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Old 30-10-2003, 06:34   #1
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A Primer on Fiberglass Construction

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.


STRENGTH.
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 and chopped glass is not terribly strong because it uses short length fibers and no interweaving of the 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 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.

Resins:
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.


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:
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:
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:
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

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.

ABRASION RESISTANCE.
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.

PUNCTURE RESISTANCE.
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.

WATER PERMEATION.
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.

WORKABILITY.
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.
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Old 30-10-2003, 07:08   #2
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This is a very interesting topic to me.I was of the mindset that cored hulls were the way to go, but after reading all of Mr.David Pascoe's remarks on the subject, I'm a bit perplexed.To me, the issue isn't about strength, it's about the possibility of water getting into a cored hull.I understand the use of new high tech.materials do make a hull stronger, and lighter, but I'm concerned about the other potential problems. Granted, I'm not an engineer, but when I read comments by a reputable marine surveyor like the following, it gets my attention.

" Once water gets into a core, a phenomenon called hydraulic erosion takes place.Due to the slamming and pounding of the hull bottom on the sea surface, water contained within a laminate or core will be compressed by the flexing laminate structure.Thus, the bottom lterally becomes a diaphragm pump.Once ply separation occurs,the impacts of hull against water creates hundreds of pounds of hydraulic water pressure within the laminate. The pressure is so strong that it will erode the plactic and shred the glass fibers."

"Should Hulls Be Cored Below Waterline? I don't hesitate for a moment in saying no. Not with any kind of material. The risk is too high that something will go wrong, mistakes either by the builder, the owners of the boat, or someone working on it. We all know that it's hard enough to keep the superstructure of the boat from leaking, but to keep water out of a core below the water line may nigh well be impossible. Fiberglass is known to be water absorbent enough as it is without adding more risk to the mix. To do it right requires a very high degree of care which can ultimately be compromised by something as seemingly innocent as running a screw through the laminate somewhere in the bilge. it's just too easy to make a mistake."

"Unfortunately,core problems are often undetectable during surveys unless the problems are far advanced.That is particularly true when the outer skins are particularly thick and neither sounding nor moisture meters are likely to give any indication of trouble."

As a buyer, if neither soundings nor moisture meter readings won't pick this potential problem up when performed by a marine surveyor, how can I be confident that the hull integrity is good?
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Old 30-10-2003, 12:27   #3
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I am quite familiar with the site that you reference. I have discussed this site with quite a few surveyor and yacht designer friends over the past couple years and while the author of that site is entitled to his opinion, at least in my conversations, the author is seen as being very fond of hyperbole, self-promotion and holding opinions that are less than universally agreed upon in the surveyor and designer industry.

A classic example of this "Once ply separation occurs,the impacts of hull against water creates hundreds of pounds of hydraulic water pressure within the laminate. The pressure is so strong that it will erode the plactic and shred the glass fibers." This is true in the case of very thin outer laminates with poor bonds to the core which is more typical of power boats and very high performance racing sailboats. It is not a realistic case for production sailboats where the lamination schedule necessary to distribute the rigging and keel loads means a particular heavy skin and a skin that can easily bridge small delaminations without hydraulic pumping.

I also think that the statement that "Unfortunately,core problems are often undetectable during surveys unless the problems are far advanced.That is particularly true when the outer skins are particularly thick and neither sounding nor moisture meters are likely to give any indication of trouble." Again I strongly believe that this is an overstatement. Any good surveyor can detect very small delaminations or wet spots in the core. I have been around surveys where delaminations as small as a 1 1/2" in diameter were detected and easily repaired. This is not rocket science.

One very important point that the author in question does not touch on is that there are very big differences in core materials and care of construction. To make a blanket condemnation of cored construction ignores that critical reality. When done properly a cored hull is preferable below and above the waterline. It may be correct to say that as a consumer it is hard tell if the core wsa installed correctly and on the older boats that are within yoru price range it may not have been, and based on that you may elect to avoid coring below the waterline.

Respectfully,
Jeff
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Old 30-10-2003, 16:45   #4
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Talking Why am I always THANKIN' this guy ?

As always your input is educational, Jeff. I KNEW there was a reason I used West System on Warchant in spite of the cost. Around here , the 60 bucks a gallon stuff compaired to 25 bucks a gallon for polyester resin. People would chortle at me using the expensive stuff, but I always replied that 60 miles offshore is no place to find out you made a mistake by saving 40 bucksa gallon.
Cored hulls may be a better, more modern construction method now, but seeing as how my boats hull is solid frp, I am , at this point, feeling assured that it will hold up to what this " Don't do anything ,stupid" ol' man will subject it too. If it creaks and groans, rest assured, I will check it out !!
Once again, your effort to educate is welcomed by this Epoxy covered fingered ol' man.
Thank you !
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Old 30-10-2003, 21:18   #5
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My turn then, Thanks for your kind words.

