Thanks for your insight folks,
From the specs on the site, it's a big weight hit to move up to a CS15 from a s12, a move from 120# to 300#. Not having seen the plans, only pics, I'd guess that some of that exponential weight increase is in heavier scantlings, structure for the mizzen, and decking.
A BRS almost fits the bill, maybe in "lightweight" trim, but I'm attracted to the more advanced hull form of the spindrift/CS series. I like the lack of a deck on the BRS and it fits my inland lake protected water projected use.
Ray, thanks for the thoughts on the tradeoffs of the designs, the flotation and utility of the fore and aft seating is one of the really attractive design elements of the S/CS series and keeps bringing me back, that weight tradoff would be worth it to me. One that might not be worth it is the additional weight for the mizzen setup with step and partners and gear and all the rest, another reason my preference is toward the spindrift cat rig. Don't get me wrong, I sail a cat ketch now and appreciate the utility of the rig, but I'm focusing on simplicity and weight on this concept.
The thought of building a CS15, (or the delicious looking upcoming Lapwing ) without decking but with the traditional Byrnes fore and aft seating arrangement appeals to me, and I'd be interested to hear what Graham would have to say about the effects of doing that on weight and integrity.
As far as the safety aspect of the lack of decking on the GIS goes, I'm an inland lake sailor would only expect to be out in fair weather on protected waters with this boat.