By Penfold

Virtually every type of rig on every type of boat can be said to work on the same aerodynamic principles. What works for one class will most likely, with tweaks and modifications, work for any other.

The basics of boat tuning are fairly straightforward; the most powerful set up required is in light to moderate conditions - when the wind has enough speed to stay attached to the sail but the crew are nearly but not yet fully hiking/trapezing. For most boats this is in the range of 7-11 knots. Below 6 knots and above 12 the shape of the sail will be flatter and more open - in the lighter breeze this is to allow the sail to keep the airflow attached and not cause stalling, and in higher wind it is to prevent the boat being totally overpowered and staggering/capsizing in gusts.

A typical example of this would be the use of mast rake to regulate power. When it's windy and a boat is overpowered, using large amounts of aft facing slope (rake) will, by virtue of the change in angle of the leading edge of that sail, cause the generated force to act on the rig with a vertical component. As the boom is to leeward this force pushes up from the leeward side of the boat so there is less resultant force pushing the boat over. Equally the more power required the more upright the mast should be set.

On most boats the position of the mast step is fixed (within a few mm) and the point at which it is fixed in relation to the centerboard dictates the most upright it is possible to set the mast. The mast may only go upright to the point where the helm has still got a little weather helm. (Weather helm is the tendency for the boat to luff up when the helm is released with the boat being sailed upright.) Much more mast rake would generally be required above 12 knots. The amount of rake will be proportional to the amount the boat is over powered.

Power
Spreaders are the key to power in the mainsail. The straighter the mast is, the more depth will appear in the aerofoil section, so increasing power output from the main sail. The amount of mast bend required will depend on the shape locked into the sail when it was built (Luff curve and fullness). If we return to the idea of maximum power required when the rig is only just up to full power and the crew not fully stretched- when the main is sheeted in hard the leech telltales should just stall. When a little twist (ease on main or ease+car up) is introduced the tell tales should start to fly. In lighter winds this may only be intermittent.

Therefore, if the tell tales need much encouragement the sail is probably to deep, if they always fly it's too flat.

From this knowledge we can set the spreaders. If the sail needs to be deeper then angle the spreader forward to straighten the mast and vica versa if the sail is to deep.

These are the basic principles for setting up the rake and power of a mainsail using the standing rigging to alter rake, and the spreaders to alter mast bend. Next time we will look at how the running controls like Vang, Cunningham, Outhall etc affect sail shape, and how you can use them to change gear on the water…