Between one-fourth and one-third of the energy in rowing is lost to the flow of water around the blade which decreases efficiency, i.e., the ratio of effective energy to expended energy. Hitherto the assessment of this hydrodynamic rowing efficiency was difficult and could not be achieved as a function of rowing angle and time. By recording the path of the blade and the forces acting on the blade a method was found, which is practical to use and renders the hydrodynamic efficiency at each instant of the rowing action. The assessment is based on the measurement of boat velocity, angular velocity of the oar or scull, and moment exerted by the rower. These parameters are measured, digitized, and stored in the boat for later computation on shore.