Hulls need stiffeners to withstand shearing, torsion, tensile, etc. forces. Stiffers break up the long spans of fiberglass into more resilant pieces. Types include bulkheads, floors, frames, stringers, and beams.
Bulkheads are athwartship usually plywood barriers inside the hull. Yachts have multiple including chain-locker, full, half, and sail-locker bulkheads. Builders glue these plywood barriers, and the best quality builders use fiberglass tabbing to structurally connect the bulkhead, deck, and hull. Floors are semi-circular shapes that support the cabin sole. Floors are often of fiberglassed over foam. Frames are athwartship stiffeners inside the sides of the hull. Stringers are longitudinal fiberglassed over foam pieces that go from sterm to stem along the inside the sides of the hull cross-hatching with the frames. Finally, beams are overhead, athwartship stiffeners which support the deck and coordinate with bulkheads or the mast.
Another option in addition or in place the above are molded liners. Like decks and the hull, these pieces have separate molds with which workers generate a fiberglass structure. Then these liners are tabbed and glued to the hull and bulkheads. Hunter Yachts makes one-piece molded liners which replace stiffening network. This use of a one-piece liner is the extreme version of composite building. It significantly lowers skilled labor costs along with speeding up mass production. There are two problems with molded liners. First, they block access to the hull. To make replairs in the case of a hull break, you have to cutaway the liner to access the hull. And even to normally access the bilge, the molded liner is limiting because of the build-in structures. Second, the glueing process is tricky and has been prone to poor bonding. Placing liners in and glueing them done must be done in one shot. If the mold is misaligned or mistimed, you can have trouble. Molded liners are to some degree to blame for the oil canning and shifting bulkheads on flim flam production boats like Hunters, Catalinas, and Beneteaus. Quality builders like Hylas and Swan do not use molded liners because of these issues.
Stiffeners prevent a hull from breaking up in serious seas. Bulkheads, floors, frames, stringers, and beams are types of stiffeners. The best example of quality supports is the illustration from Henry Mustin’s book, Surveying Fiberglass Sailboats. The picture shows the extremely strong construction of Nautor exemplified by their Swans.