Manoeuvrability analysis of double ended ferriesin preliminary designM.Insel & I.H.HelvaciogluFaculty of Naval Architecture & Ocean Engineering, Istanbul TechnicalUniversity, Maslak, 80626 Istanbul, TurkeyEmail: insel @ sariyer.cc.itu.edu.tr , helvaci @ sariyer.cc.itu.edu.tr

Abstract

The choice of propulsion/manoeuvring system of ships operating in congested waters is mainlygoverned by on manoeuvring capabilities of the available systems. A quantitative approach tomanoeuvring in the preliminary design of these ships is often requested by the operators. Such astudy has been conducted for the selection of propulsion/manoeuvring system of a double endedpassenger ferry to operate across the Bosphorus in Istanbul with the aim of reducing the journeytime in highly congested seaway traffic. Four alternative types of propulsion/manoeuvring systems are considered in the current study,namely, conventional propeller and rudder, propeller-high lift rudder, cycloidal propellers and Zdrives. An overall comparison based on the cost, marine engineering aspects, weight, speed andmanoeuvrability is presented for guiding the choice of propulsion/manoeuvrability device ondouble ended passenger ferry. Environmental forces originated from wind, waves and current aretaken into account for the manoeuvrability analysis. Critical manoeuvres have been chosen asposition and track keeping in the Bosphorus in heavy traffic, especially among transit vessels,and approach to Karakoy pier. The thrust and side force generated by each type ofpropulsion/manoeuvring device has been compared with the required forces. Finally, examplecases with polar graphs of thrust-side force have been given to show the advantages ordisadvantages of each device.

1 Introduction

Complete manoeuvrability analysis of a proposed design is often not possible due tounavailability of required detailed data such as hull, propellers, engine, rudders and appendages.Steady state techniques have been deployed to fulfil the manoeuvrability requirements in thepreliminary design such as given by Asinovsky [1]. These techniques are based ondetermination of disturbance forces and moments and investigation of required thruster forcesto balance disturbance forces and moments for a given station keeping or track keepinganalysis.

Double ended passenger/vehicle ferries are often utilized for fast turn around in closed seas.The passenger traffic in Istanbul has wide use of passenger ferries to cross the Bosphorus forcommuter traffic. A double ended, passenger only, ferry has been proposed to reduce thejourney time between Kadikoy and Karakoy which has a traffic of over 30,000 passenger a day.Possible journey time reduction by the use of double ended ferry has been calculated, and a 20 %time reduction has been found. The choice of propulsion/manoeuvering system for the double ended ferries shows variationand selection criteria are not well established [8]. A quantitative approach to manoeuvring bysteady state technique has been conducted for the selection of propulsion/manoeuvring system ofa double ended passenger ferry to operate in the Bosphorus in highly congested seaway traffic. A cost-benefit analysis has revealed the potential use of four alternative systems:a) Conventional propeller and rudder-2x2b) Conventional propeller-high lift rudder-2x1 (Figure 3a)c) Cycloidal propellers-2x1 (Figure 3b)d) Z drives-2x2 (Figure 3c)where 2x2 means two propulsion unit at both ends, 2x1 means one propulsion unit at both ends. The linear manoeuvring theory has been applied to investigate the directional stability.Environmental forces and manoeuvring forces have been calculated and compared with availablethruster forces for critical manoeuvres such as station and track keeping in the Bosphorus, andKarakoy pier approach. Considered environmental force has included wind, wave and currentgenerated forces in extreme conditions on the route. Polar thrust-side force diagrams are used todemonstrate the adequacy of the available systems.

2 Double ended ferry propulsion/manoeuvering systems

Four alternative types of propulsion/manoeuvring systems are compared in terms of marineengineering and naval architectural aspects. The available data obtained from the variouscompanies and open literature have been investigated for an overall design study. The results aresummarised in Table 1. The manoeuvring aspects of the systems will be discussed in Section 3 to7. The physical characteristics of the propulsion systems are considered first. Then the comfort,safety and overall reliability properties of the propulsion systems are compared. Lastly, initial,operating and maintenance costs of each type of device are compared. This table shows that each type of device has some advantages and disadvantages. Reaching aclear distinction among the alternative systems is highly dependent on the design synthesis of theproposed ship. To do that may require the use of multi-objective decision making techniques.

