THE CRITICAL ROLE OF ACCURATE WIND MEASUREMENT FOR WIND ASSISTED SHIP PROPULSION [7] CHALLENGES OF ACCURATE WIND MEASUREMENT ON BOARD Any deck mounted and deck operating wind propulsion technology is operating within a dynamic environment, where the incoming wind 昀椀eld is not homogenous and stable. The incoming wind properties (direction, intensity and 昀氀ow vectors) onto each mechanical sail are primarily a昀昀ected by: • Interaction e昀昀ects between adjacent and surrounding mechanical sail units; • Interaction e昀昀ects between each mechanical sail and the deck structure/superstructures (i.e. deck housings, hatch coamings, mechanical sail foundations, etc); • Interaction e昀昀ects to the mechanical sail due to the ship’s freeboard (distance of main deck level from waterline level); • The impact the incoming wind gradient (speed and direction varying from the sea level up to the maximum installation height of each mechanical sail); • Flow vortices around the ship deck, superstructures and mechanical sails tip vortices; • Turbulence and dynamic forces due to wind gusts; and • Water spray (green seas) causing 昀氀ow disruptions and air density variations. Mechanical Sails performance calculations and full scale tests (either numerical or physical) relate to a stable moving ship at sea, through calm waters and no winds, which is not the case when the ship usually encounters sea waves, currents, swell and wind-induced waves, hence experiencing roll, pitch, sway and heave motions while the wind is incoming in such heterogenous state already on to the mechanical sails. The meaningful apparent wind conditions for mechanical sails to generate net forward thrust on the ship are occurring for BF > 3, and by large probability extend (over 70%) at an apparent wind window of 0o – 90o, hence most of the time the ship will also experience close-haul to beamy waves, of various wave height, thus almost aways in a combined rolling and pitching state. Thus, mechanical sails will always work in a continuously changing Apparent Wind incoming 昀椀eld, which cannot be captured e昀昀ectively by an Anemometer or a LIDAR, as neither can measure the exact proximity of incoming wind conditions to the mechanical sail across its full height in real time. When wind propulsion technology providers develop estimations and numerical assessments for the aerodynamic coe昀케cients of their mechanical sails (such as the lift coe昀케cient CL, or drag coe昀케cient CD) they either use wind tunnel tests, CFD simulations, or stand- alone small of full-scale physical land tests, to derive their polar diagrams (i.e. the angular CL and CD performance of the mechanical sail at every incoming wind speed). Under all these methods the incoming wind 昀氀ow develops in a controlled environment or by isolating in昀氀uential parameters to reduce complexity. These Polar diagrams though (however re昀椀ned after physical tests) are used for performance predictions of mechanical sails that will work in dynamic environments, where the incoming wind 昀氀ow will be constantly changing over time, Thus the actual mechanical sail on board will deliver di昀昀erent thrust spectrum during every voyage. With local, direct, actual incoming wind 昀氀ow measurements across the full height of the mechanical sail, such information could feed di昀昀erently the operational parameters settings of mechanical sails (i.e. AoA, RPM, etc), so that they adjust to deliver the best possible net forward thrust force with the max possible tolerable side force to the ship, hence boosting performance for wind propulsion, matching more accurately performance expectations. Figure 5: Disturbed wind 昀氀ow on mechanical sails due to boundary layer in昀氀uences from the freeboard and superstructures (left) and due to interaction e昀昀ects between the sails (right) [7] 13