Exceptional operating condition – incomplete propeller immersion
Figure 3: Typical downward bending moment induced by propeller (incomplete propeller immersion)
Figure 4: Increased relative slope in way of the aft bearing (edge contact)
Figure 5: Contact pattern in way of the aft bearing
Figure 6: Exponential increase in local surface pressure
Operation with incomplete propeller immersion may induce an excessive eccentric thrust on the propeller and, consequently, a downward bending moment on the shaft. This may result in exaggerated localized loads (edge loading) and surface pressure on the aft bearing arising out of an increased relative slope and reduced bearing contact area (Figures 3, 4, 5, 6).
Localized bearing loads acting on a diminished contact area, not catered for in the design criteria, lead to total or partial loss of an effective hydrodynamic oil film of minimum thickness. Hence, the risk of prospective consequential bearing damage co-exists under exceptional operating conditions with incomplete propeller immersion.
The additional bending moment generated is a function of the degree of lack of propeller immersion, RPM and the power. Elaborating this further, the bending moment is proportional to the thrust force, which is proportional to the square of the RPM. Consequently, increasing RPM introduces an exponential degree of risk in a partially submerged propeller condition.