3 Manoeuvering equations and directional stability

In the preliminary design stage, linear manoeuvering analysis can be utilized due to the lack ofdata. Force equations of such analysis as given by Clarke [3] are :Y Y v Y v Y r Y rv v r r= + + +

� �� � (1)

N N v N v N r N rv v r r= + + +� �� � (2)

Coefficients of manoeuvering can be determined from model tests. However such tests areobviously useless in the preliminary design, hence use of empirical equations such as given byClarke [3] are often utilized.

Table 1 Various Properties of Propulsion Systems ( A: Good, B: Average, C: Poor)Propulsion/Manoeuvering System

ConventionalPropeller &

Rudder2x2

Propeller &High LiftRudder

2x1

CycloidalPropeller

2x1

Z Drive

2x2

Power requirement A A C BRequired ship geometry U-V shape U-V shape U shape U shapeTotal propulsion weight Average Average Heavy LightRequired engine room Large Large Average SmallVibration and noise B B A BMechanical safety A A C BAdditional training No Yes Yes YesReliability A B B CRedundancy B B C AFuel consumption A A C BHandling and repair cost Low Low Low HighInitial cost Low Average High AverageRunning cost Low Average High Average

Directional stability of the proposed ferry has been checked by using Nomoto’s [3] andNorbin’s [3] P indices

′ ′ + ′ = ′T r r K� δ (3)

P KT

= ′′

1 2 (4)

which gave a P value of -0.52 which does not satisfy Norbin’s suggested P value of above 0.3[3]. Since this criteria is essentially given for single screw merchant ship forms a moremeaningful approach the following directional stability criteria was adopted:

′ ′ − ′ ′ − ′ ′− ′ >Y N m x N Y mv r G v r( ) ( ) 0 (5) According to this criteria (which gives the value of 0.00116) the ferry is directionally juststable. Hence a skeg is found to be necessary for improved directional stability. Conventionalpropeller arrangements have better hull shapes for the directional stability, and the worse arethe full forms for Z drive and cycloidal propeller systems. A comprehensive simulation of shipmanoeuver should be conducted when required data is available. Examples of such cases aregiven by Wilson et al [9] and Korkut and Aldogan [6] including nonlinear effects.

4 Environmental forces and moments

Environmental forces originating from wind, waves and current are taken into account for themanoeuvrability analysis.

4.1 Wind Forces

Wind generated force and moment can be important for the ships with large superstructures. Asthe current case, double ended ferry, usually has a large longitudinal windage area, windgenerated forces above waterline have been calculated from Gould’s method [4,5]. The windspeed in the Bosphorus is assumed to be as maximum of 24.5 knots from any direction relative

to the bow. The historical database statistics indicates that winds above this speed occurs onlyduring 3 % of the year. A reference wind speed at the top of the deck is calculated by using a given wind speedmeasured at 10 m above sea level :UU

HH n

10

1

10= ( ) (6)

and an effective wind speed for the superstructure is calculated as :

( )U UHnn

2 22= + (7)

by using this effective wind speed :

F C Cos C Sin S ULW DW LW= −( ) 2

Aβ β ρ 2 (8)

F C Sin C Cos S UTW DW LW= +( ) 2

Aβ β ρ 2 (9)

M C S UW MW= 2

L AOA

ρ 2 (10)

where CDW , CLW , CMW are drag, lift force and yaw moment coefficients respectively whichare given for various ship types.Ferry ship type coefficients are utilized for the current work.

4.2 Wave Loading

Wave effects are pronounced during the winter season in the Bosphorus. Max. significant waveheight is taken as approximately 2.1 m. The direction of the waves coincides with the winddirection. Wave loading on the hull has been calculated from BSRA [2] which was derivedfrom experimental results on a cargo vessel by Wise and English. Wave drift forces andmoment are given by:F C g LL L PP Wζ ζ ρ ζ= ( )1 3

2 (11)

F C g LT T PP Wζ ζ ρ ζ= ( )1 32 (12)

M C g LM PP Wζ ζ ρ ζ= 2 ( )1 32 (13)

where CLζ , CTζ , CMζ are longitudinal, transverse force and yaw moment coefficients whichare given as functions of relative wave angle by the bow.

