3Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Testing of Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Calculation Methods for Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Series Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Lifting Line Designed Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Lifting Surface Designed Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Surface Panel Designed Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Hydrodynamic Design of CP Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Design Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Main Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Tank Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Design Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Blade Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Model Test and Full Scale Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Twin Screw 3990 GT Cruise Vessel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Single Screw 6000 DWT Chemical Tanker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Single Screw 16000 DWT Tanker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Single Screw 5100 DWT Chemical Tanker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Twin Screw Supply Vessel AHTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Hydrodynamics of Ship Propellers

4

5Designing propellers for ships hasalways been a challenge due to thecomplexity of all the factors involved.These factors are not only related tothe propeller itself but also to the hulland the propulsion system whichmust work together as integratedsystems in an optimised and reliableway.

Introduction

A century ago, Sir Charles Parsondesigned the worlds first steam turbine powered vessel named Turbinia. The vessel incorporated anumber of innovative design featuresand broke the speed record at thattime. At Turbinias first sea-trials in1894, it showed a disappointing shipsspeed of only 19.5 knots despitetrying different propeller designs onthe single screw vessel. After intensive research Parson reali-sed that the speed problem arosefrom the formation of vapour bubblesaround the propeller blades. Thisobservation was later to be calledcavitation and today it still plays asignificant role in designing propellers.

To study the phenomenon of cavita-tion, Parson built the worlds firstcavitation tunnel in which a number ofsmall model propellers were tested.Based on his experiments the Turbi-nia was redesigned by distributingthe power between three shafts andusing much improved blade shapes.He finally succeeded in setting thespeed record with a staggering 34knots, thereby outperforming eventhe Royal Navys torpedo gun boatswhich could achieve a speed of 30knots - a speed considered to beimpressive at that time.

When faced with the problem ofcavitation, Parson built his own pri-mitive cavitation tunnel and probablywithout knowing it, he contributed tothe understanding of propellers byinventing one of the most powerfultools for analysing of cavitation.

Over the years a number of toolshave been added to the toolbox -most of which are of the analytictype - but the most important one isprobably the development achievedby the appearance of increasinglyfaster computers over the last twodecades.The design tools for propellers haveevolved in two different directions,one being the empirical/testing andthe other the analytical/calculating.Through the last century they havesupported each other well and bothhave contributed to the understan-ding of the propeller and the condi-tions in which it works.

The Turbinia steaming at sea onone of her runs. The vessel wasfinally modified to achieve theimpressive 34 knots.

6Testing of Propellers

Today the testing of not only the pro-peller but also the hull takes place atwell established and recognisedhydrodynamic institutions around theworld.The testing is performed by towingand propelling 6 to 10 m long hullmodels through a 200 to 300 m longmodel tank. In two seperate tests theresistance and the power needed atdifferent ship speeds are measured.In the initial stage of the testing, thepropeller used is selected among thelarge number of existing propellers,which the institute has in its posses-sion. This propeller is usually desig-nated a stock propeller. Later theactual propeller designed for the vessel can be manufactured in modelscale and fitted on the ship model toverify its performance efficiency.

An important part of the testing, seenfrom a propeller designers point ofview, is the measurement of the wakefield, which will give the inflow veloci-ties to the propeller at any radial andcircumferencial position. The wake

field is obtained by substituting thepropeller with a pitot probe, rotatedaround the propeller shaft. The probemeasures the pressure which canlater be converted into the threevelocity components (axial, tangentialand radial).

A hull model being towed in themodel tank for determination of resistance and power.

7When observing a propeller bladerotating behind a ships hull, one willdiscover that the inflow velocitiesand pressure will change dependingon the blades angular position.Especially for single screw full bodyships the twelve oclock position cancause the pressure to drop under thesaturation pressure eventually leadingto cavitation.

One aspect that cannot be tested inthe long model tank is the cavitationbehaviour of the propeller. This is dueto difference in pressures betweenmodel and full scale and the subse-quent cavitation test is consequentlyperformed in a cavitation tunnel,where the pressure in model scalecan be adjusted to match the fullscale one.The propeller model is placed in thetunnel at the upper measurementsection and driven by its own motor.The water in the closed loop tunnel iscirculated by an impeller situated inthe lower part of the tunnel.

