replacing fixed pitch propeller - more possibilities for improvements

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Significant propulsive efficiency

improvements can be achieved by

modifying the propulsion system of a

vessel. Here we discuss the impact of

changes to the propeller’s diameter,

number of blades, rate of revolutions anddesign point. We also consider the

application of a nozzle, especially the HR

nozzle, and other energy saving devices to

the ship’s hull. The proposed modifications

are definitely worth investigating when

considering propeller replacement.

A recent article in Marine News [1]discussing the replacement of fixed pitchpropellers showed that it is possiblenowadays to design propellers with higherefficiency and consequently lower fuel

consumption without increasing thepressure pulse levels.

Significant efficiency improvements canbe achieved even when the maindimensions remain unaltered. It was

assumed that other boundary conditions,like rate of revolutions, blade number anddiameter, do not change and that nothing isadded to the propulsion system. Taking allthis into account, the following four

alternatives to increase efficiency werediscussed:n Reduce the blade arean Change the blade contourn Modify the radial pitch distribution to

optimize the loading distributionn Apply the Lips tip-rake concept.

The article also discussed the necessity toreplace the propeller in case of a change inthe mission profile of the vessel. Forexample a fixed pitch propeller on a ferry originally designed to operate on two

engines cannot be used on one of the twoengines alone.

In this article we extend the possibilitiesfor increasing the efficiency of a new propeller while still maintaining the

pressure pulse levels, i.e. noise levels.Together with the options discussed in theprevious article, this can lead to a furtherreduction in fuel consumption.

In cases where the above options cannot

be applied, it may still be possible for otherchanges to yield improvements. For examplechanging the propeller diameter, possibly combined with adapting the rate of revolutions, can lead to significantimprovements in the performance of thepropeller. As most options discussed in thisarticle have other consequences than just forthe propeller, these will be examined as well.

When designing a replacement propellerit is worth investigating the completepropulsion characteristics. The followingquestions should be considered:

n Is the propeller diameter optimum?n Is the number of blades the right choice

or is it possible to improve efficiency orreduce vibrations by altering the numberof blades?

34 - Wärtsilä 1-2005

Replacing fixed pitch propellers– more possibilities for improvements

The Ship Power Supplier

by Marcel van Haaren, Fixed Pitch Propellers, Wärtsilä Propulsion Netherlands BV

Fig. 1 - Low-speed diesel-mechanical propulsion system, Sulzer 6RT-flex68T.

Fig. 2 - Low-speed diesel-mechanical propulsion system including PTO/PTI, Sulzer 9RT-flex84T-D.



n Is the design point of the propelleroptimum with respect to the enginelayout? Perhaps the choice of anotherpower-rpm ratio will make it possible toreduce fuel consumption.

n Can the application of a nozzle or otherenergy-saving devices bringimprovements?

Improving the propulsion system

There are several factors to be considered when selecting the propulsion system of anewbuild vessel. A gearbox is rare in the case

of large fixed pitch propeller applications, sothe propulsion system is mainly thecombination of the engine and propeller.

Regarding the propeller, certain choicesare made for the diameter and number of blades. This is significantly influenced by the load range limits of the engine and thechosen design point. Of course thepropeller diameter is influenced by theavailable space in the stern frame.Moreover, the natural frequency of thevessel’s structure and/or number of cylinders can determine the choice of the

blade number. Cylinder numbers that aremultiples of the blade number might causeengine vibration problems.

The propeller diameter and bladenumber were most likely optimized duringthe design of the original propeller.However, it is worth investigating this againas some parameters might have beenchanged in the meantime. For examplereducing the design speed will require alower design power, leading to anotheroptimum diameter and/or number of blades. Wärtsilä often meets cases where for

some unclear reason these parameters arenot optimal.