4.3 Current Effects

There is a strong current in the Bosphorus which can reach upto 6 knots. However such casesare very rare and transit traffic is limited when current is above 4 knots. Hence a maximum of 4knots from North-East is accepted for the Bosphorus. The current in Golden Horn is up to 2knots from North-West which is taken as the limit in the calculations. The effects of the current on the ship manoeuvers have been calculated by BSRA method[2]. This method gives the longitudinal, transverse forces and yawing moment by

F C L T V CosLCDC

WL C= ρ β 20 9

2

.(14)

FC

L T V SinTCDC

WL C=ρ β

20 9

2

.(15)

M C L T VCMC

WL C= ρ 20 9

2 2

.(16)

where CDC , CMC are the total force and the yaw moment coefficients, β is the current forceangle which is given as functions of the angle of current by the bow. The relative angle iscalculated to take the ship speed into account. A correction due to shallow water was alsoapplied as given by BSRA method [2].

5 Sample case Karakoy-Kadikoy

An examination of manoeuvering feasibility of double ended passenger ferry, preliminarydetails are given in Table 2 and Figure 2, was conducted as requested by Turkish MaritimeCorporation [7]. Proposed ferry would operate between Kadikoy, which is the main exchangepoint from land based transportation into the sea transportation on the Asian side, and Karakoymain pier to access business centers in the European side, crossing the Marmara side of theBosphorus as shown in Figure 1. The demand for passenger transport between Kadikoy andKarakoy is over 30,000 people a day with peak times of 8:00 to 9:00 from Kadikoy to Karakoyand 17:00 to 19:00 from Karakoy to Kadikoy. Haydarpasa is the railway connection in theAsian side.

Table 2: Proposed double ended ferry preliminary characteristicsLength between perpendiculars 60.3 metersBreadth 13.6 metersDepth 4 metersBlock Coefficient 0.48Displacement 1188 tonsSpeed 14 knots

Conventional passenger ferries with two propeller-two rudder configuration currentlyoperate on the route with 15 minutes interval and 46 minutes journey time. Total time spent onthe full speed is about 28 minutes, and the rest is spent on manoeuvering. Specially Karakoyend has busy traffic with other conventional passenger ferries, cruise liners, small motor boats.The Bosphorus crossing of the route is also busy due to transit traffic. It is quite commonpractice to stop or reduce the speed for allowing transit traffic passage.

A review of the manoeuver in Karakoy (Figure 4a) has shown that a considerable time isspent on backing and turning. Proposed double ended ferry manoeuver, given in Figure 4b,reduces the manoeuver time by the half. Same tendency is observed for Haydarpasa andKadikoy. Total time of the journey is given in Table 3 reveals that a reduction in the order of 9minutes and 30 second can be achieved by a double ended ferry. The choice ofpropulsion/manoeuvering device has not an important effect on this timings. Further reductionscan be achieved due to manoeuverability in heavy traffic in Golden Horn and the Bosphorus,and by reducing waiting times in Haydarpasa.

6 Critical manoeuvers

Critical manoeuvres have been chosen as station and track keeping in the Bosphorus in heavytraffic (especially among transit vessels), approach to Karakoy pier and other consideration of

manoeuvrability device. For each critical manoeuvre environmental forces and moments arecalculated by the methods given above.X F F F FLW LC L L INERTIA= + + + −ζ (17)Y F F F FTW TC T T INERTIA= + + + −ζ (18)N M M M MW C INERTIA= + + +ζ (19)

Table 3: Estimated timings for the journeyConventional Double Ended

Karakoy departure 3 min + 1 min 30 sec +Karakoy-Haydarpasa 10 min + 10 min +Haydarpasa arrival 2 min 15 sec + 45 sec +Haydarpasa passenger unloading 30 sec 30 secHaydarpasa departure 30 sec 30 secHaydarpasa-Kadikoy 3 min 3 minKadikoy arrival 3 min 45 secKadikoy departure 1 min 45 secKadikoy-Haydarpasa 2 min 2 minHaydarpasa arrival 2 min 45 sec + 45 sec +Haydarpasa passenger loading 2 min 2 minHaydarpasa departure 30 sec 30 secHaydarpasa-Karakoy 12 min + 12 min +Karakoy arrival 4 min + 2 min +Manoeuvering Time 17 min 7 min 30 secTotal 46 min 30 sec 37 min

Then required thruster forces (Figure 5 and 6) and angles were derived as:

2*1 Thruster ArrangementTCos T Cos X1 1 2 2α α+ = (20)T Sin T Sin Y1 1 2 2α α+ = (21)T Sin L T Sin L N1 1 2 2α α− = (22) In the solution of the thruster forces, longitudinal force of forward thruster is assumed to take40 % of X force.

