VI International Conference on Computational Methods in Marine Engineering
MARINE 2015
F. Salvatore, R. Broglia and R. Muscari (Eds)

The development of “Ultimate Rudder” for EEDIYOSHIHISA OKADA*, MASAHIRO KAWASAKI*, KENTA KATAYAMA*,AKINORI OKAZAKI*, KENICHI FUKUDA*, MASAFUMI OKAZAKI**

Propeller design department, NAKASHIMA PROPELLER Co.,Ltd.
688-1, Joto-Kitagata, Higashi-ku, Okayama 709-0625, Japane-mail: yoshihisa@nakashima.co.jp, www.nakashima.co.jp

Key words: Ultimate rudder, Rudder bulb, Energy saving device, CFD, EEDI
Abstract. EEDI (Energy Efficiency Design Index) came into effect mandatory in Jan.
2013, and the ship owners definitely required a higher efficiency propulsion system than everbefore. Hence, the shipyards have been conducting an optimization of ESD (Energy SavingDevice) system in self-propulsion test for each project. As the results, the shipyards haveinstalled a rudder bulb as an effective ESD.
The rudder bulb is a popular ESD system from a long time ago. Mewis1) described that therudder bulb was developed by Costa in 1952 and the efficiency improve by the rudder bulbfor a container vessel was 1% on average. Fujii et al.2) developed “MIPB (Mitsui IntegratedPropeller Boss)” as an advanced rudder bulb. The feature of MIPB was a streamlined profile
from propeller cap to rudder. According to their paper, the efficiency improve by installing
MIPB was 2-4%.
Recently, NAKASHIMA PROPELLER Co., Ltd. developed ECO-Cap (economicalpropeller cap)3) as a new ESD with FRP (Fiber Reinforced Plastics). The strength of FRP ishigher than that of NAB (Nickel Aluminium Bronze), therefore ECO-Cap was able to adopt
thin fins on propeller caps for low resistance. Although the material used for the energysaving propeller cap was generally NAB, the research results on FRP showed that FRP could
be used as ESD due to their properties such as lightweight and flexibility.
As explained above, the authors thought that there was a possibility to evolve the rudder
bulb profile using the easily moldable FRP compared with NAB. This paper described about
the development of “Ultimate Rudder” of new design concept by FRP. The authors optimized
the profile of “Ultimate Rudder” by CFD and confirmed the efficiency increase from 4.9 to
5.4% in self-propulsion test.
1INTRODUCTION

The rudder bulb is adopted as the ESD due to the rudder for improving the efficiency. The
mainly effects of hydrodynamics by the rudder bulb were known to the following items.
a) Decrease of contraction flow behind a propellerb) Elimination of a propeller hub vortexc) Homogenize of wake distribution and improvement of the hull efficiency by wake gain
To attain the above items effectively, the bulb mounting position is thought to be as closeto the propeller plane as possible. In the case of MIPB, the bulb position was closer than pastrudder bulb by installation of the divergent propeller cap and the efficiency was improved.

Y.Okada, M.Kawasaki, K.Katayama, A.Okazaki, K.Fukuda and M.Okazaki

Thereafter, some ESD manufactures installed the divergent propeller cap for their own rudderbulb. However, it was difficult to make rounded cap profile like a conventional rudder bulb by
the NAB casting, the divergent propeller cap usually has straight outline. If the propeller cap
has the rounded profile like a rudder bulb, the efficiency will improve due to wake gain.
In this study, FRP material was used to Ultimate Rudder, and the propeller cap, which wasadopted the rounded profile shown in Fig.1 to compensate the drawback of NBA casting. The
Ultimate Rudder with FRP were developed by CFD analysis, furthermore the model test wasconducted for a confirmation of an effect on the efficiency of Ultimate Rudder.
NAB(A)

FRP

Picture of Ultimate rudder
Structure of rounded cap(A)Figure 1 Profile of Ultimate rudder
2 ANALYSIS MODEL BY CFD
NP208BC was bulk carrier that used for CFD analysis of the Ultimate Rudder, and fourblade propeller was installed. Figs.2 & 3 show hull & propeller profile, and Table 1 showshull dimension and propeller particulars.

