When you start breaking things is when you know that youre on the wrong side of the 'safety factor' curve in structural design.
A little bit of spit and polish, a wee bit more of cross-section and strength and she'll be able to scream again in 15-20kts. .... and a wee bit more spit and polish, a wee bit even more cross-section and strength and she'll be able to scream above 20+ kts.

When youre designing on the 'hairy edge' you dont know how close you really are until 'something breaks' ... and the next time you bump up the 'safety factor'.