In the following paragraphs we describeoptimization examples for diameter,number of blades and propeller rpm. Since

the thrust determines the shipspeed, thecalculations were performed for constantthrust. The optimization examples are worked out on the basis of the same case.The subject vessel is a bulk carrier with thefollowing particulars:n Service speed: 14.5 knotsn Engine power: 7061 kW n Rate of revolutions:112 rpmn Diameter: 5000 mmn Blade number: 4In the examples propulsive efficiency is theproduct of the propeller open water

efficiency times the hull efficiency.Therefore, the analysis includes not just thepropeller on its own but also how thevessel’s hull and propeller act on each other.In fact the lowest fuel consumption isachieved by the highest propulsiveefficiency.

Diameter variation,

other factors unchanged

Table 1 shows the efficiency effect of theabove case when the diameter is optimized.

As the table indicates, increasing the

diameter to an optimum 5800 mm achievesa propulsive efficiency improvement of almost 9%, which translates directly into a9% reduction in fuel consumption.

Of course, it is possible that the largerdiameter would not fit in the present sternframe or that the propeller intersects thebaseline of the vessel. We must furtherremember that a smaller tip-hull clearanceleads to higher pressure pulse levels.Nonetheless, when the diameter isincreased by 400 mm, for instance, theefficiency improvement is still 6.6%.

In this case it was assumed that the bladearea ratio is constant, leading to a lowerpower density for the higher diameter. When the blade area ratio is adapted insuch a way that the power density is kept

constant, the propulsive efficiency gain, andconsequently reduction in fuelconsumption, is increased still further.

Many investigations have been carried outon large-diameter, slow-running propellers,see for example [2], [3], [4] and [5].

Varying the number of blades,

other factors unchanged

Table 2 gives the results of performing theoptimization process for the number of blades in the same example.

Table 2 lets us conclude that a blade

number of 4 is not optimum with regard topropulsive efficiency. By applying a higherblade number the efficiency gain is a few percent. The fact that the number of bladeshas to be increased for a more efficientpropeller shows that the original diameterof 5000 mm was too small, although otherreasons like natural frequencies or numberof cylinders might have determined theexisting blade number.

Varying propeller rpm, other factors

unchanged

It might be difficult to change thepropeller rpm at the design point for anexisting installation. Assuming the rpmcould be changed, the propeller rpm wasoptimized for the same configuration. Theresults are presented in Figure 3, whichshows that by increasing the rpm to about140 the propulsive efficiency improves by almost 5%.

As we noted above, the rpm of 112 inthis case might have been selected for otherreasons, like the application of a specificengine type. In this case, however, the

increase in rpm immediately results in alower fuel consumption. Naturally,reducing fuel consumption in this manneris not valid for every situation; there arecases where a decrease in rpm yields lower

1-2005 Marine News - 35

Relative propulsive efficiency versus propeller rpm

94

96

98

100

102

104

106

102 108 114 120 126 132 138 144 150

rpm

Present situation

R e l a t i v e

p r o p u l s

i v e

e f f i c i e n c y

Fig. 3 - Effect of optimizing propeller rpm.

Diameter(mm)

Relative propulsiveefficiency

5000 100.0

5800 108.7

Table 1: Effect of optimizing diameter.

Number ofblades

Relative propulsiveefficiency

3 95.3

4 100.0

5 102.9

Table 2: Effect of optimizing the number

of blades.



fuel consumption. We also need toremember that the propeller has to beredesigned in order to match adifferent rpm.

Optimum combination of diameter,

number of blades and propeller rpm

When determining the optimumcombination of diameter, number of bladesand propeller rpm, it makes a difference in which sequence the optimization process isdone. For the above situation optimizationof diameter, followed by blade number andthen rate of revolutions resulted in theoptimum combination as presented in thesecond column of Table 3.

The propulsive efficiency improvementcompared to the original situation is 9.0%. As the diameter of 5800 mm might be aproblem with regard to space in the stern

frame, the optimization was repeated by allowing a diameter increase of not morethan 400 mm. The same sequence of optimization results in the combinationshown in the third column of Table 3. Thisconfiguration gives a propulsive efficiency improvement of 7.3%.

