Sail battens are utilised on many sailing boats. They are what keeps the mainsail in the right shape and prevents it from just becoming a flat surface. They are always at the top of sail and some boats have 1, 2 or even 3 or more battens, depending on the type of boat.
There are also two different types: full and half. The full battens are on high performance boats, so that the entire sail is held in the correct shape. Half battens are used on most other boats to keep the sail mostly in the right shape.
As you have already learnt from the physics of sailing, a curved surface is required to create the aerodynamic force of lift and so without sail battens the sail would be unable to move the boat.
The genoa does not usually have battens, because it has a natural curve to it and also the bottom of the sail is not held in a rigid position by a boom, like the mainsail is, which allows the sail to be fully curved, unlike the mainsail.
The boom, is the reason that the mainsail has battens in it, as it means that the bottom of the sail is straight, which should mean that the entire sail is straight. Of course this is not so.
The sail is made to have extra material in its centre so that a curve shape can be achieved. However without battens this extra material would flap like a flag.
The battens can be at sometimes an annoyance, because they are so hard that if you tack or gybe, they may stay in the same original position and keeping the sail curved in the wrong direction, essentially creating a sail that is stalling and may cause you to go backwards.
When rolling the mainsail up after taking it out on the water, the sail must be rolled so that the sail battens are parrallel to the roll itself. This means that you should always roll along the leech of the sail instead of the luff of the sail that you put in the mast when you hoist it up.
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