John,

Some folks added flotation (ex: air bags, foam) under the middle thwart. The reason is that many have found that the built in floation sometimes isn't enough to float the hull above the top daggerboard slot.  This means that water can come in through the daggerboard box while you are trying to bail.

I have not personally experienced this,

If I were to add flotation I would use something like this http://www.wildwasser.net/overnighter-set-large so my float bag(s) also doubled as dry bags.

Steven David Roberts

http://www.skerrybuild.com