http://www.classicseacraft.com/commu...5&postcount=31
Dave Gerr's book mentions that trapezoidal spacer requirement. The PDF linked in that post shows a floated bulkhead and elastomer in the gap and to create the fillet.
I assume it (the USCG mandated spacer for certain boats- T-boats?) looks like pic # 2 here:
http://www.bateau2.com/howto/foam_phantom.php
I understand the concept but haven't actually implemented it yet in glass and foam or wood. I did something a bit similar on a 30 ton aluminum boat- but that's more like a skin doubler.