When we are sailing to windward, the sail acts like the wing of an aircraft. Air passing over it produces a low pressure area on the lee side and a high pressure area on the windward side, which creates lift.
This lift from the sails is countered by the lateral resistance of the keel. Without a keel, a sailing boat would simply go sideways. But, as long as the total force – the centre of effort on the sails
– is countered by the lateral resistance of the keel – what’s known as hydrodynamic force – and these forces are in balance and acting in line with each other, the boat will sail forwards.
On the physics front, we also need to consider wind shear. The friction of the water on the gradient wind causes it to back and slow down the closer it comes to the surface of the water and so there is a difference in wind speed and wind angle between the top of the mast and the deck. The higher up, the greater the wind speed and the more the wind angle moves aft. The lower down, the lower the wind speed and the more the angle moves forward (backed).
So we need to be able to adjust the twist in our sails to take advantage of this.
The headsail acts as the forward driving force, and the main, affected by the sideways force of the wind, is more like a rudder, turning the bow to windward as it powers up. So we need to know what sail shape we should be making to get the most out of the wind we are given. In light airs, we want a fuller sail shape – a more powerful shape – and in heavy airs a flatter, less powerful shape. There are several elements that we can adjust for the headsail; twist, depth and draught position. And for the main; twist, depth, draught position and helm balance.
All of these can be adjusted easily with the exception of draught position, which requires us to alter the halyard tension. And as I don’t think it’s realistic for us to be running up and down tweaking halyards for every point of sail and wind strength, I’m leaving draught position alone. The controls for adjusting the set of the headsail are:
■ Forestay tension
■ Halyard tension
■ Position of the sheet lead
■ Car on the genoa track
■ Sheet tension