While not glamorous, the diagram above came from the notes shared with Sunday’s lesson on the Nacra 20 and just maybe it’ll make sense to others out there as well! There’s a good reason to have twist in your mainsail, just look at how the jib ‘pre-bends’ the air around the mainsail. At the bottom its most pronounced as the jib is widest there. At the head of the mainsail, there is no jib ahead of it therefore, no pre-bending of the air.
In the diagram above, the top two-thirds of the sail would be in complete stall – indicated by the inside telltales flowing while the outside ones would be non-flowing. While in heavy winds, the squarehead main will twist itself open it is in times of light air that the above scenario will be the case – and is what we see a fair bit!
The mainsheet adjusts the amount of twist in the mainsail, easing it out will induce more twist and conversely, pulling it in. On squareheads its of paramount importance to get the twist right to really go. To little twist and you’ll probably stall, to much twist will be a bit slower but the ‘lesser of the evils’.
The difference between twist and leech pressure… Leech pressure will give you your pointing ability while twist will give you speed. Before you sheet in/out take a look at your telltales and adjust the sheet if THEY tell you to.
Think of the mainsail in three parts but concentrate on keeping the top flowing the whole time and you should be pretty right. The situation becomes even more apparent when we add a spinnaker to the equation. Then you have something that effects the air around the main in a similar but LARGER fashion… but that can be up next.
Hope that helps some of you out there…