This topic comes up frequently. Here is my response from a similar thread comparing older to newer boats:
Obviously, one of the most obvious differences between early fiberglass boats and more modern fiberglass construction is sheer amount of weight and how it is distributed. but there are also big differences in how they were built.
There is a very popular myth that early fiberglass boats are as heavy as they are because early designers did not know how strong fiberglass actually was. That''s bunk!
During WWII the US government had done a lot of research on fiberglass composites and that information was pretty readily available. The properties were really pretty well understood. Carl Alberg was working for the Government designing fiberglass military gear when the Pearson''s hired him to design the Triton. He knew how fiberglass worked. What he knew, and as most designers of that era and as we know today, is that while fiberglass reinforced polyester laminates are pretty strong in bending, they are not very stiff. This means that when loaded like a beam, fiberglass laminates can with stand a large loading and bend without breaking but will bend farther than other materials such as the same weight piece of wood with the same loading. (That is why fiberglass fishing rods became so popular in the early 1950''s)
But they also understood that fiberglass is a pretty fatigue prone material and that flexing greatly weakens fiberglass over time and so building a flexible boat will greatly reduce the laminate''s strength over time.
Early designers understood stood all of this about fiberglass. In order to try to get fiberglass boats with close to the same stiffness as wooden boats, fiberglass hull thicknesses were increased beyond what was needed strictly for bending strength. That is why they were as thick as they were.
What was not understood very well was how to handle the raw materials, resins and fabrics, during construction to maintain the Fiberglass''s inherent strength. To achieve the full inherent strength of the various materials in fiberglass used in a fiberglass hull requires:
-Careful mixing of the resins,
-A surprisingly long cure time,
-Careful handling of the reinforcing fabrics (For example folding fiberglass mat or cloths weaken the individual fibers)
-And a proper proportion of resin to reinforcing fiber.
The difference in strength and durability between an ideal laminate and one that was laid up less than ideally can be enormous, especially if allowed to flex a lot over time (perhaps as much as 50% on a unit basis). The extra thickness in the hull might add as much as 30% to the overall bending strength of the hull but substantially less (perhaps between 5% and 10%) to its resistance to puncture (sheer).
One of the really striking things about early fiberglass boats is the almost total lack of internal framing compared to more modern design. Early fiberglass boats were a wonder in their simplicity of design and construction. Early designers viewed the fiberglass hull and deck as a monocoque structure and so really did not try to brace it with a systematic layout of longitudinal or athwartships framing.
Whatever internal framing there was used on these early boats was not tabbed into the hull with the same attention that was given to tabbing by the 1970''s. When I worked in boatyards in the 1970''s it was not all that unusual to see these 60''s era boats come in ''banana''d'', (as it was called which meant flexed until the tabbing on bulkheads, flats and risers had been loosened) by the extremely high rigging loads of that era. I spent a lot of times re-tabbing boats in those days.
Also when you work on these boats it is not unusual to find very resin rich laminations. Resin really adds almost no strength to fiberglass. It is really there to hold the fibers. In early boats, lots of resin was used because it made it easy to wet out the cloth and to get compartively smooth surfaces for layup to layup bonding. These resin rich laminates results in lower initial strength and a more fatigue prone laminate. In the 1970''s this became better understood and today even pretty inexpensive boats are careful to use better ballanced resin to fiber contents. It is quite routine to see vaccuum bagged (or injection/ vaccuum techniques like Scrimp) that produce very light, dense and strong parts within the industry.
While there were some internal elements glassed to the hull they occurred where convenient to the design and allowed shockingly large unsupported panels. When you sailed these older boats and a wave hit the hull, you would feel the vibration of the panel flexing. While this flexure does not equate to weakness, it does equate to the likelihood of more fatigue over time.
On a point by point basis I would compare early fiberglass to newer fiberglass this way:
Resins: Early boat builders tended to use a lot of accelerators in an effort to decrease curing time. The use of accelerators tends to produce a more brittle and fatigue prone laminate. In the Mid-1970''s and early 1980''s resin formulations changed producing resins that are especially prone to osmotic blistering. By the mid to late 1980''s resins were changed again reducing the likelihood of blistering. Today, it is not unusual to find more exotic resins (vinylester and epoxy) used in even mass production boats. Vinylester in particular offers a lot if used in outer laminates. Vinylester is nearly as water impermeable as Epoxy but is far less expensive. VE offers superior fatigue, and blister resistance. When used with higher tech fabrics (even higher tech fiberglass fabrics), VE dramatically increases the strength of lay-up. Boats like the new C&C 99 are using epoxy resins as well.
Early fiberglass fabrics have comparatively short fiber lengths and lower fiber strengths than current materials resulting in less strength. Beyond that they were often handled poorly (folded and stacked) so that the strength of the fibers were reduced further. In the 1970''s as better stress mapping was understood, directional fabrics were developed and even conventional materials were more properly oriented to improve their load capacities.
Today, we use higher strength conventional laminates, and have an arsenal of higher tech fibers range from Bi-axial and Tri- axial oriented fiberglass fabrics, to higher strength fiberglass fibers due to improved fiber manufacturing techniques, materials like Kevlar and Carbon fiber. (Even value oriented builders like Hunter and Beneteau are employing Kevlar in its newest boats for increased strength, stiffness and abrasion resistance.)
