Propeller Modification for Recovery of Decreased Propeller Shaft Speed

Propeller Modification for Recovery of

Decreased Propeller Shaft Speed

By Propeller and Shafting Committee

With years to elapse, ships undergo an unfavorable operationg condition-over-torque of rnain engine, by their speeds being decreased due to the deterioration of the performance by aging of hul1,main engine and propeller.

There are several measures available against such over-torque condition :

-applying a margin to the propeller speed at a propeller designing stage,-recoditioning main engine,

-cleaning up fouled hull surface,_polishihng propeller surface,

-modifying propeller particulars, etc.

Among these measures, the modifying of propeller particulars is usually practiced from viewpoints of workabi1ity, economy and effectiveness.

This paper describes modifying methods of propeller particulars, with effects on shafting, etc.,which will be a useful guide to ship operators and shipbuilders.

1. Introduction

After ships have been put into service, there are cases where main engines come to torque-rich condition such that engine speed lowers and engine torque becomes excessive due to aging such as the fOuling of hul1s, main engines and propellers.

The method which applies margins to propel1-er speed in the design stages of propellers taking account of aging has been used since about 25 years ago. According to the results of the last questionnaire survey, the amounts of propeller speed margins are decided according to values recommended by engine manufacturers and shipyards. Though there are cases where the amounts of propeller speed margins are decided taking account of the fouling of ship hulls and propellers, they are 3-4% in many cases and 4-5.5% in some cases in the concrete.

Propellers are usually modified to recover propeller shaft speed in the range where propeller efficiency is not influenced. In case of the method by washback modification on the trailing edge side of the propeller blade, the amount of modif-ication is 3-5% in propeller speed in many cases and 0.6 degree in nose tailline angle (4.5% in rotating Speed) in some cases. In case of the

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Translated from Journal of MESJ Vo1. 26, No. 10 (Manuscript received March 15, 1991) propeller diameter cutting method, it is 3-5% in propeller speed (5-7% in propeller diameter) in many cases. The method by pitch modification by twisting propeller blades is usually used in Europe and this method is applied to smal1-sized propel1-ers with smal1 blade thickness on the trailing sides to which washback modification can not be applied in some cases in Japan.

In case where a main engine comes to torque-rich condition in spite of applying a margin to propeller speed in the design stage of a propeller, measures to cope with this torque-rich condition are the reconditioning of the main engine, the cleaning of the surface of the hul1,and the cleaning and modification of the propel1-er. However, in case where lowered shaft speed is to be recovered by the reconditioning of the main engine, considerable increase in shaft speed can not be expected.

In this paper, the method of recovering lowered shaft speed is therefore explained focussi-ng on propellers for main diesel engines.

2. Factors of the Lowering of Shaft Speed

The fol1owing can be considered to be factors causing torque-rich condition, that is, the lowering of shaft speed under the same output.

(1) Increase in surface roughness of hul1

Increase in the surface roughness of the

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Propeller Modifiication for Recovery of Decreased Propeller Shaft Speed 81

hull directly increases the resistance Of the hull and ship speed lowers under the same output. marine growths, pockmarks due to rusting, and bends on the hull can be considered to be reasons for changing the surface roughness of the hul1. Though marine growths can be removed well by cledning the hu1l during docking, other two phenomena can not be recovered as before.For this reason, considerably large roughne ss remains on ships in service compared with their new states.

(2) Increase in wake fraction

Since increase in the surface roughness of the

hull results in thickening the boundary layer of flow along the hul1, the speed of advance (Va) of the propeller becomes smal1. That is, the mean wake fraction (w) becomes large.

(3) Increase in surface roughness of propeller

The surface roughness of the propeller can be increased by not only cavitation erosion but also chemical corrosion. When the surface roughness of the propeller increase

Fig. l Flow Diagram of Method by Washback Modification on the Trailing Edge Side of Blade

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82 Propeller and Shafting Committee

es, its efficiency lowers and torque absorbed by it increases.

