|Topic Review (Newest First)|
|02-26-2013 12:44 PM|
Re: Fiberglass Hulls Over Time
There are pretty massive differences in the layup on my 1970 Cal 29 that seem to be fairly logical so IMHP there was some level of understanding
|02-26-2013 11:52 AM|
Re: Fiberglass Hulls Over Time
Originally Posted by Razcar View Post
Chopper gunned hulls are started similarly, a release agent, then the gelcoat, then some resin is sprayed onto the mold and then the chopper gun is used. This chops up glass strands into smaller particles and sprays them onto the resin. A few more layers of resin and chopped up glass and you have a hull ready to be popped from the mold.
While handlayed glass sometimes leaves air and resin gaps, it is stronger because the weave of the glass better resists flexing and stretching. The chopped strands of a choppergunned hull do not have the integrity of the glass mat to fall back on.
Many builders use a hybrid. Structural areas will be handlayed glass mat and the rest will be filled in with chopped up glass to build up the hull itself
|02-26-2013 11:34 AM|
Re: Fiberglass Hulls Over Time
Thanks Overboared, this seems like the right sort of thing!
Hard to find though, it seems out of print.
|02-26-2013 11:27 AM|
Re: Fiberglass Hulls Over Time
Heart of Glass by Daniel Spurr
|02-26-2013 11:01 AM|
Re: Fiberglass Hulls Over Time
Originally Posted by deltaten View Post
|02-26-2013 08:52 AM|
Re: Fiberglass Hulls Over Time
Chopper gunned glass is also not as strong as properly laid out cloths for the same reasons listed above.. chopped up strands of glass fall in a random pattern that does not add the strength of long directional strands
|02-26-2013 08:49 AM|
Re: Fiberglass Hulls Over Time
While this repeats some of the information in my earlier post, this was a draft of an article which I wrote some years ago as an introduction to fiberglass technology.
A primer on FRP technology
FRP (Fiberglass Reinforced Plastic- the technical name for 'fiberglass' construction- sometimes also called GRP) had become the primary way that pleasure craft have been built since the late 1960’s. There are a lot of ways to build a FRP boat and a lot of variations on each method. The three most common are Monocoque, cored and framed. You often hear people use the term ‘Solid Glass Construction’. This is actually a very vague and not a terribly precise description of the structure of a FRP boat. As the term ‘Solid Glass’ hull or construction is typically used to mean a boat that does not have a cored hull. A non-cored hull can be monocoque (the skin takes all of the loads and distributes them), like many small boats today and larger early fiberglass hulls or framed as most modern boats are constructed today. A cored hull is a kind of sandwich with high strength laminate materials on both sides of the panel where they do the most good and a lighter weight center material. Pound for pound, a cored hull produces a stronger boat. Cored hulls can also be monocoque or framed.
Framing helps to stiffen a hull, distribute concentrated loads such as keel and rigging loads, and reduce the panel size, which helps to limit the size of the damage caused in a catastrophic impact. Framing can be in a number of forms. Glassed in longitudinal (stringers) and athwartship frames (floors and ring frames) provide a light, strong and very durable solution.
Molded ‘force grids’ are another form of framing. In this case the manufacturer molds a set of athwartship and longitudinal frames as a single unit in a mold in much the same manner as the rest of the boat is molded. Once the hull has been laid up the grid is glued in place. The strength of the connection depends on the contact area of the flanges on the grid and the type of adhesive used to attach the grid. This is a very good way to build a production boat but is not quite as strong as a glassed in framing system.
Another popular way to build a boat is with a molded in ‘pan’. This is can be thought of as force grid with an inner liner spanning between the framing. This has many of the good traits of a force grid but has its own unique set of problems. For one it adds a lot of useless weight. It is harder to properly adhere in place, and most significantly it blocks access to most of the interior of the hull. Pans can make maintenance much harder to do as every surface is a finished surface and so it is harder to run wires and plumbing. Adding to the problem with pans is that many manufacturers install electrical and plumbing components before installing the pan making inspection and repair of these items nearly impossible.
