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Cracked Royalex stern repair

I cracked the stern of my Royalex canoe pretty badly. On the outside, the cracks resemble the outline of a pretty wide skid plate, although it doesn't run across the centerline either at the wide or narrow end, so it's really two symetric cracks on either side of the hull that converge at the narrow end without touching:

On the inside, there are three separate cracks all along the centerline of the stern, together about the same length as on the outside:

When I press on the bottom of the hull from the outside, the outside cracks open up and the inside bulges upward.

I'm crushed. How do I fix this? Do I first bevel the cracks, then fill them with (thickened?) G/Flex? Apply layers of S-glass on top both on the outside and inside? How many? Do I glass over my existing dynel skid plate? Or is this damage beyond repair? Please advise.

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  • Well, that sucks. First I would ask how the boat sustained this damage? If it was the result of a very forceful impact, such as dropping it from a height onto concrete on its stem, or wacking the stem hard on a rock going over a ledge, then I would go ahead and repair it. If the damage resulted from an impact that seemed to be less substantial, I would question whether or not the Royalex had deteriorated. In that event, I might go ahead and repair it anyway, keeping in mind that the boat might be prone to more damage in the future.

    The good news is that the damage has occurred out toward the stem where the canoe is structurally strongest as a result of its convex form and the gunwales being close together. Almost any type of force sufficient to damage the canoe in this area is going to be a force that compresses the outer part of the hull and puts the inner part of the hull under tension. Therefore, I would consider the interior portion of the repair to be more important than the exterior, which is also good because it will show less.

    Based on the photos, I expect my plan would be to repair all interior cracks with two layers of aramid such as Kevlar cloth, applied with the lines of the weave at a bias with the line of the cracks. Aramid is extremely strong in tension, not nearly so strong in compression, but there will be virtually no compressive force applied in that location. I would probably plan to repair the outside with a single layer of 6 ounce/square yard S fiberglass, again cut on a bias to the line of the cracks. I usually try to make patches overlap the undamaged portion of the hull by 2 inches, but if the cracks extend to the existing abrasion plate or into it, I would probably simply butt the end of the external patches up to the side of the abrasion plate, then thicken the existing abrasion plate with another layer of cloth. I would probably use S fiberglass for that purpose as it is a bit stronger than Dynel. But that plan would be subject to modification based on what I saw when I removed the vinyl over and around the cracks.

    I would definitely gutter out the cracks on both the inside and the outside, then fill them in with G Flex thickened a bit with silica powder. Overfill the cracks slightly, then sand smooth.

  • Well, that sucks. First I would ask how the boat sustained this damage?

    I don't even know. The more likely explanation is that it happened in a class III rapid. I was told to go the the left of the big boulder in the middle of the river so I went to the right, hit a rock and was in the water... I don't know what happened to the boat afterwards but a fellow paddler watching did not notice any particular hard hits that would warrant such damage.

    The only other remote possibility is that it got damaged while getting lifted and moved three feet up a bank by someone else while still loaded with camping gear. Only the center was loaded though since I had 45" flotation bags in the ends. No clue if the damage is compatible with that. I suspect not.

    Based on the photos, I expect my plan would be to repair all interior cracks with two layers of aramid such as Kevlar cloth,

    Two concentric patches, first one overlapping the cracks two inches, second patch smaller by an inch?

    I would probably plan to repair the outside with a single layer of 6 ounce/square yard S fiberglass, again cut on a bias to the line of the cracks. I usually try to make patches overlap the undamaged portion of the hull by 2 inches, but if the cracks extend to the existing abrasion plate or into it, I would probably simply butt the end of the external patches up to the side of the abrasion plate, then thicken the existing abrasion plate with another layer of cloth.

    I understand you are suggesting two patches, one or each side/crack, then another one to fix the compromised abrasion plate? Would it make sense to just have one gigantic abrasion plate-shaped patch to cover all three? Except, what color would look good on a huge patch like that?

