatures related to the length of a ship of concern - the new ordinates are larger the longer the ship, and the relative difficulties of negotiating the bends are immediately

dis-played. In the bottom diagram, finally, the abscissa values are also re-lated to the length of the ship, whereby the presentation is given a special feature: the integrated areas below the curves are a direct

mea-sure of the bend angles (in radians). If the first and final legs of the

fairway centreline are parallel], the sums of areas above and below the

zero curvature line are equal, if not the area difference is a measure

of the resultant change of direction. These relations are valid also for

any plot of curvatures of actual ship tracks through the same fairway

passage, where the smooth function indicates shortcuts in the bends.

(See example in Chapter 5.) Whereas the normal behaviour of a ship in similar bends may be typical for its class and size the extent of navi-gational aids available as well as the routine of the pilot will be of

significance. In one-way passages the pilot may prefer to move over close to the inner buoys of the bend to ease the turn and to keep aware of his position.

17

-18

R2

L R,,P2

2

FIG 8. SCHEMATIC GEOMETRY OF FAIRWAY DOUBLE BEND

4. MANOEUVRING CHARACTERISTICS

Modern nautical training often emphasizes the importance of voyage planning including routing as well as port approach and berthing. The

strategy of coastal manoeuvres will be based on the use of vessel

traf-fic services provided in terms of navigational aids, piloting, tug as-sistance, and traffic information, and on the judgement of weather

conditions, etc. (Cf. Fig 1.) This planning must give room for such changes of the ship handling tactics, which may be required by shipboard

failures or by the development of the traffic situation. It is

essen-"P, S S/L m/L S. R2 L I R2

3

2

FP

19

0.6 08 1.0 P/D 1.2

FIG 9. CA AS A FUNKT ION OF PAID MEAN LINES FOR FP AND CP PROPELLERS FROM KMW CAVITATION TUNNEL TESTS

(From ref. /13/.)

tial that the manoeuvring capabilities of the ship are known and

under-stood by the mariner in charge. Simulator training has proved to advance

the understanding, ship-borne computers and interactive manoeuvring predictors will hopefully move on from prototypes to standard outfits,

/11, 12/. The simulator or the predictor may both be programmed for

the ship concerned. The discussion below will stress more general

rela-tions, which should also be kept in mind when considering the

feasabi-lity of traffic control.

Stopping and slowing down

There is a vast literature on emergency stopping manoeuvres, mostly examplified by the crash stop test from full speed included in routine

delivery trials. From the almost triangular shape of the trial speed-versus-time diagram Norrby /13/ introduced a non-dimensional expression

for the track reach ST,

g S, _(4.1)

V02 2

amean

Alternatively, of course, this expression stands for (ST/L):FnL2. The

factor 13 is a correction for the shape of the diagram "triangle",

i.e. a

measure mainly of the time history of the stopping and reversal of the

screw. (Cf. "Knight's coefficient".) From a statistical analysis Norrby

derived characteristic mean values of (3 between 1.09 for dieselpowered

6

CA 5

4

Singe.- Screw FP

B

F?

. .S

er

141111 C A

0

Twin- screw IC P PA /P Fn L2 CA ClA/Glo

(

Pa ra m et ea-R IA 6 8

FN 10. CRASH STOP TRACK REACHES FOR SHIPS A, B, C, b

Non-dimensional presentation to base of new parameter'

ships with fixed pitch propellers down to 1.02 for ships With CP pro-pellers. The individual scatter may be expected to be large. The mean

retardation factor ameah - the mean rate of change of speed (negative)

in terms of acceleration of gravity, g - is a function of engine torque

and screw characteristics in astern conditions. Experimental relations

were included for the factor CA = PA.TA/QA on base of the astern pitch

ratio PAID for FP and CP Propellers, and they are here represented in

Fig. 9. The quotient A/TA = PAA/QACA should be expected to be a

domina-ting parameter for a crash stop manoeuvre. By accepting certain ratios

of astern to forward torque for ships with certain engine-and-screw

systems the final diagrams for track reach estimates were given as

func-tions of the quotient PAA/Q0CA instead. Single screw ship statistics

showed standard deviations of some 20 per cent, or somewhat less for

the CP ships. Twin-screw ships do not fit in the diagrams,

At design forward speeds the factor C=PTo/Qo may be taken as

essen-tially constant, being of the order of 6. The forward thrust may be approximated from, the forward resistance, using a constant thrust

