Tie Down strength on Wenonah ultralight

I am planning to install tie down points with 1” webbing, gflex and fiberglass, on a Wenonah ultralight canoe. These points will tie down packs, camping gear and air bags. My question is whether, for strength, to install these on areas where there is a foam core underneath, on areas where there is not foam core underneath, or does it matter? Also, is the center area of the hull better/stronger than the chines area or the tumblehome area? Are the rib areas a good place for them? Would the areas with foam underneath have chance of delaminating under stress?

This summer, I plan some extensive practice time overturning the canoe and reentry, etc. I like tying in gear and would prefer not to debate that. These tie down points will see some significant stress keeping heavy packs in place, etc., and so I want to locate them in the strongest areas possible. Your collective thoughts would be much appreciated. Thanks.

It may not matter, but if you want to …

– Last Updated: Mar-19-13 3:50 PM EST –

... minimize producing stress in a potentially damaging orientation, putting the tie-down points a little up the sides of the hull would be best. That way, the direction of pull (which will be primarily upward) will be less "straight out" from the hull and tend to be more in-line with the hull. This will also cause your anchor patches to be stressed less in tension and more in the direction of shear, and in terms of adhesive strength this will be enormously stronger (though it may not matter).

It'll be interesting to see what others say, but I'd be inclined to avoid pulling straight up on any part of the hull that is well inside the margins of the foam core. However, I bet that the junction of the inner and outer layers along the edges of the foam core would be a pretty strong place, since it's naturally reinforced with the inner layer being oriented almost cross-wise to the outer layer right along the edge of the core. Attaching your anchor patches right along that bevel might be as good or better than partway up the sides as described above.

I would avoid placing tie points over
the foam core or its margins. An exception might be made if a tie point is only going to serve light duty.

It is possible to spread the stress of a tie point by incorporating a base of two concentric round or oval layers of Kevlar. The largest layer would be first on the hull. Even a single layer would help. Kevlar is a very light cloth and so won’t add as much weight.

Why not the margins?

– Last Updated: Mar-19-13 5:43 PM EST –

The cross-sectional shape of the hull is like half an I-beam (albeit a distorted one) right at the edge of the foam core. If the anchorage point is within the curvature of that junction of the two layers, that nearly-vertical section coming off the edge of the core will be tension, and in an orientation far more effective at providing flexural strength than adding layers to an already-flat section of hull and applying a right-angle force. In addition, the close proximity of the anchor point to the stiffest part of the entire boat seems like an advantage, rather than a disadvantage, since the load applied here would not tend to pull one layer away from the core as could happen if the anchorage were directly over the core. From an engineering standpoint, I don't understand why one would want to avoid this situation.

Confirms my hunch to avoid the foam
Both you guys post great information, and I appreciate your answering my specific questions. You are confirming my hunch to avoid the foam.

I do not understand the construction well enough to know where the curvature of the junction of the inner and outer layers along the edges of the foam core is exactly, or why that would be in tension and the stiffest part of the boat. I assume this is the location that I somewhat ambiguously identified as the chines in my original post. I also gather from the discussion that the strap should be aligned vertically for optimum grip against the shearing force.

In my original post, I neglected to included kevlar in my materials list. I was thinking two squares about 2”x2” on either end of the strap, and maybe two layers, as suggested. The idea of two concentric oval layers would be better, but I fear that I do not have the experience or skill to effectively cut the kevlar into an oval with an opening in the middle for the raised part of the strap to stick up through. I bought a rotary blade and mat for the purpose of cutting Kevlar but have not tried it yet. I’ve concluded that it really helps to get some fiberglassing skills to go along with paddling skills.

The tip to make the bottom layer, of whatever dimension, larger than the next is also good and reminds me to do that.

Very helpful, cheers and thanks.

The margins are a weakness zone, not
a strength. The chines are also a problem area. I’m assuming his tie points are going to do something significant. Now, if they’re only going to snub down his laundry basket, then he can put them anywhere.

Tie points in my little universe are for things like the lower end of thigh straps in a whitewater boat.

In any case, the margin of a foam core will have a stress riser to the outside, and foam to the inside. A rather narrow zone to place a tie point that might really have to do something.

Super high quality scissors can make
cutting Kevlar easy, but I cut Kevlar with cheap office supply scissors.

