G'day, & welcome!
Thickness isn't a measure of quality or soundness for going offshore
nearly so much as is the quality of construction in general, & in her layup
Not too long back, it was common to have hulls of some boats made with a chopper gun. Which is a spray gun that literally sprays bits of chopped fiberglass
& resin into a mold
in order to make a part. It's quick, takes much less skill to use, & until recently, was the cheapest way to build something in fiberglass
... But, the quality tends to be low, as there's about 2x more resin used than glass. And no glass with continuous fibers are used.
Then there's your basic hand layup
, still using polyester resins. Going back to when glass first was used in boats. And usually it layers of mat, alternating with woven roving, layed into a mold
The resin content's lower than the above, but still high. And the hulls still tend to be seriously thick. With them being stronger than chopper gunned hulls, but still "low".
Then you move up to something using stitched & woven fabrics, like tri-axials, unidirectional, biaxial, etc. Using any of the 3 common resins, & either laid into a mold by hand, or laid into a mold & vacuum bagged until things are cured. The latter being more common in custom boats. And the more high tech the construction & materials, the stronger the layup. With a higher cost, & they tend to be thinner hulls, though things are often cored as well.
From there you're in the realm of infused hulls, using usually Vinylester, or Epoxy
. Axial, stitched, woven, & exotic fabrics (Carbon, Kevlar, etc.). Cores are common/prevalent. And hulls sometimes get "baked", aka Post-cured.
They're the strongest, & often lightest. But most expensive, & technologlically labor sensitive. Though for custom boats & parts
, are the way to go.
How thick a hull is will depend on it's age, type of construction, materials, intended useage, etc. With their not being a stock answer. As a hull made of vacuum bagged epoxy
, & high quality fabrics can be 4-6x as strong as a hand laid hull made from polyester resin, & cheap
fabrics, thickness per thickness, or pound per pound.
In older boats things are a lot thicker, as they didn't know how strong GRP was back then, nor did they have tools like FEA. Plus resin was cheap
compared to now, or even 30yrs ago. That, & modern boats depend as much on their structural cores, bulkheads, framing, etc. for their strength, as they do on their glass skins. Where such wasn't the case 30yrs or 60yrs ago.
To some degree, hull thickness can be ascetained by tapping. And in a survey
you might ask them to pull out a through hull for inspection
as part of checking a boat out. At which point you'll have an idea of her thickness at that specific point. But hull thicknesses vary Widely
thoughout a given boat, depending on what kinds of stresses that area of the boat will see.
As for example, in my Ranger
33' she was several inches thick in some places near the keel
& in areas which supported the keel
. While in others she was 1/8" thick. Like in her topsides, back aft by the cockpit
lockers & quarter berths. With her being a "mid-level" boat quality wise from her era.
You'll have to get a feel for what boats are higher quality for their age range. And gather that they're solid. But there's no easy answer to the question. Nor are many boats inherently weak in their hulls, but more so in other ways that they were built. Such as in bulkhead attachment for example. It being a major contributing factor for & of this.
One easy to see example of it might be to look at the reports of which boats survived beachings in the big Cabo storm of approx 1980, & why. As some were barely scuffed, while others were totalled, by the 'same' conditions.
PS: And yes, your reading of Ferenc Mate will to some degree help, especially with pre 1990 boats. For another excellent perspective, download (gratis) the Dashew's Cruising Encyclopedia Vol II
along with the rest of their books
. Just don't try & read it all at once