Glassed-in shelves, bulkheads, bunk flats, and other interior furnishings can often serve as a part of the framing system. These items are bonded in place with fiberglass strips referred to as ‘tabbing’. Tabbing can be continuous all sides (including the deck), continuous on the hull only, or occur in short sections. Continuous all sides greatly increases the strength of the boat but may not be necessary depending on how the boat was originally engineered. The strength of the tabbing is also dependent on its thickness, surface area and the materials used. When these elements are wood they can often rot at the bottom of the component where the tabbing traps moisture against the wood.
The strength of laminate (in either cored or non-cored panel) depends primarily on lay-up quality, kind of fibers used, number of laminations, and orientation of cloth. But also it depends of how carefully the laminate is handled and the ratio of resin to laminate. Glass and carbon fibers before they are laid up are quite brittle, and folding the dry laminate can break some of the fibers in the laminate. This weakens the material substantially. Historically, production manufacturers would cut multiple layers of each piece of laminate to be used in the manufacture of a particular boat and then fold the pieces and store them in a pile until they were needed. This of course created weakened lines within the fabric. Most quality production builders avoid folding the laminate today.
When it comes to the actual fibers, there are a number of properties that are considered:
Strength in tension- (Tensile Strength) The point at which the fibers can be pulled apart,
Strength in compression- (Compressive Strength) the strength at which the fibers crush,
Elongation (deflection properties)- This is the amount that a material changes length for a given pull or push on the fiber. This is usually given as the Modulus of elasticity (E), which is the length in inches that a square inch of the material elongates or compresses per pound of force. When we deal with FRP there is often a different E for tension and compression. Since the resin is typically responsible for taking a large portion of the compressive loads but have almost no tensile strength, the focus is usually on the E (sub) t or the Modulus of elasticity in tension for the given fiber.
Orientation: The direction or directions that the fibers are oriented within the fabric. Also how the fabric is made. Flat fibers oriented the same direction (tows) and woven roving where the fabric is essentially straight are very strong ways to use fiber. Woven fiberglass cloth has a lot of kinks in the yarns and so are actually weaker and stretchier. Mat is not terribly strong because it uses short length fibers. Bi-axial and Tri-axial cloths use linear fibers either perpendicular or else oriented 120 degrees apart. This allows careful alignment of the fibers with the tensile stresses in the load mapping.
Abrasion resistance: The ability to withstand exposure to a rough surfaces once the resin and/or the gelcoat has worn through.
Laminate materials are chosen for their strengths and weaknesses in each of these properties, as well as, cost, of course. Because a fiber is low stretch it does not mean that it is also high strength, and just because a fiber has high tensile strength it does not mean it has high compressive strength. Resins have their own properties and, while they are far less important to the overall strength of the composite than the fibers in question, the choice of resin makes a very big difference in the ultimate strength of the part, as well as, its fatigue resistance.
The three most common resins are Polyester, Vinylester and Epoxy. Polyester is a group of resins that can vary quite extensively in their properties. It is the least expensive and the most commonly used resin. It has poor ductility, impermeability and resistance to fatique as well as being very poor in developing secondary bonds. It is often modified to increase or decrease cure times. One iof the best features for production boat building is that polyester will not fully cure until deprived air. This allows muliple laminations with a laminating resin without sanding between laminations. The last lamination is a finishing resin which contains a waxy material that foats to the surface and seals out the air permitting a complete cure.
Vinylester is a family of vinyl modified polyesters. This recent material (since the 1990’s) is a wonderful material. It has excellent ductility and memory, great fatique resistance properties, and is easy to work with. Used heavily in the helmet industry it has come down greatly in price and is being used pretty extensively on even high volume boats.
Epoxy has a whole range of extremely wonderful properties. It really shines where secondary bonds are important. Unfortuneately it is very expensive and harder to work with than the other resins and so is rarely used. It offers superior secondary bond adhesion (adhesion after the initial fiberglass has set up) and so is the preferred material for fiberglass repair.
Looking at the individual fibers.
