Is it OK to add a layer to a composite?

I’ve purchased a new Tempest 165pro and am pretty much thrilled with it. I am getting used to owning a composite boat, have already put a couple significant impact cracks into the gelcoat. A friend that does custom drag boat and racing parts in fiberglass/carbon/kevlar is teaching me and selling me what I need at bulk price to maintain my “new” kayak’s gelcoat. He’s also going to help me with a custom bulkhead/footrest.

The Tempest seems relatively light compared to many other boats, but the hull seems a bit easy to flex,(though it took some good hits already without any structural damage). Since it seems doubtful that this boat will have an easy life, I’ve started to think I should beef up the hull a tad. My composite guru thinks we could hand lay a layer of carbon fiber to the inside of most of the hull, hand roll, squeege and dab out the excess resin to add strength without much weight.

What is the collective wisdom on this?

Thanks in advance,

Price versus Bang-For-The Buck
I’m all for experimentation, innovation, and

the do it yourself attitude, but I’m not sure you’ll

get the results you want at an acceptable price point.

Stiff is not the same as impact resistant.

Flex Can Be Good
Fiberglass layups are sometimes designed to flex on impact or when under strain, as the distortion can help dissipate energy and avoid punctures, fractures or delamination. I’ve never seen a composite kayak produced by a reputable firm develop any problem caused by flexing alone.

I’ve been aboard sailboats whose hull panels pant like a dog on a hot day in a seaway, but have never heard of any failures of the panel itself. Did see one case where the forward bulkhead/galley unit was popped clear of the hull, tho - that was impressive…

Keel strip
I’d start with that and see how it works out before adding all the weight and rigidity.

You might wanna google info about
how repeated flexing will break down composites. It’s only a consideration when relatively marked flexing occurs, but it does happen.

Stay within certain limits, and there will be no “fatigue” damage. Probably the Tempest in question will not exceed those limits.

I would not waste effort and money on
general reinforcement of the hull. The builders knew what they were doing. You should just paddle the boat for a while, and check inside or outside to see where stress and even damage is occuring.

If observation suggests that inside reinforcement is needed, it will probably be under the seat, and possibly under your heels. I bought a Kevlar ww kayak cheap, and while installing a larger cockpit rim and seat, I decided to reinforce under the seat with a couple of concentric layers of 5 oz Kevlar.

When a composite boat takes a big hit, usually the highest stress on the outside of the hull puts the cloth in compression, and carbon and glass are best in compression. But the same blow will usually put the inside layers in tension. Kevlar excels in resisting tension. Carbon is good, but if distortion is extreme, carbon is more likely to show brittle failure, while Kevlar may continue to hold.

The implication is that, if you’re going to reinforce inside, Kevlar rather than carbon is usually preferred. Kevlar and carbon are about equally light, and absorb similar amounts of resin. Right now, Kevlar is quite a bit cheaper than carbon. If you check the construction of glass/Kevlar or carbon/Kevlar canoes and kayaks, the builders almost always use glass for the outer layers and Kevlar for the inner layers. (If used outside, Kevlar tends to fuzz when wearing, and has mediocre compression strength.

Incidentally, there is a suburban legend about Kevlar boats absorbing water and delaminating. Complete nonsense, I assure you. No one in my whitewater community has seen such a thing occur.

I wouldn’t unless it was absolutely…

I recently patched a hole in the front inside near the bow of a kayak, and it is next to impossible to work in that small pointy space trying to work through the hatch. Working on the inside of the stern would be the same. You can’t hold a light and do the work at the same time.

My guess is you would end up with an awful mess.

Jack L

Gel cracks
I have an Impex Outer Island which is a lighter glass layup than many of the Brit designs. The problem is, there is more flex than the gel coat can take so it has developed spider cracks all over the hull. There’s no structural issues but it’s a bit annoying. The one problem that can or will occur is when water gets behind these fractures and then freezes. It will eventually lift away and expose the glass beneath. My plan is to strip down the finish, fill in any issues then recoat with either marine paint or Krylon Fusion exterior spray paint. I’ve seen this done and it looks pretty good if done right.

A keel strip is a good idea as others have said.

I’ve done some of this.
I recently got a beater boat that has several badly repaired holes. I ground off the blobs of epoxy and repaired the holes with layers of glass. One crack across the entire bottom of the boat got five layers of very thin glass with the final layer being as wide as the whole bottom. I need the bottom to flex less right near the bulkhead and still be able to flex as it got father from the bulkhead. A little flex can help prevent a puncture from a rock or stump.

For the gelcoat I ground out the cracks and and filled with marine tex and left it at that.

After that I used regular krylon which has been working better for me than the krylon fusion.

Unless your boat has a hole in it, I’d leave the cracks alone until the off season. I can’t imagine why you’d want to add the weight of a whole layer of carbon and epoxy to the inside of your boat, but it would add some stiffness. If you want to let the boat flex and improve puncture resistance then I’d add a layer of kevlar to the inside, in any case you will add a lot of weight to your boat.

My little repairs over about a 15 by 20 inch area along with bottom paint added 3 pounds to the boat. Almost 8% to the boat weight.

I personally would not even add a keel strip until I had the boat for a season and could tell where it is wearing.

Sounds like you put the largest patch
on last. CE Wilson does that too. But Charlie Walbridge (Boatbuilders Manual) says put the largest patch on first, and so do the Gougeon bros at West. I’ve analyzed the structural issues as best I can, and largest patch first leaves no hidden weak zones. The patch edges can be leveled easily with hand sanding.

In my opinion
epoxy binds fiberglass lamina together so effectively that it makes no difference structurally whether whether patches are made progressively larger or progressively smaller.

If you plan to lay up multiple layers of 'glass in one go, having the overlying layers be progressively smaller in size allows you to feather the edges of the smaller patches after the epoxy cures without risk of sanding completely through a larger overlying patch.

If you apply the patches one at a time allowing each to cure and feather each one before applying the next, I don’t believe that it matters nor have I seen any evidence in practice that it does.

I’m interested in the largest patch last vs first. I was repairing from the out side as their was no access but I usually do put the smallest patch into the hole first and build from there. Can you send me a link about doing it the other way?

I’d have to write the link myself,
because Walbridge did not detail why to do it that way.

First, if you dish or taper the glass around the damage, then to hold the layers together better, you want a continuous piece of cloth that drops down over those layers.

Second, if you put the largest patch down first, then the fibers have a straighter run, closest to the surface of the original hull. If you put the smallest patch down first, the close layer is shorter. The fibers of the last, largest patch are taking little steps across the edges of the earlier patches. That isn’t good structure. Think of reinforced concrete. So far as possible, you want a straight run of the reinforcing rods. They should never have to take little steps or kinks around things.

If the patches go on from small to large, there are little gaps at the edges of each patch, underneath the next one. That doesn’t make sense. And if you want to sand those steps smooth, you’re going to have to cut into the fibers of the overlying patch. If you do it the other way around, you can feather the edges of the smaller patches with a bit of sandpaper under your fingers, and you will not be cutting into the fibers of the underlying patch.

I can probably think of other reasons, but it’s getting late.