Cored Laminates in GRP Boats

Posted on Friday 15 January 2010

As a caveat, I am not a laminate engineer, but a just an amateur boat builder.  As such, the information contained within this post is written to be a basic primer.

Modern GRP boats often use a cored laminate construction for several reasons.  A cored laminate can be much stiffer, stronger and lighter than a non-cored laminate.  Using cored laminates can reduce the weight of the hull and deck, allowing more weight to be concentrated in the keel, at least in monohulls, and result in a faster, stiffer, higher performing boat, than the same design would be if built using uncored laminates. Using cored laminates allows multihulls to be much lighter and more rigid than using uncored GRP, which is relatively heavy and flexible.

While core materials are often more expensive per pound than resin and fiberglass cloth, proper designed cored laminates can often reduce the cost of making a boat, by greatly reducing the amount of resin and cloth needed to make the boat.  Using cored laminates can also reduce or eliminate the need for additional stiffening or reinforcement such as an interior grid, stringers, or pan liner—simplifying the construction.

Basically, there are three commonly used traditional core materials that I will be discussing: foam, end-grain balsa, and marine plywood. There are also basically three classes of resin used in making GRP boats that will be discussed: polyester, vinylester and epoxy. Finally, there are several materials that are used to reinforce laminates.  These materials range from the basic glass cloth to high-tech materials  like carbon fiber for greater strength and stiffness, or kevlar cloth for greater abrasion and puncture resistance. I will also discuss the different forms the reinforcing fibers are found in.

One thing I will only mention in passing is chopper gun fiberglass.  Chopper gun fiberglass used a mechanical “gun” to spray chopped fiberglass strands and resin as part of a boat building process. This was a fairly imprecise and primitive way of building up laminate, and is not used in higher quality laminate boat production today. It tended to yield very variable quality and strength laminate, and was highly dependent on the operator’s skill.  While many classic boats were built using this technique, it has fallen out of favor due to the highly variable results as far as I know.

Why is a Cored Laminate Better?

A cored laminate is lighter and stiffer than a solid fiberglass panel of the same area and weight.  The core material is effectively acting as a stress web between the two layers of laminate bonded to it. This is similar to the inner perpendicular section of an I-beam, which acts as a stress web for the two outer sections of the i-beam. Basically, for a cored laminate to bend, you need to compress the skin on the impact side and stretch the skin on the opposite side… by having the core, you’ve effectively turned the bending of a single layer in to the compression of one side and the tension of the other side, with the forces being transmitted via the core material.

However, when a balsa core rots, the two skins no longer work in tandem, since they’re no longer laminated to each other. The same problem of the skins not working in tandem can occur when water penetrates between the core material and one of the skins, which can happen when water intrudes and the laminate flexes, since the flexing of the laminate can act to pump water in between the skin and core.  This is why properly potting and sealing the core off for any holes drilled through the cored areas of a deck, cabin top or hull is so important.

Core Materials:

Marine Plywood:

This was originally used as a core material to create stiffer decks and also used commonly as a backing material for high-load areas where hardware was often attached. Some manufacturers used marine plywood as a core material only in high-load areas, but others used it as the sole core material in their hulls. While, marine plywood, or in less than ideal cases plywood, had the primary advantage of being fairly inexpensive and fairly easy to get—it is not a good core material for several reasons.

First, while plywood does have a lot of tensile and compressive strength, it is a fairly heavy material, so there is little weight savings using it. The other core materials allow the laminate to be much lighter for the given strength.

Second, it has the worst disadvantages of both foam and end-grain balsa.  Like foam core materials, it allows water to migrate vast distances and delaminate large areas over a relatively short period of time.  Like end-grain balsa, it rots and breaks down when water gets into the laminate.

Fortunately, marine plywood isn’t used very much as a core material in modern fiberglass boats.

End-grain Balsa:

End-grain balsa is one of the best core materials for making a very stiff light GRP boat.  Balsa has two major advantages over the foam core materials.  First, it is generally far easier to bond the laminate skins to a balsa core due to the nature of balsa, as capiliary action helps draw the resin into the balsa and create a strong bond.  Recently, this has become less important, as many boats are now being built using epoxy resin, which has greater secondary (adhesive) bonding characteristics than polyester or vinylester resins.  Second, the use of end-grain balsa generally localizes the water intrusion and slows the spread of the water intrusion when compared to foam or marine plywood core materials.

A second major advantage that balsa has over foam core materials is that it is generally stronger in shear and compression than most of the foam core materials.

Finally, balsa also doesn’t deform or weaken at higher temperatures, which can be an issue in darker colored foam-cored laminates.  This is more of an issue with epoxy resin-based laminates, which soften at lower temperatures than polyester or vinylester resin-based laminates.  This is one reason that many epoxy based laminate products are light colored and have restrictions about using darker colored finishes on them.

