Probably none: except for the smaller boats (less than 14'), the bottoms are made of fiberglass!
Almost all our bottom panels are made of plywood with glass on each side, a fiberglass sandwich.
You may ask the question because most other designers announce the bottom fiberglass as an option but that is because they design plywood boats. Our are composite boats and the fiberglass is already there.
For the small boats
It is not necessary to fiberglass the bottom for strength or resistance to water: all parts are completely coated in epoxy. You may want to fiberglass for resistance to abrasion if you drag the boat on beaches or expect to run aground.
First check your plans: the fiberglass for the bottom may be on the plans. If not, it is very easy to estimate: look at the plans, nesting drawing. You will see the bottom panels on standard 4x8 plywood sheets. That is the area you need.
Another method is to take the length of a small boat with a safety margin. 50" wide fabric, 6 to 10 oz. woven is perfect and can be bought by the yard from our online store:
BoatBuilderCentral.com