kevlar soaking up water

This weekend I was shopping in an outfitter/paddling store and the store owner and myself got to talking about kevlar boats. He said that he has heard that kevlar boats over time actually soak up water and become heavier.

Anyone have any experience regarding this? I haven’t heard of this and would like to know your thoughts.

"he has heard"
seems to me that when a dealer has to resort to hearsay about a product he sells it’s not important information.

I agree with Ericnyre, it’s more theoretical than practical.

The repair issues with kevlar are more significant than it’s ability to absorb moisture. You can see staining on the inside of a kevlar kayak where impact has fractured and water has entered the fractured resin. For it to gain weight I think the damaged kayak would have to sit submerged in water for a few months. So if you have a damaged kevlar kayak with soft fractured resin and you store it at the bottom of a lake it could be an issue.

Let me guess
This guy has plenty of poly boats. Maybe a few glass boats. But he doesn’t sell Kevlar boats.

It’s easy for a suburban legend like
this to get started. Yet no one asks why knowledgeable boat builders would continue to use Kevlar.

Organic chain fibers have various tendencies to soak up water. Polypropelene absorbs hardly any. Polyester (Dacron etc) absorbs a little more. Nylon absorbs more still, and stretches some when soaked. Kevlar is (I have heard) chemically related to Nylon, so it is not surprising if it absorbs water. Maybe it stretches or relaxes when it is soaked, I don’t know.

But as Eric pointed out, when surrounded with resin, all fibers are limited both in their exposure to moisture, and in their ability to absorb it. Surrounding resin not only keeps moisture from the sides of Kevlar, it also limits water absorption because the Kevlar is prevented from swelling. (If it does swell…)

Kevlar has been used by builders of slalom racing boats for decades. These boats get cracked and hammered. Their owners, who paid a lot, will not tolerate weight gain. I’ve been watching reports from the 70s to the present, and have NEVER seen a report of significant weight gain in a Kevlar boat, or for that matter, any other whitewater boat. So we are dealing with a suburban legend.

Necky marketing I suspect
they made a decision to not use kevlar and go for glass and carbon. I don’t see any problem with deciding on any combo of materials but one of the things the rep du jour said when the ACL came out was that Kevlar wasnt used in aerospace because it soaked up water and the new boats were made by a firm that made aerospace products. The implication being that everyone still building with kevlar was behind the times.

With the HUGE number of kevlar boats out there,boats that sit in the water like racing boats, it’s a bit suspicious.

What you were told is normally found near the South end of a North bound bull.

The resin would need to be removed before water could be absorbed. Certainly a contact laminators sweat will soak into dry fabric and keep it from resinating, but once the resin is worked into the fabric, that isa the end of the story.

Two Misconceptions about Kevlar

– Last Updated: Mar-18-07 3:45 PM EST –

There are two misconceptions about Kevlar: 1) It absorbs water (false); 2) Its a great material to build lightweight sea kayaks with (also false).

Clarification: (3/18/07) Exposed Kevlar fibers will absorb water. I have found Kevlar fibers are easily exposed in high wear areas of kayak, specifically under ones heals.

Kevlar is not a very stiff material compared to carbon or glass. The best property it adds to a hull is impact resistance. It does this with a realtively low weight. This is good if hitting things can not be avoided. Therefore it is a great material for for lighter weight white water boats or surf kayaks.

When Kevlar is impacted it deflects a lot. Kevlar, not being stiff, does not resist deflection and if hit hard enough will often deflect well beyong the yield limit of gel coats. Therefore you can hit rocks and even possibly jump up and down on your kayak and the Kevlar may not break. However, do not be surprised to see abused or heavily usede Kevlar boats have a lot of gel coat cracks.

Another problem with all composites is that the fibers eventually creep microscopically within their resin. This creeping occurs after repeated flexing from normal use. So a boat that may be stiff enough when new will be less so after a season or two of use. I have seen this occur more readily and to a significantly higher degree with Kevlar.

Kevlar is only a mediocre material for sea kayaks, and best used with a combination of other reinforcements like glass and/or carbon that can provide the stiffness and abrasion resistance that Kevlar lacks.

Actually Kevlar can be very smartly used in a cored hull as a layer on the inside of the core. But there aren't many production boats that are cored and utilising Kevlar this way.

Kevlar is a great material for marketing kayaks. Its really not that expensive, its not rare, the supply more than exceeds the demand. Most folks including nearly every one of you reading this aspire to someday own a Kevlar kayak. You are all being fooled. 5 years ago, before I knew better, I also thought Kevlar was the thing to have.

