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Propeller blade manufacturing and materials

MAN B&W Controllable pitch propeller. Propeller blade manufacturing and materials

The international standard organization has introduced a series of manufacturing standards in compliance with which propellers have to be manufactured (ISO 484). The accuracy class is normally selected by the customer and the table below describes the range of manufacturing categories.


Class

Manufacturing accuracy

S

Very high accuracy

I

High accuracy

II

Medium accuracy

III

Wide tolerances

At MAN B&W Alpha the propeller blades are checked by computerized four-axis measuring equipment. I
If no class is specified, the propeller blades will be manufactured according to class I but with surface roughness according to Class S.

Blade material
Propeller blade are made of either NiAl-bronze (NiAl) or stainless steel (CrNi). The mechanical properties of each material at room temperature are:


Material

NiAl

CrNi

Yield strength

N/mm2

Min 250

Min 380

Tensile strength

N/mm2

590-780

600-790

Elongation

%

Min 16

Min 19

Impact strength Charpy V notch

Joules

30

21

Brinell Hardness

HB

Min 150

240-300

Both materials have high resistance against cavitation erosion. The fatigue characteristics in a corrosive environment are better for NiAl than for CrNi.
Propeller blades are, to a large degree, exposed to cyclically varying stresses. Consequently, the fatigue material strength is of decisive importance. 
The dimensioning of a propeller blade according to the Classification Societies will give a 10% higher thickness for the CrNi compared to NiAl in order to obtain the same fatigue strength.
As an example the difference in thickness and weight for a propeller blade for engine type MAN B&W 6S35MC (4,200 kW at 170 r/min) is stated in table 2.
CrNi-steel requires thicker blades than NiAl-bronze, which is unfortunate from the propeller theoretical point of view (thicker = less efficiency). Additionally, the CrNi is more difficult to machine than NiAl.
For operation in ice the CrNi material will be able to withstand a higher force before bending due to its higher yield strength and for prolonged operations in shallow water the higher hardness makes it more resistant to abrasive wear from sand.
The final selection of blade and hub material depends on the operating condition of the vessel. In general terms the NiAl material is preferable for ordinary purposes whereas CrNi could be an attractive alternative for non-ducted propellers operating in heavy ice or dredgers and vessels operating in shallow waters.


Ice class

C

1A

Material

NiAl

CrNi

NiAl

CrNi

Thickness at r/R=0.35

mm

132

146

169

187

Thickness at r/R=0.60

Mm

71

78

90

100

Thickness at r/R=1.00

Mm

0

0

15

13

Blade weight

kg

729

877

952

1053

Table 2

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