Material Data Impact and Fatigue testing from Epoxy Works

[Guest]

The skid plate test suggested in another thread may appear in a future issue of Epoxy Works. They have already done something at least partially applicable, and surprising, especially in different fabric layers resistance.

Those tests were not for skid plates, but for decks on a sidewheel steamer, using a variety of single layer and sandwiched materials (6 and 12-oz. bi-directional E-glass fabrics, 15-oz. biaxial E-glass fabric with 3/4-oz. mat, 4-oz. Polypropylene, 3.6-oz. Dynel, and 4-oz. Xynole Polyester). No fugly kevlar felt involved.

West Systems did not test their samples for abrasion resistance (important in a skid plate) or for added weight (not an issue with a sidewheel steamer, important in an ultra-light canoe where a couple pounds less is another $1000 in cost)

https://www.epoxyworks.com/index.ph...toughest-deck-coverings/?hilite='ticonderoga'

The impact resistance testing is especially noteworthy in best fabric results. I was a bit surprised that a single layer Dynel was near the bottom in performance, and I mistakenly thought that Dynel and Xynole were essentially the same thing.

Impact Resistance
https://www.epoxyworks.com/index.php/ticonderoga-testing-for-the-toughest-deck-coverings/ticon5/

Progressive fatigue (not sure how to extrapolate this for skid plates)
https://www.epoxyworks.com/index.php/ticonderoga-testing-for-the-toughest-deck-coverings/ticon6/

I may still do the half-arsed experiment myself when I find some suitable test beds (piece of Royalex, piece of glass/kev composite, or maybe just another sheet of vinyl siding, which I have).

The “dropping a guided steel rod” experiment is easily replicated, but I’d still like to know abrasion resistance and added weight.

And I need to pick up some better fabric for an under-layer sandwich.

 

[pblanc]

It seems to me that a single layer of Dynel cloth did quite well since it was being compared primarily with mixed multi-laminates consisting of Dynel over some other base layer. A single layer of 3.6 ounce/square yard did as well as a layer of 6 ounce/square yard bidirectional E fiberglass in the first test, and considerable better in the progressive fatigue test.

I have used a fair bit of both 5 ounce/square yard Dynel fabric and 6 ounce/square yard S fiberglass for abrasion plates and/or structural repair of stem damage on canoes. S fiberglass is often said to be 20-25% stronger and stiffer than E fiberglass. It has been my impression that a single layer of 6 oz. S fiberglass offers more impact resistance and overall structural strength than a single layer of 5 oz. Dynel, although Dynel is certainly not bad strength-wise and probably offers better abrasion resistance. There is little doubt in my mind that a single layer of S 'glass is stiffer than a single layer of comparable weight Dynel. Greater modulus could be beneficial in some instances and detrimental in other applications.

As for Kevlar felt, you can have it. Even if you can make the surface smooth and the edges smooth, it is still way too thick for whatever good it provides. Part of my feltophobia is due to my experience with Kevlar felt end plates on whitewater canoes. I have seen quite literally hundreds of such boats in which the Kevlar felt plate broke away in irregular chunks of various sizes as a result of impacts. Part of this might be attributed to adhesive bond strength to Royalex with the particular adhesives used, and results might now be better with the availability of G Flex epoxy. But I attribute a good deal of the poor impact resistance of Kevlar felt to the fact that it consists only of short, compressed fibers with no strong fibers running through it lengthwise for any distance. It may provide reasonable abrasion resistance if it does not break off (although there are many better alternatives, IMO) but for Royalex boats that see significant whitewater use, Kevlar felt is pure crap.

 

[Alan Gage]

I haven't read the report yet but I've never considered the stems to be a place where cloth strength was all that important. With those tight curves it must be the strongest and most rigid part of the hull and I would guess that most of the builders are adding a little extra cloth to that area knowing it's going to take some knocks. I'm not involved in any whitewater circles and don't run any myself except on trips far from home but I don't ever remember seeing anyone complain about doing damage do a stem other than abrasion resistance. So when I'm looking at skid plate material I'm just looking for something lightweight and relatively easy to install that isn't going to wear away easily and that can be easily patched or repaired should be need arise.

I also thought Xynole and Dynel were essentially the same thing. I believe I got that impression from Raka, who sells Xynole but not Dynel.

Alan

 

[Guest]

It seems to me that a single layer of Dynel cloth did quite well since it was being compared primarily with mixed multi-laminates consisting of Dynel over some other base layer. A single layer of 3.6 ounce/square yard did as well as a layer of 6 ounce/square yard bidirectional E fiberglass in the first test, and considerable better in the progressive fatigue test.

