Kevlar questions

Thinking about buying a kevlar boat because (a) it’s a great deal and (b) I’ve never owned one before and I like to experiment. The weight is of course another advantage. So, some questions.

Is it true that kevlar is a bit more fragile than glass? I’m sure there are lots of variables here, but what are they?

Secondly, what about repair? Harder to repair than glass? Enough to make it so annoying as not to be worth it? This is the thing I worry about.


– Last Updated: Feb-11-15 1:00 PM EST –

Kevlar and glass tend to be used in similar manner; same number of layers, partials, etc, so strength is about the same except weight goes down about 25% with outfitting included in Kevlar hulls.

When glass breaks, resin and glass fibre both part; we tend to swim home. Kevlar has significantly increased tensile strength, so when the resin shatters the Kev holds and we paddle home with a ducting tape patch.

Once home, glass patches easily, the edges of those patches sand smooth. kevlar patches easily too, except the patch edges do not sand well. The solution is peel ply which holds those edges down so they do not need sanding.

There are other, fiddley things; Kev doesn't bond with resin, it's hydrophyllic, soaks up water, it requires sharp shears to cut easily. Innegra may replace it in several medium quality laminated paddlecraft.

Few of us are getting stronger. Try the Kev hull, and welcome to the world of modern paddlecraft. The next step is carbon construction, and M-5 is on our horizon; stronger, lighter, more expensive.

Thank you

Kevlar is easy to repair
the only thing I find is it is harder to cut than glass.

jack L

I’m pretty rough on my kevlar boat
and have banged it around a good bit past few years on and off the water with no breaks, but lots of scraped and scratched fuzzy spots that I need to get after soon. Love the boat and the weight. R

Some Kevlar boats might be fragile
A Kevlar boat in an ultralight layup might be more fragile than a solidly built fiberglass hull.

Aramid fibers are stronger than E or S fiberglass in tension, but actually weaker than glass in compression. And aramid doesn’t bond to resins as well as fiberglass. Still, overall the strength to weight ratio is more favorable for aramid than glass so most high quality lightweight hulls have utilized aramid or carbon fiber to lighten weight while maintaining strength.

Aramid tends to work best for interior layers with fiberglass (preferably S glass) or carbon fiber as exterior layers. Having fiberglass on the outside facilitates repairs as well since it can be sanded very smooth, and the outside is what shows.

Apart from the fact that aramid is more expensive and more difficult to cut and can’t be sanded to the extent that fiberglass can, repair using the two different materials is much the same. And you can use fiberglass for interior repairs on aramid hulls as well.

There’s soooo much I don’t know about composite boat construction. I figure this is the best way to learn: jump in.

FTR, it’s a CD Cypress.

The Cypress is a fine boat.
You’ll love it.


cutting Kevlar
Here’s a tip for cutting Kevlar. Purchase cheap scissors at any store, the ones that cost less than $2. They should work fine for your needs. I’ve been using scissors like this for almost 30 years for cutting Kevlar, not the same pair mind you. If sharpened occasionally, you should get approx 1 year of usage out of one pair of these scissors.

The weak link
Keep in mind that no matter what fabric is used in the layup, the gel coat is still the weak link and will sustain damage in spite of the fabric if you are rough on the boat.

One word of caution about kevlar: It has already been mentioned that kevlar does not absorb resin as readily as glass fabric, so if the fabric is exposed by way of a deep enough scratch, etc., it is possible that the kevlar fabric will wick water. I have heard of cases where this can be quite extensive to the point that the boat could gain substantial weight.

The best policy for any composite boat is to treat it kindly; learn how to enter and exit the boat while in the water and never drag it–ever. It will probably still get some minor gel coat damage over time, but it’s much easier to keep it nice than to constantly have to fix it.

Great points
I am very careful with my plastic boats in general, so I figure I would be okay. I usually enter by going in waist deep and practicing my scramble to get in. Reverse scramble to get out, just to practice my balance.

Yeah, I use cheap office scissors too.
Another contributor to success is to pull back a little bit during each cutting stroke. Seems to saw as well as cut the fibers.

magooch, my experience with Kevlar
ww boats is that unrepaired blows do not result in wicking water or weight gain. Only if an area is smashed so badly that the resin matrix is destroyed, and the Kevlar fibers are no longer constrained by resin, is there going to be absorption. I don’t let damage like that go unrepaired.

I think the bit about Kevlar boats wicking water is 90% suburban legend. If it were true, Kevlar would not have been one of the dominant cloths in whitewater racing boat construction. Nobody buys and races a boat that gains weight just from being wet inside, against the Kevlar layers.

to the above two posts. I have found that the cheap utility scissors that Harbor Freight sells for a couple of bucks, and often offers a promo coupon to get free, work as well as anything. They have blade serrations that seem to help.

And indeed, making rather short snips and pulling back a bit as the blades close on the fibers seems to help a lot.

While you can’t sand Kevlar…
…a carbide scraper will feather patch edges just fine. Steel scrapers work fine too, but they dull more quickly.

I suspect that it’s mostly nonsense, just stories that have been repeated over and over, getting more sensational with each telling.

I have noticed
on older canoes with aramid interior layers in which the weave of the fabric was not originally filled with resin (which is most of them) with age and abrasion the resin would wear off the inside to some extent.

When this happens, the interior fabric seems to get damp and it takes some time to dry out, so I don’t think it is just surface water. I doubt that water permeates much into the fibers and I don’t know how much weight it adds (probably not much) but I don’t like it.

When they get this way I apply a coat of clear epoxy to the interior. Just make sure it is an epoxy that won’t blush when cured.

Could one use a quality marine varnish?

Don’t worry
Materials really don’t matter as much as design and method of construction.

Suburban legend
Good one