Its amazing just how a few knots more speed can induce huge additional loadings on lever points. Maybe the F20C is repeatidly going just that bit faster than the F18 designed casting can cope with.
I think the curved boards give greater lift vertically and shed some of the lateral stability on to the rudders which in turn loads the casting. Any N-20 owner knows how susceptible to failure that part is. I broke 3 in ONE beach start( Hatteras leg) during the Worrell. 5 total THAT race. They beefed them up with a side strake but for the price of the F-20C now I guess they should be Titanium. The curved board program is going to have some speed bumps for sure, this very well may be a by-product of that.