Figure 2 Hull profile of NP208BC

Figure 3 Propeller profile2

Y.Okada, M.Kawasaki, K.Katayama, A.Okazaki, K.Fukuda and M.Okazaki

A normal rudder, a conventional rudder bulb and two kinds of Ultimate Rudder wereanalyzed by CFD and Fig.4 shows each rudder profile. Two different profiles of UltimateRudder were designed as bulb diameter series. Ultimate Rudder1 (UR1) with small diameterand Ultimate Rudder2 (UR2) with large diameter.

Normal rudder

Conventionalrudder bulb

Ultimate Rudder1

Ultimate Rudder2

Figure 4 Rudder profile for CFD analysis
The RANS analysis was performed by SOFTWARE CRADLE SCRYU/Tetra Ver.10,
which was a commercial CFD code and was based on a finite volume method with
unstructured grids. The Shear Stress Transport k-model is applied to the turbulence modelof present simulations. A full hull body submerged under the design load water line ismodeled. The rotational region is introduced to simulate the propeller rotational condition.
Fig.5 shows the present computational domain and the surface mesh around the hull with apropeller and rudder. Constant velocity and zero pressure are prescribed at the inlet and theoutlet boundary respectively. The numerical mesh is an unstructured grid, and basic cells are
tetrahedral and prismatic cells are applied to near the surface for resolving the boundary layer.
The first layer thickness of the prism layer was set to a non-dimensional wall distance for awall-bounded flow y+ =1. The total number of meshes is about 45 million.
A symmetry condition (double-body model) at a still-water surface is implemented in thepresent analysis. The wave resistance coefficient cannot be calculated and is given by theresults of the resistance test. RANS simulations of the resistance test, self-propulsion test andpropeller open water test are performed to analyze the self-propulsive performance at thethrust identity condition as used in the self-propulsion test. At the self-propulsion point, thetotal resistance of the ship including an additional towing force (ex. Skin friction correction)is balanced by the delivered thrust from the propeller. The required propeller thrust isobtained by interpolating the results of three rotational rates of the propeller. The Froudenumber 0.271 is equivalent to 13.5kt at full scale.

Figure 5 Analysis model of CFD

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Y.Okada, M.Kawasaki, K.Katayama, A.Okazaki, K.Fukuda and M.Okazaki

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ANALYSIS RESULTS BY CFD

Fig.6 shows the comparison of the wake distribution of towed condition. The calculation
results of axial and tangential wake by CFD were almost similar distribution compared withmeasurement results of model test. This model and condition of CFD analysis was goodenough accuracy for the development of the rudder bulb.

Model test

CFD

Model test

Axial wake

CFD

Tangential wake

Figure 6 Wake distribution (model test/CFD)

3.1 Survey of Ultimate rudder profile
The CFD analyses for survey of Ultimate Rudder profile were carried out.
Fig.7 shows the vortex behavior by the iso-surface of Q-function, which means the second
invariants of the velocity gradient tensor. The normal rudder was confirmed the strong vortex
behavior from the rear of the propeller cap to the rudder plane. On the other hand, theconventional rudder bulb, UR1 and UR2 were reduced the vortex behaviors by the rudderbulbs.
The self-propulsion factors of each rudder bulb are shows in Fig.8. These values of eachrudder bulb are divided by those of the normal rudder. In this graph, the direction for increaseof the efficiency was presented to positive value about self-propulsion factors.
1-t, 1-w and ηr of both Ultimate Rudder were better than those of the normal rudder andthe conventional rudder bulb. Especially, 1-t and 1-w of Ultimate Rudder was remarkable.
1-w was improved by the displacement effect of Ultimate Rudder. Wake gain of UR2 wasincreased compared with UR1. This indicated that the rounded cap of large diameter waseffective than that of small diameter.
In comparison with the normal rudder, the improvement of ηh･ηr was 3.5% at UR1 andwas 4.1% at UR2 respectively. In this way, the effect of Ultimate Rudder installed roundedcap was confirmed by CFD analysis. As the results, UR2 was selected as the most effectiverudder bulb for NP208BC.

