Out sailing the F20C on Saturday in 15 knots, 4 ft waves, clocking 17-19 knots downwind, crew trapped back and chickened in, both of us grinning from ear to ear. Bear away on a 6ft wave, speed over 20 when suddenly BAM, the leeward rudder breaks away. Crew comes in, chicken attached and slightly entangled in the sheet, so can't jump forward to lower the chute. I tell him to grab the rudder and move towards the chute, only to hear the shocking reply: there is no rudder !! Quick look back, and indeed both transoms have been freed of their burdens.
The boat slows to about 11 as the chute is released, and heads up a bit as I douse. She feels nice and calm, and steadies to about 7 knots on a beam reach as we release the sheets. Good thing we're about a mile from shore, because it really doesn't look like she wants to stop. Adrenaline does wonders to the thought process, basics step in and we release the jib (as far as possible with the self tacker) and sheet in the main. OOOPS – she bears away 30 degrees and speeds up to 15 knots.
Quickly prepare the drogue we store in the tramp pocket, but realize that at 15 knots I can't deploy off the bows… hold on tightly to the shroud, lower feet into the water to windward, and gradually she rounds up a bit and slows, enabling me to deploy the drogue over the bridle, at which point she sits calmly head to wind.
In the meantime, my partner discovered that the rudder assembly was being towed behind us by a loop of shockcord, and raised it aboard as we were slowing (we have a loop of shockcord run through the hole in the upper casting and around the gudgeon so the rudder arms don't flip over when we capsize. A while ago we passed the cord through the chickenline attachment shackle on the gudgeon, and that saved our rudders). A quick inspection showed both rudder castings had sheered at the attachment points, and could not be reconnected.
Although it seemed like ages without any control at high speed, it took 4 minutes and 800 yards from breakage to stop.
A quick call for a tow, lowered the main, released the jib and we were back ashore safely.

Several observations and questions:
We have been suffering from intermittent lee helm both up and downwind. The mast is raked correctly for our weight, and we had just adjusted the rudder rake as far back as possible, which helped a bit, but the helm was still neutral at best – a bit of weather helm is not only easier to control, but would have made a real difference without rudders (remember that youtube movie of the X40 losing rudders and running straight into a wall ??). Is neutral/lee helm a common problem with other F20C boats? Could there be interaction between daggerboard height and lee helm?

The collapse process is reasonably clear – the bottom casting broke at the lower attachment point, rotated the casting around the pivot screw until it tightened, at which point the upper attachment point broke. The leeward rudder pulled back at high speed on the windward rudder, broke the lower casting attachment point, and the upper point flew off the gudgeon. But, why did a reasonably new casting break?

We reaffirmed our policy of always sailing with a drogue and cellphones.

Damage summary – 2 rudder castings, 1 small tear in the chute from the slow takedown on a beam reach, 1 short length of bungee – could have been much worse.