The Effect of Cupping on Four-Quadrant Propeller Performance

The Webb Hydroflite Propeller Series (WHPS) consists of four matching pairs of three-bladed ten-inch-diameter pleasure craft propellers with P/D ratios varying from 0.6 to 1.2. Recently, one propeller from each matching pair was modified with a uniform trailing edge cup. Cupping is a widely employed and inexpensive aftermarket alternative to repitching a propeller that is not performing in a given application. The hydrodynamic performance characteristics of each modified propeller were determined by open-water and four-quadrant model tests prior to and after cupping. Performance comparisons were made for each pair of propellers tested to determine the level of manufacturing consistency as well as the effects of cupping. Test data for peak open-water efficiency and four-quadrant performance were compared to B-Series propeller data to determine whether the B-Series can be used confidently for performance predictions. It was found that the addition of the cup could be modeled as an increase in effective pitch. For a given KT/J2 operating curve, the cupped propeller operates at a higher advance coefficient than the original propeller when running ahead. The increase in advance coefficient leads to lower shaft speed at a given thrust. When running astern, there is little change in the KT curve for the cupped propeller and therefore negligible change in advance coefficient but there is a large increase in the required torque at a given shaft speed.