Respectfully,
Jeff
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Old 30-10-2003, 22:35   #6
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Sent a link to the surveyor about this discussion, perhaps he can chip in on the subject......
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Old 31-10-2003, 05:50   #7
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In my work, we use an infrared camera to diagnose a varity of problems. One very effective use of the camera is scanning the building roof to locate leaks.The roof is made out of some type of membrane.Once water penetrates it, the area becomes saturated.After the surrounding area heats up during the day,it's very easy to locate the "cooler" area where the saturation is at while using the camera. It seems like infrared thermography would be a very good tool to use in identifying problems in regular FRP, and cored hulls. However, the infrared cameras are very expensive.Has anyone heard of,or know of marine surveryors using this technology?

Another technology that's available that I believe would be very good at identifying leaks on boats is ultrasonics.We have a ultrasonic "gun" at work also.With the purchase of the gun, we also recieved a device called a"warble generator." Yep, you read that one right..... " According to the salesman of our ultrasonic gun,the device sends out a very high frequency pitched sound.The generator can be placed in a closed area, i.e., a car,and while it's transmitting, the ultrasonic gun operator would "scan" the exterior of the car around the windows,etc.The ultrasonic gun will pick up the high frequency sound generated by the warble generator around areas that have small openings to the inside of the car, thus, identifying areas that are allowing water to get in.The salesman demostrated this technique for us,and it did work great on a car. It seems to me that it would work just as well on a boat.I've not had a chance to test the tool for a boat type of application yet, but I hope to borrow the device and see if it will work. Maybe I can for once and all stop those annoying little leaks
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Old 31-10-2003, 06:45   #8
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I/R Testing

Stede:

"Professional Boatbuilder" has an article on successful Infrared Testing (for water trapped in cored hull of a 15 year old sailboat) in their October/November Issue (#85).

The author's (David King) bottom line opinion of I/R mapping was that the method can be very accurate & useful, but also requires considerable interprative skills.

Anyone wishing a FAX copy (4 pages), can email me their fax number (sorry no scanner).

Follows a link to the company who did the described I/R immaging:
http://www.boatman.com/infrared.html
Todd & Associates Infrared Imaging

and
David Pascoe is earlier quoted as opining that < ... core problems are often undetectable during surveys, unless the problems are far advanced ...>

Although I do not purport to be a Marine Surveyor, my work has often required that I evaluate Cored Hull and/or Deck structures. I have been able to discover:
- Both minor and major discontinuities (interface dis-bonding) between the FRG Skins and Core
- Both minor and major water intrusions and core saturation (even where no liquid water was apparent when assembly opened up)
Let me reiterate:
These discoveries were made by an inexpert inspector (me), using only visual surface inspection and sounding. It can’t be that hard!

OMO
Gord
Regards,
Gord

PS: Great "Primer" Jeff !!!
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Old 01-11-2003, 05:12   #9
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Cored Sandwich Construction

Here's a 'little' more information about Cored Sandwich Construction:

http://www.diabgroup.com/americas/u_...andwich_hb.pdf

Some nice light reading.

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Old 01-11-2003, 16:23   #10
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Gord,

Thanks for the info. on the infrared. I hope to get a fax of the documents from you as soon as I get a new ink cartridge for my home fax machine. As far as that second link on the "sandwich" info., man....it made my head hurt I'll probably have nightmares tonight of me trying to work all of those equations
ha! ha! There is a lot of good information though. At this point, I wouldn't rule out a cored hull boat, but I would make sure I had a very good surveyor that understood the potential problems, and would probably opt to have an infrared scan done of the hull if possible.

Jeff,

Please forgive me if it seemed like I was unappreciative of the "Primer on FRP" you provided.It's some very good information and I made a copy of it.I got kind of sidetracked on this issue.When you provided the link to the 40' Farr, and I saw the hull was a "sandwich" type of hull, I got into all types of side tangents that I've been researching.Some were specifically aimed at understanding the construction of the hulls, and others were involved with some of the technologies I'm involved in that I thought might be a good fit for maybe a survey business. Anyway, thanks for getting involved with the topic, and providing some excellent information, as usual.
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Old 01-11-2003, 19:51   #11
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My current boat is a solid glass hull with plwood encased in the decks. It is very strong and weighs 7400 pounds. The only way to make it lighter would be to use a lighter layup or use coring. Another boat we owned a 1500 pound 21 footer had balsa core in the deck and a chemicaly made core in the hull. I sailed the boat upside down one day so it got saturated everywhere. It was November so some of the dampness froze not long after. The deck came apart on the inside with an area about 3 feet square delaminating. The hull never had a problem. The hull was stiff and buoyant. So my conclusion was that balsa coring sucks if it gets wet and the new ( 1982 ) chemical coring was great. Solid glass if layed up properly will most likely last forever ( 100 years ) but the boat will be heavier and not as quick. A well made cored hull should also last forever. BC Mike C
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Old 01-11-2003, 19:54   #12
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Stede:

I did not think that you were unappreciative. I thought that you were thinking about my comments and had raised some counterpoints. My response was intended to respond to your counterpoints and thier source.