I still paddle a glass/Kevlar composite kayak. Its not particularly light at 55 lbs. It is not nearly as fast as it used to be because it has softened quite a bit and is whippy and oil-cans. It has some cosmetic cracks in places where the flexing was too much for the gel coat to take. The good news is that it still a well designed and well built kayak that will still whoop the big name production kayaks.

I think Kevlar could be used effectively say as a strip or two of reinforcement down the centerline of the hull. It would serve as impact insurance for the accidental collising with a hard object. All new Hunter sailboats use Kevlar like this. But as a primary material for sea kayaks it is just alright.

Maybe this is why Necky doesn't use Kevlar anymore. Smart engineering, but frankly poor marketing on their part.

News to me . . .
I have never heard that kevlar soaks up water. I think they are worth the extra money because they are lighter. For me, the fact that they are also bulletproof is just a bonus. Just like my armoured SUV - While I may never need to use its full capability driving to Starbucks and back, it is nice to know that my vehicle (on road or river) can withstand a rocket-propelled-grenade.

I have personally witnessed Kevlar and Fibreglass boats absorbing water and gaining weight. It is really important to store kayaks without hatch covers on them because in the enclosed environment, water will be absorbed into the weave as it has no place to evaporate to. In a perfectly built boat, the fabric is encased, but with the demand for lighter boats and using techniques like vacuum bagging, that cloth is brought to the surface. Gelcoat absorbs water too. Just ask any mariner. A boat that has been sitting in water gains weight.

Nermal, thanks for showing up. Now
folks can see that the soak-up legend is still accepted.

But I have composite boats over 30 years old, and they haven’t gained weight.

I gained weight
and I’m not made of kevlar

I’m guessing the decision to not use kevlar is all cost/marketing related and not any short coming of kevlar as a building material.

Have you seen how Epic or Current Designs Chinese kayaks are made?

Some figures
Here are some figures I found about the percentage water-absorption (at 20 degree Celsius and air humidity 65%)

Wool: 17%

Silk: 11%

Cotton: 8%

Polyamide: 4.5%

Glasfibre: 2%

Acryl: 2%

Polyester: 0.4%

Polypropylene: 0.05%

What these figures tells me, is that not only Kevlar can absorp water, but other materials used in canoe building too. Perhaps carbon fibers will also show some water absorption? Wether this really can be a problem, is another matter, and possibly depends more on how and how well these materials are used.

Wood for instance can absorp a whole lot of water, and still many boats and paddles are made from wood…

Yes it can happen
Water will get in the fibers but it has to have an entry point, ie… a crack or hole. We have fully enclosed honeycomb core panels, flight control surfaces and such, that we find water in. Yes they are supposed to be sealed, paint coating completly intact, but then when it’s closely inspected usually you can find where the edge has a small delamination and creates a void where the water can get in. We remove the water buy vacuum and heat, then inject the area with resin, unless it’s bad enough to remove some layers and repair it. We have to on aircraft because at altitude it can freeze and really cause some damage.

On your canoe I don’t think I’d get to wrapped up in worrying about the water in the kevlar, unless your completely full and it’s upstream of you and gaining fast.

Oh, I do not have a kevlar canoe, out of budget.

I don’t stay at a Holiday Inn Express, that’s what my camper is for. But I have been working on aircraft and composites for over 25 years.


Yeah, but
what the hell do Aerospace folk kow about composite technology that sea kayak guys don’t!!! C’mon man…these guys still think their Kevlar kayaks can stop a bullet.

absorb vs. wick?
Any serious ocean sailor or powerboater could tell you that stranded electrical wire will wick moisture, but I wouldn’t say that the copper absorbs it. I saw a graet demo of this – a piece of wire with one end in a glass of salt water and salt crystals at the other end.

On the other hand, synthetics can absorb a suprising amount of water. Nylon is notorious for this in the plastics industry-- it can absorb almost 10% water by weight, and controlling the moisture level is a major processing headache.

A better question would be “How much does kevlar absorb compared to other boatbuilding materials?”

Yes, and
perhaps kevlars resistance to sticking to things in the matrix (relative to other materials)could result in interlaminar shearing which creates tiny avenues for moisture, which in turn promotes more breakdown. I suspect this was the cause of the sponginess in the surf kayaks over time. A surf kayak takes a huge amount of stress relative to a touring or race boat. Friend used to manage Northwest Composites and they built wing to body fairings out of kevlar for MDouglas. These did get heavier in time he said secondary to the above, but they were not critical structurally.

why not make kayaks out of s-glass
and core materials? Screw all this kevlar, carbon stuff.

For sea/touring kayaks that might be OK
but for whitewater craft, a couple of inner layers of Kevlar keep the boat in shape for repair after the outer layers of S-glass have been cracked.

That would be good!
I’d rather have that than an all kevlar boat. But that’s just what I’d prefer.