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This paper focuses on the forces acting in a controllable pitch propeller (CPP) mechanism and the manifestation of unwilling, small amplitude, vibrating motion (fretting) in a blade bearing caused by seaway. If this happens, fretting can result in fretting fatigue of components, fretting wear, and increase of friction in bearings. An attempt has been made to define the circumstances leading to fretting in a CPP and to describe the influence of sea state on fretting behaviour. Using hydrodynamic tools it is possible to determine the theoretical conditions for fretting. A combination of theoretical and experimental results provides a better understanding of the fretting phenomenon in a CPP. The first part of the paper gives an introduction to the working regimes and forces acting on a CPP. In the second part the presence of fretting is defined based on the calculation of forces and moments during one revolution of a blade in the non-actuating regime for the design condition of a specific propeller. The third part investigates self-propulsion tests in irregular seas and hydrodynamic forces acting on a CPP in those conditions. The results indicate a decrease of fretting in higher waves and higher speeds. It has been shown that it is not wise to look only at the load mean value as design criteria.