Framing, liners and Coring:
Early boats rarely had cored or framed hulls. They also rarely had either structural or cosmetic liners. This is an area that is a bit more complex with good and bad aspects to each of these options. To breifly touch on each type of construction, there is cored and non-cored and framed and non- framed with specialized types of each. 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'' construction is typically used it means 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. They can also be 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 weght center material. Pound for pound, a cored hull produces a stronger boat. Cored hulls can also be monocoque or framed construction. While cored decks are almost universally accepted in one form or another, cored hulls tend to be a very controversial way of building a boat. Done properly , pound for pound there is no stronger, stiffer, more durable way to build a boat. It''s the "done properly" that mekes coring so controversial. Ideally a hull is cored with a closed cell, non-out-gassing, high density foam, that is vacuum bagged into place. Thru-hull orface and bolting areas are predetermined and constructed of solid glass or reamed out and filled with epoxy. All of that makes proper coring expensive to construct. There is almost nothing better than a properly cored hull, and almost nothing worse than a poorly constructed cored hull.
Decks are typically cored with end grain Balsa. End grain balsa offers excellent sheer resistance for a given weight and cost. The orientation of the cells theoretically promote good adhesion with the laminate and also resists the spread of rot. Early boats often had plywood decks with glass over. This is the worst of all worlds. Because of the orientation of the cells plywood tends to distribute rot very quickly once rot starts. Plywood tends to be heavier than other deck cores and does not have as good adhesion to the laminate as other core choice. Plywood was a cheap but not very good way to go.
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). Used in combination, all of which combined provide a light, strong and very durable solution but one that is expensive to manufacture and require higher construction skills to build precisely.
Molded ''force grids'' are another form of framing. In this case the manufacturer molds a set of athrwartship 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 or durable 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 electical and plumbing components before installing the pan making inspection and repair of these items nearly imposible.
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. Continous 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.
Most early boats were non-cored hulls with minimal framing, this allowed a lot of flexure and really put a lot more stress on the minimal framed connections within the boats. Most had balsa or plywood cored hulls.
Hull to deck joints:
Early boats typically had a number of hull to deck joint. Most simply had an inward turning flange on the hull and that was bolted through the deck and toe rail. These thru-bolts were seen as the primary bond and varied widely in size and spacing. They rarely had backing plates even from the best builders of the era. Between the hull flange and the deck was either some form of bedding compound, such as polysulfide (like Boatlife) or organic compounds (like Dalphinite) or more commonly a polyester slurry. All of these are comparatively low adhesion and lifespan solutions.
In the 1970''s some offshore intended cruisers started glassing the joint from the interior but the big change was to higher adhesion caulking/ adhesives in the joint. 3M''s 5200 became a common adhesive for this purpose. Bolt spacing was increased as builders often considered the 5200 to be the primary connection. Outward facing flange connections became more popular because they permit quicker turn around time for the molds and less labor to prep the mold for the next boat. They are inherently weaker and more vulnerable.
Today, most manufacturers seem to be using any one of the earlier techniques with the ''Big Three'' using extremely high adhesion adhessives engineered for the aerospace industry. These produce extremely sturdy joints that should outlive most of the other joint types that have preceded them. You never hear of hull deck failures any more which back in the 1970''s seemed to be a fairly frequent occurance.
Early glass boats tended to use extremely stiff spars and extremely high rig tensions. Without adjustable backstays these high loads were imparted into the hull on a routine basis. They really can take a toll on a boat. It was not unusual to find these early boats so distorted by rigging loads that doors in passageways would not close on a beat.
In the late 1970''s and into 1980''s there was a real shift in turnbuckle design. Some of the more popular turnbuckle designs really had comparatively short life spans and resulted in lost rigs and rigging. By the 1990''s turnbuckle design had changed yet agaib and seemed to have moved toward a more durable engineering.
Over time rigs got lighter and more flexible. This is a mixed blessing. A slightly flexible rig imparts less load into the hull and deck and bend can be increased to depower sails. Taken to the extremes seen in late 1970''s through early 1990''s race boats, they make a rig that is hard to keep in the boat. In the early 1990''s IMS recognized this problem and shifted the ratings a bit to encourage stronger rigs and so rig losses in newer IMS type race boats are compartively uncommon these days. Some of this improvement is the use of Carbon Fiber spars. Carbon Fiber makes a really stiff and shocking light spar material but is very expensive and the jury is still out on the long term life expectancy of carbon spars.
Early fiberglass boats were really engineered as if they were a wooden boat built out of fiberglass. They ended to be more flexible and although heavy, the poorer strength of materials that came from material and handling choices meant that they had very high stresses but they were not as sturdy as they appear. By the 1970''s designers better understood how to engineer fiberglass as fiberglass, but were faced with historically poor resins that resulted in real blister problems. By the 1980''s resins improved, as did fiberglass material handling techniques and rigging design and strength of materials. The blister problem was better understood and higher tech resins and fibers entered the industry. Today''s baots tend to be lighter and stronger than earlier boats. This weight savings is used to produce higher ballast ratios and to produce greater stability or carrying capacities. Hull deck joints have improved in some ways, but I hate the fact that outward flanges are becoming popular again. Blister problems have been reduced greatly and rigs are becoming easier to operate. That said I see popularity of inmast furling mainsails to be a serious negative trend.
At least that is how I see it.