It sounds like you are unclear on the basics of fiberglass construction. This is a draft of an article that I wrote for another venue that might be helpful. It is a little dated but the basics are still correct.
A Primer on FRP
FRP (Fiberglass Reinforced Plastic- the technical name for 'fiberglass' construction- sometimes also called GRP) had become the primary way that pleasure craft have been built since the late 1960’s. There are a lot of ways to build a FRP boat and a lot of variations on each method. The three most common are Monocoque, cored and framed. You often hear people use the term ‘Solid Glass Construction’. This is actually a very vague and not a terribly precise description of the structure of a FRP boat. As the term ‘Solid Glass’ hull or construction is typically used to mean a boat that does not have a cored hull. A non-cored hull can be monocoque (the skin takes all of the loads and distributes them), like many small boats today and larger early fiberglass hulls or framed as most modern boats are constructed today. A cored hull is a kind of sandwich with high strength laminate materials on both sides of the panel where they do the most good and a lighter weight center material. Pound for pound, a cored hull produces a stronger boat. Cored hulls can also be monocoque or framed.
Framing helps to stiffen a hull, distribute concentrated loads such as keel and rigging loads, and reduce the panel size, which helps to limit the size of the damage caused in a catastrophic impact. Framing can be in a number of forms. Glassed in longitudinal (stringers) and athwartship frames (floors and ring frames) provide a light, strong and very durable solution.
Molded ‘force grids’ are another form of framing. In this case the manufacturer molds a set of athwartship and longitudinal frames as a single unit in a mold in much the same manner as the rest of the boat is molded. Once the hull has been laid up the grid is glued in place. The strength of the connection depends on the contact area of the flanges on the grid and the type of adhesive used to attach the grid. This is a very good way to build a production boat but is not quite as strong as a glassed in framing system.
Another popular way to build a boat is with a molded in ‘pan’. This is can be thought of as force grid with an inner liner spanning between the framing. This has many of the good traits of a force grid but has its own unique set of problems. For one it adds a lot of useless weight. It is harder to properly adhere in place, and most significantly it blocks access to most of the interior of the hull. Pans can make maintenance much harder to do as every surface is a finished surface and so it is harder to run wires and plumbing. Adding to the problem with pans is that many manufacturers install electrical and plumbing components before installing the pan making inspection and repair of these items nearly impossible.
Glassed-in shelves, bulkheads, bunk flats, and other interior furnishings can often serve as a part of the framing system. These items are bonded in place with fiberglass strips referred to as ‘tabbing’. Tabbing can be continuous all sides (including the deck), continuous on the hull only, or occur in short sections. Continuous all sides greatly increases the strength of the boat but may not be necessary depending on how the boat was originally engineered. The strength of the tabbing is also dependent on its thickness, surface area and the materials used. When these elements are wood they can often rot at the bottom of the component where the tabbing traps moisture against the wood.
The strength of laminate (in either cored or non-cored panel) depends primarily on lay-up quality, kind of fibers used, number of laminations, and orientation of cloth. But also it depends of how carefully the laminate is handled and the ratio of resin to laminate. Glass and carbon fibers before they are laid up are quite brittle, and folding the dry laminate can break some of the fibers in the laminate. This weakens the material substantially. Historically, production manufacturers would cut multiple layers of each piece of laminate to be used in the manufacture of a particular boat and then fold the pieces and store them in a pile until they were needed. This of course created weakened lines within the fabric. Most quality production builders avoid folding the laminate today.
When it comes to the actual fibers, there are a number of properties that are considered:
Strength in tension- (Tensile Strength) The point at which the fibers can be pulled apart,
Strength in compression- (Compressive Strength) the strength at which the fibers crush,
Elongation (deflection properties)- This is the amount that a material changes length for a given pull or push on the fiber. This is usually given as the Modulus of elasticity (E), which is the length in inches that a square inch of the material elongates or compresses per pound of force. When we deal with FRP there is often a different E for tension and compression. Since the resin is typically responsible for taking a large portion of the compressive loads but have almost no tensile strength, the focus is usually on the E (sub) t or the Modulus of elasticity in tension for the given fiber.
Orientation: The direction or directions that the fibers are oriented within the fabric. Also how the fabric is made. Flat fibers oriented the same direction (tows) and woven roving where the fabric is essentially straight are very strong ways to use fiber. Woven fiberglass cloth has a lot of kinks in the yarns and so are actually weaker and stretchier. Mat is not terribly strong because it uses short length fibers.
Abrasion resistance: The ability to withstand exposure to a rough surfaces once the resin and/or the gelcoat has worn through.
Laminate materials are chosen for their strengths and weaknesses in each of these properties, as well as, cost, of course. Because a fiber is low stretch it does not mean that it is also high strength, and just because a fiber has high tensile strength it does not mean it has high compressive strength. Resins have their own properties and, while they are far less important to the overall strength of the composite than the fibers in question, the choice of resin makes a very big difference in the ultimate strength of the part, as well as, its fatigue resistance.
The three most common resins are Polyester, Vinylester and epoxy. Polyester is a group of resins that can vary quite extensively in their properties. It is the least expensive and the most commonly used resin. It has poor ductility, impermeability and resistance to fatique as well as being very poor in developing secondary bonds. It is often modified to increase or decrease cure times. One iof the best features for production boat building is that polyester will not fully cure until deprived air. This allows muliple laminations with a laminating resin without sanding between laminations. The last lamination is a finishing resin which contains a waxy material that foats to the surface and seals out the air permitting a complete cure.
Vinylester is a family of vinyl modified polyesters. This is a wonderful material. It has excellent ductility and memory, great fatique resistance properties, and is easy to work with. Used heavily in the helmet industry it has come down greatly in price and is being used pretty extensively on even high volume boats.
Epoxy has a whole range of extremely wonderful properties. It really shines where secondary bonds are important. Unfortuneately it is very expensive and harder to work with than the other resins and so is rarely used.
To be continued.