Improving Fuel Efficiency in Ro-Pax Design , RoRo2004-NAOS-SVA

7/27/2019 Improving Fuel Efficiency in Ro-Pax Design , RoRo2004-NAOS-SVA

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Improving Fuel Efficiency in Ro-Pax DesignPresented at RORO 2004 byRoberto Prever, NAOS srl. Ship & Boat Design

Rainer Grabert, SVA Potsdam GmbH, the Potsdam model basin

1. Why Owners or Operators should invest in Hydrodynamic?

Let's assume that, on a Ro Pax vessel, fuel cost can range between 20% and 45% of thetotal daily costs, formed by capital costs & running costs.

By observation of different design, the scattering of fuel consumption per unit of cargotransported (expressed in ton) and nautical miles is + / - 40%! Whilst much of this is due to

general design options, a part is due to hydrodynamic optimisation.

Let assume that the portion due to hydrodynamic is about +/- 10% compared to the average.The saving in power requirements involve

A) Cost investment for installed propulsion power

B) Maintenance costs

C) Fuel costs

Speed kn

18 20 22 24 26 28 30 32

fuelg/tDWn

m

5

10

15

20

25

30

35

40

Fig. 1: Spread of fuel consumption per unit of Dwt and nautical miles


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All together those costs may range in the region of 25 % of the operation cost of a Ro PaxFerry. For a medium size one, with a LOA of about 180 m, suitable to transportapproximately 2000 lm and 1000 passengers, those costs may be assumed as 35.000 perday; power installation deriving costs shall be in that case about 11.000 per day.

Assumed to be 10% the average hydrodynamic optimisation, this means 1.100 /day, i.e.about 3 % of the total costs. Although this percentage is not huge, it is still attractive beingclose to the current rate of interest. The cost of a good hull form design, relative CFDcalculation and tank testing could be in the region of half year of power installation relatedcosts (A, B, and C above mentioned).

During the last years, NAOS has had care of hydrodynamics integration with the general Ro

Pax Ferry design aspects. Since the cooperation with SVA Potsdam started 4 years ago,encouraging results have been achieved, and those will be shortly presented in this paper.

2. FLEx Bow

2.1 Resistance

This registered trade name has been assigned to a new bow design. This design can besubstantially applied to any vessel, does not require change in dimension neither extra costs.Potential flow calculation has shown very interesting improvements, on different hull formand for different speed. The improvements achieved were in the range of 8 12 %.

Fig. 2: Wave elevation - conventional bulb portside, FLEx Bow design starboard


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Even if Potential flow calculation shows a tendency to overestimate this difference, tanktesting has shown a difference of 8% at the design draft and 4.5% at a partially loaded draft.

There are no problems in applying this design to new buildings and a retrofitting of existingvessel is under evaluation.

2.2 Sea keeping behaviour of the FLEx Bow design

Comparative model tests with both bow forms were carried out in irregular waves related towind forces of 5, 7 and 9 Bft. The wave parameters were the following:

HSig Ship HSig Model T1 Ship T1 Model

Bft 5: 2,15 m 0,085 m 6,52 s 1,297 s

Bft 7: 3,75 m 0,148 m 7,82 s 1,556 sBft 9: 6,10 m 0,241 m 9,01 s 1,793 s

The model was free-sailing and self propelled with powers, necessary for ship speeds ofV1 = 12 knots, V2 = 18 knots and V3 = 24 knots in calm water.

14

15

16

17

18

19

20

21

22

23

24

25

5 6 7 8

Bft [-]

VS0[kn]

M1168Z011 V1 M1168Z012-V1

M1168Z011 V2 M1168Z012-V2

M1168Z011-V3 M1168Z012-V3

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Fig. 3: Comparison of the ship speeds in waves

In Fig. 3 the speed losses of both designs are compared. At the operating ship speed (18 to25 knots), the new design has higher speed in waves. Only with the lowest speed in a windforce of 7 Bft the new design has a lower speed than the initial design.

Fig. 4 shows the heave motion of both designs. In all cases the heave motions of the newbow are smaller than those of the initial bow. At the highest speed and at wind force 9 Bft the

difference of the heave amplitudes is 0.4 meters.


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0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

5 6 7 8 9

Bft [-]

Heave[m]

M1168Z011 V1 M1168Z012-V1

M1168Z011 V2 M1168Z012-V2

M1168Z011-V3 M1168Z012-V3

Fig. 4: Heave motions

The difference of the pitch motions between both designs is not big, but there is a tendencyto lower pitch angles of the new design for all measured points. In Fig. 5 the pitch angles

between 7 and 9 Bft are plotted.

