Both possible outcomes happen quickly and are slow in terms of time around the course!
|Bow up feedback loop being allowed to continue.
Image credit unknown
So as the boat comes out of the water, the vertical component of the foil force INCREASES, causing the boat to want to rise further.
|Instability in pitch is combined with instability in ride height
due to the foils effectively becoming more horizontal with increasing ride height
In other words acceleration replaces pitch angle as the determining factor in the amount of lift produced.
|Sailing downwind with an AoA ‘reserve’ allows the boat to accelerate.
Higher speed and course/sheet adjustments can compensate for the reduction in AoA caused by increased drive force.
It can be thought of as analogous to riding a unicycle: It can be difficult to master and laborious but it is possible within limits.
Time and again one reads in forums and hears sailors comment that in certain conditions the foils need to be partially retracted to limit the amount of vertical lift and bring the boat back under control.
This is a serious limitation: retracting the foils simply reduces dihedral angle, giving up the full benefits of vertical lift just at the speeds where the greatest advantages occur (remember that as speed rises foils get more effective and hull drag becomes more expensive).
|Retracting a curved foil (constant radius) reduces effective dihedral|
Yet this limitation is accepted as inherent in the current winning configuration.
One manufacturer has even increased the radius of curvature of their updated foils.
They have made the newer foils straighter partly because they were getting ‘too much lift’.
Surely a better solution would be to improve dynamic stability and avoid giving up the advantages of foil assistance, just as speed rises sufficiently for the advantages to become significant.
That is the current trend and will be the subject of the next post.