de-duction. Another roughly constant thrust deduction may be assumed for

the astern condition'. In Fig. 10 track reach data from crash stop trials

with the four ships A, B, C and D have been plotted on base of a new

parameter,

'pup

_A F_nt2 ST/1 'CIA/Q (RID ) 20 (4.2) 16 ST 12 10 4 -2 -8

Values of IS and CA have been taken from above and from Fig. 9 respec-tively - cf. /13/ - and the values for R/A have been read from Fig. 6. Although the results must not be generalized the diagram indicates the

superior stopping capability of twin-crew ships.

In port approach areas, where traffic services are mostly offered, ships may be assumed to move at reduced speed, and a further reduction may

next be required before passing another waypoint. If speed regulations

are stated in knots, the speed-length ratios are widely different for small and large ships, and the slowing-down manoeuvres will be diffe-rently affected by the variation of hull resistance. On the other hand, differences in engine characteristics are of less concern, as long as "stop engine" is not asked for.

In theoretical predictions of stopping or accelerating manoeuvres the relation between resistance and speed is conveniently expressed in ex-ponential form as

R = R1.(Y-)P Vi

The value of the exponent to be chosen depends on the approximation

aimed at. In case of a crash-stop p= Pio will be used to define a

rela-tion such as to cause the same average resistance over the full stopping

time as does the original curve. More general, if the average

resist-ance Rmean identity is to be achieved in a limited speed range between V1 and V2, then the appropriate value of p=p12 may be obtained from

V9 -1

v2 p+1

Rmean/R1 = (p+1) -=-) [1-(--) ]

V1 Vi

where plo is a special case, again. These "average p" values should not

be mixed up with the "derivative" values p=pi used when calculating small changes of speed due to small changes of resistance. Here the approximations of equation (2.1) and Fig. 6 have been applied to derive the new diagram of Fig. 11, showing the three exponents pi, pi° and

P12 (for V2 = 0.6 V1) for two extreme values of the length-displacement

ratio.

The retardation of a ship may be calculated from

(l+k)A

V T(1-0 - R(v)

dS

If the thrust T can be expressed as a function of V (or T/A as a

func-tion of FnL) the retardafunc-tion distance from V1 to V2 may be obtained by

straight-forward integration. So is the case when propeller RPM is

mo-1 mentarily reduced to a value corresponding to a new and lower constant

speed. When moving at "full manouevring speed" in the port approach

(4.3)

(4.4)

21

22 1:1 Ri FnL

)

11 Fn 111 01 02 FnL

03

FIG iL ALTERNATIVE RESISTANCE CURVE EXPONENTS, FOR SHIPS OF

DiFFERENT LENGTH-DISPLACEMENT RATIO For definitions, see Chapter 4 3

2

1

R

(

a call for "slow ahead" or "dead slow ahead" may be expected to half

the constant speed. By use of empirical relations for the normal

KT(J) curve it is possible to approximate the variation of thrust

during the slow down, finding

T/Ti = 0.75 - FnL/FnLi

(P12 =

2) 1

T/T1 = 0.55 - 0.86 FnL/FnL (p12 = 3)

At the initial stage of the manoeuvre the thrust is negative and adds

to the resistance, but in the final stage an effective thrust-resistance

equilibrium is gradually approached. For practical purposes the true

new steady state will not be awaited, and it may be of more interest to

have an estimate of the distance to V2 = 0.6 Vi.

The alternative square and cubic resistance curves give results which

differ by a few per cent only, the average being

F 2

1 nL

(s/L)0.6

FnLi 0.58 (R/,01

If applied to ship B the formula will show that the "dead slow ahead" manoeuvre will reduce speed from 11 to 6,6 knots in a distance corre-sponding to some 13.5 ship lengths, or 2 700 m.