I have been pleased with the skid plate results using a single layer of Dynel and a 50/50 mix of West 105/206 and G/flex, and for non-experimental purposes continue to use that, with graphite powder and black pigment for aesthetic reasons, hand compressed under peel ply to reduce Dynel’s tendency to swell with epoxy.

I am not certain how to extrapolate the Fatigue Test data in a skid plate application. I have yet to crack/break or even seriously dent a single layer Dynel skid plate, but I am not a whitewater boater and try (sometimes unsuccessfully) not to smack rocks.

I have used a fair bit of both 5 ounce/square yard Dynel fabric and 6 ounce/square yard S fiberglass for abrasion plates and/or structural repair of stem damage on canoes.
S fiberglass is often said to be 20-25% stronger and stiffer than E fiberglass. It has been my impression that a single layer of 6 oz. S fiberglass offers more impact resistance and overall structural strength than a single layer of 5 oz. Dynel, although Dynel is certainly not bad strength-wise and probably offers better abrasion resistance. Greater modulus could be beneficial in some instances and detrimental in other applications.

I have read the same about Dynel vs S-glass impact resistance, but never seen any real comparative data. I use S-glass for repairs in areas not subject to abrasion, and Dynel for skid plates. I am convinced about Dynel’s superior abrasion resistance.

A couple years ago we experimentally repaired several composite sea kayaks that are highly abused scraping across Everglades oyster bars, limestone and worm rock. Unless you habitually drag your boat across broken glass that is as abrasion abusive as it gets.

We added skid plates or keel strips using either single layers of Dynel, S-glass, E-glass and even thick as heck 18oz bias weave tape (E-glass I suspect), all peel ply’ed.

After a single season of abuse on guide client boats in an abrasion harsh environment the S-glass and thick bias tape had largely been destroyed and the E-glass was worse. The areas with Dynel were still going strong.

I know I want at least an outer layer of Dynel, but would like to try a two layer sandwich, incorporating a (single) piece of more impact-resistant material underneath. I looked into Xynole, and found suppliers, as well as bewildering Dynel vs Xynole discussions on different boat builder forums.

http://bfy.tw/MiAn

I have no idea what Epoxyworks tested “4oz Polypropylene” material is and, thought that polypropylene adhered poorly with epoxy (apparently not). The material listed as “BTI XM 1508 Biaxial” looks interesting and sandwich tests very high, but the nomenclature is good as Greek to me.

I have S-glass I can cut on the bias as an underlay, and some E-glass that I will never use anymore. Maybe a strip of heavy-weight bias glass tape up the center where a vee stem would most sharply take impacts.

And some kevlar felt. What the heck else and I going to do with a yard of it?

As for Kevlar felt, you can have it. Even if you can make the surface smooth and the edges smooth, it is still way too thick for whatever good it provides. Part of my feltophobia is due to my experience with Kevlar felt end plates on whitewater canoes. I have seen quite literally hundreds of such boats in which the Kevlar felt plate broke away in irregular chunks of various sizes as a result of impacts. Part of this might be attributed to adhesive bond strength to Royalex with the particular adhesives used, and results might now be better with the availability of G Flex epoxy.

I too have Feltaphobia. But, back in the day, aftermarket kevlar felt kits, installed without excess resin, edges faired down and raspy felt surface smoothed, were better than nothing, and anyone could follow the instructions and install at worst a sloppy DIY a skid plate. And folks did do their worst.

The principal advantage was that the short unwoven fibers would accommodate complex stem curves easier than woven fabrics, which require bias cuts and more attention to detail.

That complex curve gets worse the higher the skid plate is positioned. Some of the kev felt kits recommended starting the skid plate 6” down from the stem, which on canoes with recurved stems extended above where one might smash headlong into a sheer vertical rock face. I’m not saying I never did just that, but it was infrequent.

I have likewise seen too many fugly cracked and broken kevlar skid plates. See the skid plates on DougD’s $1 Mad River ME from 1984. Those puppies need to be chiseled or ground off and replaced with Dynel. But, who knows how old or how badly installed those skid plate may have been.

I have not yet seen a 20 year old Dynel skid plate, but the anniversary isn’t that far off.

I agree that kevlar felt is a terrible material choice, but the abundance of failures seen may be attributable to there being 10,000 kevlar skid plates out there on old canoes, done with who knows what resin and kit or DIY “technique”, and far fewer done with Dynel or modern materials. I’ve had a couple dozen canoes with ancient kev felt plates come through the shop, some comically over-rich installed, and regrettably still have them on three of our family boats. 3 ½ actually.

for Royalex boats that see significant whitewater use, Kevlar felt is pure crap.

