Cam, The debate about the longevity of fiberglass is not in question. It basically boils down to whether the vessel was " rode hard and put away wet " so to speak. As to hearing about hulls cracking up, a trip through your local boat yard will show you that a lot of 60s and 70s fiberglass boats are under going major refits or meeting with the chainsaw.
The biggest example of drastic material fatigue was probably the Honolulu flight that had it's cabin ripped open due to metal fatigue. Most older sail boats die of far less famous death. They are usually on their forth or fifth owner when they bounce off the dock one time to many, or the cleats let lose in a thunderstorm and they pound onto the shore. It follows the same rule as used cars, the new owner purchases the boat because he doesn't have enough money to buy something newer. He also doesn't usually have the money for the best materials to up keep the vessel.
Still the biggest reason older boats die, is not that their glass fails, but other cascading equipment failures. On a ten to fifteen year old boat, the new owner will probably look at replacing the sails, some if not all the lines, and giving the engine a good tune up. On a thirty year old boat you'll have all that plus; Resealing all the ports, replacing all the through hulls, replacing the sheaves and standing rigging, replacing the engine or transmission, and resealing all the deck connections. Of coarse most of these and other problems will dealt with as they acur, which means fixing the collateral damage that has been caused. Such as replacing cabinetry because a leak sprung in the shrouds and de-laminated your cabinets.
There will always be exceptions to the above. A mechanics boat per say, or the boat equivalent of a little old ladies car, one the has been stored inside for a long time and maintained reasonably well. This argument can go on forever, but unless you have an outside influence that forces you to continue using old boats, most people are going to opt for newer less maintenance intrusive vessel.
Just look at all the old Packard and studebakers driving around Cuba. You can keep anything going if you really want. We still have collectors who buy and maintain classic cars in this country, but few if anybody drives these old cars on their daily commute.
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