Manoeuvring in a wind

(4.6)

(4.7)

A slow-down distance of 1,5 nautical mile may be acceptable in open

water and moderate winds, but it may be unrealistic in other places,

where a good helm control must always be retained. The stopping

dis-tance is not made much shorter by first stopping the diesel engine;

RPM will drop rapidly by some 60 per cent and the propeller will

then continue windmilling, slowing down with the ship until

fric-tional losses bring it to a stop. At that stage steering control is

seriously impaired. (Reducing pitch to zero in a GP ship may be even

worse, as the flow to the rudder is then blocked at the same time as

dynamic stability is lost.) A short engine-astern manoeuvre may mean an

initial loss of helm control, but when forward RPM is back, the

pro-peller loading will be such as to favour steadying on course again.

The diagrams in Fig. 12 illustrate two alternative manoeuvres with ship

B in the SSPA simulator. The ship is in both cases proceeding at full

"manoeuvring speed" in a true 15 m/s wind from 450 on the port bow. The

"steady state" speed is some 10,5 knots, and the mean helm angle is

around 20 or 30 to starboard - the ship has a tendency to luff in the

wind - and the track over ground is set off about 1,80 to windward. With

the engine set on "dead slow ahead" speed is falling off slowly only,

and manual helm control is more difficult, especially as there is all

the time a demand for a slowly increasing residual helm not so easily

23

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W1ND EFFECTS ON PASSENGER FERRY (SHIP A) $N 30° COURSE

CHANGING MANOEUVRES WITH A 25° RUDDER LIMIT

From computer simulations using a wind force model by

which is duplicated the lee helm requirement at low'

speed known from prototype

25 ,Err. -2,00 8 20 0 600 10 0

26

TWin-screw / Single-rudder v Do, as rebuilt

Twin- screw / Twin-rudder 0 Do, as rebuilt

Twin-screw / Single -rudder Trans -Allorit:c poss.-Oner,

Bow thruster tiktedl

al +97,5 19911 198.1 197,5 1977 ₁₉₆₆ 5969' 1973 ₁₉₆₆ 1955 (ShIp ip,A750 f936 50 1001 150 L Cm] 200

FIG 14. _{RELATIVE WIND AREA (SIDE WIND AREA OVER DRAUGHT SQUARED) FOR}

PASSENGER SHAPS AND MODERN PASSENGER ,& CAR FERRIES, SHOWN AS

FUNCTION OF SHIP LENGTH AND YEAR OF DELIVERY

felt by the helmsman. If the speed is _{more rapidly' brought down by an}

initial braking manoeuvre, control _{is safer during the final part of the}

manoeuvre (In the example the wind effect helps the ship from _sheering'

to starboard' when the propeller is momentarily reversed..),

Whenever

relative,

_{wind Velocities are high or wind areas are excessive,}

the full contrail of a ship is at danger. In _{a recent paper /14/ Martin}

designed a number of "control boundary" diagrams for typical ships, for

which wind tunnel results were earlier available'.

3973 4 4

Most ships carry a 'weather helm to avoid turning up against a beam wind.

In some modern passenger ships the large _{funnel aft will take the role}

of a mizzen. Ferry A was known to require a lee helm at low speeds

_in

strong winds, however, probably due to the aftward _{shift of the}

non-linear hydrodynamic force at large angles at drift in conjunction with

a small trim by the stern. The three _{curves in' Fig. 13 compare the track}

plots of three simulated manoeuvres, in a 20, m/s true wind, all made to

change the course made good by 300 to windward using, no more than 250

tato A

4

1991 150 11,0'0 so A) AL/72

helm, but all at alternative initial speeds. It _{is obvious that a} 10 knot

speed was much too low for the required manoeuvre, and that ships of this type must keep up speed in fairways with frequent bends.

