While the originals were built without glass, they did have ribs. The fiberglass replaces the ribs. You do need it on both sides because the glass locks the wood in place and you don't want it locked on just one side. Wood either needs to be free to move, or completely locked.

You can make the boat lighter by using lighter glass and/or thinner wood. Both techniques will make the boat weaker, but if you are treating the boat with care, it might not be a problem. You could easily get a way with 4 ounce glass on 3/16" strips and may be able to go lighter. Also consider using softwoods for the gunwales, etc.

: This past spring I built a sized down version (9 1/2') of a Wee Lassie. Using
: 6 oz cloth, it ended up weighing about 18 lbs. Now I want to try to build
: a Sairy Gamp, and would like to keep the weight as low as possible. I was
: wondering if it is absolutely necessary to fiberglass both inside and out.
: I think I can lower the weight by about 3 lbs by using 4 oz cloth, but I
: would like to get it lower yet. Would it be a very bad idea to fiberglass
: the outside only, while coating the inside with 1 coat of Epoxy to seal
: it?