3. Method of Recovering Decreased

Shaft Speed

The fol1owing methods are commonly used for recovering decreased shaft speed.

3.l Method by Wa8hback Modification on the

T railing E dge Side of P ropeller Blade

Aerofoil sections are commonly used for propeller blade sections. When washback is given to the trailing edge side of this blade section by slightly cutting and shaping this side, the effective pitch of the prope1ler decreases, since the attack angle decreases due to a change in the nose tail line and a decrease in the camber of the blade element. Consequently, the rotating speed rises under the same output.

For the method of shaping the trailing edge side of the blade for washback, there are two methods. The first one is to give washback by cutting the trailing edge side of the blade, and the second one is to give washback by bending the trailing edge side of the blade toward the suction side after uniformly heating it with a special burner. The latter method is not adopted currently from the viewpoints of the accuracy and workability of washback modification and the necessity of special jigs, though this method was adopted in the past in Japan.

The amount of washback modification on the trailing side of the blade is limited in the range where propeller efficiency does not lower and a change in propeller blade area does not badly influence the performance of the propeller against cavitation. The yardstick of the amount of modification is the cutting of about 3% of blade width. The rising of rotating speed by about 4% can be expected by this modification. The further rising of rotating speed can be expected by applying the method by washback modification on the trailing edge side of the blade together with the propeller diameter cutting method. However,careful study is necessary when applying these methods together, since propeller efficiency lowers and torsional vibration characteristics are influen-ced.by a decrease in polar moment of inertia. Of the propeller Fig.1 shows the method of estimat-ing the rising of rotating speed by applying the method by washback modification on the trailing edge side of the blade. Fig.2 shows an example of cutting according to this method. Figs.3-1 and

Fig. 2 Example of machining According to Washback Modification on the Trailing E dge Side of Blade ( 1/2)

3-2 show estimated rates of the rising of rotating speed by applying this method. It is desirable to use these figures as yardsticks in the early stage of study because of the considerably large dispe-rsion of measured values.

Merits :

(a) Work can be done while propellers are

installed on ships.

(b) The term of work for shaping the trailing

edge sides of propeller blades is short and manhours for it are fewer than those for other methods, since work is easy because of the small thickness of this part over whole range of shaping.

shafting is smal1, since a decrease in the polar moment of inertia of the prope1ler is smal1.

(d) Propeller performance and self propulsion

factors hardly change.

(e) Influence on propeller performance against

cavitation is almost none.

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Propeller Modifiication for Recovery of Decreased Propeller Shaft Speed 83

(f) There is no influence on propeller meterial.

Demerit :

(a) There is a limit of rotating speed after

rlslng.

3.2 Pr0peller Diameter Cutting Method

When the periphery of a propeller is cut, its diameter decreases, and its pitch ratio, developed area ratio, boss ratio and blade thickness ratio increase. It is well-known that the rotating speed of the propeller of which periphery is cut increases in self propulsion condition under the same speed or the same output absorbed in comparison with the original propeller. On the other hand, the efficiency of this propeller naturally lowers in comparison with the original one if the latter is nearly optimal. For the propeller diameter cutting method, cutting range is determined according to the propeller speed after rising which has been decided beforehand. It is necessary to study how the propeller speed after rising changes when the periphery of the propeller blade is cut. Fig.4 shows a flow diagram for calculation. Fig.5 shows the estimated increase in propeller speed by the propeller diametr cutting method. It is desirable to use this figure as a yardstick in the early stage of study because of less measured values and their large dispersion.

Merits :

Fig.3 Example of Machining According to

Washback Modification on the Trailing E dge Side of Blade (2/2)

(a) Propeller speed after rising can be set at

higher values.

(b) Work for cutting the peripheries of propelL

er blades can be done while a propel1er is installed on a ship.

after cutting the periphery of the blade makes propeller speed higher and the size of an area to be shaped hardly influences the amount of reduction in ship speed.

