I've got a number of questions:
- How is a hull built? Is there a thickness that must be met?
- What is the average useful life of a fiberglass hull?
- Extrapolate this to blue-water sailing. How thick should the hull be, and how stiff? And, how could you tell on an older boat how much life is left in the glass?
Dan Dickison responds:
Your question is multi-layered, like many sailboat hulls, and thus the answer is a little complicated. The standards for hull thickness vary depending upon the method of construction, what part of the boat you're talking about, the size of the boat, the intended use of the boat, and the material used.
For boats that are constructed with solid fiberglass and no core, the hull will be thicker near the keel, the hull-deck joint, and any other areas where the design involves an angle or the need for reinforcing (like the partners around the mast, the mast step, or winch bases). Over the broad expanses of the hull, like most of the topsides, the hull will be thinner. For a small boat like a 12-foot Laser, you could expect the hull to be roughly 1/8-inch thick in those areas. A larger boat built in the same fashion, say a J/35, would have appropriately thicker sections in these areas, maybe up to 1/4-inch. You'll also find that older boats, like those from the '60s and '70s, are generally built with thicker lay-ups of fiberglass.
Boats that are built with composite sandwich construction ordinarily have thinner fiberglass skins, but are thicker all together because of the core between the inner and outer skins. Again, larger boats are likely to have thicker hulls than smaller boats. Some manufacturers can actually show you a core sample, which will give you an exact idea of the thickness. I've seen core samples from some core-built sailboats in the 40-foot range that measure more than two inches thick, but those were taken from the turn of the bilge where you would expect the boat to be thicker because of the needed reinforcement to carry the weight and the stress of the keel.
You also ask about the life span of fiberglass hulls. The answer here too is conditional. If the boat was well built, with the correct ratio of resin to glass, and it has been well maintained and not abused, there's no reason to think you couldn't get 40-plus years out of it. There are plenty of boats afloat today that were built in the late '60s, and even before that. That said, fiberglass applications, like all materials, are subject to deterioration due to the normal stress and wear and tear they experience while being put to use. One way to determine how much stiffness is left in a particular boat is to measure the hull deflection when the rig is stressed. You'd really need to have the boat out of the water to do this properly, but you can always tighten the backstay or the headstay and measure the amount of "banana" (fore and aft) deflection as an indication of the stiffness. For blue-water sailing, all of the above information still applies, but it's rarely a bad idea to go to sea in an overbuilt boat, except for course if the added weight makes that particular design unweildy.