Load Generation Strategy for Propeller Induced

Pressure Fluctuations

Thomas Stoye1), Karsten Werner1), Jan Tellkamp1), Stefan Kruger2) Wilfried Abels2)

1) Flensburger Schiffbau-GesellschaftFlensburg, Germany

2) Technische Universitat Hamburg-HarburgHamburg, Germany

Abstract

Market demands have lead to a significant improve-ment of the vibration and comfort level, especially forcomplex ship types such as RoRo and RoPax ships.The need to optimise the design for efficiency doesnot allow to spend extra material with the singlepurpose to keep the vibration levels at the requiredvalues. The better strategy is to minimize the ex-citing forces as such, where the propeller is the mostimportant source for discomfort, noise and vibration.Whenever a new ship design had vibration problems,the propeller could be identified as the main reason,and changes related to the propeller are a substan-tial cost factor. To minimize the propeller impacton the hull, the wake field has to be optimised onone hand and the design of the wake adapted finalpropeller on the other. The demand for low pressurepulses has lead to propeller designs characterized bylow cavitation volumes, leading to the situation thatin some cases the non- cavitating part becomes dom-inating for blade rate. Furthermore, the designs arecharacterized by the fact that the higher harmonicsdo not necessarily decrease, but some of them cantake comparable values to blade rate. This makesit difficult to decide if a specific propeller design isacceptable or not if the decision is only based onthe judgement of the pressure pulses, which are typ-ically measured in appropriate testing facilities (e.g.HYKAT). Therefore, two major development needsare obvious: Firstly, methods are needed to evalu-ate propeller designs and their impact on the hullstructure before a blade is tested. This requires acombination of numerical methods and correlationtechniques. The propeller induced pressure fluctua-tions have to be calculated with sufficient accuracy,taking into account the effective wake on one handas well as the interaction with the rudder. Theseloads, which have to be generated during the initialdesign stage before the contract is made have then tobe transferred to an FEM-Model of the steel struc-ture which then allows to optimize the steel design to

fullfill contract specification values. Secondly, meth-ods are needed to transfer the measured values fromthe HYKAT to the steel structure model to evaluatewhich of the propeller designs tested is most able tocope with the ship design. The paper describes theessentials of these methods and strategies. Futher-more, the correlation with full scale data recordedfor a large RoRo vessel is discussed.

Keywords

Propeller Induces Pressure Fluctuations, Vibrations,Structural Analysis

Introduction

Todays RoRo- and RoPax market asks for taylor-made, sophisticated designs in all technical areas.The more advanced a design becomes, the more pre-cisely all parameters describing the ship need to beevaluated and should be available as soon as possible.For a shipyard, it is mandatory to quantify these pa-rameters with cost-efficient first principle methodsin the early design stages. These methods have tobe reliable for the given set of parameters that areavailable in that design stage. In this case, the cal-culation results can give reliable input for appendedcalculation methods.

The main source for vibrations and noise on ships areusually main engine and propeller. To quantify thevibration level caused by the propeller in the earlydesign stages, it is neccessary to make use of capa-ble numerical tools in both CFD- and FEM-domain.Model tests are hardly applicable in the very earlydesign phase, as they are too time consuming. How-ever, model tests can serve for the purpose of vali-dating numerical tools and in turn increase their ac-curacy and reliability. Since direct calculation toolshave been in use at FSG for several years, the inter-disciplinary link between the hydro- and structuralcomponents allows the yard to quantify the noise-and vibration level within the first design steps. This

9th Symposium on Practical Design of Ships and Other Floating StructuresLuebeck-Travemuende, Germany© 2004 Schiffbautechnische Gesellschaft e.V.

ensures high quality products with a high efficiencyand guarantees to meet the owners requirements. Inaddition, the technical (and financial) risk can beminimized.

If the information about the amplitude and exten-sion of propeller induced pressure pulses on the hullhas once been determined, a first guess can be madeabout the expected vibrations that will be caused bythe propeller. However, the information about theamplitude and extension of pressure pulses on thehull surface alone can only give a very rough ideaabout the expected comfort level: The resulting vi-brations at specified locations on the ship stronglydepend on the underlying steel structure that willtransport the pressure fluctuations through the ship.To take this circumstance into account, an interfacebetween the calculation output of the Vortex-LatticeMethod for the calculation of the pressure pulses tothe preprocessor for a global FE-Analysis has beenimplemented into FSGs method data base. This en-ables the yard to get an idea of the vibration levelthat is expected already in early design stages and ata time, when there are still enough degrees of free-dom available to take countermeasures and to im-prove the design and comfort level. On the otherhand, the interface makes it possible to achieve reli-able calculation results and vibration amplitudes inlater design steps when more detailed informationsare available on the hydro- as well as on the struc-tural side.

Since the optimization potential of the propeller de-sign in order to reduce pressure pulse fluctuations ismore or less limited and modern propeller designsare already very sophisticated (compared e.g. withpressure pulse fluctuations of the Wageningen B-Series), another optimization potential can be foundin the arrangement of the steel structure: In or-der to achieve low vibrations levels at all locationsof the ship without producing high costs, it is use-ful to have a good idea about the expected excitingforces and their subsequent consequences onto thesteel structure already in early stage ship design. Inthis design stage, vibration minimizations and op-timizations caused by structural modifications aresome decimal powers less expensive than modifica-tions after the sea trial or delivery of the ship.

Numerical methods

The calculation of propeller induced pressure pulsesis carried out by a quasi continuous Vortex-Latticemethod for propellers in unsteady flow that has beendeveloped by HSVA. This method has been imple-mented into FSGs method data base and the pre-processing has been connected to the ships data base.Beside the propeller induced pressure pulses of firstand second order, the total thrust and torque in alldegrees of freedom is another useful output.

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P022: x-thrust BladeP022: x-thrust Prop

Figure 1: Thrust of single blades and whole propellerfor different propeller designs as calculated by theVLM

Since cavitation can be predicted by the Vortex-Lattice method that has been implemented intoFSGs method database, the prediction accuracy ofthe propeller induced pressure pulses is further im-proved. Cavitation can have a non-negligible influ-ence on the pressure fluctuations and with this affectsthe vibrations on the whole ship structure. Cavita-tion can not allways be avoided and is not to beregarded as a problem as long as it is not erosiveand its extension on the propeller blade is small andconstricted.

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Figure 2: Cavitation distribution for different pro-peller designs

Since the rudder is acting in the slipstream of thepropeller and is not regarded as part of the hullform,it has been excluded from this kind of calculations.The rudder is accounted for by its effect on the effec-tive wake, which influences the inflow to the propellerand the pressure pulse generation by the propeller.

Figure 3: Propeller induced pressure pulses on hull-form

The propeller pulses on the hullform can be obtainedfor the first and second order blade rate frequency.The phase angle of these two frequencies is not unim-portant, as can be seen in fig. 8: They can have apositive as well as a negative influence on the result-ing total pressure.

Propeller design

It is not usual for a shipyard to consult a propellerdesigner within the first design steps. The reason forthis is the missing of input parameters like a reliablewake field, a final hullform and with this a preciseidea of the desired thrust on the propeller.

To get a good idea about the achievable propellerperformance concerning vibrations and cavitation,a comparable propeller design from FSGs propellerdatabase is taken and scaled and modified for thespecific purpose. This results in a propeller designthat is already relatively close to the final designbut leaves some safety margin for further modifica-tions. Changes in the boundary conditions of theship (and with this for the working conditions forthe propeller), as they are usual in early design steps,can easily be applied to the propeller design.

Figure 4: preliminary propeller design

A discretization of the propeller can be generated

automatically for the purpose of performing a VLM-calculation.