As one may conclude in this case theoptimum propeller rpm is close to thepresent rpm, meaning that no specialadaptations for the engine are needed. Applying 112 rpm instead of 110 rpmyields no significant difference in efficiency.

The change in blade number might havemore impact on the propulsionconfiguration – number of cylinders – andthe ship’s structure.

Design point of the propeller

Changing the design point of a propellercan achieve significant improvements infuel consumption. When changing thedesign point, however, this should fit within the allowable load range of theengine. On the other hand if the enginecharacteristics are not changed, it is

generally known for a 2-stroke engine thatan increase of rpm at the same power leadsto a lower fuel consumption. This effectcan be seen in Figure 4, where the redcurves show rpm-power combinations atconstant fuel consumption. The curverepresenting the lowest fuel consumption ison the far right. So, when aiming for lowerfuel consumption, it is more favourable todesign at higher rpm, which is the same asapplying a higher light running margin.

There are two restrictions to increasingthe light running margin. Firstly, the rate of

revolutions in the design point may notexceed the overspeed limit of the engine.Secondly, the propeller can no longerabsorb maximum power in ‘clean’condition. This should not become a

weakness in the operation freedom of thevessel.

The lie of the propeller curves whenthe rpm for the design point is increased, isoutlined in Figure 5. In the case of the ‘toolight’ designed propeller, the trial propellercurve intersects the maximum rpm limitbefore reaching maximum power. Underservice conditions, however, the propeller

curve shows that it is just possible to apply the maximum power to the propeller.

In replacement cases it is feasible toimagine there is no further need to usemaximum power. This can be caused by achange in the mission profile of the vessel.It is also possible that the engine can nolonger deliver the maximum power becauseof the age of the engine. In these or similarcases the propeller can be designed lighterin order to have a lower fuel consumption.

In a lot of cases it is worth investigating whether a redefinition of the propeller

design point is interesting and whatimprovements with regard to fuelconsumption can be achieved. Wärtsilä hasa lot of experience in this field and canprovide sound professional advice.

We noted previously that changingthe rpm yields differences in fuelconsumption. In the above example it wasmore favourable to apply a higher rpm. Weshould not forget the effect on the propeller when changing the design point by increasing the rpm, or else theimprovement obtained by changing thedesign point is cancelled out by the effect

on the propeller itself.

Pressure pulses and vibrations

In the previous section it was shown thatsignificant savings in fuel consumptioncould be achieved by selecting anotherdiameter, blade number or rate of revolutions of the propeller. Besides theseoptions, the increase in the rate of revolutions in the propeller design pointmay give an additional reduction in fuelconsumption.

The modifications as discussed cannot

be applied without consideration of otheraspects than just efficiency. Somerestrictions or consequences were already indicated in the previous section. Thissection goes further by dealing with some



rpm

Power110 %

40 %

50 %

60 %

70 %

80 %

90 %

100 %

1 0 8 %

1 0 4

%

1 0 0 %

9 0 %

8 0 %

7 0 %

Fig. 4 - The characteristics for fuel consumption on low-speed engines (2-stroke).

Originalsituation

Optimumsituation

Situation for limiteddiameter increase

Diameter (mm) 5000 5800 5400

Blade number 4 5 5

Rate of revolutions (rpm) 112 105 110

Relative propulsive efficiency 100.0 109.0 107.3

Table 3: Optimum combination of diameter, number of blades and propeller rpm.



consequences regarding pressure pulses andvibrations.

Tip-hull clearanceIt is important to include pressure pulselevels and vibrations when optimizing thepropulsion system. The pressure pulse levelsmay increase, especially when applying a

larger diameter, simply because of thesmaller tip-hull clearance.

