|Topic Review (Newest First)|
|05-26-2006 09:56 PM|
|PBzeer||Though I'm no expert, if I could have bought new, I would have bought the Tartan 3400.|
|05-26-2006 08:06 PM|
Cored hulls aren't necessarily a bad thing. If the manufacturer has taken the care to put the thru-hulls through non-cored areas, then the hull should be fine.
One thing about cored hulls, they are more complicated to repair, compared to solid laminate ones.
|05-26-2006 05:21 PM|
You are right on - Tartan. While I was looking around I was warned about their cored hulls. That is what brought on this discussion threat --- I've learned quite a bit. I'm still a little unsure but after I reread these the replies to this post for the umpteenth time I'm hoping it will sink in.
Any thoughts on the newer 1990+ Tartans and/or C&Cs for primarily coastal and some Carribean sailing?
|05-26-2006 07:58 AM|
|PBzeer||Bernie....it sounds like you're talking about a Tartan or C&C. The builder would be a good indication to how well done the process is.|
|05-25-2006 11:35 PM|
This is not to say that epoxy is not a good or proper material for patching, strengthening or re-inforcing GRP boats, or making repairs to GRP boats.
Epoxy, in the relatively thin and small quantities used in most repairs will cure properly. The statement about requiring an oven for proper curing has to do with the quantities of epoxy that are used in building an entire hull as a single piece.
|05-25-2006 10:58 PM|
Good points all. I did want to touch on one thing that you mentioned. While it is true that epoxy does lose strength with elevated temperatures, (as does polyester and vinylester but to a proportionately lesser extent), I did some research on this topic a few years ago. From what I was able to turn up, the data that I saw suggested the better grades of epoxy formulated for marine use, if properly cured (and you and I are in agreement on epoxy curing) really does not begin to lose significant amounts of strength due to high temperature until the laminate sustains prolonged exposure to temperatures in the very high 200 to mid 300 degree range for an extended period of time. While temperatures in that range can occur in the marine environment, they are not likely to occur in the marine environment, even on dark hulls. From what I was able to tell the problem of strength loss due to heat should be considered more relevant on boats that have not been properly post-cured.
If I had to pick a resin to build a composite boat today, I would probably chose vinylester resin with epoxy secondary bonds.
|05-25-2006 10:03 PM|
Unfortunately, the different resins have some drawbacks, which don't appear to be mentioned above.
Polyester resin is the most common and cheapest, but is the most prone to blistering and osmosis problems.
Vinylester resin is a bit more expensive than polyester resin, binds well to polyester and vinylester resins, and is much more resistant to osmosis problems.
Epoxy resin is the most expensive of the three resins, and it binds well to both polyester and vinylester resins, and is quite resistant to osmosis problems. However, most epoxy resins will lose structural strength at higher temperatures, and this can be a serious problem if you want a dark colored deck or topsides. It is extremely unadvisable to haven anything other than white if you're using epoxy. Epoxy generally requires oven curing to ensure a complete and consistent cure.
Vacuum infusion is a good way to get a low resin, high strength laminate. However, vacuum infusion has to be done properly, with the right resin consistency and proper pressure for the piece being laminated. One caveat, the surface left by a vacuum infusion process is a bit more difficult to tab additional fiberglass to later on from what I have read/heard from people in the industry.
Cored hulls are generally lighter, stronger and stiffer than solid laminate hulls. A properly laminated cored hull is going to make a much stiffer and lighter boat than one with a solid laminate of equal strength. The real problem with cored hulls is what can happen if the cored hull is not properly designed where the through hulls penetrate the laminate surface.
On better boats, the area around the through hulls is either solid glass, or has had the core removed and been filled with thickened epoxy, to seal the core and prevent water penetration and migration into the laminate. Anyplace a cored laminate is breached, the core should be sealed from any possible moisture penetration, or the moisture entry can lead to delamination. This is true of both the hull and the deck.
Some boat manufacturers avoid this by either using solid laminate below the waterline, or leaving a section adjacent to the keel solid laminate. Some only core the deck, where the ight weight and high stiffness are more critical, than in the hull.
|05-25-2006 09:08 PM|
A lot of details -- I really appreciate you pulling all this together. While it is lengthy, I will take time to absorb it! Your wealth of knowledge on the subject is clearly evident!
|05-25-2006 02:37 PM|
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.
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.
Hopefully you will find this helpful. I appologize for its length.
|05-25-2006 02:35 PM|
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.
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.
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