Sails can propel a boat in two ways. The first occurs simply when sailing with the wind (downwind) where the sail is set perpendicular to the wind to proved the greatest surface area. The air decelerates as it hits the sail omitting a force onto the sail. As a result, according to Newton's 3rd, there is an equal and opposite force that propels the boat forward. The second, occurs when sailing upwind or perpendicular to the wind. Here the sail acts as an aerosol and redirects the wind. The wind arrives at an angle to the sail at the front of the boat and gets redirected to its rear (See image) My question: How can I calculate/predict the end resultant force that propels the boat forward? - I have designed an experiment where I measure the resultant force of the sailboat at different points of sail, and hence, I would like to identify the accuracy and reliability of my measurements and could possibly find the efficiency. For a downwind sail this is easier as I know the volume and mass of the wind acting on the sail per second... How would I approach this problem when sailing upwind or perpendicular? Thanks in advance.