Rudder Stock Failure.

Here's a side view of the stock. And here's the complete bottom bracket removed from the rest of the stock, viewed from below.
Side view. Top view.
My thanks go to the gentleman who found the piece that was left floating in the water. I'm pretty sure I know what went wrong, and I don't think I'd necessarily have figured it out without the whole thing to look at. It failed on the leeward side while I was making a rapid change of course to track a gust. I initially assumed that the failure was associated with the weakness that's inevitably caused by the rudder pin, and I was quite surprised because I'd thought that I had plenty of carbon in that area.
I now suspect that the initial failure was at the other end of the detached piece, about half way along the foil. In this area the sideways loads on the foil are doing their best to splay the two sides of the stock apart, and I think that the outer skin simply failed in tension. With this gone the resulting leverage at the front would have been more than any sensible structure could have stood.

On the positive side the gantry and the top of the stock seem fine, with no sign of any consequent damage, and the bottom bracket was hitting the wash from the stern more than I would like, so I've got the opportunity to reshape it to deal with that too. If this is the only significant structural failure I get on the boat I shall be very pleased.

By the way the strange lighting effects on the pictures are because I just put the bits straight on the flat bed scanner and scanned them in directly in a dark room!

I've talked to Andy Paterson about this, and Bloodaxe are building similar stocks with a solid carbon bottom bracket, about 5mm thick. I shall rebuild my stock with a solid bottom bracket like this, suitably shaped. Because its shallower there should be less of a problem with the wash, especially if I angle it up about 15 degrees from front to back, and the solid construction should have plenty of reserve strength.

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