Designing a CP propeller blade is a complex process, requiring an extensive range of expert knowledge in the specialised fields of fluid physics and mechanical engineering.
In addition to hydrodynamic blade design, the calculation of hydrodynamic loads and their effects when the blade pitch is changed and in various operating conditions is of great importance. In order to provide advanced blade shapes and satisfy ever-heightened requirements, use is made of state-of-the-art calculation methods, refined continuously by means of research projects carried out in cooperation with research institutes.
High skew
To suppress cavitation induced pressure impulses even further, a high skew blade design can be applied. By skewing the blade it is possible to reduce the vibration level to less than 30% of an unskewed design.
Because skew does not affect the propeller efficiency, it is almost standard design on vessels where low vibration levels are required.
Today, the skew distribution is of the “balanced” type, which means that the blade chords at the inner radii are skewed (moved) forward, while at the outer radii the cords are skewed back. By designing blades with this kind of skew distribution, it is possible to control the spindle torque and thereby minimize the force on the actuating mechanism inside the propeller hub.
For high skew designs, the normal simple beam theory does not apply and a more detailed finite element analysis must be carried out.
Blade and propeller hub materials
Propellers 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/mm
min 250
min 380
Tensile strength
N/mm
min 630
660-790
Elongation
%
min 16
min 19
Impact strength Kv at -10oC
Joules
21
21
Brinell Hardness
HB
min 140
240-300
Both materials have high resistance against cavitation erosion. The fatigue characteristics in a corrosive environment are better for NiAl than for CrNi.
