As much as neither of us would ever expect to see these words, I agree with Seabreeze97 100%. I do not believe that early fiberglass boat designers did not know the properties fiberglass. (I apologize that most of what follows was cut and pasted from drafts of articles that I had written for other purposes.)
As I have said here before, earlier boats had heavier hulls for a lot of reasons beyond the myth that designers did not know how strong fiberglass was. Designers knew exactly how strong the fiberglass of that era actually was. The US government had spent a fortune developing fiberglass information during WWII and by the early 1950ís designers had easy access to the design characteristics of fiberglass. (Alberg, for example, was working for the US Government designing F.G. composite items when he designed the Triton and Alberg 35) Based on conversations that I have had with designers from that era, I believe that the reason that the hulls on these early boats were as thick as they were had more to do with the early approach to the design of fiberglass boats and the limitations of the materials and handling methods used in early fiberglass boats.
Early designers and builders had hoped to use fiberglass as a monocoque structure using an absolute minimal amount (if any) framing which they felt occupied otherwise usable interior space. Along with decreased maintenance and reduced potential for leaks, the added useable interior volume was a major selling point in the fiberglass boat literature of the day.
But without framing, there are important structural reasons why these early boats has such thick hull laminate. On its own, fiberglass laminate does not develop much stiffness (by which I mean resistance to flexure) and it is very dense. If you try to create the kind of stiffness in fiberglass that designers had experienced in wooden boats, it takes a whole lot of thickness which in turn means a whole lot of not very useful weight.
Early fiberglass boat designers tried to simply use the skin of the boat for stiffness with wide spread supports from bulkheads and bunk flats. This lead to incredibly heavy hulled boats and boats that were still comparably flexible compared to earlier wooden boats or more modern designs. (In early designs that were built in both wood and fiberglass, the wooden boats typically weighed the same as the fiberglass boats but were stiffer, stronger, and had higher ballast ratios)
The large amount of flexure in these old boats has proven to be a real problem over the life of the boat. Fiberglass hates to be flexed. Fiberglass is a highly fatigue prone material and over time it looses strength through flexing cycles. A flexible boat may have plenty of reserve strength when new but over time through flexure fiberglass loses this reserve. There are really several things that determine the overall strength of the hull itself. In simple terms it is the strength of the unsupported hull panel itself (by 'panel' I mean the area of the hull or deck between supporting structures), the size of the unsupported panel, the connections to supporting structures and the strength of the supporting structures. These early boats had huge panel sizes compared to those seen as appropriate today and the connections were often lightly done.
This fatigue issue is not a minor one. In a study performed by the marine insurance industry looking at the high cost of claims made on older boats relative to newer boats and actually doing destructive testing on actual portions of older hulls, it was found that many of these earlier boats have suffered a significant loss of ductility and impact resistance. This problem is especially prevalent in heavier uncored boats constructed even as late as the 1980's before internal structural framing systems became the norm. The study noted that boats built during the early years of boat building tended to use a lot more resin accelerators than are used today. Boat builders would bulk up the matrix with resin rich laminations, and typically used proportionately high ratios of non-directional fabrics (mat or chopped glass) in order to achieve a desired hull thickness. Resin rich laminates and non-directional materials have been shown to reduce impact resistance and to further increase the tendency towards fatigue. The absence of internal framing means that there is greater flexure in these older boats and that this flexure increases fatigue further. According to this study, apparently, there are an increasing number of major marine insurance underwriters refusing to insure older boats because of these issues.
Although many European and Australian and NZ manufacturers had used internal framing very earl on, by the 1970's US manufacturers began experimenting with a variety of internal framing systems, with Ericson, being one of the first US companies to produce a production boat with a molded "force grid" internal framing system. With the common use of internal framing, the weight of hulls were able to be reduced while their strength and stiffness increased.
Like any other period in boat building, the 1970's has its own issues. While many manufacturers were adopting better material handling practices, better lamninating schedules and adding internal framing, not all were, but most were beginning to lighten laminate schedules so that many of the issues of the 1960's can be found more extremely on some boats from the 70's.
Probably the biggest issue with boats from the 1970's was the blistering issue. By the mid-1970's this was an across the board problem within the US built fleet. The problem remained serious well into the mid- to late 1980's, depending on manufacturer, with the worst period for blister prone laminates being the early 1980's.
Once you get past the 1980's, the basic engineering and lamination of boats improved greatly across the board. But then you start getting other issues, which depending on make or model might include such dubious practices as glued structural components rather than tabbed, outward flange hull to deck joints, full interior liners being used to transfer working loads to the hull, poor qualitycontrolled cored hulls, and under-engineered hull to keel structures.
Which is to say, that using broad generalities there is no one period that automatically better than all the others. Each had period had some well built boats and a whole lot of not so well built boats. The good news is that you are only buying one boat. If carefully research the makes and models that had reasonable build practices, and find a good clean example, you should be fine. There are no easy answers here.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay