If we look at our headsail sheets it will become clear that rather than just run back to winches, they are first run through strong points on the deck known as ‘cars’. These cars are usually fixed onto a track allowing them to be adjusted forwards and backwards and locked in place. Why is this?
Well, the reason for a car is twofold. First of all, the car provides the correct run of the sheet to the winch, thus stopping the line from jumping off the winch or riding over itself. It is adjustable so that the angle of the sheet from the car to the sail can be adjusted.
Moving the car aft will reduce the vertical moment of the sheet and pull back the foot of the sail. This allows the upper part of the sail to twist open at the luff. Moving the car forward will increase the vertical moment of the sheet and thus close the twist in the upper part of the sail.
A car should always be set when a different headsail is bent on or reefed as the position of the clew will move as the sail area / shape is changed. Car positions may also be changed when the point of sail changes. The trimming of sails is a dark art and a science and requires an understanding of the tools available to you to change sail shape and efficiency.