Only at very few institutions is it pos-sible to place the whole ship modelin the cavitation tunnel and as a consequence the correct wake fieldneeds to be modelled by placing adummy model upstream of the pro-peller. It requires considerable skillfrom the personnel at the institutionsto achieve the same wake field in thecavitation tunnel as was measured inthe model tank. Therefore carefulmeasurements must be carried out toverify that this is the case.

Today, cavitation tests are not carriedout for the same reasons as in theday of Parson, who discovered apronounced drop in propeller effici-ency. Parsons initial propeller wasprobably what today would becharacterized as a super cavitatingpropeller, fully surrounded by air bubbles, which reduces the efficiencydrastically and creates loud noiseand vibrations in the aft body of theship.

Propellers for merchant ships oftoday are much more limited in theirextent of cavitation on the bladesresulting in only a marginal drop inefficiency, but the noise and vibrationproblem still remains.

23

54

1

4 m

10.5 m

7 m

Cavitation tunnel with:

1. Test section

2. Thrust and torque

dynamometer

3. Propeller motor

4. Axial flow impeller

5. Impeller motor

8Cavitation will be present on mostpropellers of todays merchant ves-sels especially when operating atmaximum power. Compared to theearlier hull designs the ones of todayare much more full bodied (high blockcoefficient ) which unfortunately willcause that the wake field experiencedby the propeller blade through onerotation gets much more uneven.This makes it almost impossible todesign a propeller for modern fullbody ships without any cavitation,unless a pronounced drop in efficiencyis accepted.

The true nature of cavitation has stillnot been fully understood, althoughmuch more insight has been gainedover the last decades.Cavitation is associated with thegeneration of air bubbles which areformed at the propeller blades wherethe pressure gets below the satura-tion pressure of the water. The for-mation of bubbles is in itself not ofmuch concern, but when the bubblessooner or later enter into a high pres-

sure region they will implode. Thiswill not only result in high levels ofnoise and vibrations in the aft part ofthe vessel but the cavitation can alsobe of an erosive nature if the implo-sion takes place at the propeller orrudder surface.The objectives of testing propellers ina cavitation tunnel is twofold. On onehand, the test should reveal that noharmful cavitation erosion exists onthe propeller blade and on the other,that the pressure impulses measuredat prescribed locations on the hullsurface do not exceed a limit beingacceptable to the type of vessel tested. The pressure impulse level is recordedby pressure transducers mountedflush with the hull surface above thepropeller and the value measured inkPa (kilo Pascal) is used as a criterion for the performance level.

Calculation Methods for Propellers

Since the invention of the screw propeller there has been a strongdesire to be able to analyse and inparticular to design propeller bladesby applying hydrodynamic theories.

Series PropellersThe first serious attempt was madeby designing a series of model pro-pellers with different blade numbers,blade area ratios, pitch distributionsetc and subsequently testing all different propellers in an open watercondition - ie uniform inflow withoutthe influence from the hull.One significant example of seriespropellers is the Wageningen propellerseries [1] which constitute a compre-hensive set of data for the propellerdesigner. To facilitate the daily use,charts have been worked out fromwhich a propeller can be selectedbased on the design and the ope-rating data it is supposed to match.Today, propeller series such asWageningen are still used in the project stage to optimise the globalparameters such as diameter, bladearea ratio, number of blades etc withrespect to efficiency.

One drawback of the series propelleris its cavitation performance whenoperating in the wake field behindthe actual ship. Once a propeller hasbeen selected the pitch and camberdistribution is fixed - parameters whichare of the utmost importance for itscavitation performance.Many vibration problems on vessels,which were designed before intro-duction of the computer designedwake adapted propellers, can beascribed to series propellers beingapplied without sufficient insight intotheir cavitation behaviour when operating in the uneven wake fieldbehind the ship in question.

A cavitating propeller.

9Lifting Line DesignedPropellers

In 1952, H W Lerbs [2] introduced anew method for calculating propel-lers named lifting line. Lerbs propo-sed to substitute the propeller bladewith a so-called lifting line along which the radial distribution of lift iscalculated under the influence of anumber of trailing vortices.

This was a new step forward in whichthe radial distribution of circulation(resembling lift or thrust) could bespecified in order to obtain the opti-mum efficiency. An important diffe-rence from early days was the possi-bility to include the influence of thewake field. But the most consequen-tial difference was the option to selectprofile section at each radius whichcould not only result in the optimumcirculation/lift but which could alsobe selected with a combination ofcamber and pitch to achieve opti-mum cavitation performance.