Carbon: Carbon has two very important characteristics, 1.Carbon has a comparatively high tensile strength but 2. an extremely high Modulus of Elasticity in tension and moderately high compressive E. This means that Carbon fiber composite parts have a lot of strength in bending but more significantly they can take big loads without much changing shape. It is this property that makes carbon so ideal for masts and other spars. It is also a reasonably light fiber. Carbon has some big negatives as well. Carbon is only moderately in resisting fatigue and so can breakdown in situations where it alternatively flexed and un-flexed. One characteristic that is often overlooked is that Carbon fiber conducts electricity and can be electolytically active (i.e. subject to electrolysis) (One popular theory on why Coyote lost her keel was that there was problems with the grounding of 24 volt generator and the carbon fiber attachment of the bulb keel bolting plate was weakened.) Carbon is also not very good in resisting abrasion. These properties makes it an ideal material for short lived race boat parts and light weight spars like windsurfers and spin poles but not so good for a cruising boat hulls or long life items.
Kevlar is one of my favorite materials. This is one very tough material. It has very good tensile strength properties (but not as great as Carbon or S glass). It also has a large E. Unlike carbon it has excellent fatigue resistance and abrasion resistance. It is extremely light and will actually float out of the resin. You must either vacuum bag kevlar or use a fabric with both glass and kevlar in it. You can’t sand a laminate with kevlar in it. Trust me I have tried. The kevlar balls up. The way I have dealt with repairs over kevlar is to cut the kevlar strands with an Exacto and then finish with a layer of F.G. cloth. Kevlar is amazingly tough to cut or work with. If you drill though a Kevlar boat (Rugosa had a kevlar hull and deck) and you don’t use a sharp drill the kevlar will not cut and will wrap around the bit and drag the drill to a stop. To me it is an ideal material for the exterior laminates for boat hulls. Kevlar is not too great in compression, so it is best used in concert with S-glass, so that the S-glass can take help take compressive loads.
S glass is a type of fiberglass. There is a lot that distinguishes S glass from E glass, but basically, when glass fibers are made there are a variety of ways of doing it. All of the methods result in glass fibers that are not smooth on the surface when seen in a microscope. The roughness is actually small cracks in the surface of glass fiber. The fewer breaks the stronger the tensile strength of the fiber. Also the longer the fiber the fewer the un-restrained ends of fiberglass fiber and therefore the stronger the composite. The process that produces S-glass produces longer, less fractured fibers and then uses that fiber in fabrics that minimizes crimps in the fiber. S-Glass has really good tensile strength but does not come close to carbon or kevlar with regard to elongation. It is a good alternative for the interior of cored hulls where
E-glass is the run of the mill everyday fiberglass laminate. E glass is used in virtually all production boats and has reasonably good properties for most applications. It is the least specific specification and can vary very widely in quality. All early fiberglass boats were made of E-glass. E-glass can have especially poor fatigue qualities and only fair Tensile strength. It has terrible E properties in tension and only so-so E-properties in compression. In other words it is very flexible. While it is initially true this flexure has little to do with the bending strength of the material, in a material that is not very good in fatigue, flexure can be a significant problem.
Kevlar is harder to laminate than the other fibers. It is hard to cut and floats to the surface. It dulls cutting tools and is hard to tool. The key is to use sharp tools to cut the laminate vacuum bag the lamination and use glass mat buffer laminates. Both carbon fiber and Kevlar require Vinylester or epoxy resins to get any real advantage out of them.
OLD VS NEW FIBERGLASS
One statement you see a lot is “Early boat builders did not know how strong fiberglass was and so made it very thick.” Horse Feathers! This is just plain bunk. The federal government had done a lot of research on Fiberglass and the information was widely available in the 1960’s. As a kid, I had literature on fiberglass that pretty clearly analyzed its properties. Guys like Carl Alberg, who was working for the government designing fiberglass ammo boxes when he was hired by the Pearsons to design the Triton, knew exactly what fiberglass would do. They knew that the e-glass of that era was pretty poor quality and was especially prone to flexing and to fatigue. They attempted to design fiberglass boats to be as stiff as wooden boats of the era. This took a lot of thickness since F.G. was very flexible compared to wood. This was especially true on a pound for pound basis. They also knew that if the boats were not as stiff as wood, there would be major fatigue problems. This put early designers in a bind. If they made the glass boats as thick as a wooden planked hull they would be impossibly heavy. If they did not, fatigue would condemn them to a short life. They mostly chose to compromise. By that I mean they chose to do boats that were not as stiff as the wooden boats they replaced but were heavier. Early glass interpretations of wooden boats were generally heavier and carried less ballast than their wooden counterparts. They were much stronger in bending but not as stiff. As fatigue took place some of these early glass boats became even more flexible which leads to more fatigue, which can lead to a significant reduction in strength.
