jp, 

i have played around a lot with light-builds with some pretty good results.

i recently completed a petrel kayak that came in at 30 lbs fully rigged and have studied various layups described in nick shades book 'building strip-planked boats' where he lays out tables of different layups (core and skin combinations and their strength characteristics).

in my 30 lb petrel project, i did both the inside and outside glass with 4 oz s-glass on a 3/16 core and very careful epoxy work.  my inside seams were executed with 2 inch kevlar tape.  careful fillets and the use of light-weight fillers where appropriate were also an important piece of the equation.

professionally builts single skulls used in competition have a minimum weight of about 30 lbs.....and no professionally built boat has a problem coming in at the minimum legal weight.

in the oxford shell, probably the easiest way to get it at the 30 lbs target would be to build it as a strip-built boat. in a strip built there is not going to be the weight of sheer clamps and nails....and a cedar strip core is significantly lighter than a okoume core.    another approach that may be practical is to do it as a hybrid with a okoume hull in plywood and the deck as strip.  again, this would allow you to dispense with nails and sheer-clamps and have a very light deck.  at worst, you can build as is and do a good job of cutting the weight by minimizing the size of the sheer clamp by routing it down to look like quater round and dispensing with the bronze nails and using duct tape to temporarily hold the deck in place while the epoxy sets.  careful mitrework with a block plane to to allow the bottom and side panels to mate perfectly will also minimize the epoxy use and save a bit of weight.

hope these ideas help

h