Mast bend is the most effective tool to change the shape of the main when sailing, so its really important that the luff curve of the sail fits the mast bend. Of course you need a backstay adjuster to do this – most boats have one, and can be fitted easily enough if not.
On boats with fractional rigs with swept-back spreaders, the mast will be bent to some extent all the time – called pre-bend. This is the way that these masts set up properly and securely, but the backstay can give more bend, and so still affect the shape of the main.
The outhaul controls the lower third of the sail (mainly) and applies tension to the clew of the sail. This reduces the camber of the foot, which reduces the camber in the lower part of the sail.
The traveller operates only in the one plane – it lets the boom in our out, but with no change in twist. Its disadvantage is that its distance of operation on most boats is limited to 10-20 degrees of the centreline. With the twist set up correctly with the sheet the use of the traveller makes for easy and controlled fine-tuning.
The mainsheet controls the boom position, in two planes. The horizontal plane – the boom in or out – is the obvious one, but it also lets the boom up, and twists the mainsail – a more “open” leech. Some twist is good trimming, and the use of the telltales lets you know the correct twist.
The leechline should not be overlooked – although its main function is to prevent fluttering, it can be used to round up the back of a main in light airs, by pulling the batten tips to windward.