You see the statement a lot that “Early boat builders did not know how strong fiberglass was and so made it very thick.” Horse Feathers! This is just plain bunk.
During WW II the US federal government had done a lot of research on fiberglass and the information was widely available by the later 1950's and 1960’s. As a kid in the 1960's, I had detailed literature on fiberglass that pretty clearly analyzed its properties and which included accepted design approaches including published strength and flexure values.
Guys like Carl Alberg, who was working for the government designing fiberglass ammo boxes when he was hired by the Pearsons to design the Triton, knew exactly what fiberglass would do.
They knew that the e-glass of that era was pretty poor quality and was especially prone to flexing and to fatigue. They attempted to design fiberglass boats to be as stiff as wooden boats of the era. This took a lot of thickness since F.G. was very flexible compared to wood. This was especially true on a pound for pound basis. They also knew that if the boats were not as stiff as wood, there would be major fatigue problems. This put early designers in a bind. If they made the glass boats as thick as a wooden planked hull they would be impossibly heavy. If they did not, fatigue would condemn them to a short life.
They mostly chose to compromise. By that I mean they chose to do boats that were not as stiff as the wooden boats they replaced but were heavier. Early glass interpretations of wooden boats were generally heavier and carried less ballast than their wooden counterparts. They were much stronger in bending but not as stiff. As fatigue took place some of these early glass boats became even more flexible which leads to more fatigue, which can lead to a significant reduction in strength.
A large amount of flexure, as is typical in these older boats, was 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. By the early 80's, most and certainly the better boat builders understood this issue and had systematically added framing systems that reduced the spans of the panels.
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 (approaching 50/50 ratios rather than the idea 30/70), 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.
Over the years, the combination of the methods used to handle fiberglass fabrics within the factories, the improved methods usedto make the fiberglass fibers and turn them to fabric, the formulation of the resins used, and the precision with which resins are mixed, the careful balance of resin to fiber ratios and orientation of the fibers within the hull combined with better stress mapping has improved the strength and durability of the matric enormously. The closer spaced framing and structural attachment means that the physical stength of the modern boats far exceed the strength of the earliest thick skinned boats.
But more to the point, the 1970's was a period when boat builders had begin looking for ways to lighten boats. With the price of resin rising rapidly, one natural way was to reduce hull thickness. They simply lightened scantlings but had not yet made the leap to proper internal framing. Boats like the Catalina 30, have skins that are no thicker than modern boats of a similar size, but lack the structural advances that make the even medium quality modern boats much stronger than their predecesors.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 01-12-2010 at 10:06 AM.