With the publication of the NACA air-foil sections of the 16. and 66. seriesat almost the same time, a powerfultool was emerging, which for the firsttime could give the propeller designera possibility to design a propellerwith respect to cavitation aspects.

MAN B&W modified NACA airfoilsections of the 16. series.

Lifting Line Trailing Vortex

Principle of the lifting line calculation model.

10

Evaluation of the cavitation was facilitated by the work of T Brockett[3] who, for the above NACA airfoilsections, calculated a series of chartsfrom which the onset of cavitationcould be determined.

Only the onset of cavitation could becalculated - not the degree of thechord-wise extension over the bladesurface. As already mentioned a certain level of cavitation mustregrettably be accepted on mostmerchant vessels in order not tocompromise the efficiency too much. To overcome this obstacle, a methodwas devised by MAN B&W [4] tocalculate the chord-wise distributionof lift by using a conformal mappingtechnique. As a result the cavitationcould be a truly integrated part of thedesign process for the first time.

One disadvantage of the lifting linemodel lies in the nature of the method.The substitution of the blade with alifting line implies that the chord-wiseextension of the lift is not directlyincluded in the solution.After the appearance of the liftingsurface method, a set of correctionfactors has been published [5,6] which can be incorporated into thelifting line model to improve the calculation accuracy.

Cavitation extension on a propeller blade.

Sheet cavitation inception curves.

11

Lifting Surface Designed Propellers

An improvement of the lifting linemodel was developed in the 1980s(Greeley and Kerwin [7]) in an effortto overcome the shortcomings of theinadequate treatment of chord-wiselift of the lifting line method as wellas to include the influence of skewand rake.

The method distinguishes itself fromthe lifting line, in the way it modelsthe propeller blade. The surface ofthe blade is subdivided into a numberof elements describing the surface,and on which a boundary conditionof no through-flow is prescribed. Tomodel the strength of the circulation/lift a distribution of vortices arelocated on the mean surface, and toinclude the effect of induced drag, a

number of free trailing vortices areshed from each element.The method proved valuable in con-tribution to the understanding ofskew and in particular its influenceon pitch, camber and thickness whichthe lifting line had failed to do,without the inclusion of lifting surfacecorrection factors.

As with the lifting line method, the lifting surface method is sensitive asto how the trailing wake is modelled.This is especially important for heavily loaded propellers.

Trailing Vortex

Principle of the lifting surface calculation model.

12

Surface Panel Designed PropellersAt the beginning of the 1990s, MAN B&W joined for-ces with two other companies (degaard & Danne-skiold Samse and the Danish Maritime Institute) todevelop a new computer program for calculating ofpropeller performance and to adopt the latest publis-hed technique called the Surface Panel method. Thecomprehensive project, which was financially sup-ported by the Danish Ministry of Industry took 4years and 13000 man-hours to complete. The pro-gram and the results were first published at a RINAconference in 1995 [8].

The method is unique in the sense that it does notonly include the propeller blades but also other ele-ments such as propeller hub and nozzle. Even thehull surface can be a part of the solution. Needlessto say the calculation requires a fast computer workstation especially equipped for this purpose.

The program is mainly developed for analysing ofpropellers and not as a daily design tool for new pro-peller blades. But the program has already given adeeper insight into a number of new areas such ashub and blade interference and novel propeller bladedesigns such as tip-fins.

Calculation of cavitation and its extension is a part ofthe program. The inclusion of the hull surface abovethe propeller enables the program to determine theforces in the aft ship originating from the cavitatingpropeller. The results can form the basis for a subse-quent Finite Element calculation of the complete hullto disclose if any undesirable resonance or vibrationexist.

The surface panel method for:Panellised propeller hub and blades (upper)

Panellised trailing wake (middle)Chordwise velocity distribution (lower)

13

HydrodynamicDesign of CP PropellersIn the past it was said that designingof propellers is partly a science, partlyan art. This statement is still validtoday although the tools describedpreviously provide a valuable help.But one has to keep in mind that notonly does the propeller form part ofthe propulsion system, it also inter-acts with the hull.

Design ConditionsPrior to entering into the actualdesign phase, knowledge should beacquired of the operating conditionsof the vessel as well as of the propul-sion engine.The first step is to agree with thecustomer on the conditions for whichthe propeller should be optimised.The following cases outline a numberof possibilities:

The propeller is to be optimisedfor 85% engine load with a 15% seamargin at a draft of 8.5m. Propellerrevolutions will be determined accor-ding to the combinator curve.

In this case the ships speed will bedetermined during the design phaseand based on tank test resultsincluding the 15% sea margin, or ifnot available from a ships speedprognosis calculated by MAN B&W.The revolutions will be derived fromthe actual combinator curve.

The propeller is to be optimisedfor a ships speed of 14 knots whileoperating at a draft of 8.5m andincluding a sea margin of 15%. Pro-peller revolutions will be determinedaccording to the combinator curve.This case is identical to the aboveexcept that the engine load will becalculated, instead of the ships speed.

The propeller is to be optimised

for 90% engine load including a ser-vice load on the shaft generator of450 kW with a 15% sea margin at adraft of 8.5m. The fixed propellerrevolutions to be determined fromthe synchronous speed (correspond-ing to 50 or 60 Hz) of the step-upgear for the shaft generator.

As in the first case the ships speedwill be determined during the designphase, but since the vessel is equip-ped with a shaft generator, the poweravailable for propulsion will be redu-ced with the shaft generated power.It should be noted that it is the antici-pated service load of the shaft generator, which is used and not therated power of same.

Due to the requirement of keeping aconstant frequency, a constant shaftand propeller speed must be obser-ved and manoeuvring is accomplis-hed solely by changing the pitch -unless a frequency converter isforeseen. The propeller revolutionsare determined from the requirementof keeping the said frequency of 50or 60 Hz and the gear ratio in thestep-up gear for the shaft generator.This revolution is usually 2-5% lowerthan the one corresponding to theMCR revolution of the main engine.

The three different examples of howto optimise a propeller and its way ofoperation are illustrated in the Pro-peller operating diagram below.

1

2

3

Propeller operating diagram showing three different design condition examples as the basis for propeller optimisation.

14

It is of vital importance that the opti-misation conditions are fully under-stood and agreed upon with thecustomer before starting the designprocess.

Besides the optimisation conditionsthe propeller must be designed anddimensioned for sufficient strength atthe maximum power (MCR) conditionaccording to the rules & regulationsof the actual classification society.Other conditions such as trial (lowerdraft and no sea margin) and offdesign ( ie prolonged operation atlow pitch settings at max revolutions)may be included.

Main ParametersAn important part of the optimisationis to determine the main parameters- propeller diameter and rate of revolution - to match each otherbased on the optimisation conditionagreed upon.At first this is done in a project stageas a part of the MAN B&W projectservice and thus forms the basis fora quotation. After the order has beenplaced, it often pays off to make afine tuning of the final propeller diameter if the revolution is fixed (inthe case of direct driven low-speedtwo-stroke plants) or the gear ratio(in the case of medium-speed gearedfour-stroke plants).This optimisation should be based onthe result from the model tests andincludes an evaluation of the cavitationinduced noise and vibration to the hull.

Measured wake field in model scale.

Full scale wake field calculated from model wake field.

15

Tank Test ResultsThe test performed by one of thetank test institutions forms an impor-tant basis for the design of the pro-peller. It is commonly believed thatthe propeller designer only needs thewake field measurements to concludethe design, but to ensure that an opti-mum design is reached, a completeset of reports should be forwarded.At MAN B&W, the reports are used toanalyse the wake field to compensatefor the difference in propeller diameterand skew distribution as well as scaling from model to full scale.In the resistance part of the reports,the resistance is modified in order toaccount for the sea margin, and fromthe self propulsion part of thereports the propulsion factors (wakefraction, thrust deduction and relativerotative efficiency) are used to arriveat the correct thrust for a given shipsspeed.

Design ObjectivesOnce the optimisation condition andtank test results have been analysed,the actual design of the propeller blade can proceed. To achieve the optimum solution, adesign should evolve that has thehighest possible efficiency and at thesame time maintains low noise andvibration characteristics.

Propellers are traditionally judged bytheir open water efficiency withoutpaying attention as to how they perform behind a hull. Restricting oneself to select a propel-ler from the open water efficiencyalone could lead to false conclusions.