Get the Kevlar well back in the jaws of the scissors. Use short cutting strokes, and with each stroke, pull back a little bit during the stroke. This helps the scissors jaws saw through the fibers rather than just pushing against them.

This is a somewhat tedious process, but you don’t have much to do.

The reason I specify oval patches rather than square or rectangular is that a straight edge leaves a stress riser. A lot of times it may not matter, but when one wants the best patch, the layers should be round or oval.

First put down your one or two layer Kevlar patch. Then fray the end(s) of your webbing. Probably the epoxy in the patch will have hardened. If so, clean it with soap and water or rubbing alcohol to remove amine blush. Then epoxy the frayed webbing end(s) down. You may want to tip the boat hull so the webbing ends want to stay down, and you may need to put some food wrap or wax paper over the wet frayed ends and apply weight to compress the fibers.

Okay, but how are ultra-lights used?

– Last Updated: Mar-20-13 9:11 AM EST –

This is an ultra-light hull, so even 20 pounds of pull applied at a right angle to any flat or nearly-flat portion of the hull will deflect it almost an inch, so I figured that the extreme amount of pull exerted by thigh straps and the like would be pretty much out of the question, before even considering the fact that I've never seen ultra-light hulls used in water that makes it necessary for such outfitting. Though the flat, un-reinforced parts of the hull are very flexy under very light load, the curved areas are much stronger and the the margin of the core is stiffest of all. I suppose you could cause a break at the joint if you stressed it enough, but that amount of force applied somewhere else would be destructive too, and I can't imagine that happening in flat water. The stress applied by float bags to a boat that's simply swamped (not pinned against a rock in current) is pretty negligible when spread across many connection points. After all, the only thing the straps which connect to the stems and bottom of the hull do in that case is keep the bag from slipping out from under the cage, and in a flatwater situation that doesn't take much. Tying in gear in a flatwater situation isn't much different.

And here's a thing about the kind of stress riser that you describe which you are not considering, and it's something that's well illustrated by the practice of welding stiffening members such as angle iron to sheet metal. It's easy to create a stress riser at the edge of the weld which attaches the stiffening member (analogous to the one you refer to where one edge of fabric comes to an end a couple inches away from the foam core), but in that case the stress riser is irrelevant. It would only be a problem if one could cause a very strong and isolated flexing stress crosswise to the stiffening member, such as by cutting out a section and stressing it in that direction. In the complete structure, the stiffening member functions to spread the load a great distance longitudinally away from the point of load, such that the amount of flexing stress in the cross-wise direction at any given location remains small (of course, in such a situation it's best to orient the member in the most effective direction for the type of stiffening that is needed, but the idea remains the same in all situations). You can illustrate this for yourself on any ultra-light canoe. Set it upside-down on sawhorses and push as hard as you can on the margin of the foam core. The only flexing you will see is in the sidewalls, where the force is transferred to the sawhorses through that very weak area of the hull. Any flexing across that foam margin will be too small to observe. On the other hand, bending any non-reinforced, non-curved portion of the hull as much as two, sometimes even three inches can be done very easily just by pushing with your thumb (pushing really hard will deflect the material even more). If it were me, I'd put my anchor points where the hull is stiffest, not most flexible.

I’m curious why the chines would be weak
To my thinking the chines - being an arch - would be the strongest place for attachment. What am I missing?

well, whatever you do -
don’t ask Wenonah for thier opinion.

One might suppose that they would have the best idea of strengths and weaknesses in thier hull design - straight from the horses mouth, so to speak

much better to rely on speculation and opinion from the internet

I agree with you
Along the same lines as a stiffening member, the curved chine will be effective at transferring a point load forward and backward some distance away from the point where the force is actually applied, effectively spreading the load over a much larger area, and thereby reducing stress per unit of area.

Then where?
The sloped transition from the tapered core floor and between the ribs has a doubled layer of cloth that is quite rigid.

Foot brace rails are installed on the core ribs through a 1"x2" aluminum plate and two rivets so the core ribs themselves are a prefered anchor point.

Seems to me glued or glassed in d-rings in 3" diameter patch will be spreading the stress around well enough.

Nothing like calling Wenonah

Is Kevlar really necessary?

A couple of follow up questions
If I am reading you correctly, you are suggesting that the Kevlar patch should go under the strap tie-down. My simplistic thought was to put the Kevlar over the strap ends. I assume putting the Kevlar under is to reinforce the hull there, yes? Putting the Kevlar over the strap ends is overkill for holding them in place and just the epoxy is plenty sufficient, yes?