By a well known rule of thumb the wind pressure on the side of a ship

is calculated as W2/20 tonnes per 1 000 m2 area, where W is wind speed

in m/s. (This rule corresponds to the assumption of a drag coefficient

close to 0.8.) In normal sailing the force and moment due to wind of

any direction must be balanced by the hydrodynamic forces on hull and rudder; most of the beam wind pressure will appear as a drift force on the hull. By use of the low-aspect-ratio wing analogy, referred to

already on page 3, and empirical data from /8/ it is possible to derive

an approximate formula for the hull of draught T m:

Lateral force per degree} T2

tonnes (4.8)

drift angle at 10 knots

If the two simple formulas above are combined the limiting wind veloci-ty W (in m/s) for operating the ship at a given speed Vs (in knots) and

drift angle 6 (in degrees) is obtained from

672

W < 5 - V

(4.9)

AL s

Let it be assumed that 6 = 100 is a practical upper limit of an

accept-able sideway drift, and that traffic regulations may limit the speed to 7 knots: formula (4.9) will now limit operating conditions for ship A

to wind speeds below 15 m/s! Again,

if

wind speeds of 20 m/s shall be

accepted, then the ratio AL/T2 shall not exceed a value of about 30. While most passenger ships do present higher values the recent trend

for sea-going ferries is worth

of

consideration.

(Cf

Fig 14.)

Propeller thrust has an important bearing on the handling capabilities of a ship. For ships with similar stern arrangements the maximum rudder

force is also more or less proportional to propeller thrust _{or bollard}

pull at low speeds, and a significant figure will, therefore be indicated

by the quotient of power installed and maximum speed. If different ships

are to be compared with respect to low speed manoeuvring in winds of a certain absolute magnitude the proper ratio will not be non-dimensional

but have the dimension CI. For practical reasons the figure included

in Table I is written (KW0/V0)/AL,expressed in "kilonewtons per square

meter". (When ships of similar speed are compared the ratio of "horse-power per square meter" will have the same relative signficance. The

analogue ratio, of course, is appropriate for judging the relative size

of lateral thruster units.)

1.0 0. 28 T

+L

= K'6 ds' R R (4.11) 2.0 100 Ail IT

FIG 15. CONTROL EFFECTIVENESS K-/T" - EXAMPLE OF RESULTS FROM FIRST-ORDER ANALYSIS OF FULL-SCALE ZIG ZAG TESTS WITH THREE 40 000 TDW TANKERS, SIMILAR EXCEPT FOR STERN ARRANGEMENTS

(From ref.

/8/.)

Steering and turning

At a certain forward ship speed and screw loading the rudder force and the turning moment produced are proportional to the rudder deflection, and in the dynamics of the ship they appear in the lateral force and

yawing moment equations, respectively. The two degrees of freedom of

immediate concern are the athwartship sway (v) and the yaw (or rate of

change of heading, tl) = dtP/dt) with respect to a suitable reference

point along the ship. Under certain conditions it is then possible to study the resulting time history of the heading angle lp by use of the linear first-order equation (Nomoto, /15/)

+ = Kd (4.10)

in which the proportionality factor for the rudder force is included in the gain K. A certain change of heading per unit time realized at a given speed V corresponds to.a certain relative curvature of the

path of the ship, L/R = tb' = LtP/V. In this non-dimensional scheme time is represented by the number of ship lengths sailed, s' = s/L . L-1fVdt, and equation (4.10) reappears as

K

-200 - 1.0 L/R rn 0.8 300 0.5 400 500 0.4 1000 - 0.2 5000 Deep water Shallow water (h =16.5m) Shallow water So = 29 20 30* 46 0.02 0,05 01 015 02 "Frequency" L/S,,

a) Steady turning radius and b) Path curvature amplitudes

turning circle curvature in sinusoidal motion

FIG 16. TURNING CHARACTERISTICS OF A 60 000 TDW TANKER (SHIP B) IN DEEP AND SHALLOW WATER

(11 m draught.)

where R is the momentaneous radius of turn and where 6 is to be

ex-pressed in radians. In a state of constant motion R = L/(K'6) within

the linear theory. In Fig. 16 the steady turning curvature L/R is shown

to

a base of helm angles for ship B, as calculated using a much more

complex mathematical model.