However, it has two major disadvantages over some of the foam core materials.  The first is that balsa, like marine plywood, will rot when continually exposed to water.  Preventing water intrusion into a balsa-cored laminate is a priority. This means that using balsa as a core material in the hull really isn’t a good idea, unless the construction methods are meticulous and avoid any exposure of the core to water ingress.  The second is that end-grain balsa will transfer the forces from an impact from the outer skin to the inner skin, and in the case of a collision will generally rupture.

PVC Foam Cores:

There are two types of foam core materials—ductile and rigid. Examples of the two are Airex—which is a ductile PVC foam material; and Divinylcell—which is a rigid PVC foam material—both of which have a long history of being used as a core materials in GRP boats.

Both types of foam have the advantage of not rotting when exposed to water and the disadvantages of allowing water to migrate and delaminate large areas in relatively short order, and can present some difficulties in bonding it securely to the laminate skin layers. One other disadvantage is that foam core materials can deform at elevated temperatures, and this has become more of an issue as epoxy resin based laminates become more common.

Ductile foams are best used for hull laminates.  The reason for this is that the ductile foams will often compress in the case of an impact and will protect the inner laminate from rupturing, helping preserve the water-tight integrity of the hull.  Combining ductile foam core materials with layers of kevlar cloth in the hull laminate make for a very durable hull with high resistance to being holed.

Rigid foams are often used for deck, cabintop and interior laminates.  The rigid foam cores make laying up the large, relatively flat panels easier and create very rigid and stiff structures. Unlike ductile foam core materials, rigid foam core materials will transfer most of the impact force through the laminate, which makes them less desirable for hull construction.

Depending on where in the boat structure the laminate is being used, different density foam core materials can be used to provide greater strength as needed.  However, end-grain balsa is generally stronger in shear and compression than either ductile or rigid foam core materials.

GRP Resins:

Polyester-based Resin:

Polyester-based resins have the advantage of being the least expensive of the three types of resin used in modern GRP construction.  However, time has shown that polyester resins have some issues with osmosis that are less of a problem with the more expensive vinylester and epoxy resins.  Polyester resins can also have some issues with bonding to the core layer in cored laminate construction.  Polyester resins also have the lowest tensile strength and poor secondary (adhesive) bonding characteristics, which make it less than ideal for repairs, especially of a structural nature.

Vinylester-based Resin:

Vinylester based resins came into use as a response to the osmosis problems that have been associated with the polyester resins.  Vinylester and polyester resins are very similar and are often used together in production boats, where the vinylester resin is used for the outermost layers to provide greater resistance to osmotic blistering in modern boats.

However, vinylester resins are a bit more expensive than polyester resin. Like the polyester resins, vinylester resins have poor secondary (adhesive) bonding characteristics, making it, like polyester resin, less than ideal for repairs.

Epoxy-based Resin:

Epoxy-based resins have the highest water resistance of the three most commonly used boat building resins.  Epoxy hulls tend to have far fewer osmotic blistering issues than polyester or vinylester resin-based boats.

Epoxy resin also has the highest tensile strength of the three resin types. This often means that an epoxy resin-based GRP hull can be lighter than one that uses polyester or vinylester resin. It also has the best secondary (adhesive) bonding characteristics of any of the resins, making it the best suited for structural repairs and modifications.

The two big disadvantages of epoxy resin are the cost of the resin and the thermal characteristics of the resin.  Epoxy resin is the most expensive of the three types of resin.  However, the expense of epoxy resin is somewhat offset by the greater tensile strength.

The greater problem is the fact that epoxy resin-based GRP laminates soften or deform at lower temperatures than polyester or vinylester resin-based laminates.  Some manufacturers use a combination of vinylester resin over epoxy based laminates to help offset this issue. However, it is generally a good idea to finish epoxy resin based laminates in very light colors.

Reinforcing Fibers:

Glass Fibers:

The most common and least expensive reinforcing cloth is fiberglass cloth.  This is the original material used in GRP construction, but has been supplemented by other, more expensive and specialized materials in modern boats. There are two types of glass fiber used in boat construction:  E-glass and S-Glass.  There isn’t that much of a difference between these two types of glass fibers, but cloth made with S-glass is generally about 20% stronger than regular glass made from E-glass.

Kevlar Fiber:

Kevlar is an unusual choice of materials for reinforcing GRP hulls, in that it is has little strength in compression.  It is primarily used in areas where puncture resistance and abrasion resistance are required. Combining an interior laminate that has Kevlar in its makeup with a Airex foam core can make a hull very durable and highly resistant to being holed.  It is generally only used in hull laminates.