Grid generation

Hydrodynamic pressure pulses are usually calculatedand displayed as a pressure distribution over a dis-cretization of the hull surface. On behalf of a re-duction of calculation time and -power, the hullformrepresentation is limited to the relevant part of theship, where propeller induced pressure fluctuationsare expected. In exchange, a higher grid resolutioncan be achieved in the interesting areas around thepropeller.

Figure 5: Grid for the pressure pulse calculation de-rived from the hullform describtion

The grid generation is relatively high automated: Ifonce defined, an adequate hullform grid can be de-rived from the modified hullform describtion withoutmanual modifications. The grid definition consistsof a number structured grid patterns, that can beadapted and refined locally to their specific require-ments.

Figure 6: Steel model for FE-grid generation

A reliable FEM-grid can be generated from the steelstructure model which is already available during theinitial design phase.

Correlation techniques

As the numerical determination of propeller inducedpressure pulses is extremely difficult due the low val-ues, there is a need for correlation between measure-ment and calculation. This is more or less the samesituation compared to experiment and full scale mea-surement. The only difference is that model basinshave a wide experience with the correlation betweentheir experiments carried out in a cavitation tun-nel (e.g. HYKAT), whereas no or limited knowledgeexists for the correlation between numerical calcu-lations and mesured data. Of course, some statis-tical prognosis methods exist that are also basedon mesured data (e.g. Holden’s method), but thesemethods require a few input data only and can notreflect changes in propeller design or wake field mod-ifications. Therefore it is important that the loadgeneration on the steel structure is carried out inseveral steps during the design phase:

• Initially, the propeller excited pressure fluctua-tions are derived from numerical methods thatneed certain correlations. These data are thenused as input for the FEM-Model.

• Then, the measured values from the HYKATcan be taken and fed into the FEM- Calcula-tions.

Threfore, a present research project sponsored by theGerman BMBF aims on the development of correla-tion techniques that allow for the better predictionof propeller excited pressure pulses. The error to becompensated by the correlation may be

• a methodical error

• a random error

The correlation is of course linked to a specific cal-culation method, in this case the Vortex-Lattice-Method. Fig. 7 gives an example for such a type ofcorrelation between numerical simulation and mea-sured data.

Figure 7: Example of Correlation between calcula-tion and experiment

CFD-FEM Interfaces

For the purpose of optimizing the steel structure,the boundary conditions in form of the propeller in-duced pressure pulses have to be applied onto theFE-Model. As described above, a grid for global FE-Analysis has a completely different structure than ahullform grid for hydrodynamic calculations. Thisis because both grids serve for a completely differ-ent purpose. A converting algorithm has been devel-oped and implemented into FSGs method databasefor the purpose of converting the hydrodynamic pro-peller pressure pulses to forces acting on nodes of aglobal FE-Grid.

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Figure 8: Propeller induced pressure pulses on apanel of the hydrodynamic grid

This was achieved by converting the amplitude andthe phase of the propeller pressure pulse for eachblade rate into complex forces, acting on the center-point of each hydropanel. These forces are in turnweighted by their distances to the next node in theFE-grid and added together to forces acting onto the

FE-nodes. An integration of the pressure over thediscretized hydropanel surface results in a total forcethat should be equal to the accumulated node forcesfor each phase angle. With this, the accuracy of theconversion algorithm as well as the compatibility andaccuracy of both grids can be checked.

Figure 9: Pressure pulses applied on the FE-Model

The resulting node forces can then be used for furtherinvestigations, parameter studies und optimizations.With this kind of automated preprocessing, a param-eter study can be performed, e.g. the assessment ofdifferent sailing conditions. This affects for exampledifferent loadcases, several points on the combinatorcurve (in case of a cpp) can be investigated, includingdifferent thrust loads as well as different pitch anglesof the propeller. Furthermore, different propeller de-signs can be assessed. With such investigations, thecritical operating point for the propeller can be iden-tified and modifications can be performed in order toimprove the numerical ship.