The reason for the tie-downs is to secure portage packs when tripping, and air bags for daytripping/ basecamping, or a mix of packs and air bags. This is for safety in the event of swamping or other mishap on large cold lakes, especially solo.

I know from experience that a fast and easy system is important for all kinds of reasons, and I want to minimize the number of straps in use. I hope to avoid the cage lacing altogether when using just portage packs. This means that the tie-down points need to be located as low down toward the hull as possible, and need to be strong. For the air bags at least, I will need one or two tie-down points in the center of the hull, even if that means locating them on the foam area.

So, I expect these tie down points will experience some serious stress, during warm water practice swamp/capsize/reenter sessions, if nowhere else. A 40 lb portage pack is going to throw its weight around, so to speak, in a 35-45 lb. canoe. Ultimately, I want to make sure to avoid the disappointing if not tragic result of having the tie-down points fail to hold if and when I really needed them to hold. Of course, I also want to avoid damaging the foam on my expensive ultralight canoe.

I was fatigued last night when posting and got turned around about fiberglass and kevlar. I thought that I must have neglected to have fiberglass in my original post but didn’t double check and then misstated as to using Kevlar. I hope that makes sense but it doesn’t matter. The point is that I have used fiberglass but not Kevlar. I did not have good luck cutting fiberglass cloth with scissors. I have not tried cutting Kevlar. I will try the technique of pulling back a little bit during the stroke of the scissor.

Reading between the lines, I think you are suggesting that kevlar is a better material to use than fiberglass for this application, yes?

Much thanks again.

Unnecessary and rude
Mattt, if you’ve read even a few posts by g2d and guideboatguy, you would know that they know as much as anyone about this stuff. I highly value their opinions. Moreover, they are generous enough to take the time to answer questions in writing, with considered, follow up dialogue.

Also, I like to think that this information will be accessible and helpful to others. Would you prefer to post back and forth here about whether the prism or magic is the better solo canoe, or whether a bent shaft or straight shaft is better?

I have called Wenonah in the past for advice on certain appropriate issues, and gotten good information. I called them about the size of the air bags they sold at that time. I called Bell long ago and was lucky enough to have David Yost answer the phone. My outfitting questions here do not lend themselves to a phone call to the manufacturer. Who knows how knowledgeable the person answering the phone will be at Wenonah, whether they can take the time to fully understand what I am asking, and whether their engineers have ever thought much about it, and anyway, business prudence dictates that they have to be careful what they say and stay positive about their product.

I don’t know why I even rose to the bait and bothered to take the time to answer your rude post.


– Last Updated: Mar-20-13 7:36 PM EST –

Well, g2d knows way more about attaching tie-downs and other modifications than I do, and I'd never question his choice of materials or adhesives or working methods. However, the ways in which sheet-like materials can be made more rigid by means of adjusting their shape or adding "beams" of specific shape is not a matter of speculation. The basic principles are pretty simple. It only gets complex when they are designed for a particular type and magnitude of load. From what you say, I don't imagine you "see" these things when you look at the world, but examples are everywhere.

Manufacturer's won't or can't always tell you what you want to know about additions or modifications. Here's a perfect example: I had a 1980 Subaru years ago, and Subaru specifically told me that under NO circumstances should a trailer hitch be installed, because the body of the car "wasn't suitable" for hitch installation and because the car "wasn't capable" of pulling a trailer. We built a trailer hitch anyway, from leftover scraps of steel that were at hand, and designed it to attach at three of the best-reinforced locations on the car body, with the hitch frame between those locations being strong enough to insure that any stress was spread between the attachment points, all using the same principles I applied in my post above. Not only did the car pull trailers weighing 700 or 800 pounds with ease (it turns out that Subaru's official statement about this was dead wrong!), but the hitch became a handy place for jacking up the whole rear end of the car when necessary (Subaru was dead wrong about this part too, though in actuality I realize they have to assume that some people are like you, and can't figure out how to do such things properly so their official stance has to be "don't even try"). Related to that, near the end of the car's life it got rammed from behind hard enough to drive the drawbar of the hitch nearly through the bumper of the other car (the other car was mid-size, but large by today's standards, and bumpers were made of steel in those days too, not plastic). The drawbar of the hitch was slightly twisted by the impact, but there was no damage whatsoever to the body of the Subaru (I checked extremely closely - NOTHING on the car got even the slightest bit bent).

