The current generation of masts is more flexible across the long (fore/aft) axis than in the short (port/starboard) axis of the cross-section.
They can be set up to flex significantly under mainsheet and downhaul (Cunningham) tension going upwind, when the long/soft axis is closely aligned with the centreline of the boat.
Downwind, when the mast is rotated approaching 90 degrees (so the long/soft axis is across the boat), and mainsheet and downhaul tension are less, the mast remains straighter, making the sail deeper.
The deeper sail allows an increased lift coefficient. This is important for downwind sailing since sail area cannot be increased.
Automatic depowering is desirable because it reduces the workload on the skipper.
One consideration is to raise the centre of effort (CE) sufficiently to induce enough heeling moment to fly a hull early in the wind range.
The above key factors lead to a very efficient boat with some unique traits:
– The displacement to length ratio of each hull is orders of magnitude greater than most other classes.
– Hulls also have a very high length to beam ratio, so wave making resistance is less dominant and wetted area is important.
– The hulls have to work through a 100% increase in displacement (from taking half the weight of the boat when upright to supporting all the weight when sailing on one hull) over a wide range of boat speeds.
– With advances in rigs and sailing technique, the boats are spending more and more time on one hull. One of the extremely rewarding challenges inherent in the class is learning to ‘do the wild thing’, sailing downwind on one hull.
– Construction favours platform stiffness within the minimum weight. Beams are increasingly unsupported (strikerless) and bonded to the hulls.
This is often justified as being connected with angled and curved foils but much confusion exists regarding the physics involved.
It is therefore worthwhile to examine first the effects of hull shape and then the story of curved foils to fully understand the latest A cats.
Stay tuned for Part 2…