Analysis of fretting inception for marine propeller by single blade loads measurement in realistic operating conditions. Straight ahead and turning circle maneuver

Controllable pitch propeller (CPP) is a versatile solution to fulfill high hydrodynamic efficiency and low environmental emissions for different operative scenario of a seagoing vessel because it allows the most suitable matching between the prime mover and the propulsor. In contrast, this configuration is a complex mechanical system and, therefore, it may be subjected to a wide range of failures related to the automation system of blade pitch variation, strength and fatigue life of the various mechanical components. This paper focuses on blade fretting, that is a small amplitude vibratory motion of the blade with respect to its lodging in the hub; this motion causes the wear between the contact surfaces, reducing the fatigue life of the material as well. The prediction of this phenomenon in realistic sailing conditions during the design stage is of utmost importance to prevent damages and failures during ship operations, by developing suitable control laws. The analysis is based on single blade loads measurements, obtained with a novel set-up installed on a free running maneuvering, self-propelled model of a twin screw ship. The experimental campaign was aimed to characterize single blade loads during different ship operations, with particular effort on maneuvering (standard and safety maneuvers) and motion in waves. This paper is focused on the straight ahead motions as well as steady and transient phases of the turning circle maneuver, performed at different rudder angles and speeds. The results show that the onset of fretting is sensitive to the wake evolution, and, hence, to the rudder angles; moreover, consistently to the asymmetric behavior of the propellers, fretting is differently triggered on the port and starboard propeller.