hobie17intheuk, When you say sheered off at the top, do you mean that the integrated hooks have been broken off? If so I had a similar problem when I first started sailing my H17. I made the distance (about 5 inches) between rope handle and the upper stop knot (allowing the boards to extend no further than 14" below the keel). When I hit some rough water the knot slipped pass the stop all the way to the handle. This extended the board way too low, the front hook sheered off and spring went to Davy Jones Locker. Now I make the 14" distance from where the handle is at the stop. Mbounds is right both are pricy to replace (new boards are ~ $350.00 ea and springs over $50.00). I had to buy a new spring, but I was able to rebuild the board.
To rebuild the board hook I drilled a 3/4" hole approximately 4" into the end of the board starting from where the hook used to be. I then used fiberglass resin to glue a 6" 3/4" threaded fiberglass rod http://www.indelco.com/pc-10424-1252-aic1.aspx
into the hole (resin in hole first, then use rod to force out excess, no air pockets
). I used a threaded rod for both physical and holding strength. Let harden. Then wrap the first inch or so of the exposed rod with resin inpregnetated fiberglass knitted rope http://china-gasketmaterial.com/en/fiberglass_and_ceramic_fiber03.htm
to build up size and shape. Let harden. Use saws, files (or my preference, a Dremel) to cut hook shape to match original. I have used the rebuild board for 4 seasons of hard sailing with no problems.