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Big Crack... OUCH

Discussion in 'Strippers, Stitch-n-Glue, and Other Wood Composite' started by pklonowski, Feb 6, 2006.

  1. pklonowski

    pklonowski Unrepentant Canoeist

    I finished building my stripper 5 years ago, and it's been a great little solo canoe. I've given it TLC as needed (2-3x/year, complete sanding & refinishing), and have repaired some dings where the rocks hit too hard. Hey, it's a boat, not a coffee table.

    Recently, I found a fairly large crack running about a foot long, parallel to the strips, in the port quarter. It's in the inside 'glass only; the outside 'glass is still watertight. I'm pretty certain the crack was not there prior to early December, last time I checked it over, and it hasn't been on any rocky rivers that could have dinged it like this. I also noticed that my nicely bookmatched decks are both split on the glue line, which also wasn't there in early December.

    The only thing I can think of that's different since early December is that, on Jan 1st, it was trailered about 50miles on a home-built tralier, rated for 2000lbs, but not loaded anywhere near that. I've heard that underloading trailers is hard on composite boats, and was wondering if that's where I got this crack? Anybody have any ideas?

    Also, as I'm sanding it out, there's a bit of a canyon around the crack, where some of the 'glass cloth has delaminated & chipped out (see attached pic). Is it better to fill the crack with glass fibers & epoxy, then patch over the whole thing, or should I sand it down to the wood level, and patch across it that way? Other ideas?

    Many thanks!
    Paul
     

    Attached Files:

  2. yelnif

    yelnif another little project

    water freeze ?

    hello Paul, it is possible that you sustained a hard 'blow' on a previous trip and thus may have created a hairline crack-sometimes very hard to detect- which allowed water to enter and accumulate. When the water froze it created pressure and 'pushed' the glass away from the wood leaving the visable foot long crack. It appears that the glass cloth is visable in your pic. which usually means that the resin is thin in that area- this could be a contributing factor. I believe that you will have to 'feather' out the damaged area and then apply a glass cloth and resin patch out @ 2+ inches around the damaged area to regain hull integrity.I have never heard of an underloaded trailer being responsible for this type of damage- if the canoe was properly secured for transport (inverted,padded at the gunwales,tied down). just my 2 cnts-good luck Lee.^.
     
  3. Todd Bradshaw

    Todd Bradshaw Sailmaker

    I don't think water intrusion is a big player in this case. Usually, if water gets in it starts to turn the wood dark long before it does structural damage. This looks like a pretty typical impact fracture or a fracture from flexing the hull.

    When you apply pressure to the outside of a stripper, the glass on the outside gets a bit of compressive force, but the glass on the inside is put in tension. The thickness of the core material, holding the two skins apart magnifies the stress on the inside. Essentially, to avoid breaking, the inside glass has to move a whole lot more than the outside glass does and it often can't move or flex enough to survive. The core itself has very little cross-grain/cross-strip strength and splits easily. The outside glass may get creased, gouged by whatever you hit and may even show a small white stress fracture line, but will probably stay together and usually stay stuck to the core on boats built with epoxy resin. The inside glass layer is where the real battle is taking place. It's likely to break open along the core split and also delaminate on either side of the break.

    The same type of effect can also sometimes happen without hitting any obstacles. Bottom bounce in normal use due to building the hull a bit lighter than it should be can do it and it will usually happen in the area where the flexible floor meets the turn of the bilge - a place where the hull curvature starts to naturally stiffen the boat.

    Contributing factors are also possible. If, for example, you acidentally sanded the core a bit thinner in that area, it may be more prone to flexing or less able to survive impact than the surrounding wood. Another very common one is people skimping on the weight of the interior fiberglass layers in a effort to save weight. They think that since the inside is not coming into contact with rocks or being dragged on a beach, it's safe to reduce that part of the sandwich and save a few pounds. What they fail to understand is that this drastically weakens their hull and that they're removing strength from the part of the laminated hull that's actually going to need the most strength. Unless you're building an honest-to-goodness race boat and are willing to accept the fact that it is borderline flimsy and treat it as such, skimping on interior glass weight is usually a big mistake. Strip-building forums are constantly loaded with the typical "Rather than follow the directions on my first boat, I'm going to use 1.5 layers of 6 oz. cloth on the outside and a single layer of 4 oz. on the inside to save weight". This is usually followed-up a few months later with "Hey, my new boat split open on the inside, how do I fix it?"