To fulfil the design objective of hav-ing the highest possible efficiency, itis necessary to introduce the con-cept of Total Propulsion Efficiency(TPE) which constitutes the followingpart efficiencies:

Open water efficiency (h o)The efficiency of the propeller whenoperating in open water without theobstructing flow from the hull.

Hull efficiency (h h)A fictitious efficiency - can exceed 1- and is a measure of how well thehull and propeller perform in com-bination.

Relative rotative efficiency (h r)A fictitious change in efficiency - canexceed 1 - showing how well thepropeller is operating in the behindthe hull condition compared to theopen water condition.

Mechanical efficiency (h m)The efficiency of converting mechani-cal power measured at the flywheelof the main engine to the propeller.

The Total Propulsion Efficiency is theproduct of the above part efficiencies and is defined as

TPE= h o x h h x h r x h m

Compared to the more well-knownQPC (Quasi Propulsive Coefficient)which does not contain the mechani-cal efficiency, The Total PropulsionEfficiency (TPE) forms a more consi-stent and stringent way of evaluatingdifferent propulsion systems.The advantage of using the TPEcompared to the QPC is its directcorrelation with the power and fuelconsumption needed for propulsion.

The low efficiency of diesel electric propulsion plants is remarkablebut the figure is composed of the following efficiency elements:

GenSet efficiency 96 %

Transformer and converter 98 %

Electric motor for propulsion 98 %

Reduction gear from electric motor to propeller shaft 98 %

Shaft line and losses in bearings 99 %

At MAN B&W the following mechanical efficiency figures are used:

Two-stroke propulsion plants without reduction gearbox 99 %

Four-stroke propulsion plants with reduction gearbox 97 %

Geared diesel electric propulsion plants 89.5 %

16

Blade DesignWith all preconditions set the bladedesign can proceed. The main objectives within the con-straint mentioned earlier is to obtainas high a TPE as possible and tosuppress the cavitation to an accep-table level. However, for a fixed pro-peller diameter the only part-efficien-cies being influenced by the bladedesign are the open water efficiency

and the relative rotative efficiency.It is a common belief among propel-ler designers that the two designobjectives are in contradiction toeach other and consequently mustbe balanced to get a compromiseddesign. But today some design features areavailable which can be applied toreduce the cavitation without sacrifi-cing the efficiency.

To build up a propeller blade, thecomplicated 3-dimensional form isusually reduced into 2-dimensionalelements which are then adjustedduring the design process.

Blade area The blade area should be kept as small as possible in order to reduce the frictionlosses when turning in the water, but to suppress the cavitation extension acertain area is needed. A measure of the blade area is the so-called blade arearatio (Ae/Ao) which is the ratio of all the blades compared to the area of thecircle circumscribed by the propeller diameter.

Blade shapeThe blade shape can be varied to even out the cavitation along radius and inthe case of a nozzle propeller, it is advantageous to have wide-chord length atthe tip (Kaplan shape).

Different blade area ratios:Left: Ae/Ao=0.40Center: Ae/Ao=0.55Right: Ae/Ao=0.70

Different blade shapes. Max chord location varied fromcenter of blade to tip of blade.

17

Skew angleA powerful tool to suppress propellerinduced noise and vibration is theapplication of skew. For modern CPpropellers, the skew distribution is ofthe balanced type, which means thatthe blade chords at the inner radii areskewed (moved) forward, while at theouter radii the chords are skewed aft.By applying this type of skew it ispossible to control the forces (spindletorque) needed for pitch settings. Inmost cases the blades will be balan-ced in such a way that the forces inthe design pitch setting will be zero.

Skew has the advantage of reducingthe pressure impulses emitted frompropeller to the hull surface to asmuch as one third of an unskeweddesign without sacrificing the effici-ency, which will remain unchanged.

RakeThe noise and vibration level in theaft ship depends on the distancebetween the propeller tip and hullsurface - in particular exactly aboveand in front of the propeller.

A way of increasing the distance is torake (incline) the blade towards aft. As with skew the efficiency remainsunchanged. However, the blade isexposed to higher stresses originatingfrom an increase in the centrifugal for-ces which must be counteracted byan increase in blade thickness.

High skew.

Aft raked propeller.

Non skew.

Low skew.

Medium skew.

