Most sail concepts don't work with today's open deck cargo ships, that problem is solved by the OutSail concept.. but it still faces many more problems: - Is the connection enough to transmit the sail forces? - How will a ship with no sailing keel behave? - How will it affect balance and righting moment? - How will it behave after years of poor maintenance? - What happens if the wind picks up and the sails are not retracted early enough? ( The wind forces acting on a sail will at some point prevent you from hauling it in, but with traditional sails an axe can solve the problem )