The non-dimensional static gain K' is a measure of the rudder-on-ship

effectiveness, and relatively high numerical values are desired. (Due

to the sign convention in a right-handed system of reference K' is

negative, giving a positive (starboard) turn when negative (starboard)

rudder is applied.) The non-dimensional time constant T' is a measure

of the effective inertia of the ship, and a small value should be ex-pected to further good manoeuvrability. As a kind of effective or aver-age values of K' and T' can easily be derived from trials they are

frequently used for comparative purposes. In general ship design K'

and T' are influenced in the same direction, and it is then reasonable

to try to achieve a high quotient K'/T' at least. The first-order theo-ry is strictly valid for dynamically stable ships only, and the formal analytical criterion for the appearance of instability on straight course will make both K' and T' change sign. The quotient K'/T' has been shown still to have a significant meaning in the analysis of tests with ships that are marginally stable, /16/. The first-order equation

L/R 0.5 05 0.4 0.3 0.2 0.1

has been extended in various ways to account for non-linear effects, but modern prediction and simulation methods usually resort to the use of the full dynamics model.

Fig. 15, already referred to on page 9, makes use of the quotient K'/T' to illustrate the benefit of a stern arrangement, where each rudder

operates in a behind-screw position, /8/. Undoubtedly unorthodox arrange-ments with three propellers and one or two rudders may add additional advantages in low speed and harbour manoeuvring.

5. MANOEUVRING IN FAIRWAYS

In coastal areas and port approaches ships usually move in more or less

narrow fairways, as commented upon in Chapter 3. Due to the lateral

blockage of under-keel flow the yaw damping or resistance to turning is always increased, and this increase is generally accompanied by an

increase of the dynamic stability. In a way the ship is harder to get

around in the turn, but she is also more predictable. (Due to the

kinectis of the yaw-and-sway system dynamic stability may fall off in an intermediate range of depths just below twice the draught, depen-ding on the characteristics of the ship.)

Fig. 16a illustrates the decrease of steady turning circle curvature

that results from a finite water depth, here examplified by results for ship B, the 60 000 tdw tanker. This ship operates in narrow fairways, where the turning rate develops only slowly and where the steady state

turning capability is often still insufficient. Frequent use of "RPM

kicking" must therefore be exercised.

These fairways often include double bends of the type shown above in Fig. 8. Whenever possible adequate bend radii and straight reaches are

incorporated in the lay-out, and channel widths are widened to comply with experience and international recommendations, /17/. The Swedish Administration of Shipping and Navigation and the Swedish Transport Research Delegation cooperate for the improvement of fairway safety standards. Within a study for the Administration SSPA recently comple-ted an extensive comparison of two existing and alternative fairways, mainly based on real-time simulations with the participation of expe-rienced pilots from the area. The diagrams displayed in Fig. 17

high-light some of the results.

The lower part of Fig.17 defines the nominal fairway centre-line and

its curvature, the approach from the sea being to the right. The

fair-way is surrounded by skerry side banks and shallows which may give rise

to serious interaction in places. Each of three pilots navigated the ship through the passage under a number of alternative specified condi-tions of wind, current pattern and speed on approach, using engine

manoeuvres at will. The dotted curve will just give an example of one

individual run, randomly selected from among more than 80 transits through this passage. The curve in the upper diagram shows the total

2

0

Width between

swept area envelops

Fairway centre-line curvature

Typical track curvature of individual run

36 32 28 21. 20 16 12 8

s/L

0

Distance along fairway centre-line

FIG 17. RESULTS OF A REAL-TIME SIMULATOR STUDY OF A 60 000 TDW TANKER (SHIP B) IN REPEATED TRANSITS THROUGH A TYPICAL COASTAL FAIRWAY Normalized presentation of fairway, track curve and width of

swept areas. (Cf. Fig 8.)

width

made use of during all these runs, defined by the width between

the starboard and port envelops of swept areas for all the runs. This

width is likely to come close to the proper width of a "manoeuvring lane" in each part of the passage.

The full information hidden in the many original track plots and time records as well as in diagrams of the type just shown can only be ex-tracted with access to the detailed charts and soundings and to the special comments offered by the pilots. The results of that analysis is outside the scope of this paper, of course, but there are still some general conclusions to be drawn.

