The impact of ice on a propeller is a complex process, which most likely results into a milling or crushing process, or a combination. The highly dynamic forces during an impact can change rapidly in amplitude as well as direction. Little information can be deducted from classic test setups with rigid propellers, especially forces on the individual blades are seldom successfully measured. The presented paper will outline the challenges which need to be overcome to measure these impacts more accurately and elaborate on a measurement setup which has proven to capture the impact loads of actual ice impacts in great detail.
Classic designs for propeller testing involve rigid propellers with force measurements located at the base of the propeller. From this type of setup, immediately two problems arise. First of all, when impacting relatively large ice blocks, it will be hard to derive which part of the measured forces contributes to one blade and which part to the other. Secondly, in practical test situations the natural frequency of the entire propeller mounted on a force transducer can be relatively low. Impacts are suspected to contain forces with high frequency components, causing the entire propeller assembly to vibrate and thereby obscuring a correct measurement of the impact loads.
A measurement setup has been designed to overcome both problems. Rather than measuring the forces on a complete propeller, only a single blade is mounted on a force transducer. This particular transducer is capable of measuring forces in x-, y- and z-directions as well as moments around the x-, y- and z-axis. The other propeller blades are mounted directly on the shaft drive. This measurement setup reduces the first problem, any impact visible in the measurements signals are solely due to forces on the instrumented ‘key-blade’.