Kevlar cloth can be a difficult material to work with, since it tends to “float” in the resin, and is hard to wet out completely for that reason. As a general practice, it makes sense to have at least one layer of glass cloth over the Kevlar layers, since this helps the Kevlar wet out properly.  Another reason to have a layer of regular glass cloth over kevlar is that Kevlar tends to sand poorly, making finishing the laminate difficult if the exterior layers are Kevlar.

Recently, Spectra cloth has replaced the use of Kevlar.

Carbon Fiber:

This material is very expensive and often difficult to get, since the aerospace industry tends to use much of it. It is often used in areas where extra stiffness and strength in the laminate is required. It is one of the few laminates that is used for spar construction, due to the requirements of high strength and stiffness.  Carbon fiber has several issues that make it less than desirable a material to work with.

First, carbon fiber can cause galvanic reactions if any metal hardware is attached to it. This is because graphite is a very noble material and special steps need to be taken to isolate any metal hardware from the carbon fiber.

Another issue with carbon fiber is that it is relatively brittle and subject to fatigue-related failure if it is not properly engineered to prevent the material from flexing cyclically under load.  Carbon fiber is also very directional in nature and if the laminate is not properly laid up, it will fail from loading that are not properly oriented to the fibers. Finally, carbon fiber-based laminates can often fail if the laminate is scratched or damaged. This was an issue that was first seen in the early carbon-fiber bicycle frames, where a tiny scratch in the laminate would lead to sudden failure a short time later.

Reinforcing Fiber Forms:

Fiberglass is sold in several forms for use in laminate construction.

Chopped Strand Mat (CSM)

The weakest material sold is chopped strand mat or CSM.  This consists of short strands of fiberglass held together by a binder matrix which dissolves in the resin as the mat is laid up.  Originally, CSM was not recommended for use in epoxy-based resins, as the binder material that held the CSM was only soluble by the styrene found in polyester and vinylester resins.  However, as epoxy resin has become more commonly used, newer, epoxy compatible, forms of CSM have come onto the market.

CSM is often used between layers of stronger material to help bind them, since it conforms to the uneven textures of roving and heavy weight cloth fairly well.  It is also often used in the outermost layer of a laminate to prevent the texture of the cloth from “printing through” the surface finish of the laminate. It is also used to build up thickness in a laminate quickly. While not as strong as longer-fiber materials, it does have the advantage that it is equally strong in all directions due to the random orientation of the chopped strands.


The next most common material is fiberglass cloth. This is exactly what it sounds like, a cloth woven from fiberglass strands.  This is stronger than CSM as the longer continuous fibers provide the finished laminate with greater strength than then short fibers in CSM.  However, the weave of the cloth makes it weaker than the stitched mat products. The bends of the cloth weave weaken the fibers except in Kevlar or Spectra materials.

Directional Roving

The strongest material is directional mat.  This is available in uni-directional, bi-axial and tri-axial variants, which describe how many layers there are and what orientation the strands of fibers are in. Rather than being woven, like cloth, the fibers are basically held in bundles that are either chemically bound or “tied” together.  In biaxial or triaxial, each direction of fibers is a separate layer and ties between the layers hold the bundles oriented properly.

Because these materials have the fewest “bends”, they tend to be the strongest.  However, they do not conform to complex shapes as well as CSM or cloth.  For this reason, they’re generally best suited for longer, relatively simple sections of the hull and deck.

However, cloth and roving has to be oriented properly for the strength of the material to work properly.  Cloth and roving are strongest when the loads are along the length of the fibers and weakest where the loads are oblique to the direction of the fibers.

Kevlar, Spectra and Carbon Fiber materials are usually available in cloth form.  Carbon fiber is usually also available in directional roving forms.

My Ideal Boat Construction:

My ideal boat would have hulls made of Airex-cored hulls with Spectra reinforcement along the bow and keel.  The deck and cabin top would be Divinylcell-cored, with carbon fiber reinforcement of the mast step area and the akas.  The carbon fiber reinforcement would be glassed over with an exterior fiberglass layer to provide galvanic isolation for hardware installation.

Areas that were highly loaded, like the chain plate and deck hardware attachment points would be solid glass with carbon fiber reinforcement.  Areas in the hulls where the through-hulls need to be installed would also be solid glass.

The boat would be constructed using epoxy resin for the hulls and vinylester resin for the deck and cabin top.

12 Comments for 'Cored Laminates in GRP Boats'

    February 7, 2010 | 5:22 pm

    [...] Marine Plywood is a horrible choice for core materials, as I've written about in this post. I would recommend going with Airex, which is a ductile PVC foam. It is probably flexible enough [...]