(d) Less expense can be expected tompared

with the production of a new propeller.

(e) There is no influence on propeller material.

Demerits :

(a) Propeller efficiency lower

(b) Propeller efficiency becomes lower than

that of an optimally designed propeller for which the propeller speed after cutting has been used as a design point.

takes considerably much time. Special care should be taken, since this work is liable to be incomplete.

(d) Even though the blade area to be shaped

after cutting is made wide to some extent,propeller efficiency is hardly improved.

(e) Propeller performance against cavitation should carefully be examined, since it sometimes lowers due to a decrease in blade area af ter c utting.

(f) Torsional vibration of the shafting should

carefully be examined, since the polar moment of inertia of the propeller greatly decreases and torsional vibration is greatly influenced. (Resonant speed of torsional vibration rises.)

(9) There are cases where static balance tests

are needed.

3.3 Method by Twisting Propeller Blades

The method by pitch modification by twisting propeller blades is usually adopted in Europe. This method has been applied mainly to large-sized propellers in Japan during the period from 1970 to the first half of 1975. This method can be outlined as fol1ows. The area near the blade root (0.3-0.4R) is heated by a soft burner and a jig is fixed to the part near the blade tip (near 0.7R). Loads are imposed on the pressure side and suction side of the propeller with jacks. The blade is twisted and permanently deformed. Thus, the blade pitch decreases. Though considerably large pitch correction is possible by this method, the amount

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84 Propeller and Shafting Committee

of correction is actually 3 through 5% in propeller speed in many cases.

After that, this method twisting propeller blades has been out of use because the method mentioned under 3.1 which enables propeller speed to be recovered by simple work has been developed nd put into practical use. However, this method is sometimes applied to smal1-sized propellers with small blade thickness on the trailing edge sides to which the method mentioned under 3.1 is hard to apply.

Merits :

(a) Considerably large pitch correction is possL

ble in comparison with other methods.

(b) Even if blade pitch changes, blade area

does not decrease. Though added mass of water may change a little due to pitch correction, its change is not so large that it influences torsional vibration.

er performance against cavitation is hardly inf lue nced .

Demerits :

(a) Special jigs are needed.

(b) Makers who execute this work are limited and propellers must be carried in factories.

(d) Labor for carrying propellers in and out fromfactories and for the work for twisting and correcting blades considerably increases.

(e) The term of work for twisting and correcting becomes considerably 1ong.

(f) The exact control of heating temperature is necessary during work for twisting and correc tlng.

(9) Care should be taken, since propeller mat erial may be inf luenced.

(h) The measurement of pitch at each radia1 position is necessary after twisting blades.

4. Calculation of Recovery of Shaft

Speed by E ach Method

The recovery of shaft speed has been calculated for the method by washback modification on the trailing edge side of the propeller blade under 3.1, the propeller diameter cutting method under

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Propeller Modifiication for Recovery of Decreased Propeller Shaft Speed 85

Fig. 4 Flow Diagram for P ropeller Diameter

Cutting Method

3.2, and the method of twisting propeller blades under 3.3. The results of calculation are as f ol1ows .

Main engine MCR : 16800ps x 122rpm

Propeller 5 blades x Dia. 5700mm X

Pitch 5360 mm X Developed area ratio 0.74

In the above case, the amount of correction necessary for increasing propeller speed at 80% MCR by 2.4rpm is as fol1ows.

In case of 3.1 : 15-50mm in blade width

(0.5R-0.95R)

In case of 3.2 : About 170mm in diameter In case of 3.3 : About 125mm in pitch

Table l shows particulars of the propeller modified by each method.

