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Bernie K 05-25-2006 11:05 AM

Hull Construction
In search of the perfect boat...I've read pros and cons of various hulls but am confused when I see a hull advertised as "vacuum bagged, oven cured epoxy hull, variable draft, high lift low cg bulbed lead keel". The confusing part is the "vacuum bagged, oven cured epoxy hull" -- what is this? Is it a core that over time could possibly absorb water if the vacuum bags crack/rip?

The same goes for the deck "vacuum bagged, infused vinylester composite" -- is this also a water absorbant material that has been bagged?

Any inputs would be appreciated.


Johnrb 05-25-2006 11:22 AM


Rather than me explaining vacuum bagging I’ll excerpt this article (linked below). I expect that vacuum bagging is a separate entity from "oven cured epoxy". The purpose of this construction method is to reduce the amount of resin by squeezing it out of the uncured laminate.

"An ideal fiberglass layup minimizes the amount of resin because resin by itself adds weight without adding strength. When doing fiberglass layup by hand, it is hard to get the minimum amount of resin because you need enough to soak the cloth and the cloth doesn't lay completely flat against the surface. The solution to this is vacuum bagging."

"Vacuum bagging uses atmospheric pressure to press the cloth tightly against the surface being covered so that the excess resin is squeezed out and soaked up in a disposable outer wrap. This technique requires a vacuum bag, vacuum pump capable of pulling a significant vacuum (at least 25 inches of mercury) and various accessories and supplies. Thus vacuum bagging has been mostly restricted to large commercial use and a few enterprising hobbyists."

"Vacuum bagging requires that the part being laminated be covered with cloth (as in hand layups). The part is wrapped with a thin film which is porous and will not stick to the epoxy, called "release," and a thick layer of absorbent material, called "breather." This whole assembly is then inserted into the vacuum bag and the air inside is removed by a vacuum pump. Because the air inside the vacuum bag is removed, the air pressure from the atmosphere outside the bag pushes tightly from all sides, pressing the bag against the breather. The excess epoxy is squeezed out of the cloth, passes through the release and is soaked up by the breather. The breather also allows the air to flow away from the tube and out of the bag."

I expect this is a fairly costly construction technique for larger boats. It’s interesting to note this method has been used for some time for high quality touring kayaks (my kayak is constructed in this manner).

Bernie K 05-25-2006 11:31 AM


Appreciate your quick reply. The boat in question is 34ft but I believe their entire lineup through 55 ft uses this process. From what I understand from above is that this process is complicated and appears to be a good technique. Since no mention is made of a "core", is the "epoxy hull" understood to mean there is no core that could absorb water? Perhaps after reading so many threads on hull problems, I'm being extra careful. But...I don't want a problem down the road and am trying to look at all possibilities.

Jeff_H 05-25-2006 11:41 AM

This is partially cribbed from draft that I had written for another purpose but....

"Properly engineered "vacuum bagged, oven cured epoxy hull" and "vacuum bagged, infused vinylester composite" describes the resin being used and the method of controlling proper resin ratios to ensure an optimum resin to laminate ratio and in the case of oven curing, the best way to insure a proper cure in Epoxy.

Vinylester and epoxy are the best resins currently available for yacht construction. Both have excellent fatigue resistance resistance to moisture penetration. Both are immune to blistering. Depending on formulation each have substantially higher strength capabilities than conventional resins. When coupled with carefully sellected laminates (directional fiberglass, kevlar or carbon fiber) these resins can produce the highest impact resistance and flexural strength per pound of any boat building material out there. Vinylester is slightly more expensive than conventional polyester resin and Epoxy is significantly more expensive than vinylester. Vinylester is widely used in crash helmets and armoring vehicles and the vinylester developed for that purpose is actually pretty inexpensive and offers tremendous advantages in terms of resistance to damage and post deformation strength retention over other resins.

Vacuum bagging is a process in which the laminate is laid up and wet out typically as a whole, and then a temporary 'peel coat' absortive layer placed over the laminate. Then an airtight membrane is placed over that and placed under a vaccum. The vacuum compresses the resin and reinforcing fabric creating a dense evenly wet out layout. The peel coat, absorbs excess resin. By adjusting the vacuum pressure the ratio of resin to laminate can be adjusted resulting in a maximum strength at a minimum weight for any given panel design. These are the best ways of producing a composite hull currently in mankind's arsenal.

Oven curing is somewhat unique to epoxy. Left to its own devices, epoxy cures over a very long period of time and can cure un-evenly. Oven post curing is a way of precisely curing epoxy to achieve a higher ultimate strength and to achieve it sooner. It too adds cost but it is an improvement on standard epoxy construction.

The current data suggest that when coupled with moderately high density, crosslinked, closed cell foam cores, and proper laminates, these techniques will produce the highest strength and impact resistance per pound hulls and will produce the most durable constuction technique that we have available. Of course like anything that is high tech, this comes at a price in terms of higher material costs, the need for proper engineering and the need for more care during construction."


Bernie K 05-25-2006 12:06 PM

Jeff & Johnrb

Thanks for the details -- all these go into my nugget! A couple last questions on the subject.

If the "vacuum bagged, oven cured epoxy hull" and "vacuum bagged, infused vinylester composite" processes are good (given the manufacture is capable of accomplishing the task), where do the cored hulls come into play?

Are there manufactures that are making these cored hulls and simply laying fiberglass over them anticipating they will only be around so long before the core starts absorbing water?

On the flip side, the non-cored options -- are these simply the traditional layed fiberglass for strength? Sorry if these seems like basic questions --- I'm still in the learning phase?


Jeff_H 05-25-2006 02:30 PM

The two snippets that you referred to have no reference to coring at all. Coring is an entirely separate issue.

Properly done, a cored hull should have a much longer life than a non-cored hull. Fiberglass is a very fatique prone material, meaning over time, the more it flexes the weaker it gets. Coring produces a much stiffer hull than can be practically achieved without coring and therefore a cored hull is less prone to fatique. A coring with 'memory' can withstand enormous impacts without damage, permanent distortion or delamination. Coring with memory helps to absorb and distribute shock loads into a larger panel area thereby reducing the localized unit stresses.

Most manufacturers of boats without cores laminate a heavier panel than the outer laminate of a cored hull in an attempt to equal the stiffness of a cored panel. To keep laminating costs down, this is generally done with lower cost resins and lower tech laminates, and substantial qualitites of non-directional laminate (mat). Non-directional laminates are much more prone to fatigue and have been shown to greatly reduce impact resistance especially over time. From an engineering standpoint, and durability standpoint this is a very inferior way to build a boat.

Which is not to say that all cored hulls represent the best construction technique. We have all encountered boats with delaminated coring and to a great extent that is what gives coring its bad reputation. BUT, coring varies widely in quality and workmanship has to held to a higher level when a hull is cored. Early coring techniques were not very good and so to one degree to another core failure is pretty common in older boats. Similarly the power boat industry has been using poor quality low density foams, with minimal skins for years leading to the kinds of dramatic failures that David Pascoe tends to focus on.

Jeff_H 05-25-2006 02:33 PM

It sounds like you are unclear on the basics of fiberglass construction. This is a draft of an article that I wrote for another venue that might be helpful. It is a little dated but the basics are still correct.

A Primer on FRP

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.

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 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.

To be continued.

Jeff_H 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.

Jeff_H 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.


Bernie K 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!

Thanks again,


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