18

Profile sectionFor each radius, the blade is built-upof 2-dimensional airfoil sections. Theairfoil used in propellers is mostlyfrom the NACA family series whichhave proven successful in havingboth low drag and good cavitationcharacteristics.A NACA profile is characterised by abasic thickness and a camber distribution which can be changedindependently of each other. Thisfacilitates the design of profiles withspecific properties at each radius.

In addition to the profiles own 2-dimensional thickness and camberdistribution, their distribution alongthe radius is also varied to achievean optimum thrust distribution.

Radial camber distribution.

Typical profile sections at different propeller radii.

Pitch distributionAn important parameter in propellerdesign is the distribution of pitch asa function of radius which need notbe constant as is the case with ascrew or bolt thread as the propelleris moving its way through water.Using pitch reduction at both the huband the tip can be advantageous inreducing the cavitation extension, although care must be taken not toapply excessive reductions which willresult in a decrease in efficiency.

The optimum propeller combines allthe parameters mentioned into adesign, which apart from the twomain hydrodynamic objectives, alsowill fulfil the requirements of sufficientmechanical strength, low pitch chan-ging forces, sufficient space betweenblades to allow pitch from full aheadto full astern, etc.

All the parameters mentioneddepend on each other and no theoryexists on how to combine them to anoptimum design. Consequently, it isimportant that a software package isat hand to allow calculation of theinfluence of all factors. A highly educated staff with many years ofexperience combined with in-housedeveloped software makes MANB&W enjoy a leading position in pro-peller design and manufacturing formerchant ships.

19

Radial thickness distribution.

Radial pitch distribution.

20

Model Test andFull Scale Results

Through the years MAN B&W hascarried out a large number of testswith propellers of their own design atmost of the independent tank institu-tions around the world.Apart from verifying the calculations,the tests play an important role inassuring the ship owner, consultantand shipyard that the propellerperformance is acceptable. As thetests usually are carried out beforethe ship is built, all partners in theproject have the possibility of modi-fying not only the propeller designbut also the hull form before theactual manufacturing is started.

In contradiction to full scale tests,model tests have the advantage thatthey can be conducted under thesame conditions, thereby excludingthe influence of sea, wind and hullsurface condition. The results obtainedfor different designs or modificationsto either the propeller or hull are thusreadily comparable. This is especiallytrue with regard to power and effici-ency evaluation.The hull vibrations originating frompropeller induced pressure impulsescan be verified from both model testsand full scale measurements.

CasesTo exemplify the level of performanceincrease that can be expected from awell-designed propeller, the followingcases are given.All figures are as measured by thehydrodynamic institution mentioned.

Twin Screw 3990 GTCruise VesselThe vessel has a MAN B&W four-stroke medium-speed propulsionplant developing 1960 kW per shaftfor a ships speed of 17.8 knots.Tests carried out by Marin Wagenin-gen, The Netherlands.

Main propeller parameters

Propeller diameter mm 2850Blade area ratio - 0.55Skew angle deg 45

Measured performance

Propeller open water efficiency % 70.2Pressure impulses kPa 0.9

The efficiency (TPE) turned out to be2.8% higher than for the stock pro-peller selected by the tank institution.The pressure impulse level was evaluated as very low even based onthe high demands required by a cruise ship. In reference [9] the performanceobtained during testing and sea trialis presented by the consultant.

Single Screw 6000 DWTChemical TankerThe vessel is powered by a MAN B&Wlow-speed two-stroke propulsion plantdeveloping 3600 kW for a ships speed of 14.0 knots. Test carried out by Marintek, Norway.

Main propeller parameters

Propeller diameter mm 4100Blade area ratio - 0.52Skew angle deg 35

Measured performance

Propeller open water efficiency % 59.8Pressure impulses kPa 1.4

The efficiency (TPE) turned out to be6.7% higher than for the stock pro-peller selected by the tank institution.The high gain in efficiency comparedto the stock propeller was obtainedthrough a slight decrease of the propeller diameter. Even though intheory the open water efficiencywould suffer, this was more thancounteracted by a substantial increase in hull and rotative efficiency- indicating that the propeller is welladapted to the hull.The pressure impulse levels werevery low for this type of ship and can partly be ascribed to a large propeller/hull clearance.

Model test of a high skew propellerfor a twin screw 3990 GT cruisevessel.

Model test of a high skew propellerfor a single screw 6000 DWT chemi-cal tanker.

