76 Grampian 28?
I would not think that fiberglass has a life span per se. Neither concrete nor fiberglass truly breaks down or looses strength on their own. They require other causes. In the case of fiberglass loss of strength can result from one or more of the following,
-The surface resins will UV degrade.
-Prolonged saturation with water will effect the byproducts formed in the hardening process turning some into acids. These acids can break down the bond between the glass reinforcing and the resin. (Grampian used a lot of accellerators and in the one I owned and the ones I helped work on this results in micro crazing which concerns me structurally over time.)
-Fiberglass is prone to fatigue in areas repetitively loaded and unloaded at the point where it is repetitively deflected. High load concentration areas such as at bulkheads, hull/deck joints and keel joints are particularly prone.
-Salts suspended in water will move through some of the larger capillaries within the matrix. Salts have larger molecules than water. At some point these salts cannot move further and are deposited as the water keeps moving toward an area with lower moisture content. Once dried these salt turn into a crystalline form and exert great pressure on the adjacent matrix.
-Poor construction techniques with poorly handled cloth, poorly mixed or over accelerated resins, and poor resin to fiber ratios were very typical in early fiberglass boats. These weaker areas can be actually subjected to higher stresses that result from much heavier boats. Its not all that unusual to see small spider cracking and/or small fractures in early glass boats.
-Of course beyond the simple fiberglass degradation there is core deterioration, and the deterioration of such things as the plywood bulkheads and flats that form a part of the boat''s structure.
There are probably other forms of degradation that I have not thought of but I think that the real end of the life of a boat is going to be economic. In other words the cost to maintain and repair an old boat will get to be far beyond what it is worth in the marketplace. I would guess this was the end of more wooden boats than rot. I can give you a bit of an example from land structures. When I was doing my thesis in college, I came across a government statistic which if I remember it correctly suggested that in the years between 1948 and 1973 more houses had been built in America than in all of history before that time. In another study these houses were estimated to have a useful life span of 35 years or so. As an architect today I see a lot of thirty five year old houses that need new bathrooms, kitchens, heating systems, modern insulation, floor finishes, etc. But beyond the physical problems of these houses, tastes have changes so that today these houses in perfect shape still has proportionately small market value. With such a small market value it often does not make sense from a resale point of view to rebuild and these houses are therefore often sold for little more than land value. At some level, this drives me crazy, since we are tearing down perfectly solid structures that 35 years ago was perfectly adequate for the people who built it, but today does not meet the "modern" standards.
The same thing happens in boats. You may find a boat that has a perfectly sound hull. Perhaps it needs sails, standing and running rigging, a bit of galley updating, some minor electronics upgrades, a bit or rewiring, new plumbing, upholstery, a little deck core work, an engine rebuild, or for the big spender, replacement. Pretty soon you can buy a much newer boat with all relatively new gear for far less than you''d have in the old girl. Its not hard for an old boat to suddenly be worth more as salvage than as a boat. For example, a couple years ago a couple friends of mine were given a Rainbow in reasonable shape. She just needed sails and they wanted an auxiliary, but even buying everything used the boat was worth a lot less than the cost of the "new" parts. When they couldn''t afford the slip fees, the Rainbow was disposed of. She now graces a landfill and the cast iron keel was sold for scrap for more than they could sell the whole boat for.
Wooden boats represent the difference between a maintainable construction method versus a low maintenance. A wooden boat can be rebuilt for a nearly infinite period of time until it becomes a sailing equivalent of ''George Washington''s axe'' (as in "that''s George Washinton''s axe. It''s had a few new handles and a few new heads but that is still George Washington''s axe".) But fiberglass boats are much less rebuildable. You can replace many of the components but when the major structure reaches the end of its life there is nothing that can be done.
I think that you can find good solid boats from the 1960''s and 1970''s that were built by quaility builders and well maintained by prior owners. These are the best deals because that owner has spent the money to buy new and taken the hit on the limited market value of these boats. Grampians of the era you are considering were pretty junky and so would not necessarily fall in that category.
And finally if you buy an old fiberglass boat, paint the bilges white. It does nothing for the boat, but if you ever have to sell the boat, then someone may look in your bilge and say "Lets buy her because any man that would love a boat so much that he went through the trouble to paint the bilge white must have enjoyed this boat and taken great care of her no matter what her age."