Located a small distance up the luff from the tack is the cunningham, generally a pressed-in ring reinforced with webbing straps that is used to adjust the amount of tension on the luff of the mainsail, which in turn controls the location of the camber in the sail. You can also use the main halyard to adjust luff
"... more gains can be made from learning how to set and trim the sails you have than simply buying new sails. You need to know how to get the most out of them"
These old 12 meters are sailing with loose-footed mainsails. You can see the lens of light fabric hanging below the boom.
tension, and indeed some racing sails on smaller boats no longer have cunninghams. But it is easier and less likely to cause a problem if you take up on the cunningham, since among other things, adjusting the luff with the halyard requires a lot more tension, not only because you have to raise the whole sail fighting against gravity and the tension of the mainsheet, but also because there is friction in the luff attachments. The compression load exerted by the main halyard on the mast is also something many sailors wish to minimize. In short, feeding a line through the cunningham ring achieves the same results as taking up on the halyard without the complications.
The foot of the sail is one of the most versatile edges of the sail when it comes to manipulating sail shape, since small adjustments can make big changes in the overall depth of the sail. In moderate winds or choppy conditions you can ease the foot out to add shape to the bottom of the sail, thereby providing more power. Or you can tighten it up to flatten the sail in either light air or heavy conditions. The foot shelf is shaped like a lens and once you pull on the clew it collapses and flattens out, pulling down on the middle part of the sail, removing shape from the bottom of the sail and flattening the overall profile. Many conventional mainsails are still attached to the full length of the boom by a foot shelf. And in fact, old sailing rules such as the IOR rule dictated that the sail had to be attached to the boom using either a bolt rope or slides, but that has changed. With more modern booms and hardware it's no longer necessary to have the sail attached unless you are a cruising sailor that plans to use the foot shelf for collecting rain water. Not only that, modern sailing rules like IMS and PHRF no longer require the sail to be attached to the boom, which has resulted in many more mainsails being loose-footed, that is they are attached at the tack and the clew only. This allows for more versatility in the sail and omits a
potential problem area, i.e., the foot attachment. In the past the bolt ropes would chafe through and slides break allowing the sail to rip free. Today, simply applying tension to the outhaul achieves the same result as collapsing a foot shelf. Many sailmakers still add a piece of light, lens-shaped fabric to the foot of these loose-footed sails, but this only serves to add sail area when sailing downwind. Once the shelf is not needed it can be rolled up and secured with a light Velcro strap.
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