Fig. 5: Pitch motions

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

7 8

Bft [-]

Pitch[]

M1168Z011 V1 M1168Z012-V1

M1168Z011 V2 M1168Z012-V2

M1168Z011-V3 M1168Z012-V3

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Accelerations were measured at three locations, in the centre of gravity, at the forward andaft perpendicular. The accelerations at all three places are less or the same for the new bowdesign as shown in the figures Fig. 6 to Fig. 8.

0

1

2

3

4

5

6

7

5 6 7 8 9

Bft [-]

AccelerationFP[m/s*s]

M1168Z011 V1 M1168Z012-V1

M1168Z011 V2 M1168Z012-V2

M1168Z011-V3 M1168Z012-V3

Fig. 6: Acceleration at FP

0,0

0,5

1,0

1,5

2,0

2,5

3,0

5 6 7 8 9

Bft [-]

Accel

erationCG

[m/s*s]

M1168Z011 V1 M1168Z012-V1

M1168Z011 V2 M1168Z012-V2

M1168Z011-V3 M1168Z012-V3

Fig. 7: Acceleration at CG


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1

2

3

4

5

6

5 6 7 8 9

Bft [-]

AccelerationAP[m

/s*s]

M1168Z011 V1 M1168Z012-V1

M1168Z011 V2 M1168Z012-V2

M1168Z011-V3 M1168Z012-V3

Fig. 8: Acceleration at AP

3. Twin Lifting Skeg

Although is not new, the twin lifting skeg has been successfully introduced on MerseyViking and Lagan Viking class vessels in 1995. The idea was to move as aft as possibleengines and reduction gears, to enlarge the lower hold.

Thus, giving to the skeg also the function to support the ship when docked, the central skeghas been completely eliminated. This form has been tested on 10 already built ships, with aL/B value of 6.6 and a block coefficient of about 0.55.

Fig. 9: Twin Lifting Skeg


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The course keeping ability was always good, as well as the ability of the ship to changecourse in a short time, due to the open form. Propulsion coefficients are something inbetween a single screw and a twin screw propeller ship, while resistance values are low.

In few worlds it can be stated that a good hull efficiency (ETA-H) is combined with a ratheruniform wake field. The advantages obtained on the arrangement of machinery space, theshorter and cheaper shaft line and good manoeuvring characteristics shall be enough toseriously consider such option.

4. IRP (Integrated rudder propeller)

Has been studied by NAOS and presented to a renowned propeller manufacturer well beforeWartsil introduced the Propac Rudder system. Although the system was considered veryinteresting, no further development has been carried out due to different non-technical

reasons. One was the great commercial expectation from the electric podded solution - aftersome years we can see that not many Ro-Ro ships or ferries has adopted electric drivenpods mainly for:

1) initial high cost

2) lack of overall efficiency.

Fig. 10: IRP system in various applications


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The main difference between our IRP system and Propac Rudder is that the propellerbearing is behind and not forwards the propeller, housed by a kind of rudder horn thatincludes a Costa bulb.

This shall encompass the following advantages:

Better wake field because of no brackets forward the propeller

OD for propeller pitch astern elimination of the costly machining to make a long holein the propeller shaft

Hydrodynamic advantages deriving from the Costa Bulb

That combination already existing technologies has never been used on board of ships, andwe are still looking for a partner that wishes to undertake this.

5. Optimisation of appendages

5.1 Stabiliser fins

Especially for ferries and cruise vessels roll damping is an important design point. Usuallythese types of ships are fitted with bilge keels and folding stabiliser fins. Also the housingswith retracted fins produce resistance. This fact is well known and considered in the design.

Fig. 11: Geometry of a stabiliser fin Fig. 12: Pressure distribution

The resistance addition of a retracted stabiliser fin as plotted in Fig. 11 is usually in a rangebetween 1% and 4% of the total resistance. It depends on position and on design and iscaused mainly by the edges. Fig. 12 shows the problematic zone.

Following the stream lines from the trailing edge of the fin housing, you can see that there isan influence on the wake field as shown in Fig. 13 and Fig. 14. Each of the both regions offlow lines, plotted in red and blue colours, produces a hump in the wake field. Additional thewake field pulses in these regions. In Fig. 14, four different time steps are plotted,


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representing the minimum and maximum of both humps. The wake field pulses because ofunsteady conditions at the fin housing. Both humps pulse with a different period.

