that's exactly what I meant... but I still don't see how it works if the ropes are tied to the back of the hull 
sorry we are going off topic....let's go private and then post the result for those interested 
It's a single rope (or sheet in nautical terms) tied at each end to the stern and runs through a pulley (or block) on the boom, the three points (the fixings to the stern and the position of the block) forming a triangle. As the wind veers, the stern sail and boom swing from side to side with the rope running through the pulley. The looser the ropes, the more the boom can swing; the tighter the ropes, the less the boom can swing. In real life, the tightness of the ropes would be adjusted by a member of the crew to take account of the strength of the wind.
Try it by pushing the end of the boom from side to side with the rope fastened to the stern to form a triangle.
Here are two pictures of the layout in real life which is slightly more complicated than in Playmobil but the principle is the same.