Coring allowed the hulls to be made much thicker without the weight penalty. In calculating the stiffness of a section, the thickness is to the third power and so small gains in thickness result in big gains in stiffness. Coring allows a boat to be very stiff and strong and thereby reduces fatigue. Its not that coring comes without problems. The core is primarily subjected to horizontal sheer. To visualize Horizontal sheer, (Take a deck of cards and bend them. As you do you’ll feel the cards slide one over the other. That slippage is horizontal sheer.) The core material must be able to withstand the reversing horizontal sheer loadings without fatigue. That is what Balsa core does best. But balsa core can and does rot. It takes a higher density foam to equal the sheer strength and fatigue resistance of Balsa. That said, if you are building for durability, nothing beats medium density foam coring.
There is an oft-quoted statement floating around the internet “Cored laminates are stronger in flat panels, but are weaker when used with curved surfaces.” There is no scientific basis for that statement. When cored materials are applied to curved surfaces the core materials are designed with small stipes that allow the compound bending. When the core is properly vacuum-bagged into place, these stipes fill with resin and greatly increase bonding and the horizontal sheer of the panel. So, while cored laminates are stronger than solid panels on the flat, they are much stronger than solid panels when used on a curved surface. The author of that statement also has some dramatic photos of delamination problems on cored hulls but all of those photos appear to be low-density foam coring, which is almost never used in sailboat construction.
Mat vs. oriented fabric:
Mat (or chopped glass) does a number of things. First and foremost, almost all fabrics are directional. Mat and Chopped glass are not. Directional fabrics are weaker at bias angles that bisect the primary load directions. With good stress mapping you theoretically could use all directional material carefully oriented but because boats are subjected to loads from all different directions there needs to be an offsetting fiber orientation across the bias. Since mat has equal strength in all directions mat helps resist those loads that do not align with the direction of the directional materials. Mat also serves a more practical purpose. Course materials like woven roving, which have a lot of strength and which represent an easy way to build depth quickly have rough laminated textures. Due to this rough surface it is difficult to get a proper adhesion between course laminates without using too much resin. Mat is able to contort to the texture and make a good connection between the course laminates. Mat has another function as well. Resin shrinks as it cures and resins cure over very long periods, as much as years. If you put roving against gelcoat, the thicker resin in the course laminates shrinks proportionately to the thickness of the resin. This results in “print through” where the pattern of the fabric can be seen by sighting down the hull.
We are just now starting to understand the problems with non-oriented materials. In actual testing performed by the US Naval Academy (from a paper presented at the 2002 SNAME Chesapeake Bay Sailing Yacht Symposium), non-oriented fiber reinforcing fabrics were found to be the primary mode of failure in point impact situations. This paper outlined that Naval Academy cutters, which are used in training exercises, are subjected to frequent collisions, but the Academy cannot afford to take them out of usage for long repair periods. As a result, impact resistance was very critical. In order to test the impact resistance a large pendulum with a massive weight was constructed. On the leading edge of the pendulum was a steel replica of the bow and stem fitting of a Naval Academy cutter. Test panels were constructed that matched both known (prior cutter lay-up schedule and J-24 topsides) and conjectural hull panels. The panels were aged and then tested warm (some resins lose strength when warm). The tests consisted of retracting the pendulum with a forklift and then releasing the restraint cable. The results were very dramatic.
