Kevlar versus fibreglass

[Brent]

I have a small fleet of wood strip canoes.  Wonderful craft, but when we go creaking they get horribly scratched up.  I have been sanding and applying fresh epoxy each year to pretty them up, but am getting tired of that.  I am thinking about laying on a layer of kevlar - just enough to cover the wetted part of the hull. 

How well will this resist scratching and gouging from bumping into rocks in a running stream?

[Guest LeeH]

I've never used Kevlar, but I've read that some builders apply a sacrificial layer of fiberglass on top of it so they never sand into the Kevlar. Supposedly it is very hard to re-smooth if you rough it up.

[PAR]

Kevlar has several things that make it not the best choice for abrasion resistance.

Epoxy coatings alone don't offer much protection from abrasion. It doesn't take much before you're through the epoxy and into the wood.

'Glass cloth increases abrasion protection substantially. If you want to have a natural wood finish visible, then 4 ounce or light fabric. 6 ounce fabric is just barely visible, if the light catches it right. The heavier the fabric, the better the protection and the less transparent.

Dynel Is a polyester fabric that is 3 times more abrasion resistant then regular 'glass cloth. It's not going to permit you to have a natural wood finish, as it dries with a cloudy look. It requires a lot more resin to wetout and can be a pain in the butt to apply if you've used too much resin (it floats). Dynel is about 15 to 20% more costly, but a far better product for the bottom of boats.

Xynole, is a modified acrylic fabric and about 6 times more abrasion resistant then regular 'glass. It isn't cheap, also needs lots of resin but out performs most other fabrics. It too will not permit clear finishes and can be difficult for the novice to apply, but performance of this stuff is well accepted.

Kevlar is a difficult fabric to recommend on home built craft. It has some wonderful qualities, but also has some butt kicking draw backs, cost not considered. It is self abating on flexible structures and frankly I wouldn't recommend it for any home built boat.

[Brent]

Thanks for all the input. 

Maybe I will add a couple of extra layers of glass to the hull and recoat with epoxy.

I was also thinking of adding some carbon to the final epoxy layer, but I hate that it will hide the wood.

[PAR]

Unless carbon is a high percentage of the laminate, you're wasting money. Both carbon and the other "high tech" fabrics are very narrow focus materials, which need to be specifically incorporated into the laminate to gain whatever benefit they may impart. A "sprinkling" of these materials here and there is just fodder for the marketing team to use in advertisements.

If you want to add some toughness, use multiple layers of cloth or Dynel and the most economical "bang for your buck".

[JeffM]

I put Xynole on the Birder 2 I will someday finish.  Rough, ugly stuff.  My grinder seems barely to scuff it.  But it drinks epoxy like there's no tomorrow: by using it I'll bet I added several pounds to the weight of the boat.

[PAR]

Both Dynel and Xynole suck up resin like no one's business, but they are quite effective.

[ScottWidmier]

I bought Dynel at www.duckworksmagazine.com and put it on the bottom of the kids mouseboats.  Really sucks up the epoxy and requires a lot of work to smooth.  However, regular fiberglass, even multiple layers, won't protect from abrasions.  So, if you want a worry free bottom...

Now on Little Gem it seems I lucked upon a bulletproof bottom.  I did put a strip of Kevlar down the V bottom but I think what has really helped was my ineptitude in applying a graphite epoxy finish.  I just couldn't get it right so ended up applying a lot of layers.  Seems to have handled tough beach landings well.  If I had it to do over again, I would have used Dynel and plan on using it for my newest build.

[Tom Lathrop]

I'm not sure what the mystery stuff is that Ray credited me with but my choice of fabric for abrasion resistance is Xynole. There are 4 fabrics I am aware of for common use that resist abrasion well.  Vectra, Dynel, Xynole and Kevlar.  Kevlar is the toughest but it's drawbacks are already listed.  Vectra is easier to get a smooth finish on than the other two.  Dynel has nearly the same abrasion resistance as Xynole but has not faired as well over time on my boats or in my testing.  When weight due to the greater amount of epoxy required is not a controlling issue, I prefer Xynole.  Fiberglass is better than nothing but not in the same league with the others listed.  Carbon as a practical choice, has no place on this list.

In addition to using these materials on boats, I conducted some objective tests some years ago and the rankings offered are mainly the result of these tests.

I offer the following weights of various materials for information:  One thing to know is that the weight of a coat of epoxy on the various materials will vary greatly according to the material.  Dynel works and weighs about the same as Xynole.

One coat of epoxy  =  0.036 lb/sq ft

Two coats of epoxy  =  0.059 lb/sq ft

Three coats of epoxy  =  0.081 lb/sq ft

6 oz FG cloth w/3 coats epoxy  =  0.142 lb/sq ft

10 oz FG cloth w/3 coats epoxy  =  0.195 lb/sq ft

XYNOLE cloth w/3 coats epoxy  =  0.36 lb/sq ft

KNITEX biaxial (non woven) FG cloth w/3 coats epoxy  =  0.46 lb/sq ft

These sheathing weights are approximate and will depend on how the individual applies the materials but may be helpful for those interested in these details.

[wkisting]

Kevlar felt is a popular choice for skid plates on canoes and kayaks, but on a bright-finished strip canoe, it might be too unsightly compared to another option that wets out clear.

The trick with kevlar felt is getting it cut neat to begin with, which takes a brand new, ultra-sharp pair of scissors. Then you wet out the felt and it will drink a LOT of epoxy in. Later, it can be shaped with a block plane and the experience is a lot like cutting into the skin of a cantalope. The others are correct that sanding will be ineffective, as all you'll do is raise the fuzz of the felt and then the stop the sander cold as it catches the fibers. It is ultra-abrasion resistant for precisely that reason, and one skid plate of kevlar should last a lifetime of scrapes, bumps, etc., though it may occasionally need a little dab of epoxy to smooth it out again if you ever hit rocks sharp enough to slice in and raise some fibers.

I bought some kevlar felt for my kayak a long time ago, but chickened out on applying it. Later, I used it as a skid guard on the leading edge of my centerboard on our CoreSound 20 and discovered that it is relatively easy to shape by block planing. The only really important and tricky thing about applying it is that it is hard to know when it is truly epoxy saturated because the felt is so thirsty and if you don't let it drink in enough, it may starve itself and prevent a good bond to whatever you're applying. For that reason, I think it is best applied by cutting it to shape, then dunking it in a container of epoxy and letting it soak for a couple of minutes to make sure it drinks in enough. Then clean up the work area after application.

FWIW, I like the results and the protection it affords.

[Edit to add: BTW, you only want to use kevlar felt for protecting the keel/centerline in a 2" or 3" wide strip, not coating the whole bottom or else it would weigh a ton. If you're looking for full bottom protection, go with Xynole or Dynel as others mentioned.]

[ScottWidmier]

I like the block plane approach!  Thanks for the suggestion.