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 Epsilon and one for more flexible masts like the SuperSpar M7 or the Proctor Stratos. An Epsilon-style mainsail is cut with minimum luff curve, so if you put it on a flexible mast the chances are the mast will bend too much for the sail and you will see nasty creases coming away from the centre of the luff.

These are so-called starvation creases and they are a strong sign that all is not well with your set-up. This can happen even if you are using the right sail and mast combination but allowing too much bend to creep into the mast set-up, either through setting up your spreaders incorrectly or through insufficient chocking at deck level.

On the other hand, putting a sail designed for a flexible mast on a stiff section like the Epsilon will create lots of power in medium air, full-power conditions, but will give you bad gust response and will leave you severely overpowered in strong winds. So this stiff-mast, full-sail mismatch is not much better either.

So once you have ensured that your sail was designed to fit your mast, or vice versa, now you can get on with setting it up for maximum efficiency. Your sailmaker should be able to give you the range of mast bend that your sail is designed for. Broadly speaking, if you are at the heavy end of the weight range for your class, then you should set up your mast at the straightest setting, and if you are light you should err towards the more prebent end of the mainsail's working range.

Remember, however that it is not what you mast and sails look like when you see them on shore but how the rig looks as you are moving through the water. This is why we set up the same mast/sail combination straighter for the 420 boys than the girls. Although the boys set up on shore with 3cm of prebend to the girls' 3.5cm, both rigs ended up on the same sailing bend of 4cm, due to the boys being heavier than the girls.

Don't straighten the mast beyond the advised parameters as while this may power up the sail, the entry angle of the sail at the luff is affected, and your ability to point will suffer. On the other hand, if you veer too far the other way and exceed the advised mast bend for your mainsail, you will also affect your performance. The most obvious symptoms of overbending the mast are known as 'starvation creases', emanating from the centre of the luff down towards the clew. In this situation, which you see quite a lot on Fireballs and 470s, the sail is effectively folding in half. The wind blows onto the leech, the leech gives up and levers open, and the wind simply falls off the back of the mainsail.

Another symptom of luff curve starvation is when you see the inboard end of the bottom batten start to poke to windward. You see this a lot on big boats when they've got too big a jib or genoa up. Sometimes the answer is to ease the vang until the batten falls back into line with the sail.

So use the sailmaker's tuning sheet as a starting point, and then use your own observations of the sails and your performance relative to other boats to determine where the best mast bend is for your given weight and sailing style. To summarise again, a straighter mast will give you more power - ideal for medium conditions - and more mast bend will depower the sail - ideal for strong winds. But always stay within the working luff curve range of your sail.