Trans RINA, Vol 153, Part A1, Intl J Maritime Eng, Jan-Mar 2011


The loads at the bearings’ position are estimated both with and without considering the hydrodynamic loads in waves and in oblique flow.


Common practice when designing bearings for the


propeller shaft is recommended by shaft specialists that the resultant vertical force including an equivalent force converted from the bending moment at


the propeller


centre should be maintained approximately less than 50% of the propeller weight upward in the opposite direction of the propeller weight (Molland.et al., ITTC. 2005). In Figure 27, a typical arrangement of the shaft bearings used for an azimuth thruster is shown. Based on this layout, the contribution of the propeller weight on the bearing can be found:


At bearing C - 2W in z direction At bearing A W in z direction


  p


When the thruster operates at large azimuth angles or in waves, there will be significant side forces and bending moments induced on the shaft by the propeller. These hydrodynamic imposed forces and moments will contribute to the bearing loads in addition to the propeller weight. Thus, the total bearing forces can be evaluated by converting the vertical and horizontal hydrodynamic lateral forces and bending moments on the propeller to forces on the bearings.


Expressions for the resulting total bearing forces * *


z


F at A and C are given below. At bearing A:


y


Ff W , F fy  


zz p


And at bearing C: my


** mz    


  y


my   zz p 0.3


Ff W F  22 ,  


** mz  


  


 


And the radial force at A and C: *2


Fr *  Fz Fy *2  pp


0.3DDf2 y y


0.3


  


0.3DDpp


 


(1) (2) (3)


Dp is the propeller diameter, my and mz are hydrodynamic imposed moments around the y and z-axes respectively, and fy and fz are hydrodynamic imposed lateral and vertical forces on the propeller. Note that we have assumed that the bearing at C takes only the radial forces and the bearing at A takes only radial and axial forces – the bearings don’t take bending moments.


From Figure 28 and Figure 29, it is found that the total bearing loads including hydrodynamic side forces and bending moments for


high advance coefficients and heading angles are three times higher for bearing C and F and p Again from Figure 30 and Figure 31, it is found that


bearing loads including hydrodynamic forces in waves are two times higher for bearing C and three times higher for bearing A than when we consider only the propeller weight in the design of the bearings.


Therefore, it is seen that the bearing loads are critical even for low advance coefficients and small heading angles.


Hydrodynamic imposed bearing loads may also be


important for the stern tube bearings in conventionally shafted propellers, since this system will also experience significant side forces and bending moments caused by waves and oblique flow when the rudder-propeller system operates in a turning situation. The type of stern tube bearings used for conventionally shafted propellers is especially sensitive to the propeller shaft having an angle relative to the straight, undisturbed shaft direction. Even though the magnitude of the bearing loads in waves is relatively smaller than in high oblique inflow, the high number of load cycles experienced by operation in waves may cause fatigue problems.


J=0.2-Hull J=0.6-Hull J=1-Hull J=0.2-Open J=0.6-Open J=1-Open


0 1 2 3 4 5 6 7 8


-40 -30 -20 -10 0 10 20 30 40 Heading angle


Figure 28: Radial bearing loads in different oblique inflows as a portion of the propeller weight on bearing C (Pulling mode)


four times higher for bearing A compared to the


consideration of only the propeller weight in the design of the bearings. The loads in positive heading angles are bigger than in negative heading angles for both open water and behind conditions.


Figure 27: Layout of the propeller shaft bearings and hydrodynamic force and moment of a typical azimuth thruster


©2011: The Royal Institution of Naval Architects


A- 19


Fr*/Propeller weight at C


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