21

Single Screw 16000DWT TankerThe vessel is powered by a MANB&W four-stroke medium-speed propulsion plant developing 4800 kWfor a ships speed of 15.3 knots. Tests carried out by Marintek, Norway.

Main propeller parameters

Propeller diameter mm 5400Blade area ratio 0.48Skew angle deg 45

Measured performance

Propeller open water efficiency % 64.4Pressure impulses kPa 1.6

The efficiency (TPE) turned out to be3.4 % higher than for the stock pro-peller selected by the tank institution.The gain in efficiency compared tothe stock propeller was obtained bya pronounced increase in the openwater efficiency.The pressure impulse level is consi-dered low for this type of ship and ispartly due to a large propeller/hullclearance.

Single Screw 5100 DWTChemical TankerThe vessel is powered by a MAN B&Wlow-speed two-stroke propulsion plantdeveloping 4200 kW for a ship speedof 14.2 knots. Test carried out by the Danish Maritime Institute, Denmark.

Main propeller parameters

Propeller diameter mm 4050Blade area ratio - 0.52Skew angle deg 45

Measured performance

Propeller open waterefficiency % 60.9Pressure impulses kPa 0.7

The efficiency (TPE) turned out to be4.3% higher than for the stock pro-peller selected by the tank institution.The gain in efficiency compared tothe stock propeller was mainly obtained by an increase in the openwater efficiency.The pressure impulses were very lowfor this type of vessel.

Model test of a high skew propellerfor a single screw 16000 DWT tan-ker.

Model test of a high skew propellerfor a single screw 5100 DWT chemi-cal tanker.

22

Twin Screw Supply Vessel AHTSThe vessel is powered by a four-strokemedium-speed propulsion plantdeveloping 2400 kW per shaft for aships speed of 14.5 knots. Test carried out by the Danish Maritime Institute, Denmark.

Main propeller parameters

Propeller diameter mm 3150in nozzle

Blade area ratio - 0.55Skew angle deg 5

Measured performance

Propeller open water efficiency % 59.6Bollard pull ton 87.0

The main design objective for at supply vessel is a high bollard pullwhile still maintaining a satisfactoryfree sailing efficiency. In this particularcase the bollard pull turned out to be3.6% higher than for the stock propeller.

Model test of a ducted propeller fora twin screw AHTS vessel.

The AHTS model propeller.

Conclusion

The propeller is an important part ofthe propulsion plant. The propeller must be carefullydesigned in conjunction with eachspecific vessel in order to obtain notonly a high efficiency but also a highlevel of comfort.It has been demonstrated that a sub-stantial increase in efficiency can beattained by a careful design of thepropeller without sacrificing the noiseand vibration level or other operationalparameters.Independent of the type of ship, ahigher propeller efficiency can betranslated into a proportional decreasein fuel consumption which over thelifetime of the ship can accumulate asubstantial saving.The knowledge of complete propul-sion plant technology possessed byMAN B&W ensures the customer thatthe propeller is designed in an opti-mum manner with due regard to allparts of the propulsion plant, shipshull and operating profile.

23

References

[1] Kuiper, G 1992. The WageningenPropeller Series. Marin publication92-001.

[2] Lerbs, H W 1952. Moderately loa-ded propellers with a finite number ofblades and an abritrary distribution ofcirculation. SNAME Transaction 60:73-117.

[3] Brockett, T 1966. Minimum pres-sure envelopes for modified NACA-66 sections with NACA a=0.8 camberand Buships type I and type II sections. David Taylor Model Basinreport 1780.

[4] Nielsen, J R, RELIFT programdocumentation. MAN B&W internaldocument, 1992.

[5] Morgan, W B, Silovic, V, Denny, S.B. 1968. Propeller lifting-surface corrections. SNAME Transaction 76:307-347.

[6] Cumming, R A, Morgan, W B,Boswell, R J 1972. Highly skewedpropellers. SNAME Transaction vol. 80.

[7] Greeley, D S, Kerwin, J E 1982.Numerical methods for propellerdesign and analysis in steady flow.SNAME Transaction 90: 415-453.

[8] Rasmussen, U M, Weis-Fogh, K.1995. Numerical analysis of marinepropellers for noise & vibration control purposes. RINA internationalconference on noise & vibration inthe marine environment.

[9] Kanerva, M 1991. Luxury yachtships of Finish design. Cruise Business 1991: 48-49