    February 10, 2010 | 1:23 pm

    [...] plywood for a while, but it really is not an appropriate core material, as I've written about on my blog. The problem with end-grain balsa is that it tends to rot if water gets in… then as it rots, [...]

    February 13, 2010 | 4:30 am

    Great post and I appreciated your bringing all the “strands” together.

    What to you think would be the differences in weight, strength, and cost between your “ideal construction”, conventional GRP construction, and say, steel?

    February 13, 2010 | 4:33 am

    One other topic occurred to me: do naval architects (or insurance people, or anyone) now have a good base of knowledge for knowing where GRP boats fail?

    February 17, 2010 | 12:56 am


    I don’t think that there is a good base of knowledge, since the construction techniques used in GRP boats is so varied. Even among boats of the same design and from the same manufacturer, especially with factories that do hand layup of the laminate, there can be a significant disparity in the weight of the boats. Up to a certain point, the lower the percentage of resin in a laminate, the stronger and lighter it will be, provided there is sufficient resin to properly wet out the fibers.

    As for costs… I think that there are too many factors to say what the costs would be. You’d also, probably, want to look at the costs over the lifespan of the boat. Steel has serious maintenance issues and if the wrong steel is used, the problems can quickly destroy any value to the boat. Maintenance is also an important issue. A properly designed cored GRP laminate is going to be far stronger, stiffer and lighter than non-cored GRP construction and far less expensive for the same given strength of laminate. A good cored GRP laminate is also going to have far better fatigue resistance characteristics as I understand it.

    Size of the boat also plays a major part. Boats under 40′ LOA really aren’t suitable for construction in steel IMHO. Steel is also a very lousy material for multihull construction, where weight is such a primary consideration. Given how much stronger carbon fiber, kevlar and spectra based laminates can be than steel per weight, there really is no comparison.

    February 28, 2010 | 11:31 pm

    [...] Marine plywood is the worst of all core materials for cored laminates, as you can read about HERE. If you do decide to recore the deck, it would be my recommendation to cut the top laminate layer [...]

    March 23, 2010 | 5:42 pm

    Interesting stuff. My wife and I are actively shopping for a Gemini 105Mc, and I was curious about the balsa cored deck. Some people think the Geminis are too lightly built, and have a ‘springy’ feel. Those people sometimes prefer the heavier UK built cats like the Catalac and Endeavors.

    Nice blog, by the way. I found it researching Gemini’s. I spent twenty years living on Cape Cod, working in offshore and underwater field. Now I live in the Turks and Caicos Islands, and it does my heart good to see people freezing in Buzzards Bay, ha ha. ( Just kidding)

    March 24, 2010 | 6:27 pm


    It really depends on what you’re looking for in a catamaran. The Endeavors and Catalacs are considerably older designs than the Gemini 105Mc. I know of two Geminis that have ridden out hurricanes on moorings or at anchor, so I don’t really feel that the Geminis are that lightly built. One thing to be aware of is that Geminis, like my Telstar, are built with a specific price point in mind, and that the fit and finish of the boat reflects that. While the boats don’t have the elegant fit and finish of a Lagoon or Quorning Dragonfly, neither do they have the prohibitive price tag.

    Both the Gemini and the Telstar give you a lot of boat for the buck. They have solid bones, but do have some shortcomings. However, they give their owners a chance to get out there and go, far sooner than they would be able to do so on another brand of boat in many cases.

    I’d point out the relatively high resale value of the Geminis says a lot about them as a brand. Be aware that Performance Cruising recently split into two companies. Performance Cruising now only handles the Gemini catamarans, and is partnered with The Catamaran Company, and owned by a management company that is run by Laura Hershfeld, Tony Smith’s daughter. The Telstar 28 and its operations are now owned by a new company called Performance Sailing, and that company is owned by Tony Smith. There has been a delay in resumption of producing the Telstar 28. IIRC, Tony, along with his wife Sue, are going to be sailing a Telstar 28 down the Pacific coast of North America as a demonstration of the boat’s capabilities next year.

    April 12, 2010 | 2:50 pm

    Hello I was wondering if I could talk with you for a moment as I have many questions regarding a telstar 28 that I have just put a down payment on (hull 315) in Michigan. I still have to sign the final loan docs this week. I know this is last minuet but I am a little nervous about a few things and you knowledge of this vessel is deep. My email is granfury84 {at} hotmail {dot} com


    August 26, 2010 | 3:51 pm

    [...] want to read this primer I wrote on Fiberglass Composites for boats… __________________ Sailingdog Telstar 28 New England You know what the first rule of [...]

    October 18, 2010 | 4:18 pm

    [...] I'd recommend you read the article I wrote on Cored Laminates, as it should give you a pretty good overview about core materials and such. Properly designed and [...]

    May 28, 2012 | 2:43 pm

    [...] [...]

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