To begin with. Solid hulls did far worse than cored hulls. In examining the panels after the collisions, the failures almost always occurred in the non-direction material being used and not in the core materials. The test sample that faired best used an oriented glass laminate, NO non-oriented materials, vinylester resin, and a high-density foam core. The pendulum never entered the outer laminate and microscopic analysis further destructive testing showed that core was still fully adhered to the skin and that the deformation was within the elastic (memory) properties of the core.
This is bad news for those with older heavier hulls. Through actual testing it has been known that these heavy solid hulls did not have the strength of newer lighter hulls but the failure mode was not completely understood. As mentioned above, it was generally believed that the issues were inferior resins and fibers, poorer handling of the materials, poor resin ratios, and the extensive use of accelerators and fillers. What is implied in the NA testing is that the problem may also lie in the extensive use of non-oriented fiber type laminates. These old heavier so-called solid glass hulls actually used an enormous proportion of non-oriented materials greatly reducing their impact resistance, stiffness, and tendency to resist fatigue.
Everything else being equal, twice the laminates take twice the time to abrade, but heavier cloths are not more abrasion resistant than lighter ones. Kevlar is enormously more abrasion resistant than any other laminate. The other factor is the force of the impact. A lighter boat hits with less force than a heavier boat so the rate of abrasion is greater on a heavier boat. On the other hand there is typically more material to resist this greater impact and abrasion. As far as I know resin has little bearing here.
If one had to design a boat solely to abrade for a day or two against rock it might be thick steel. If that was not your only criteria for designing a boat (in other words you were concerned about sailing ability and motion comfort), then it makes sense to build in FRP with outer layers of kevlar over a medium density foam core over more layers of S-glass and Kevlar.
Here more laminates is not necessarily better. Fiber type and fabric type is most crucial. Proper load distribution is crucial. This means reasonably small panel sizes, good fiber orientation and a bit of luck. Kevlar helps. Resins again have can have a major impact on performance. In the US Naval Academy testing mentioned above Vinylester Resin of a type used to build military and motorcycle crash helmets performed much better than less ductile resins. The high tech fibers, Carbon and Kevlar, need resins that can withstand higher tensile loads without developing small stress cracks. Epoxy and Vinylester can deflect more without getting the microscopic fractures that are the beginning of the end for FRP.
Polyester is the cheapest and most common resin and as laid up is not impermeable to water. Polyesters vary widely in quality and performance. They are more prone to fatigue problems than other resins. One source of water penetration is the microscopic passages created as polyester fatigues. Early polyesters were particularly brittle and fatigue prone. This problem was further aggravated by the tendency by early boat builders to use accelerators and retardants depending on temperature and the nature of the operation. Another issue is with accuracy of the metering. Early boat builders used pretty imprecise methods to proportion resin. Today metering pumps make precision metering a piece of cake, but back then mixing was more hit or miss. For example I installed an instrument through hull in a Triton and found a pocket of uncured un-reinforced resin probably a decade after the boat was built.
Vinylester resin does better than polyester so many better boat builders are now using it in the outer laminates and with high tech fibers. Epoxy seems reserved to custom builders and secondary bonds, because it is expense rather than some other flaw.
In any production boat there are several possible barriers to permeation as follows;
Gelcoat: Gelcoat is a thin molded laminate layer that is the first lay-up in a female mould. It consists of a dyed resin. Historically gelcoat was polyester resin, with an opaquing element and pigment that gave the boat its color. Gelcoat is typically sprayed or rolled on the mould. It has no reinforcing in it and so is brittle and should not be applied too thickly. Because it is applied as a continuous membrane and does not have reinforcing to wick moisture, it forms a barrier protecting inner laminations from water penetration. Because polyester is slightly porous, it is not a perfect impermeable membrane.
A recent improvement on polyester gelcoat is vinylester gelcoat. Vinylester is far less permeable than polyester and so water does not pass through a Vinylester Gelcoat as easily. To further improve the impermeability of the laminate, vinylester gelcoat is often combined with a vinylester veil-coat (the veil coat is the first reinforced laminate in the lay-up which occur between the gelcoat and the structural laminate. Since resins shrink after layup, but the reinforcing materials do not, the purpose of the veil-coat is to keep the pattern of the reinforcing weave from ‘printing through’ as the boat cures over a period of time.). Better manufacturers also use vinylester for the last lay-up as well in order to prevent bilge water from permeating the lay-up.
