I have read articles regarding Chinese junks which utilise rudders made of rubber, and they usually have diamond shaped holes in them. These articles claim that the holes provide better handling of the junks compared to a full rudder without holes in them. What are your thoughts regarding how sails with holes would affect the speed of the sail craft? Apparently the wind cannot maximise the work done on a sail when it has no holes on them, as the wind currents will rebound back onto oncoming wind currents. Is there any real physics explanation behind these two phenomena?