Fiberglass Layups/Manufacturing Techniqu

Can anyone out there enlighten me on the different types of fiberglass layups / manufacturing techniques?

I have heard of some of them but don’t really know what the difference is nor the advantages / disadvantages of them.

For instance, I have seen some criticize NDK for the use of chopped mat…but I am not sure what the disadvantages of it would be (although would probably guess that weight is the issue given that their boats weigh a ton)

I’m not looking to buy another glass boat…just curious.



It’s a broad topic, but I’ll try

– Last Updated: Nov-27-07 9:06 AM EST –

There are three basic forms of construction:

- Hand Layup - where the cloth is laid in the mold and the resin is applied by hand and worked into the cloth. When done properly, it can result in a very nice boat, but if done poorly, the boat ends up weak and heavy. It's the least consistent method of laying up a boat, leading to jokes about the "Monday/Friday" syndrome.

- Vacuum Bagging - In this process, the layup is done as above, but a layer of plastic is laid over the layup, sealed at the edges and a vacuum is drawn which pulls the plastic tight and results in even pressure being applied across the layup. Excess resin flows toward the pump and is drawn out of the layup (a special material may be laid under the plastic to encourage excess resin absorption and flow). The result is a layup with a much better resin to fabric ratio that is lighter, stronger and much more consistent than a hand layup.

- Resin Infusion - This method is similar to vacuum bagging, except that the layup is done dry and when the vacuum is drawn, resin is allowed to flow into the layup at one or more points until in completely infuses the cloth. At that point, no more resin is allowed in and the vacuum is maintained until the resin cures.

There are three types of resin used to build boats:

- Polyester resin is the least expensive, weakest and most widely used resin in kayak construction. It does a good job and many fine boats are made with it.

- Vinylester resin is somewhat stronger, more expensive and is often used in higher quality boats. It’s often combined with vacuum bagging and resin infusion to produce a superior boat.

- Epoxy resin is by far the strongest and the most expensive and can produce exceptionally strong, durable and light layups, but it’s rarely used in commercial fiberglass kayak construction due to its cost. It’s used widely by home boat builders and commercial builders of wood/fiberglass boats, as it’s the only resin suitable for laminating fabrics to wood.

There are many types of reinforcing materials used in kayak layups (fiberglass, Kevlar, carbon fiber, Dynel, Nylon, etc.), but only three forms of them are commonly seen in kayaks:

- Chopped Strand Mat is inexpensive, weak (due to its short fibers) and absorbs large amounts of resin, as it has a lot of airspace in it. On a positive note, its short fibers and omnidirectional nature allow it to conform to tight, complex curves, which makes it ideal for forming deck fittings, compass recesses and similar structures. It’s also quite stiff and is sometimes used to increase the stiffness of large panels, but it does so at a significant weight penalty. When used on an entire hull, the result is a heavy, weak, stiff structure that is prone to localize failure when stressed.

- Woven Cloth is very strong (due to it’s long fibers) and more expensive than mat. Boats constructed with it are strong, reasonably light and less prone to failure when stressed, as they tend to flex and distribute stresses over a large area. In some cases, it’s necessary to stiffen large panels with other materials to reduce excessive flexing. While chopped strand mat can be used for this, carbon fiber or structural foam are better choices that do the job without the excessive weight penalty of mat.

- Unidirectional Material, in the form of tape or bundles of strands, consists of strands of fiberglass (or other material) with most or all of the strands running in the lengthwise direction. It’s used for localized reinforcement and is not suitable for building an entire boat, since it only has significant strength and stiffness along its length.

Hopefully, this has provided you the information you need.

That’s great information! Thakns a lot.

I guess my next question would be, what type of layup does Valley use? I know NDK uses the chopped mat.



Excellent summary, but having owned
boats made with both vinylester and epoxy, and having used both for patching and minor building projects, I think the strength advantage of epoxy over vinylester cannot be very large. I favor epoxy boats, but will cheerfully buy a vinylester boat if it is expertly laid up with good choices of reinforcing cloth.

Chopped mat???
When I sold N.D. kayaks a few years ago they were all cloth lay-up, not chopped glass, or as some call it ‘chopper gun lay-up’.

Mat is different than chopper gun.

– Last Updated: Nov-27-07 11:49 AM EST –

From NDK

"We use woven mats and chop strand mat in the hull"

You may not see it but it's there.

lamination options
Nice overview

Chopper Gun laminate
Been around for many, many years - I would guess from teh 50s. Used where weight is not a factor (possibly in Corvette bodies?) it uses a air-operated gun to spray a mixture of E-glass chopped strands and polyester resin into the female mold. This may still be a common method of manufacture for powerboats.

A couple of friends built an old Yugo C-1 in a Left/Right mold (1/2 layed-up at a time) where the first layer after the gel coat was a chopper gun layer. This was followed up by a layer of woven roving. Heavy boat. Circa 1968.


Not chopper gun
NDK uses mat, and not a chopper gun. Here’s a link where you can see some picture of chopped strand:

It’s very hard to wet-out, uses a ton of juice, but it is easy to work with and comforms to complex shapes nicely. I don’t care for the finish, so if I’m going to use it, I’ll put some weaved glass over it for a better finish. The nice thing is that mat builds up the thickness quickly.

b1, here is a thread about Valley layups. Peter Orton describes his process.

Chopper gun is a spray technique…
…that’s generally used for larger, thicker fiberglass products. Short strands of fiberglass mixed with resin are sprayed into a mold and built up to the desired thickness. I’ve never heard of anyone doing that with kayaks.

VSK now uses all cloth…
…except for deck fittings and compass recesses. At least that’s what I was told by Peter Orton.

To see epoxy in action…
…check out the videos at:

That’s what an all cloth (s-glass, IIRC) layup with epoxy can do. I doubt you could equal it with vinylester resin, but perhaps Patrick can elaborate if he’s here.

What about epoxy vinylester resin??

I admit not understanding exactly
what that is. Have heard of it, but that’s it. And Salty, have you heard anything recently about the relative costs of epoxy and vinylester?

I knew what chopper gun lay-up is folks
that’s why I questioned the ‘chopped mat’, assumed the poster was talking chopper gun. Never heard of chopped mat before. Again, all the N.D. boats I saw come through my shop (year 2000 or so) where glass cloth on decks and hull. When did the change occur, and where did/are they using chopped mat in the build, the deck fitting and recess areas? If you can’t see it, where is it used?

BNystrom, you really ought to write a
comprehensive kayaking book that has to do not only with GPs, but how to change out a seat, how kayaks are made, et cetera. All the nitty gritty stuff that you are quite knowledgeable about, while we–the book purchasers–are clueless.

According to Bnystrom …
in a Sept post “they use mat everywhere…not just in the bow and stern”.

That’s the first time I had read that NDK used “mat everywhere”. However When asked the source of that information he did not respond so You can draw your own conclusions on that one.


The deck structures
of the older Valley boats were fabricated with chopped matt. I believe this technique was used not so much as a cost saver but to make the decks rigid under a heavy load such as might be encountered in certain two-boat rescue situations.It does make for a very heavy boat and I didn’t think that it was being used any longer. John

chopped mat in kayak decks
Mat is often used in kayak decks because it more easily conforms to hatch rims, sunken bungi fittings, compass stanchions and other irregularities.