2*2 Thruster ArrangementT Cos T Cos T Cos T Cos Xp p p p s s s s1 1 2 2 1 1 2 2α α α α+ + + = (23)T Sin T Sin T Sin T Sin Yp p p p s s s s1 1 2 2 1 1 2 2α α α α+ + + = (24)T Sin L T Sin L T Sin L T Sin LT Cos b T Cos b T Cos b T Cos b Np p p p s s s s

p p p p s s s s

1 1 2 2 1 1 2 2

1 1 2 2 1 1 2 2

α α α αα α α α

− + −

− − + + =(25)

It is assumed that port-starboard thrusters has the same thrust and direction for both forwardand stern thrusters i.e. T Tp s1 1= , T Tp s2 2= . Available thruster propulsion/manoeuvering thrustand side force characteristics derived from available sources are given in Figure 8.

6.1 Approach/Leave Karakoy pier

The most congested part of the Bosphorus is the entrance of the Golden Horn. The currentconventional ferries has a backing and a turning manoeuver to leave the port (Figure 4a).Arrival manoeuver is simpler, the ship goes along the pier, is moored at the stern then the shippulls itself to the pier. Proposed ship would go along the pier then approaches the pier bycrabbing manoeuver. Environmental conditions in this manoeuver has been chosen as• Winds up to 24.5 knots at any direction• Current up to 2 knots from the West• Waves up to 1.0 meters significant wave height at the same direction as wind. Ship speed for arriving and leaving is 7 knots which is reduced to 2 knots during thecrabbing manoeuver. The forces and moments due to environmental conditions and manoeuverwere calculated at 7 stages of the manoeuver, such as Figure 9, then the thruster forces werecalculated as given above. Required thrust and side force were compared with available thrusterforces as shown in Figure 12 and 13 for arrival and departure. The required force for eachthruster is given in all the stages. As the ship speed is at 7 knots X force and yaw moment is dominated by current-forwardmotion force. Wind force has substantial effect on the Y force. Polar diagram indicates that allthe advanced propulsion systems can produce the required forces for this manoeuver. Howevercycloidal propeller has the best manoeuvering characteristics.

6.2 Station keeping in the Bosphorus

Heavy traffic conditions are often observed in the Bosphorus. Transit traffic has the priority topass, hence ferry has to stop and wait for traffic and keep their position. A station keepingsteady state analysis was performed for the conditions of :• Winds up to 24.5 knots at any direction• Current up to 4 knots at any direction• Waves up to 2.1 meters significant wave height at same direction as wind. As the ship stationary, ship speed is zero, and inertial forces are neglected. Environmentalforces are calculated and an example is given in Figure 10. Required thruster forces arecalculated as given above and polar plots of required and available thrust and side force aregiven as Figure 14. As the current direction is not known before hand, forces for 7 current anglefrom 0 to 180 degrees have been calculated and plotted. As the ship speed is zero, wind, wave and current has almost the same share on X force.However Y force and moment is dominated by the current. As it was expected, the current hasthe most substantial effect when ship heading is above 30 degrees to the current direction.Required side force for current angles between 30 degrees and 150 degrees are very high, andthe ship can not be expected to perform position keeping without any drift. Again cycloidalpropeller has the edge to perform best manoeuvering (Figure 14).

6.3 Track keeping in the Bosphorus

It is also possible that the ship would keep a track and advance with a reduced speed in the badenvironmental conditions. This case is similar to station keeping as :• Winds up to 24.5 knots at any direction• Current up to 4 knots at any direction• Waves up to 2.1 meters significant wave height at same direction as wind.

However ship speed is reduced from 14 knots to 10 knots. Required thruster forces arecalculated as given above and polar plots of required and available thrust and side forces aregiven as Figure 15. Track keeping study is fully based on the current-forward motion forces as given in Figure11. Forward thrust is substantially increased comparing to position keeping, meanwhilesideforce requirement is similar. Forward thrust can be supplied by cycloidal or Z drive withoutspeed reduction. High lift rudders have the advantage to be able to keep track with higherspeed. Again current heading angles above 30 degrees can not be compensated without anysway.