Fig. 13: Flow at the fin cover Fig. 14: Influence on the wake field

To overcome it, the part of the housing containing the joint of the fin was covered. The coveris concave formed for better flow conditions. With this solution the pressure distribution andthe added resistance could be improved (see Fig. 15 and Fig. 16).

Fig. 15: Optimised geometry Fig. 16: Improved Pressure distribution

In Fig. 17 and Fig. 18 the improvement of the wake field is shown. The higher wake numberswere shifted outside of the propeller disk.


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Fig. 17: Initial wake field Fig. 18: Wake field with optimised housing

Also in extended position the fins can influence the propeller. In the following case the finswere located extremely aft and it was checked how that will influence the propeller inflow. InFig. 19 stream lines coming from the stabiliser fins are shown.

Fig. 19: Flow lines behind a stabiliser fin

In the next figures you can compare the wake fields of the ship with extended fins andwithout fins. The wake of the stabiliser fins is not steady, because of their motions andshould be outside of the propeller disk. In Fig. 20 und Fig. 21 both, wake field with andwithout stabiliser fin are plotted. It can be seen that the fin does not influence the propellerinflow.


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Fig. 20: Wake without fins Fig. 21: Wake field with fin

This example shows that with accurate CFD study of this problem it is possible to find theoptimum position of fin stabiliser system.

5.2 Bilge keels

Whether a ship has stabiliser fins or not, bilge keels should be fitted in any case. Great care

should be taken to ensure that the keels run along streamlines. Its a standard to measurethe bilge keel line in a paint flow test. If the bilge keel follows a streamline its resistance is aminimum. Otherwise vortices occur which may come into the propeller plane. The othereffect is of course an increasing resistance. On the next figures an example is shown.

Fig. 22: Bilge keel is not in a stream line Fig. 23: Stream lines for a new bilge keel

Fig. 22 shows an incorrect aligned bilge keel. Following a streamline from a certain startingpoint forward and back, the correct bilge keel line can be determined. Using CFD it is alsopossible to design twisted bilge keels using three or more streamlines like shown in Fig. 23.That may be useful for deeper bilge keels and high speeds. Fig. 24 shows that the new

designed bilge keel follow well the streamlines. In Fig. 25 the initial (red) and new (green)bilge keel are compared.


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Fig. 24: Flow around the new bilge keel Fig. 25: Initial and new bilge keel compared

5.3 Shaft brackets

The alignment of the shaft brackets into the flow is important for resistance and wake field. Itcan influence the power demand, noise and cavitation. One method to find the correctalignment is the experimental one. The pressure has to be measured at both sides of theprofile. They have to be adjusted as long as the pressure at both sides is equal.

For many projects CFD calculations are carried out before the tests. In that case its only ashort step with a fraction of the costs of the experimental method to align the shaft brackets.

For the alignment of shaft brackets using CFD it is important to take under consideration the

propeller effect. That can be done using an actuator disc. Two calculations are necessary,first a calculation without shaft brackets because the exact alignment is unknown andincorrect aligned shaft brackets would influence the flow. With the second calculation thedetermined angle have to be checked. For this calculation the shaft brackets have to beincluded.

The determination of the angles usually is done in planes orthogonal to the brackets. In Fig.26 the CFD model including the appendages is shown together with the orthogonal planes.The green lines display two planes intersecting the shaft brackets at their half-lengths. Inthese planes the velocity vectors will be plotted.

Fig. 26: CFD model of stern arrangement


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Fig. 27: Determination of the angle Fig. 28: Aligned shaft bracket

In Fig. 27 the velocity vectors of the flow without the shaft brackets are shown in such aplane. The angle between the velocity vectors and the blue line has to be used for thealignment. Then the shaft bracket is well passed by the flow like shown in Fig. 28. Its alsopossible to create twisted shaft brackets. Therefore three or more orthogonal planes have tobe used to determine the certain angles at the different heights.

6. ConclusionWe have tried to demonstrate how Hydrodynamic can be economical advantageous forOwners and Operators. At the same time we are convinced that Hydrodynamic shall alwayssupported by a good general design knowledge to be able to provide real practical solutions.

We are convinced that much is still possible to do in the shipbuilding market, where notalways many effort are devoted to Hydrodynamic efficiency.

Less fuel consumption means less costs and better environment. We shall remember alsothat to produce a bigger engine we also need a greater amount of energy and related costsand pollution!

7. References

[1] Frhlich, M. Comparative Sea Keeping Tests, Ro Pax Ferry, M1168Z011,M1168Z012, SVA report 2976

Improving Fuel Efficiency in Ro-Pax Design , RoRo2004-NAOS-SVA

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