As the number of wind assistance installations in commercial shipping grows and the industry matures, the need for fullscale verification of the performance increases. Standard procedures or guidelines for conducting such full-scale trials of are still lacking. One strategy is proposed and discussed here. The method is demonstrated using a speed trial conducted with Scandlines’ hybrid ferry Copenhagen equipped with a rotor sail. The trial result is extrapolated to yearly power saving using a statistical route analysis. With this approach, the result can be derived at a feasible cost, within a limited time frame and using commercially available tools and established procedures.