The pressure pulse levels acting on thehull increase in inverse proportion to thetip-hull clearance. Normally, older vesselshave tip-hull clearances which arerelatively large and therefore theconsequences caused by a larger diameterare acceptable. Often, these vessels are alsoof heavier construction than nowadays, which means that higher pressure pulsescan be absorbed without leading tounacceptable vibrations or noise.

HarmonicsRegarding pressure pulse levels, it is notdifficult to design a propeller with adifferent blade number but with the samevalue of the first-order blade harmonic. When the blade area ratio is not modified,the power density remains the same,leading to a similar sheet of suction sidecavitation. Suction side sheet cavitation isthe main parameter that determines thefirst-order harmonic pressure pulse level.

Of course, the frequency will change for

another blade number. This can createproblems for the vessel, as the frequency might come closer to the natural frequency of parts of the ship’s structure, leading toserious vibration problems in the vessel.

The higher order harmonic pressurepulse levels do not influence the ship’sstructure so much, but have more impacton the machinery on board. Consequently,it is necessary to keep the higher harmonicpressure pulses sufficiently low.

As the higher order harmonic pressurepulses are mainly caused by the strength of

the tip vortex, one can imagine that adifferent number of blades will influencethe higher order harmonic pressure pulses. A different number of blades affects thetip-loading and consequently the strengthof the tip vortex. When the higherharmonic pressure pulse levels are critical, itis generally better to apply a higher numberof blades.

Number of bladesIn combination with the engine thenumber of blades is also important. The

number of cylinders and number of bladescan strengthen certain frequencies especially since most fixed pitch propellers are drivendirectly. For example it is not advisable toapply a 4-bladed propeller with an8-cylinder diesel engine as this combinationmight have a negative influence both on thecavitation/sound behaviour and on thetorsional vibrations in the engine-shaftsystem.

Propeller rpm Within certain limits the propeller rpm

does not have much impact on levels of pressure pulses and vibrations, but doeschange the frequencies. As mentionedearlier, this might lead to situations wherethe frequency comes close to the natural

frequency of parts of the ship’s structure orother machinery and should therefore beavoided.

Secondly, the rpm affects the tip speed,calculated as:

V RPM D

m s tip = ×

602

2 p [ / ]

where D is the propeller diameter

The tip speed influences the cavitationbehaviour and the tip vortex in particular.Normally, it is recommended that the tipspeed does not exceed 40 – 45 m/s for anopen propeller. As can be seen from theformula, the diameter also determines tipspeed. So, when changing the diameter of areplacement propeller, the tip speed shouldnot be forgotten either.

Application of nozzles and

other energy saving devices

So far only modifications to the propellerand engine have been discussed. If no furtherimprovements, i.e. fuel savings, can beachieved, we still have some possibilities left.

The HR nozzle

The application of a nozzle, for example,may result in (additional) efficiency improvements. In general, people have theidea that a nozzle can only be applied inbollard situations but this is not the case (see[6]). In particular, the HR (High Efficiency)nozzle (Fig. 6) is not only beneficial for extra

thrust in the bollard condition, but can alsobe useful for speeds up to 15 – 20 knots.

Wärtsilä’s experience with replacementpropellers is that most of these vessels haveservice speeds within this range. It is

1-2005 Marine News - 37

Engine diagram for a typical diesel engine

0

20

40

60

80

100

120

40 50 60 70 80 90 100 110 120

rpm (%)

Engine curve

Trial curve for light-running propeller

Service curve for light-running propeller

Trial curve for 'too light' propeller

Service curve for 'too light' propeller

M a x i m u m

r p m

MCR

5% Light-running margin

Fig. 5 - Effect of optimizing propeller rpm. Fig. 6 - Lips HR nozzle.



therefore very worthwhile investigating whether a nozzle can achieve (extra) fuelsavings.

An additional advantage of anozzle/propeller combination is that theoptimum propeller diameter is smaller thanfor an open propeller. This makes it

possible to fit the nozzle/propellercombination in the existing aftship.Generally, fitting a nozzle requires a new propeller design with higher pitch.

