This topic has come up many times on this forum. The following was my response to a set of questions asked a few years back that discusses some of the fiberglass issues. This is a very long one.
1) "Isn't heavier and thicker always better?"
Yes and no....There are a lot of factors that add to the weight of a boat beyond its hull. Earlier boats were heavier for a lot of reasons beyond simply having thick hulls. Simply focusing on the hull for a moment. There are really several things that determine the strength of the hull itself. In simple terms it is the strength of the unsupported hull panel (by 'panel' I mean the area of the hull or deck between supporting structures) itself, the size of the unsupported panel, the connections to supporting structures and the strength of the supporting structures.
On its own, Fiberglass laminate does not develop much stiffness and it is very dense. If you simply try to create stiffness in fiberglass it takes a lot of thickness. Early fiberglass boat designers tried to simply use the skin for stiffness with wide spread supports from bulkheads and bunk flats. This lead to incredibly heavy boats and boats that were comparably flexible. (In early designs that were built in both wood and fiberglass, the wooden boats typically weighed the same but were stiffer, stronger, and had higher ballast ratios)
Fiberglass hates to be flexed. Fiberglass is a highly fatigue prone material and over time it looses strength through flexing cycles. A flexible boat may have plenty of reserve strength when new but over time through flexure fiberglass loses this reserve.
So back to your original question, all other things being equal a thicker panel should have more stiffness but typically these early thickened panels were just not that stiff and as a result they are prone to losing more strength over time.
2) Were those boats not made of the same polyester resin (and fiberglass) used in today's boats?
Not Really. While the basic chemistry is the same, there is a lot that makes up polyester resin. Prior to the fuel crisis in the 1970's polyester formulations were different and were comparatively brittle (but more 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'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.
This was known in the early days of fiberglass boats but resin and labor was cheap so it was easier to just pour a little more in and avoid dry spots. When I have cored older boats I have generally been amazed how much resin compared to cloth I have found, certainly compared to later boats.
Lastly, comes the controversial issue of coring. Solid glass is heavy. No two ways about it. So it is hard to achieve much bending strength or stiffness without incurring a major weight problem except in very small boats. (It is the same problem with metal construction.) If you try to keep weight down you end up with a boat that flexes a lot and flexing causes fatigue that greatly weakens the laminate. The trade off is typically some mix of internal framing and/or coring which allows smaller panel sizes and therefore less movement,
And before you say, "So just build it heavier". (As I am sure a lot of people are tired of hearing me say) Weight does nothing good for a boat. In and of itself it does not add strength or room, or comfortable motion, but it sure adds additional stresses to every working part of the boat, and it certainly slows a boat down.
Coring allows the depth of the section to increase and significantly strengthens and stiffens the section, reducing flexing and fatigue. While the outer skin is thinner and easier to pierce than a thicker uncored hull, the combination of outer skin, and core work together where the core acts as a crush zone absorbing energy and distributing it to a wider area. Even with the outer skin breached there is a relatively high likelihood that the inner skin will be intact and after the thicker laminate of the same way has been broached. Where coring does not do as well in is situations where the boat is subject to long term abrasion and in situations where a boat spends a lot of time bouncing off a dock. Even here, research suggests that certain kinds of coring is less likely to fatigue or delaminate in such applications than non-cored heavy glass.
3. What things do I look for as far as strength?
Up to now we have focused on the strength of the fiberglass materials themselves. But boats behave as a system. As I said early on there are a number of factors that determine the actual strength of the boat. We've discussed the strength of the hull panel itself but in many ways its the size of the unsupported panel, the connections to supporting structures and the strength of the supporting structures that really determine more about the strength of the boat.
You generally just don't hear of sail boats that are sailing along and a section of hull falls apart. What you do hear about are hardspot failures, hull/ deck joint failures and failures of the framing systems.