Calculations for a single screw RoRoship

For a validation of the developed methods, severalfull scale measurements have been carried out on thesea trials of a newbuidling of a single screw Roro-ship. The influence of the propeller induced pressurepulses for the global vibration behaviour can be seenin fig. 10.

(Design condition)

Propeller blade rate, 123 rpm

Propeller blade rate, 116 rpm

Figure 10: Vibration measurements on bridge forspeed up of main engine from 68 to 123 rpm

These measurement results correlate well with thepredicted vibration behaviour from the propeller in-

duced pressure pulses. A calculation result from aforced vibration analysis can be seen in fig. 11.

Figure 11: Results of forced vibration analysis at123rpm

Calculations for a twin screw RoRovessel

For a twin screw ship, the phase angle between thetwo propellers has a strong effect on the global vibra-tion: Depending on the phase of the two propellers,the vibrations can be either increase or nearly com-pletely vanish. However, the idea of synchronizingthe two propellers is not realistic. But for the calcu-lation of noise and vibrations, the worst case shouldbe taken into account in order to achieve reliable re-sults.

Figure 12: Velocity amplitude calculation from pro-peller pulses, propellers acting in worst phase

Figure 13: Velocity amplitude calculation from pro-peller pulses, propellers acting in best phase

Furthermore, a slight difference between the rpm ofthe starboard and the port propeller can result in aglobal beat, resulting in a variation of the vibrationamplitude with a very low frequency. Neither thebest nor the worst case occured when the propellerswhere acting in phase, therefore such an investigationwas determined as neccessary and useful. The worstcase can be seen in fig. 12, while the vibrations arestrongly decreased for another phase angle (fig. 13)between the propellers. This fact has been observedon several ships and cannot be avoided, but the max-imum amplitude of this beat can be decreased byoptimizing the vibrations for the phase angle thatrepresents the largest vibration amplitude.

Further investigations

As the main engine is another important source forvibrations and noise, the interaction of the vibrationsof the main engine and the propeller is of interest.Since FSG has been building series of equal or nearlyequal RoRo-vessels in the past, there have been slightdifferences in the vibration characteristics on the dif-ferent newbuildings of the same design. One expla-nation for this fact is the angle between the propellerand the main engines crank shaft. The shaft has beenmounted to the main engine randomly as it was de-

livered by the manufacturer. For future newbuild-ings, this angle can be optimized with respect to theminimization of vibrations.

Conclusions

Propeller excited pressure pulses may lead to vibra-tion levels that can result in delivery problems forcertain types of ship. To minimize the technical risk,the problem must be dealt with already during theinitial design stage. As numerical methods are avail-able today for both the propeller analysis as well asthe steel strucure response, these tools need to be in-tegrated in the design process in such a way that botthe propeller design as well as the steel structure canbe optimized. These calculations need correlationsbased on measurements, but the paper has demon-strated that the design relevant effects can be treatedwith sufficient accuracy.

Acknoweledements

The authors wish to thank the German BMBF forthe support of the a.m. developments. The resultswere achieved by several BMBF-funded projects re-lated to the integration of First Principle Methodsinto the Design Process.

References

ABELS, W., KRUEGER, S.:(2004): CorrelationStrategy for Propeller Excited Pressure FluctuationsCOMPIT 2004, Madrid

BOHLMANN, B., KRUEGER, S.:(1996): Integra-tion of FE-Analyses in the Shipbuilding Design andConstruction Process The 1st European NISA UsersConf., Manchester

BOHLMANN, B., (1997): Entwurf der Stahlstrukturunter Benutzung neuer DimensionierungsmethodenJSTG Bd. 91

STRECKWALL, H.: (1997): Description of a Vor-tex Lattice Method for Propellers in Steady and NonSteady Flow, HSVA Report No. CFD 18 / 97, Ham-burg