How much mast bend?

Paul Brotherton advises a Fireball sailor on how to match mast to sail
First, get in touch with your sailmaker to find out the working mast bend range for your mainsail. This is fundamental, although even a lot of good sailors do not understand the importance of doing this. Your sailmaker should be able to tell you what sort of mast bend you should be setting up for the sail that they have produced. Unfortunately, that is only a starting point, because setting up static mast bend on the shore is not the crucial measurement. What really matters is the mast bend you have while you are actually sailing, and that can vary depending on different factors, such as crew weight. For the 2000 Youth Worlds in the 420, we set up the boys' rig with a straighter mast than the girls, although when they were sailing the resulting mast bend would be the same. There are two fundamental controls on the Fireball for mast bend - the spreaders and the deck-level control, either chocks or a strut, like in the picture above. As far as setting up mast bend is concerned, by far and away the most important control is the angle of the spreaders. The deck control should hold the mast in column but should not be forcing the mast straight. On the Fireball, with the rig tension on, take up the slack on the deck control so the mast is firmly supported but not actually pushed back against its natural bend. This should be your maximum point of deck chocking. If overpowered however, ease the deck control so the mast can bend a little and flatten off the mainsail. It is important to find out the flexibility of the mast. For instance, with the 470 many sailmakers produce two distinct types of mainsail - one for a stiff section like the Proctor