A Cunningham is an adjustment line used to increase the performance of the mainsail by changing its shape.
Basically, it is a rope that acts as a downhaul, which is often connected to a cringle in the luff of the mainsail between the tack and first reef point.
It is used to increase luff tension by pulling the luff down which changes the draft of the sail, bringing it forward or aft. It is hard to get the same amount of tension on the luff using the main halyard alone, as it is a straight pull onto a winch therefore the Cunningham line was invented.
The Cunningham can be set up as a four, six or eight to one purchase so that the luff tension can be tweaked depending how serious you want to be. The Cunningham in the first instance is good for getting creases out of the luff. The next use is to fine tune the depth of the sail when sail trimming.
Using the Cunningham will always leave a little bag on the foot of the sail however try to remember the following:
- Increase tension for strong winds and rough seas
- Decrease tension for light winds in flat seas