5. Influence on Shafting

When decreased shaft speed is recovered by the modification of the propeller, the polar moment of inertia of the propeller decreases in case of the method by washback modification on the trailing edge side of the propeller blade under 3.1. The polar moment of inertia of the propeller further decreases in case of the propeller diameter cutting method under 3.2 in comparison with the method under 3.1. Though the polar moment of inertia does not decrease, added mass of water

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86 Propeller and Shafting Committee

changes a little in case of the method by twisting propeller blades. Since the rising of the resonant speed of torsional vibration of the shafting due to a decrease in propeller mass can therefore be considered, influence on the torsional vibration of the shafting should be examined beforehand.

6. Points to be Noted When Applying

These Methods

(1) In case of the method by washback modification on the trailing edge side of the propeller b1ade It is desirable to use the effective pitch for the calculation of propeller speed]l;+>6E (Knots) P = pitck (m) N = propeller speed (rpm) of the ship, since the effective pitch changes by providing washback on the trailing edge side of the propeller blade though the designed pitch (Geometric pitch) does not change. In order to omit the static balance test of the propel1er, each blade should be machined so that the amount of cutting becomes even, and weight balance among all blades should be confirmed by measuring the weight of chips of each blade.

(2) In case of the propeller diameter cutting method Since propeller performance lowers and ship speed lowers accordingly by cutting the periphery of a propeller in some cases, it is necessary to determine the amount of cutting by taking the relation with the operating speed of the ship into consideratL on. Each b1ade should be shaped with a template so as to equalize the radius and contour of each blade when cutting its periphery.

(3) In case of the method by twisting propeller blades Since the amount of twist for each propel1-er blade (amount of pitch correction) largely changes from the root of the blade, special care must be taken when working so that the pitch of each blade at each radial position is even and is within the manufac turing tolerance of each pitch. Special attention must also be paid to the control of propeller heating temperature when twisting blades so as to keep it within the range where propeller meterial is not

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Propeller Modifiication for Recovery of Decreased Propeller Shaft Speed 87

influenced (500-8001C for HBsC1 (Mn Bro nze) and 750--9501C for AlBC3 (NiAl Bronz e)

7. Conclusion

As methods of recovering decreased shaft speed, that is, measures to cope with torque--rich condition, the reconditioning of the main engine,the cleaning of the surface of the hul1, and the cleaning and modification of the propeller to date.Among these methods, the modification of the propeller is commonly adopted, since it is effective and economica1.

As measures to cope with torque--rich condi tion by propeller modification, the method by washback modification on the trailing edge side of the propeller blade has been adopted in many cases by taking account of workability, economy and effectiveness among the before-mentioned three methods of modification, though the propell er diameter cutting method and the method by twisting propeller blades, which can make propel1 er speed after rising higher, are effective.

The author shall be pleased in this paper is useful to study the recovery of decreased shaft speed in case where a main engine comes to torque-rich condition and becomes to be the guidance of the method of recoverying decreased shaft speed by propeller modification for ship operators and shipyards.

(BY GENJI KAIZU, NAKASHIMA PROPELLER CO., LTD.)

Nomenclature

N : Propeller speed (rpm)

n : Propeller speed (N/60) (rps)

D : propeller diameter (m)

DHP : Delivered horsepower (PS)

Kq : Torque coefficient

Va : Speed of advance of propeller (m/s)

J : Advance coefficient

N : Amount of rising of propeller speed

(rpm)

p : Propeller efficiency

: Density of seawater (kgf . s2/m)

References

1) Yazaki A. Okamoto H. and Okada K On performance of propellers with cutted tlpS.

Journal of the kansai Society of Noval Architects, Japan.Vo1.106, 1962.

2) Okamoto H. and others.Method of correcting a Number of Revolut-ion of a screw propeller.

Japanese patent No.P47 : 036589. (1972)

3) Okamoto H. and others.Method of correcting a Number of Revolut ion of a screw propeller.

Japanese patent No.P1095751. (1982)

4) Terai S. and others.Method of correcting a shape of propellers,etc, made of High Thermal Conductivity Material and its a Apparatus.

Japanese patent No.P51-081772. (1976)

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