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Old 02-11-2003, 02:48   #13
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The numbers can be intimidating, but (for us amateurs) it’s the relationships that are most useful.
1. Sandwich Cored construction provides huge strength/stiffness returns from relatively small weight increases.
2. The core & skins must be homogenous - the interlaminar bonding interface must be perfect.
3. So called solid FRG constructions are not (solid) - they are an assembly of laminations, which must also become homogenous.

Jeff’s deck of cards analogy (from the ‘Primer’) is actually a closer representation of a solid lay-up than it is of a cored assembly - which might be better described by a “sandwich”. Nontheless, his analogy describes shear.

I think we’ve pretty much described the numerous & important benefits of sandwich core construction. The question remains as to it’s general utility, for decks, and for hulls (above & below waterline).

I think we have to accept that most modern boats will have some sort of cored deck; and our only debate is what kind of core material? I would not discount any well-designed and properly constructed hull, no matter the design philosophy (cored, solid, whatever).

In short - for me I would evaluate each specific example (potential boat) on it’s own merits. A well-built and properly maintained “Cored” hull will be much preferred over a lesser “Solid” hull (or deck).

More from PASCOE on CORED HULLS (www.yachtsurvey.com)

<< For the most part, Hatteras built balsa cored boats. In earlier years, up through about 1980, the hulls were solid glass, and then cored hull sides appeared. Decks and house tops have always been balsa cored. In fact, were it not for Hatteras, Baltec would probably not be in business today. If you want to know how good balsa is as a core material, try to find a Hatteras with a core problem. Out of thousands of boats produced, there are only a few. And speaking of balsa, if you've ever noticed that Hatteras yachts are notably quieter inside than most others, that's because of the wonderful acoustic properties of balsa.

When it comes to building good quality, consistent and reasonably priced motor yachts, no one had been able to hold a candle to Hatteras.
>>

Not entirely consistant, is he?

The best discussions are those that engender a true debate, which always elicits the most useful information. Personally, (I think like Jeff?) I get the most satisfaction when my comments generate counter-argument & supplementary information. It’s very difficult to compose comprehensive postings (on even the simplest subjects), which are directed to the readership’s needs. Hence, point-counterpoint helps direct the discussion.

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Gord
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Old 02-11-2003, 05:52   #14
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First of all, I absolutely agree with Gordon that point and counterpoint make for a richer exchange of ideas. I come here like everyone else to learn and to share information. It is these back and forth discussions that I learn where I have gaps in my knowledge, discover things that I did not know, or have misconceptions corrected. That is a good thing.

In that regard, I had some comments, questions and additional points about BC Mike C's post. In terms of comments, properly constructed and maintained, a balsa cored deck should have been able to have remained submerged for years without absorbing enough water to have been damaged by freezing. Balsa cored hulls are normally constructed differently than balsa cored decks and so are even less prone to water intake. It sounds like the problem with the 21 footer's deck was poor construction and not a problem with the balsa coring itself.

My question is "What is a chemically cored hull?" I am not familiar with that term.

One additional point is that the worst posible coring is plywood. Besides for the minimal weight savings with most plywoods used for deck coring, the real issue is rot and bonding. To explain, Balsa coreing is actually little blocks of crosscut balsa oriented so that the open ends of the cells are in contact with the laminate. When balsa coring is installed the resin from the laminate actually soaks into the ends of the cells and creates a very strong mechanical connection in addition to the chemical connection (gluing). The resins also seal the ends of the cells from water intrusion. Because balsa, like most wood species, only moves water and rot along the length of the cells, the only way for rot to occur is for water to enter the end of the cell bundle and travel along the length of the cell. This means that, in a properly constructed boat, where the cell ends are sealed, rot may occur where the cell bundle is pierced but the rot is limited in its spread. In the case of plywood, in sliced veneer plywood, the cells are oriented parrallel to the deck and in two directions so that rot spreads quickly and four directions away from the source of the leak. Most plywood is not sliced veneer but is rotary cut. Because of the cell structure of rotary cut veneers used in plywood the sides of the cell wall are also open allowing rapid movement in all directions at once.

Respectfully,
Jeff
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Old 02-11-2003, 07:09   #15
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Chemical cores?

Jeff asks:"What is a chemically cored hull?" I am not familiar with that term."

I suspect that he is referring to the various “Poly-something” foams (ie: the PVC’s - Divinycell, Klegcell etc. , and maybe Polyurethane, Polyisocyanurate, Polystyrene, Polymethacylamid...pls. pardon my spelling)

Jeff also notes:
"One additional point is that the worst posible coring is plywood. Besides for the minimal weight savings with most plywoods used for deck coring..."

I agree!
Although plywood is often used to reinforce Cored Decks at hardware mounting locations, it is not the only option.
I would prefer to see Aluminum reinforcements at heavy loading points, such as winch and cleat bases.
When mounting new hardware, I’ve mostly used the Epoxy Plug method with excellent results.
When remounting hardware at existing locations, where the existing reinforcement is plywood, I’ve also utilized the Epoxy Plug method.
OMO
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