Under pressure: why mainsail track cars are key
The mainsail batten cars which broke on Virbac-Paprec 3 have a double role. Just like the normal mainsail track cars they hold the luff of the sail to the mast, but the batten cars also have a secondary and very important role in supporting the battens, which control mainsail shape and tension.
The main problem which forced the Wellington pitstop for Virbac-Paprec 3 was damage to some of the boat's mainsail track cars. These cars secure the battens of the most important sail on the yacht. In order to understand what these are and why these apparently simple parts (known as 'batt cars' in the world of sailing) are so important, one must look at the evolution of the mainsail track.
Historically, the luff of the mainsail (the forwardmost vertical edge of the sail, which runs along the mast) was secured using a simple rope that was inserted into a groove along the mast, much like today's jibs run along the forestay. This system was then replaced by sliders (small plastic parts tied to the sails that run along a groove, which also give the batten cars the name of sliders), which made hoisting and dropping the sails much easier.
Nowadays, on high performance yachts that system has been replaced by a track on the mast with cars with bearings (which the sail is attached to) which run up and down the mast, similar to the type used for the mainsail sheet. More recently, in order to boost the wing profile of the sail, cars have been introduced with another secondary function: to anchor the battens.
The mainsail battens are responsible for giving the sail the desired aerodynamic profile. However, they often generate problems. They support a huge amount of pressure at the front tip and often deteriorate or even break the edge of the sails where they are attached, which is cased in reinforcements. That is why cars were created to not only hold the luff to the mast, but to anchor the battens.
Whilst the cars work well for their primary purpose, they also hold the luff to the mast in normal sailing conditions and under a 'regular' force successfully. The problems arise when they're in unusual situations such as when a sail is flapping or flogging. In these cases, the car rattles along the track like a train being jostled from side to side. On an IMOCA Open 60 the violent forces of a large mainsail flogging doesn't make the “train” derail, but instead is liable to cause the cars to fail.
Other difficult situations occur when the mainsail needs furling up, or when gybing downwind. In both cases, the great pressure on the centre of the batten is multiplied and transferred like a lever to the edge where it is anchored. That is when the batten pushes on the side of the car with the sail, towards the bow, as well as the other side towards the stern. A twisting force is created between the car and the track which can make either of the two or the tip of the batten itself break.