Most mariners prefer a fairway alignment, in which the ship can be set

down steady on the range lines before entering the next bend. The length of the straight reach is recommended to be 3L at least. A width equal

to 3B is often considered to be adequate for one-way traffic in the

reach, with additional requirements in the bends. From Fig. 17 it is

obvious that the length of the reach is far too short in a case where nominal fairway curvatures are of the order 0,3 - 0,4

31 0.6 L/R 0.2 0.0 6 0.4 4 - 4 4 - - -4

-According to Fig. 16a the steady turning curvature for ship B in water

of fairway depth is L/R =

0,45

at 200 helm. If the rudder is oscillated

in slow sinusoidal motion with a "period" corresponding to some 10L, the curvature amplitude turns out to be less than 0,36, however. (Cf. Fig 16b.) Experienced pilots will therefore choose to let the ship short-cut between the bends, and the "statistically documented" ma-noeuvring lane may well be almost as wide on a straight reach as in

the bends.

The results from this and other similar studies performed at SSPA all underline the importance of a proper positioning of buoys and marks with a view to help the mariner to check his exact progress along the fairway and to make the best use of the manoeuvring capabilities of

the ship.

6. REFERENCES

Wepster, A.: "Developments in Marine Traffic Operations and Research. An Introduction", Proc. Third International Symposium

on Marine Traffic Service (Supplement volume), Liverpool 1978,

Liverpool Polytechnic Press.

Price, R.I.: "Marine Traffic Engineering - A New Discipline",

Marine Technology,

17(1980):2

(April),

p.199-202.

Paffett, J.A.H: "Ship Manoeuvring Characteristics", Conf. on

Marine Traffic Engineering, London

1973, p. 153-162,

RINA/RIN.

Hermans, A.J.: "A Stochastic Model of Ship Maneuvers", Proc. International Symposium on Marine Traffic Systems, The Hague

1976, p.376-385,

Delft University Press.

Abdelgalil, E.M.: "Shipping Casualties and Ship's Domain", Proc. Third International Symposium on Marine Traffic Service,

Liverpool

1978, p. 95-107,

Liverpool Polytechnic Press.

(IMC0): "Protocol of 1978 Relating to the International Convention

for the Prevention of Pollution from Ships,

1973",

Final Act of

the International Conference on Tanker Safety and Pollution

Pre-vention, 1978.

Vantine, W.H., Makibbin, T.C., Kirkby, T.M. and Christian, P.: "Ship Bridge Design Criteria and Other Design Features Relating to

Safe Navigation", Panama Canal Pilots Association, August 1975.

Norrbin, N.H.: "Theory and Observations on the Use of a Mathema-tical Model for Ship Manoeuvring in Deep and Confined Waters", Proc. Eight Symposium on Naval Hydrodynamics, Pasadena, Calif.

1970, p.807-904,

SSPA Publ.

68, 1971.

32 -3. 5.

8.

(DnV): "Rules for Classification of Steel Ships", Det Norske

Veritas, Oslo 1964.

Williams, Ake.: "The SSPA Cargo Liner Series - Resistance,"

SSPA Pub].

66, 1969.

Norrbin, N.H.: "Ship Manoeuvring with Application to Shipborne Predictors and Real-Time Simulators", Proc. International Symposium

on Directional Stability and Control of Bodies Moving in Water,

Journ. Mech. Engng. Sciences,

14(1972):7,

suppl. issue,

p.91-107.

van Berlekom, W.B.: "Simulator Investigations of Predictor Steering

Systems for Ships", Trans RINA,

120(1978): p. 23-34.

Norrby, R.: "A Study of Crash Stop Tests with Single Screw Ships", Chalmers University of Technology, Dept. of Ship Hydromechanics,

Gothenburg 1972.

Martin, L.L.: "Ship Maneuvering and Control in Wind", Paper No. 9

presented at the SNAME Annual Meeting, New York 1980.

Nomoto, K., et al.: "On the Steering Qualities of Ships",

Inter-national Shipbuilding Progress,

4(1957):35

(July), p.

354-370.

Norrbin, N.H.: "An Integrated Criterion for P-Number and K-/T-Requirements Based on Step Response and Limit-Cycle Steering

Analysis", Thirteenth 1TTC, Berlin-Hamburg 1972, Materials for

Report, p.

181-191.

(P1ANC): "Optimal Lay-out and Dimensions for the Adjustment to Large Ships of Maritime Fairways in Shallow Seas, Seastraits and Maritime Waterways", International Commission for the Reception

of Large Ships, Supplement to PIANC Bulletin No 35, Bruxelles

1980.

33 9.