4Y.Okada, M.Kawasaki, K.Katayama, A.Okazaki, K.Fukuda and M.Okazaki

Normal rudder

Conventional rudder bulb

Ultimate Rudder 1

Ultimate Rudder 2Figure 7 Vortex behavior by the iso-surface of Q-function.

Figure 8 Self-propulsion factor at Fn=0.134
3.2 Comparison of normal rudder and UR2
The detail results of UR2 were compared with the normal rudder.
The turbulent energy of normal rudder and UR2 were visualized in Fig.9. The stream lineof UR2 was smoothly and the turbulent energy of UR2 was smaller than the normal rudder.
Therefore, UR2 was confirmed the elimination of the propeller hub vortex.

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Y.Okada, M.Kawasaki, K.Katayama, A.Okazaki, K.Fukuda and M.Okazaki

Fig.10 shows the pressure distribution of the normal rudder and UR2. The negative
pressure generated at the propeller cap rear in the normal rudder, and this pressure induced thethrust of aft direction. The leading edge of rudder of UR2 was increased negative pressure andthis pressure induced the thrust of fore direction. Therefore, total thrust of UR2 was increased.
Ultimate Rudder 2Normal rudder

Figure 9 Turbulent energy of normal rudder and UR2
Ultimate Rudder 2 from bottom side

Normal rudder from bottom side

Ultimate Rudder 2

Normal rudder

Figure 10 Pressure distribution of normal rudder and UR2

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Y.Okada, M.Kawasaki, K.Katayama, A.Okazaki, K.Fukuda and M.Okazaki

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MODEL TEST RESULTS

Self-propulsion test was carried out for confirmation of propulsion efficiency inSRC(Shipbuilding Research Centre of Japan). The hull dimension and propeller profile inmodel test were used the same as CFD analysis.
The comparison of model test and CFD was shown in Fig.11. Each values of selfpropulsion factor for UR2 were divided by the normal rudder. The improvement of selfpropulsion factor was confirmed by model test results. The ηh･ηr of UR2 at each Fn wereincreased from 4.9 to 5.4%.
In comparison of model test and CFD, the improvement rate of 1-w and ηr in model test
was almost same as CFD analysis. However, the improvement rate of 1-t in model test was
about 1.3-1.7% higher than CFD analysis.

Figure 11 Self-propulsion factors by model test and CFD

5CONCLUSIONS1) The improvement of efficiency by Ultimate rudder installed rounded cap wasconfirmed by CFD analysis. The improvement of ηh･ηr by CFD analysis was 3.5% at
UR1 and was 4.1% at UR2 compared with the normal rudder.

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Y.Okada, M.Kawasaki, K.Katayama, A.Okazaki, K.Fukuda and M.Okazaki2) The following effects of Ultimate Rudder were confirmed by visualization of CFDanalysis.
・Decrease of the turbulent energy due to the propeller hub vortex
・Revision of the pressure at the propeller cap rear and at the leading edge of rudder
3) The improvement of self-propulsion factors was confirmed by model test results. Theηh ･ ηr of UR2 at each Fn were increased from 4.9 to 5.4%. The tendency ofimprovement of self- propulsion factors by Ultimate rudder could be estimated by CFD.

REFERENCES[1] F.Mewis and F.Deichmann, “Power and Cost-Savings for Container Vessels byHydrodynamic Energy Saving Devices”, SHIP EFFICIENCY by STG, Sep.,2013.
[2] A.Fujii, “New Energy Saving System -Mitsui Integrated Propeller Boss-”, ISME

Yokomaha1995, July, 1995.
[3] Y.Okada, “Numerical Analysis of the Propeller with Economical Cap by CFD”

NuTTS2013, Sep.,2013.

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