Often a barrier coat is added below the waterline to further seal the hull. Barrier coats are typically painted over the gelcoat. They are typically either epoxy for its combination of adhesion and impermeability or vinylester which is cheaper, slightly less permeable than epoxy and easer to work with.
|02-26-2013 08:44 AM|
Re: Fiberglass Hulls Over Time
I think that the expression 'hand laid' was in contrast to glass that was applied with a chopper gun. Most boats until fairly recently were hand laid. These days a large percentage of boats are 'infused' with the glass put onto the mold dry, and the resin pumped through it and the vaccuumed down. Done properly this is a super way to build a hull. It means that the glass is properly 'wet out' and yet there is a minimum of surplus resin the laminate. This results in a very dense and incredibly strong lay up for the materials involved. It also means that there are no weaker 'secondary bonds' as was the case with many hand laid boats.
The short coming of infusion is that there are not the kinds of 'eyes on' of the individual laminations as occurs with 'hand-lay-up'. So that if infusion is poorly done, there can be voids which need to get cut out or injected to achieve full strength.
|02-26-2013 08:31 AM|
Re: Fiberglass Hulls Over Time
I apologize that this is from an earlier article that I had written and edited for an earlier discussion on this topic, but hopefully it starts to address your questions.
Frequently asked questions about boat buying: old vs new fiberglass boat Construction
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 loses 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 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 application of 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 or ease of construction 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 reasonably full 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. Before lamination, 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. Non directional fabrics (mat) have been shown to be even more fatique prone, fail more easily on impact, and get more brittle than woven materials over time. 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. In early boats there was a much larger proportion of non-directional fabrics used and that has added to the deteriorated strength in these earlier boats.
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 core drilled older boats I have generally been amazed how much resin compared to cloth I have found, certainly considerably more as 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 in a comparably small boat. (It is the same problem with steel 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.
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. Cored hulls also need more attention at penetrations to avoid water infiltration and the damage that can cause.
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 concentrated load failures (keel and rig attachment for example), hardspot failures, hull/ deck joint failures, dislocated frames and bulkheads, and failures of the framing system itself.
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. It is sometimes said that starting in the 1980's the boats were better constructed 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 breaking up the hull into smaller panels and distributing loadings better around the hull. (The largest panel on my 1983 38 footer is about 14 by 22 inches. My 1960s era C&C designed 22 footer had panels as large as 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 right off of Ft. Lauderdale.)
And this is even more critical on some of the newer boats where frames and bulkheads are being glued into place. Modern adhesives allow glued joints to develop as much holding power as traditional tabbing over a much smaller contact area (called faying surface). The manufacturer’s point out that these glued joints are stronger than the glass it is glued to and that the laminate fails before the glue joint. But that gets to the heart of the problem.
The purpose of a tabbed connection is not only to ‘glue’ the part in place; it is also to spread out the load to a large enough area that the laminate is not torn and that the connected hull and bulkhead act as a system. The modern glue joint allows the stress to be concentrated in a very small contact area, which, over time means that there is likely to be fatigue and that the full load transfer will cause the hull laminate over time.
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 well constructed and 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 ended up buying, 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.)
Most people have very ambitious goals and not much money. People will give you a mix of good and bad advice but you will also want to understand why you were getting that advice. 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, walk through boat yards and look at boats being worked on and in storage, talk to people who operate repair facilities, 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! Members of sailing forums are 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 connect into a bulletin board and just like turning over a magic 8 ball, the exact right answer to your question came your way.
|02-26-2013 08:20 AM|
Re: Fiberglass Hulls Over Time
slightly tangential; but....
Just how would one NOT "hand lay" 'glass ?? Serious question, as I see this phrase used often. Was there some sort of machine that "laid" fiberglass?
I suppose you *could* consider a chopper gun blown glass hull as 'machine laid' but other than some interior parts and shower stalls...how many hulls were done thataways?
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