  • Yes, I usually make sure that one patch overlaps the intact hull by 1" and the other overlaps the first by 2 ". With aramid, the edges of patches tend to soak up resin and the fibers stand proud and are hard to feather out because the fibers tend to fuzz up. What I usually do is have all the patches cut out as well as some peel ply that is big enough to overlap the larger patch. Apply the smaller patch first and make sure that it is thoroughly wet out. Aramid can be a little tricky in that it is a little less obvious when it is fully wet out. It does not become transparent like fiberglass but it does change color.

    Apply some resin to the area of repair and lay on the smaller patch first. Use a bit of excess of resin to wet out the first patch and immediately overlay it with the second larger patch and wet that out thoroughly, then lay on the peel ply. The larger patch will make sure that the edges of the smaller do not soak up a lot of resin and stand proud. You should have enough resin on the second patch to make the peel ply damp throughout. Thoroughly squeegy out the excess resin to beyond the boundary of the larger patch.

    As for the outside repair, I would first gutter out completely the full extent of all cracks inside and out even if the outside cracks extend into the existing skid plate. Smooth out the filled cracks and complete the interior repair. After the resin cures, the stiffness of the hull should be restored. At that point, the best way to repair the outside will be more obvious. Take some photos at that point and post them if you like.

  • Thank you very much @pblanc. A few more questions regarding the inside repair, which I'll do first.

    What's a good tool for beveling the edges? File? Chisel? There aren't any leaks.

    Why use thickened rather than unthickened G/Flex for filling the cracks? Wouldn't unthickened G/Flex penetrate the cracks better?

    You suggest that I apply both patches together, rather than waiting till the first patch cures to a green state before applying the second. Is that related to getting smooth aramid edges?

    I was going to order the aramid from Sweet Composites: https://sweetcomposites.com/Kevlar.html Which style would you choose? Any tips on cutting Kevlar?

  • edited January 11

    The most convenient tool for beveling the edges is a rotary tool like a Dremel with a small, tapered grinding bit. If you don't have one but have an old "church key" beer can opener, or can find one, you can sharpen the edges on that or look for an inexpensive wood carving tool with a V shaped tip. I have never used a chisel for that purpose, but with care I'm pretty sure a narrow wood chisel would work. Once you get an approximate V shaped gully started, you can use some sandpaper and a small, wooden sanding block.

    All of the epoxy manufacturers recommend thickening their epoxy with a filler when using it to bridge gaps or fillet. Here is an article from West Systems on the use of various fillers. I can only assume that they have done tests of the bond strength with and without fillers:

    http://www.westmarine.com/WestAdvisor/DIY-Using-Epoxy-Systems

    I use the colloidal silica powder (West 406) and thicken the epoxy to approximately a "mayonnaise" consistency. If the thickened epoxy seems to be having trouble settling, warm it a bit by wafting a hair drier or heat gun over it to reduce its viscosity (and accelerate cure rate). You can also warm the cracked surface up a little before you apply the epoxy. Be careful not to overdo it.

    If you apply an aramid patch and wait until the epoxy stiffens, the fibers at the edge of the patch will soak up resin and stand proud. I would apply the second patch while the epoxy on the first is still quite malleable. The second patch will act like peel ply for the first. You will probably also get a somewhat better chemical bond between the two patches the less cured the epoxy on the first patch is.

    I have used a lot of the style 500 5 ounce/square yard plain weave Kevlar sold by Sweet composites. It is what I would recommend for your repair. Just makes sure that it is thoroughly wet out. As for cutting that variety of Kevlar, I find it is quite possible to do so with ordinary scissors. In fact, I would buy the cheapest pair I could find since you will definitely make them dull. Mark out your fabric cut and cut only one or two strands at a time. I find that pulling back very slightly on the scissors as you close them seems to help.

  • Is it a good idea to drill small holes at the end of the cracks? ezwater recommended that over on www.songofthepaddle.com and I've heard of that before in non-canoe contexts.

  • @melenas said:
    Is it a good idea to drill small holes at the end of the cracks? ezwater recommended that over on www.songofthepaddle.com and I've heard of that before in non-canoe contexts.

    You can do so if you want. It certainly won't hurt anything. The rationale is to reduce the tendency for the crack to grow lengthwise. I really don't think it is necessary if you gutter out the full length of the existing cracks, bond them with good epoxy, and cover them inside and out, with cloth that overlaps the cracks on each end.

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