Watching these boats hooning along downwind I notice that the boom swings up and down a lot and the sail shape changes.
There is no vang to keep it tensioned.
Obviously there's a reason for this, anyone know what it is?
not sure if this answers your question
however I had the pleasure to be shown over Mapfre by its skipper Zabi Fernandez while they stopped over in melbourne
I asked hi a few questions regarding their mainsail trim and as a little surprised by his answers
They do have a vang , but he said "both the vang and the traveller are not used much at all " ... their speed and its effect on the apparent wind angle means that they are sheeted on almost all the time.
IMHO perhaps what you are seeing is the main sheeted to the appropriate power possible given the conditions and the leech of the main moving as the load changes while the boat is surfing down or punching into the swells.
I must say that the crew of Mapfre won me with their spirit and generosity with my short encounter with them and i hope they can go on to win in this edition of the VOR.