Regarding pressure pulse and vibrationlevels it is known that these are better thanfor an open propeller. A nozzlehomogenizes the waterflow, resulting in lessvariation. The nozzle between the propellerand hull also reduces the forces acting onthe ship’s hull.

To show the possible fuel savings, weassume the case of a chemical tanker with

the following properties:n Service speed: 14.5 knotsn Engine power: 5500 kW n Rate of revolutions:131 rpmn Blade number: 4The propeller, originally designed for thiscase, had the optimum diameter of 5150 mm. When the configuration ischanged with a propeller in an HR nozzle,the optimum diameter can be reduced to4250 mm and a fuel saving of almost 6%can be achieved.

The wake equalizing duct We must also bear in mind that modifyingan open propeller into a propeller/nozzleconfiguration has some impact on theaftship. When the possibilities to change

the aftship are limited, there are still someoptions for installing energy saving devicesto the hull and significant improvementscan be achieved particularly in cases wherethe wakefield is relatively bad.

Most fitted devices aim to improve the wakefield, which is beneficial for the

propeller design. Moreover, energy savingdevices can affect the flow around the aftbody of the vessel. A wake equalizing duct(WED) is a good example of achieving fuelsavings (see [7]). An example is shown inFigure 7.

A WED is designed carefully to improvethe wake distribution and to reduce flow separation and suction drag on the ship’sstern. The WED is a ring-shaped flow vane with an aerofoil type cross section. It isfitted to the hull in front of the propellerapproximately 25% x D down from the top

of the propeller, in the form of twohalf-ring ducts. The two half rings are set atdifferent inclination angles for both sides,so that the total configuration is notsymmetrical to the ship’s centre line.

The basic principle of the WED is tocreate a flow around both half ducts. Any flow separation is reduced, which leads toan improvement in hull efficiency. Thismay result in considerable power savingsespecially for vessels with high block coefficients.

Fitting WEDs is also beneficial for the

propeller since the wake field ishomogenized by the WEDs. Logically thiscan be used for the propeller design so thatadditional efficiency improvements can berealized. When, for example, the wakefield

becomes more favourable, the blade arearatio can be reduced.

Beside nozzles and WEDs there are moreenergy saving devices possible, like theefficiency rudder, rudder bulb system andpropeller boss cap fin (see [8]). Wärtsilä hasplenty of experience in this field and is

happy to advise. n

References

[1] Haaren, M.J. van, “Replacement of fixed pitch

propellers”, Marine News No. 1-2002

[2] Hadler, J.B., et al., “Large-Diameter Propellers

of Reduced Weight”, Presented at the annual

meeting of the Society of Naval Architects and

Marine Engineers, 1982.

[3] Jiang, C., et al., “Investigation on Resistance

and Propulsive Qualities of Large Full Ship with

Low Revolution Large Diameter Propeller",

Shanghai Jiao Tong University, 1990.

[4] Weznicki, W., “Components of PropulsiveEfficiency as a Function of Block Coefficient and

Screw Diameter for Full Ships", Gdansk Technical

University, Ship Research Institute, 1990.

[5] Oossanen, P. van, “Toepassingsonderzoek

grote diameter lage toeren schroeven:

samenvattend eindrapport", Marin, 1985.

[6] Oosterveld, M.W.C., and W. van den Berg,

“Research in a Depressurized Towing Tank on

Ducted Propeller-Hull Interaction", 3rd Lips

Propeller Symposium, 1976.

[7] Schneekluth, H., “The wake equalising duct”,

Applications of New Technology, 1989.

[8] Ouchi, K., et al., “Research and developmentof PBCF (Propeller Boss Cap Fins -Novel energy-

saving device to enhance propeller efficiency-",

Journal of the Society of Naval Architects of

Japan, Vol. 163, 1988, and Vol. 165, 1989.


Fig. 7 - Example of a wake equalizing duct application.


replacing fixed pitch propeller - more possibilities for improvements

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