The changing material trend is a result of many things. Early polyester resins and gel coats were primarily "orthophthalic" resins. Gel coats (tinted, non-reinforced outer shell material) of 1/16" and 3/32" were common. As boat builders in the late 70's found thick gel coats cracking and also in trying to reduce construction costs, thinner gel coats (about 1/32") were used. This led to a proliferation of hull blisters because the gel coat has better moisture barrier properties than laminate. New materials with better moisture barrier properties were developed to combat the blisters. These included isophthalic polyester and a new breed of resin - vinylester. Both isophthalic polyester and vinylester were more expensive resins and thus many boats were and are being built with multiple construction techniques. The more expensive, but better moisture barriers, are used for the gel coat and outer hull materials, with less expensive materials inside or above the water line. At the same time, advances in reinforcing materials like carbon fiber, kevlar, E-glass and others have led to stronger hulls (or equivalent strength) with less resin and less reinforcing material. Most hulls today use a cored construction for the stiffness that it provides. This construction utilizes a skin of laminate over balsa or foam, followed by laminate on the inside to form a section like a "box" girder. While the core offers little inherent strength, it transfers any force perpendicular to the hull, into compressive and tensile forces in the "skins". One disadvantage of this construction, is that whenever one of the "skins" is compromised by a scrape on the bottom, a loose through-deck fitting or whatever, water can leak or seep into the core and become a major repair headache. In addition to the new polyester and vinylester materials, epoxies have come a long way since the 60's. While they are still a lot more expensive than polyester, they do offer several advantages. Epoxy is generally stronger than polyester and has much better adhesion qualities but has an inherent disadvantage in that it does not make a good temperature stable system to use it over gel coat, (two different rates of temperature expansion) nor is there a good way to build-in the surface color like gel coat. Fortunately, at the same time, linear, or multi-component, polyurethane paints have come into use that actually have some better appearance and care characteristics than gel coat. The two part polyurethanes are confined to above the water line use, so combined with epoxy barrier coats and bottom paint, epoxy construction can be a great performer. Therefore, epoxy combined with other new construction techniques, has become a viable alternative to traditional polyester construction.
Regulatory agencies like EPA are also forcing some of the changes in boat construction that we see today. One of the voc's (volatile organic compounds) released in the process of molding polyester fiberglass, is styrene. This is the chemical that you generally smell when you get near a boat building shop. The EPA has been getting tougher on styrene release, prompting some manufacturers to use a totally enclosed process like "SCRIMP". See TPI Composites to understand the process. The "SCRIMP" process (or any vacuum bagging process) has a side benefit of producing very dense reinforcement (glass) - resin ratios. This provides stronger, more moisture-resistant hulls with less resin and thus less cost and weight. Other manufacturers are switching to epoxy construction for parts of the boat requiring high strength as well as blister protection. There are no styrenes released from the curing of epoxy and thus it is an "environmentally friendly" material to use. Because the vacuum bagging closed processes like "SCRIMP", save quite a lot of material, using them with epoxy also can make epoxy a more cost-competitive process.
With all that being said for material development, there was also a lot of variance in how different designers and manufacturers used the materials available to them. Some builders, not familiar with the strength of fiberglass, used the same scantlings that were used for wood construction. Several of the boats constructed in the early 60's have hulls that are more than an inch thick of solid fiberglass. There is an interesting story about the first Bill Tripp designed Block Island 40, "Seal" that took a big chunk out of a concrete fuel dock in Bermuda without any damage to her hull. That hull was reported to be in excess of 2" thick with solid glass construction. After designing the Block Island 40, Bill Tripp was experimenting with materials by any method he could find. One reported "test" method was using his car (Just as an aside, that car happened to be a Jaguar XK-140!!) to drive over samples of laminate in his driveway.
Design Rule Implications
Fiberglass Parts and their Assembly
The hull "halves" were bonded together on the centerline and reinforced with wide strips of woven roving. Six rolled, woven roving stringers, running nearly the full length of the hull, were added for longitudenal stiffening. The deck was molded with the cockpit but without the coach roof. It was cored along the side decks, in the foredeck, cockpit sole and lazarette with a type of urethane foam although I don't know today what it was exactly. Extra laminate was used around the mast entry for stiffness and strength. The deck and cockpit were assembled to the hull before any bulkheads were installed. The entire hull-deck joint was then glassed-in with strips of woven roving. After the hull-deck joint was completed, bulkheads were installed at both ends of the main salon, aft of the chain locker and forward of the head. Partial bulkheads were installed aft of the cockpit, in the vee berth and for the hanging lockers.
After the bulkheads were glassed solidly to the hull and deck, (Note that in today's boats, most bulkheads are not glassed to the overhead.) the engine and furniture sections were installed through the open coach roof. With the furniture installed, the coach roof with hatches, was added and glassed to the deck section (on the cabin trunk). This left a mold line all the way around the cabin trunk that was covered by a wood trim piece. All of the external wood trim was fastened from inside the hull so that there were no exposed fasteners or bungs on the exterior. The ports were then installed and the interior overhead was coated with what I can only describe as a material like gelcoat/plaster. It had a glossy white finish to compliment the mahogany interior. The jointer work on the inside of the cabin was very nicely done. See Javelin 38 Line Drawings (PDF) for the interior layout, and see Javelin 38 Sales Brochure (PDF) for the standard equipments.
Additional Layout Notes