Framing systems are a key part of the strength of a boat. A section of fiberglass laminate that might be extremely strong and stiff when spanning say 12 to 16 inches is really in trouble when trying to span 24 to 30 inches. One of the key elements in evaluating how strong a boat is the frequency of framing. Bulkheads, bunk and shelf flats, engine beds, athwartship frames, and longitudinal stringers all reduce panel size and, in doing so, distribute loads and help limit the size of a tear in, or flexure of, the skin.
But the connection between the framing system and the skin is a really important component of the system as well. The joint between the skin and framing members (either tabbing or flanges) need to be wide enough to provide a good contact area for adhesion and to prevent a concentrated load on the skin where our old adversary 'Fatigue' can go to town.
Beyond that the framing members themselves need to be sturdy enough to take the loads being superimposed on them. So to answer your question, if I walked on a boat that knew nothing about, the way I would judge the strength of the boat would be to look for small panel sizes, wide tabbing and structural flanges and framing that looks appropriately sized for the job.
I would also look at high stress areas. Hull/deck joints should have wide contact areas. Mast steps and rudderposts should have large longitudinal and athwartship, knees, frames or bulkheads. Keels should have closely spaced, well glassed-in, athwartship frames (called floor frames) that minimally start at the forward edge of the keel and stop one frame aft of the end of the keel. There should be well glassed in longitudinal (which is often formed by the face of the berths) that occurs over these athwartship frames and act to distribute loads and these should occur reasonably close to the centerline of the boat (within a few feet).
Rigging loads should be tied into longitudinal and athwartship frames, bulkheads or knees.
4. You mentioned that in the 80's the boats were better made but lighter... can you explain that?
In the 1980's, better boat builders began to use better resins and use them properly, handle fabrics better, and use fibers oriented to better stress mapping. Over resin rich laminates became rarer. Framing systems became more sophisticated. (The largest panel on my 1983 38 footer is about 14 by 22 inches. My 1960s era C&C 22 had panels 2 feet by 6 feet in size.)
5. I wonder why is it going beyond the design limits of a coastal cruiser to sail from Florida to the Bahamas? The same is true for most of the Caribbean? However, to make my point... wouldn't almost any production boat 30-34 feet long be safe enough to make those trips?
This is about risk management. In good weather and with a little luck you'd be amazed how minimal a boat can make the kinds of passages that you are talking about. But if your luck runs out, and you get hammered, things happen. Boats will flex bulkheads and stringers loose. At which point, rigging loads are no longer acting on a glassed in bulkhead, which pries the deck up. Perhaps a portlight cracks from being torqued and pretty soon you have something that looks like a boat, but which no longer is a boat. (I have repaired a boat that just what I described happened to and it happened off of Ft. Lauderdale.)
6. I am trying to get the handle on what I should look for, model, weight, price, year, or what?
There is no simple answer here. The real answer (with all due respect) comes from experience. It comes from being able to get aboard a boat and look for those subtle clues that tell you how strong a particular boat is and how hard it has been used and how well it has been maintained. It comes from really researching a boat.
(In my own case. when I was narrowing my search for the boat that I recently bought, I talked via email to people in South Africa, the Caribbean and in New Zealand, who had sailed on sisterships in a wide range of conditions. I spoke to Bruce Farr's office (the design firm). I spoke to people who had sailed on the boat years before. I went through the boat with a fine tooth comb and then had a surveyor do the same thing to keep me honest with myself. Only then was I ready to buy a boat in confidence that the boat would do what I needed it to.)
You have very ambitious goals and not much money. People have given you good advice but you want to understand why you were getting that advice. That's good. But you have a long way to go (and I don't mean that as a put down). My best recommendation is that you allow yourself the time to look at a lot of boats, talk to a lot of people, get out on the water when ever you can and you will be able sort all this out and learn as you go.
7. Can anyone shine a light on my questions?
Yes, You can! We're here to help but this is your puzzle. Even if we could, and even if we did, give you all the answers, you couldn't learn enough or enjoy the ride as much, if all you had to do was dial into a bulletin board and just like turning over a magic 8 ball, the exact right answer to your question came your way.
Hang in there!