Altough water is flat it renders the same result as grass when impacted with rotor downwash. That would be that the energy of the downwash causes some of the imidiate water to be displaced and reduces what we call ground effect. When ground effect reduces power required increases. There is a whole lot more happening…

Makerofthegames. I am a commercial helicopter pilot and GR 2 Instructor. I have been flying for 10 years and training newby pilot since 2009. Vortex seems to be the culprit here indead. Opperation near water increases the recirculation load on the main rotor system and adds to the power required curve.