The draft is the section of the sail that has the maximum depth (i.e. the greatest distance from the chord to the sail). Its measurement has two attributes:
Draft itself is the amount of depth that there is in the sail. This is given as a percentage compared to the chord. For example a flattish sail might have a draft of 5% whilst a fuller sail would have a draft of 10%. To create more power (and therefore speed) we have a deeper sail, whereas if we would like less drag we make the sail flatter. Scenarios where we want to have more power would include:
- When we want to sail through medium sized waves
- When we want to accelerate (i.e. immediately after tacking)
Draft Position is the distance along the chord from the luff that the draft is located at. Generally speaking a mainsail would have a draft of around about 40% in average conditions whilst a genoa would have a draft of about 35%. By moving the draft forwards our sails will have less drag but this will be at the expense of our pointing (i.e. it is a good idea if our boat is up to speed and we will be able to make the windward mark with ease). If we move the draft further aft then our pointing improves and we also gain more power.
Already we can see that careful thought should be given to the horizontal positioning of our draft. In addition to this we also need to consider how much draft we have at different heights of our sail. In heavy winds this is simple to define. We do not want to have much depth at the top of our sails because the increase in power will cause excess heeling. In heavy airs we want to flatten off the top of our sails so that the air is spilled from them. In lighter winds we want to have as much depth as possible at the top of our sails. There are lots of reasons why this is a good idea:
- There are stronger winds higher up so we want to tap the strength of those winds.
- Because the chord length is shorter higher up the sail less drag is created if we have a deeper sail.
- Less of the air is lost vertically off of the sail.