This is a long one:
Twenty years ago (early 1980''s) was an intersting time in boat building. It was a time of big changes. The boating industry had gone through some big changes in the 1970''s in reaction to the fuel crisises of that period. Resins had changed and in doing so this became a period when blisters had really become a major problem. The worst period for blister problems occurs from the mid-1970''s through the mid-1980''s. A lot more was understood about the properties of fiberglass. From the beginning designers and builders knew that fiberglass has a lot of strenght for its weight but is not terribly stiff. They also knew that Fiberglass was very prone to fatigue so that over time flexing would result in a reduction in strength of the laminate. In early fiberglass boats designers tried to reduce flexing to the levels expected from a wooden boat and so these early boats had thick hull sections that were still a compromise as they tended to flex more than was idea and yet were exceedingly heavy in weight.
By the 1980''s designers understood that they needed to find ways to reduce flexure in order to produce stiffer, theoretically longer lived boats that also offered better performance.
By the early 1980''s, boat builders were building fiberglass hulls that were cored, thinner shell glass hulls with a structural frame or a solid glass hulls with no framing. None of these methods are inherently more durable, or better or worse than the others. They all depend on proper design and engineering.
Let me explain. Lets start with the so called solid glass hull with no framing or coring. If you build a solid glass hull without framing it is either very heavy or very flexible. If you consider boats all of similar weights, this means that a solid glass hull with no framing or coring will be considerably more flexible. Fiberglass hates flexing, and as I mentioned over time it results in fatigue of the bond between the resin and laminate, and a general reduction in strength. This problem is particularly acute where flexing occurs adjacent to rigid structures like bulkheads and bunk flats. I have looked at older solid glass boats where you stand on the ground and could pick out every bulkhead and glassed in structure. In boats that have been sailed hard, these areas are often marred by spider cracks and other signs of distressed laminate. By the mid 1980''s quality builders of solid glass hulls placed soft material between the hull and bulkheads. This reduced print through but does not solve the more global problem of repetitive flexing. Increasing the hull thickness can help some but at the price of greater weight and also greater stresses due to that greater weight. Greater weight in the hull reduces stability and, for a given weight, means less ballast and/or carrying capacity.
Prior to the fuel crisis in the 1970''s polyester formulations were different and were comparatively brittle (but resistant to blisters). As a result of the fuel crisis, the resin formulations used in marine applications were altered, and they were altered again in the early 1980''s as a result of the acute blister problems caused by the 1970''s reformulation.
Beyond that, there is the way that resins were handled. In the 1960 through the 1970''s and well into the early 1980''s mixing proportioning, temperature control and even apply resins was pretty haphazard. Various additives were pretty casually added to the resins, such as extenders, bulking agents and accelerators. Each of these offered some cost advantage, but did nothing for strength.
Probably the worst offenders were accelerators, which increases the brittleness of the resin and weakens it over time. The idea behind accelerators is that tooling for boats (moulds) are expensive. The quicker you can pop out a hull the more frequently you can use a mold. In the 1960''s fiberglass normally took a period weeks to reach a state of cure (i.e. reach something approaching full strength) that it was acceptable to remove the hull and not risk distortion. If you simply over catalyze the resin it will cure more quickly but it will also go off too quickly to have a useful pot life. So in the 1960s accelerators were used to allow a reasonable pot life but speed up the cure time.
The other component in the laminate is the actual reinforcing fabrics. In its infancy, fiberglass fibers were quite short, brittle and needed to be handled very carefully to avoid damage to the individual fibers. In production facilities in the 1960''s this was simply not well known and so fabrics were cut and folded into tight little bundles. In a plant you would see small stacks of these tightly folded and carefully labeled fiberglass fabric bundles around the perimeter of a boat being laminated.
Then there was the cloths themselves. Woven fiberglass is comparatively stretchy and weak because in the weaving process the geometry results in fibers that are folded over each other and need to elongate in order to really absorb a big load. Fiberglass fabrics also take the greatest stress in the direction that the fibers are oriented. In the 1960''s there was no effort to minimize the use of materials that reduced the strength of the fiberglass fibers because of the way that the fabric was woven and there was little or no effort to orient the fibers to the direction of maximum stress.
Then there is the ratio of fiberglass and resin. Except in compression, resin is a very weak material. Resin is very poor in tension, can''t stand elongation and is not too good in sheer. Resin is only there to glue the fibers together and to keep the fibers in column so that the laminate does not fail. The ideal fiberglass resin has no more resin than is absolutely necessary to hold the fibers together and not a tiny bit more.