And crap on the other end of the spectrum as well. With a high-end ultra-light canoe a couple pounds lighter in material can run on the order of $200+ a pound. I have no doubt that even lightly impregnated kevlar felt skid plates are by far the heaviest of the possibilities, including two-layer multi-laminate bias cut sandwiches.

I’m thinking that putting a second layer of bias material under the Dynel would take 60 seconds once everything was cut to size, taped out and epoxy mixed.

My skid plate material curiosities/criteria, in order importance for my uses:

Abrasion resistance
Thickness/edge transition after peel ply compression
Added weight
Impact Resistance

Those last two are possibly a tie, depending on the initial boat weight.

 

[pblanc]

I haven't read the report yet but I've never considered the stems to be a place where cloth strength was all that important. With those tight curves it must be the strongest and most rigid part of the hull and I would guess that most of the builders are adding a little extra cloth to that area knowing it's going to take some knocks. I'm not involved in any whitewater circles and don't run any myself except on trips far from home but I don't ever remember seeing anyone complain about doing damage do a stem other than abrasion resistance. So when I'm looking at skid plate material I'm just looking for something lightweight and relatively easy to install that isn't going to wear away easily and that can be easily patched or repaired should be need arise.

I also thought Xynole and Dynel were essentially the same thing. I believe I got that impression from Raka, who sells Xynole but not Dynel.

Alan

I have seen plenty of stem damage on whitewater canoes due to impacts, Alan, some of which were my own. So now you have heard of it. Stem impacts well above the water line occur not infrequently in whitewater canoes as a result of various mishaps including misjudging entries into tight eddies created by large boulders, or coming down off of steep drops onto partially submerged rocks.

 

[pblanc]

Mike, I had the same thought regarding the polypropylene material. I have tried using G Flex with some polypropylene and the bond was poor, although it did stick well enough that it would have been possible to test it with perpendicular impacts if there was no significant sheer or bending stress applied.

In canoes with severely damaged stems in which I had to basically replace lost Royalex, I have used either single layers of 6 oz. S 'glass, double layers of 4 oz. S 'glass, or sometimes a layer of each, and then covered it with a single layer of Dynel. That has worked very well. S fiberglass is pretty amazing stuff with excellent epoxy bonding characteristics, tensile strength that approaches aramid of comparable weight and thread density, and compressive strength that far exceeds that of aramid, which is an important property for applications at the stems. I have felt no need to look for more "exotic"materials for that purpose. I have used bias cut, plain weave S fiberglass cloth for that type of work. Like any plain weave fabric, one has to be careful to avoid snagging, shape-shifting, and fraying during application, as you know.

My understanding is that with Innegra and some other fibers, the specific gravity of the fibers is less than 1 and they tend to float on epoxy requiring vacuum bagging techniques. I have never attempted using Xynole. I did do extensive repairs to one whitewater boat constructed with interior blankets of Spectra which is a polyethylene material. I used both fiberglass and aramid for that repair and G Flex epoxy after preapplication flame oxidation of the base with a propane torch. That has held up very well.

 

[Guest]

when I'm looking at skid plate material I'm just looking for something lightweight and relatively easy to install that isn't going to wear away easily and that can be easily patched or repaired should be need arise.

Likewise. Although foam core Royalex, and I guess now T-Formex, is dentable. If improved impact resistance is a consideration, and it’s as simple as a bias cut underlayer of S-glass, it may be worth the minutes extra time.

Stem impacts well above the water line occur not infrequently in whitewater canoes as a result of various mishaps including misjudging entries into tight eddies created by large boulders, or coming down off of steep drops onto partially submerged rocks.

I meant where the skid plate kit instructions suggested “Start 5 inches below the deck plate”, especially on canoes with recurved stems, which put the top of the skid plate not just above the waterline, but above the most forward part of the bow.

Actually, in consideration, having the stern skid plate positioned higher might have another benefit; I have done (minor) damage high on the stern stem dropping over rocky ledges, where that end of the canoe sometimes takes a whack.

I put a hellacious dent there on my old Grumman, doing just that.

 

[pblanc]

Before I purchased the Mad River ME that I currently have, a tried to pick up a damaged ME cheap that had been sitting outside Endless Rivers Adventures in Wesser, NC for some time. That boat had hit a rock so hard with its front stem just below the deck plate that the Royalex hull had split vertically all the way from the top down to the water line. The screws holding the onlay deck plate had pulled out of the gunwale on one side, and the wood gunwales had caused the split hull to gape open at the front, somewhat akin to the bow doors on an LST.