We recently add a brace to help stiffen up the hull.  The brace runs from the chaiplates to just forward of the mast, and another from the foredeck straight down to the mast step, just in front of the mast.  While doing this, we noticed how much the boat was flexing.  We stepped the mast and had nearly NO tension on the rig.  Maybe 125 lbs on the shrouds.  The vertical piece had a gap of at least 1/8 inch.  This was of course before anything was bolted in, just in place for fitting. Kind of eye opening.

    The parts we used are a bit heavy, all together its about 3.5lbs.  The brace is made from 7/8inch anodized aluminum tubing with a .050 wall thickness.  We used a conduit bender to shape it.  And a pair of tube ends mounted between the chainplates and the guy cleats.  The whole mess meets at the underside of the foredeck in a solid oak block.  The vertical strut and cross brace are pinned to the oak block.  The vertical strut runs down and into the mast step. We had to squash the strut a little to get it to fit into the step.  The mast was also moved back 1 notch, to 9'11.5" from the transom.
 

This is the underneath part of the foredeck.  The Oak piece is bolted to the foredeck and the cross bar is passed through and pinned to the oak,  to the opposite chainplate.  The vertical piece is piece is pinned to the oak and then down to the mast step

 

Here is the Chainplate area. The outer bolt hole of the tube end is through bolted right through the gunwale.  The inner is through bolted into the tank.  Tough to reach up there.  The tube is pinned to the tube end here also.

 

Here's a shot at the verical brace going down to the mast step.

And Just a couple of overall shots.