By the early 1980''s builders were beginning to understand a lot more about how to handle and laminate fiberglass materials. They were still using a lot of non-woven, non directional fabrics (chopped glass and mat). These were cheap and used near the core of the layup had little impact on bending or flexure. BUT these are comparatively weak materials and have been shown to greatly weaken a laminate in puncture, sheer, fatigue, and where impact loads are encountered.
Then there are boats with solid glass hull with internal framing systems but no coring. During the era in question this was very popular with boats build in England. Ericson really popularized this construction in the States with their force grid. Essentially this was a molded frame system consisting of longitudinal and athwartship frames molded in a grid and then glued in. In concept this is a very strong way to build a boat. (It was my favorite at the time.) It reduces flex and in doing so reduces fatigue. BUT it too has its problems. The grids were generally installed with a polyester resin slurry. Polyester used in this manner is brittle and does not form great secondary bonds. In normal conditions, that is no problem but over time, or when exposed to impact, the bonds can fail and can be hard to repair. A better solution is a frame that is hand glassed in while the boat is being built. This is quite labor intensive and requires extra care since it is hard to install a liner over that construction and so the inside of the hull is often exposed in painted form within lockers.
Another problem is with impact in a frame and skin hull. If we are considering boats of similar weight, the weight of the framing by necessity is made up for by reducing the thickness of the skin. This works fine in most ways because the thickness of a fiberglass hull is actually determined by the need to have adequate stiffness rather than by a requirement for absolute tensile, compression or bending strength and the frame work provided the necessary stiffness. The smaller panel sizes result in reduced spans for the skin so the thinner skin does not have to resist as much bending stress as well. The problem comes with point impact. If the impact occurs between the frames, the skin is more prone to fail not only because the skin is thinner but because of the sheering action between the relatively stiff frames and the point of impact on the skin. (Sort of like a scissors). If the impact occurs on a frame, it can break the bond between the frame and the skin. This is often hard to detect and can be very expensive to repair. Still and all in a properly engineered boat, this is can be a very strong light way to build a very durable and repairable hull. (I chose to buy a 1983 boat that was built with closely spaced hand glassed in frames.)
Lastly, there is a cored hull. Cored hulls provide a lot of skin stiffness. Cored hulls were often combined with internal framing providing the best of both worlds. In theory, cored hulls have nearly the same laminate thickness as a framed hull. In an impact a cored hull behaves a bit differently than a solid hull. In theory, in a sold unframed hull some of the energy of the impact is absorbed in localized flexing of the hull. In a framed hull this does not happen as readily. In a cored hull, the coring acts as a crush zone. So while the outer skin may puncture, a certain amount of energy is absorbed by the core and by the compression of the outer skin against the core. This absorption of energy may actually result in some chance that the inner skin may remain intact, albeit delaminated, during an impact that might have pierced an un-cored framed hull.
Most people are aware of the problems with cored hulls. If they are abused or poorly maintained they could have areas of delamination. While this can be a problem in a recent US Naval Academy study researching the most durable way to build a boat that will withstand a lot of abuse, a cored hull with proper framing was found to be the toughest in withstanding damaging forces including impact.
Still and all cored hulls do have a thinner outer skin that is more easily damaged in an impact with a sharp object. They also don''t like water intrusion. This is more true of balsa coring than the higher quality-higher density closed cell foams. That said balsa is lighter and is more sheer resistant than foams and, if used properly, is reasonably durable. I have owned a 25-year-old boat with cored decks that had no signs of delamination in the decks. (Decks are more prone to problems than hulls, which have fewer horizontal surfaces to trap water and fewer penetrations.)
In the end, when I am buying a boat,I always like to think that I am not buying a whole model line, just the one boat that I am going to end up purchasing. Generalities about one type of construction or another, are helpful in focusing on general models to consider. In the end, you are only really looking for the one individual example of that model that is either in good shape or priced sufficiently low to cover the cost of the repairs, your time and your efforts to make it right. In this discussion, I think its a mistake universally rule out a cored hull in preference to either a solid glass hull with a glued in structural frame or a solid glass hull with no frames. None of these methods are inherently more durable, or better or worse than the others. They all depend on proper design and engineering.
You are only looking for one boat in good shape. With a cored hull, a good certified NAMS or SAMS surveyor can tell whether or not the boat is healthy. You need to take some care to maintain that health. On my prior boat, a 15 year old cored hull, I have the hull sounded out every 4 or 5 years. On a previously owned cored hull, I had an area that sounded different. I actually took a couple small corings of the suspicious area and found nothing wrong but it was easy to do and was a great relief.
In buying a cored hull, its not that hard to tap out a hull for any major glaring problems. You''d be surprised how easy it is to hear a big problem and that would save the cost of a survey on a major lemon. Of course you should still have the boat surveyed since a qualified surveyor has the experience and equipment to catch things you or I would tend to miss.
Anyway, good luck in your search.