My "speculation" about the effect of a material's shape on its strength didn't just allow my car to tow light trailers and make it convenient to lift the back end of the car, it made the difference between the car being totaled in the wreck compared to having no damage at all, other than to the exposed portion of the hitch itself (that's a pretty good result simply from the "thoughtful" addition of about 25 pounds of scrap steel, huh?). But oh yeah, it's always better to put all your faith in the manufacturer and their official advice rather than rely on your own ability and common sense.

Some thoughts

– Last Updated: Mar-20-13 3:37 PM EST –

Two of my canoes have tie-downs for packs, but the method for attaching them to the hull is different so I'll leave that part out. I'm actually pretty certain that the area in the chine of of the hull (where the greatest curvature is) will be a good, strong location. Coincidentally, in most parts of the boat that will also be right along the edge of your foam core.

I use four tie-downs per pack location, in a square pattern. I use rope, looped over the pack in the form of a diamond-hitch. It's secure, tightens with sort of a "pulley" action, and the packs stay in place pretty well. With four attachment points contributing, the tightness of the rope at each attachment point isn't all that great (shall we call it "moderate"?). It need not be as tight as you can make it - just "sort of snug" works pretty well. Don't forget that your packs will float, and in a swamped canoe they won't create nearly as much force on the tie-downs as would be the case if you were rolling the boat back and forth on dry land. The canoe, which is very light, will be "the rag doll shaken by the dog", with your packs being the dog. In other words, the packs will be the major mass you are handling, and the canoe will just be along for the ride, as far as weight and stress goes. It won't be near as bad as you expect.

If you want, you could avoid anchoring your bag straps over the foam core. You could use two anchor points, on opposite sides of the boat, and have the strap split into an inverted "Y" where it comes off the end of the bag. The split "Y" could be made of rope more easily than strapping material. As I mentioned above, I don't think your float-bag straps will see that much stress. I'd probably use two anchor points (not over the foam core) just to be safe, but that might be overkill. Even providing two tie-downs ON the foam core would cut the stress at that location by roughly half.

You’re looking for a stiff attachment
point, but stiffness doesn’t matter as much as choosing a place where the tie won’t get in synergy with other stresses to cause damage.

The stiffness of the chine, and its contour, make it vulnerable to two kinds of damage, blows from outside and closure of the chine angle if the bottom of the hull is forced upward. For longevity of the chine, one might not want to have tie pads laid onto the inside of the chine. I have two composite boats with thin hulls, foam reinforcement of the bottom, and one with Spheretex ribs up the sides. I wouldn’t put tie points on the chines of either one. I would prefer to put tie points up above the chines on the sides, or in one case to mount them on the bottom of the triple saddle.

As Mr. Guide pointed out, an ultralight canoe shouldn’t see the sort of use that would make tie point location critical. But if one ever has to do serious repairs on an ultralight, it will be much easier if there aren’t tie points epoxied to places like the foam core, the core margins, or the inside of the chines.

Dave, Kevlar is often a better “inside”
cloth because of its strength in tension. Another benefit is that while glass may yield tiny shards, Kevlar will not. But glass cloth can be used for inside tie points. I’ve used it for thigh straps in decked c-1s and have not gotten itchy knees.

I admire your ambition in planning dumping, swamping, emptying and reboarding drills. I roll c-1s and kayaks, but I don’t bother with whitewater open canoes. And with tandems on lakes, my strategy is to avoid swamping or capsizing. It’s worked so far.

Perhaps it won’t matter where you install your tie points, as long as you consider possible maximum stresses. Though I have a Bluewater tandem with a foam core, I don’t think much of that construction. If one wants a lake canoe, the maker should arch the bottom more markedly and brace the bottom vertically against the thwarts. They sometimes do that in marathon racing canoes.

I was thinking about that last night…
… and I could envision situations were anchoring inside the chines might not be best, as it would create a zone of compression rather than tension and the hull might be less resistant to crumpling than stretching. Also, it might tend to not involve the full contact area of the anchor pad. For this particular boat though with its ultra-light hull, I think we may now be in agreement that the parts of the hull which are not stiffened (as well as the part over the top of the foam core, even though overall that’s a stiff location) are likely to be quite “floppy” when pulled on by tie-downs.