6.4 Other considerations

Other manoeuvering considerations are :Station keeping in the case of forward propulsion unit break down: The thruster configurationswith 2x2 arrangement are beneficial as there will not be a manoeuverability loss. 2x1configuration will have to sail with a yaw, hence reduce speed considerably.Acceleration : Calculations indicates the ship would accelerate to 7 knots within 30 secondsand less than 40 meters.Deceleration: The ship could stop at 45 second in 100 meters from 7 knots. It is possible to summarise the results of manoeuverability analysis as shown in Table 4:

Table 4:Summary manoeuvering characteristics of alternative systemsManoeuver Propulsion/Manoeuvering System

Con.Prop +Con.Rudder

2x2

Prop+High LiftRudder

2x1

CycloidalPropeller

2x1

Z Drive

2x2Karakoy arrival and departure B A A AStation keeping in Bosphorus C B A BCrash Stopping B B A AAcceleration A A B BBreak down of forward unit A B B ADirectional Stability A A C CTurning Radius C B A B(A: Good, B: Average, C: Poor.)

7 Conclusions and recommendations

It is quite apparent that use of double ended ferry for congested waters can result in substantialjourney time reductions. In the current case the time reduction is about 9 minutes 30 secondswhich is 20 % of the total journey time. However the choice of propulsion/manoeuveringdevice has not have an apparent effect on this saving. The steady state manoeuvering analysis has proved to be a useful preliminary design tool forthe selection of propulsion/manoeuvering system. It is also used for the determination ofcurrent system capabilities. The sample case has resulted in a detailed analysis of critical manoeuvers, both station andtrack keeping analyses. Port approach can be handled easily by Z drive, cycloidal propeller andhigh lift rudders. Cycloidal propeller has advantage for position keeping, meanwhile high liftrudders are beneficial for both thrust and side force for track keeping manoeuver.

Further consideration is necessary to include the different hull forms, such as skeg variations,required for the accommodation of available systems and influence of these should be includedin the manoeuvering analysis. Thrust-side force polar diagrams can also be extended speciallyon the reverse manoeuvers.

ACKNOWLEDGMENT

This study was partially funded by the Turkish Maritime Corporation, thanks are due to lateMr. Nurettin Alptogan. The authors are also thankful to Prof. Dr. A.Yucel Odabasi for hissupport and guidance on the subject.

REFERENCES

1. Asinovsky, V., Huang, K-N., Oakes, M.C., Ship Manoeuverability Analysis Using Steady-State Techniques, Marine Technology, 1991, vol 28 (3), 163-180.

2. BSRA, Manoeuvering, Chapter VII, Ship Design Manual, BSRA, 1984.3. Clarke, D., Gedling, P., Hine, G., The Application of Manoeuvering Criteria in Hull Design

Using Linear Theory. The Naval Architect, 1983, 45-68.4. Gould, R.W.F., Measurements of the Wind Forces on a Series of Models of Merchant

Ships, National Physical Laboratory, NPL Aero Report 1233, April 1967.5. Gould, R.W.F., The Estimation of Wind Loads on Ship Superstructures, Maritime

Technology Monograph No:8, RINA, 1982.6. Korkut, E., Aldogan, A.I. The Solution of Equations of Ship Manoeuvers by the Computer,

Bulletin of the Istanbul Technical University, 1995, vol 46 (1), 71-86.7. Odabasi, A.Y., Salci A., Insel M., Ozsoysal O.A., Choice of Propulsion System for a

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Figure 1: Karakoy-Kadikoy Line

Figure 2: Proposed double ended ferry

Figure 3a: Conventional propeller Figure 3b: Cycloidal propeller Figure 3c: Z driveand high lift rudder (2x1) (2x1) (2x2)

Figure 4a: Current manoeuvre Figure 4b:Proposed manoeuvre

Figure 5: 2x1 propulsion system Figure 6: 2x2 propulsion system

Figure 7: Forces acting on the hull Figure 8: Polar available thrust-sideforceforces

Figure 9: Enviromental forces for Karakoy approach (X,Y forces, and N moment)

Figure 10: Enviromental forces for position keeping (X,Y forces, and N moment)

Figure 11: Enviromental forces for track keeping (X,Y forces, and N moment)

Figure 12: Polar plot of required and available thrust-side force (Karakoy arrival)

Figure 13 Polar plot of required and available thrust-side force (Karakoy departure)

Figure 14: Polar plot of required and available thrust-side force (Position keeping)

Figure 15: Polar plot of required and available thrust-side force (Track keeping)