    In any case, everything you see in the photo that has become whitish in color is no longer doing any good. It's either badly fractured or delaminated. You need to remove it. Then push the core strips back into shape if needed, glue the split back together, fill any place where wood is missing and apply a fiberglass patch over the inside, overlapping onto the old glass by at least a couple inches.
     
  4. OP
    OP
    pklonowski

    pklonowski Unrepentant Canoeist

    Big Crack

    Wow, great comments -- many thanks! Here are my responses:

    The canoe is always inverted & properly tied down for transport. On the car top, it has suspension providing the padding. Padding on the trailer, however, is zero. That’s why I’m concerned about tuning the suspension for the actual loaded weight, rather than the existing rating.

    The white you see in the pic is sanding dust; since the crack got wider due to delaminating as I sanded, the white from delaminated areas was already peeled off when I took the pic. Once I blew the dust out, the wood is pretty close to original colors, though there was some minimal darkening as you described.

    I won't rule out the possibility that I originally had sanded that spot too thin -- I know there are at least two thin spots, but don't recall where they are.

    I’ve carefully inspected the outside of this cracked area, and don’t see any impact marks that are any more than minor varnish scratches, really. I did have one hard ding a couple years back (different area), which broke the inside but not the outside glass, but it left a serious mark on the outside, so I think I know what I’m looking for… “I think…” so the “Bottom Bounce” you describe is an interesting concept. The crack is right in the curve of the bilge, per your comments.

    The cedar strips in the canyon are no longer glued together, though they are not misplaced, so this would further support the Bottom Bounce discussion. Also, this is one of several places between strips where light is visible through the resin layup, where I either missed with the resin/sawdust putty, or sanded it all off before glassing. Again, maybe sanded too thin…

    After posting the original here, I also noticed tiny cracks in the resin, radiating about 1” from the canyon. When building the boat, I didn’t squeegee out the resin inside as well as I should have, so there are “puddles” where the glass seems to have floated off the wood while curing, and this is definitely one of them. I expect that would weaken the area.

    RE: the comments about saving weight: I built it with one layer of 6oz on the outside, and one layer of 4oz on the inside, which is how the instructions said to do it (honest!). So I was set up for this to happen, it just took 5 years to get there. And, I can probably look forward to doing it again… :-( ….. bummer… How many layers, & what weights of cloth, should be on a boat?

    So I’ve sanded it flush to the wood, & feathered it out nicely. I’ll work some resin between the strips, then put the patch on; probably go 3” either side of the split, for safety’s sake.

    Many, many thanks for your comments… I gained a lot of insight into my boat!
     
  5. Todd Bradshaw

    Todd Bradshaw Sailmaker

    Unless the boat is awfully soft and flexible or it's badly supported (like sitting upright in the back of a pickup truck with a good portion hanging off in space, or with it's weight being concentrated on a couple of small bunks against the hull buttom) I can't see the trailer causing the problem. In any case, having it sit on the gunwale structure has the best chance to spread strain out over a large area when trailering and generous padding certainly never hurts.

    There really isn't a single, perfect layup in terms of glass weight. Bigger boats benefit from heavier glass than small boats need and wide boats may well need more glass weight than narrow ones to generate adequate bottom stiffness. If we're talking about regular, common weaves of fiberglass cloth and boats in the 15'-18' range, I've always had good luck with a full layer of 6 oz. inside and out with a second layer over the bottom, both inside and out (both layers applied at once with the "bilge-cookie" partial layer next to the wood core). It doesn't make an ultralight boat, but is usually lighter than most other canoe materials and has enough durability to survive hitting that occasional hidden rock with little more than a scratch to show for it. On really big canoes, I've used up to ten ounce cloth (doubled on the bottom) with half-ribs glassed into the bottom to keep bounce down. Solo boats, racing canoes and little boats like Wee Lassies can also use the "standard" six-ounce, double-bottomed layup, but you can save some weight by going to lighter fabric. This does come at a durability price though, so the builder needs to be a little more careful with those lighter boats.

    You also want to minimize bounce. There is nothing wrong with a little flex if you hit something (though stripper construction in general doesn't make particularly flexible hulls) but hull flex and bottom bounce in open water is bad. Every time a hull flexes or bounces, fibers are stressed, stretched and/or broken - both glass fibers and wood fibers. They don't grow back. Each time the hull bounces or flexes, a few more die and the hull gets a little bit softer (and more prone to more bouncing). The old fiberglass whitewater kayaks were a great example of flex damage. When you would slide over a rock or ledge, the area right under the seat (where your weight was concentrated) really got flexed inward big-time. After a couple seasons of shallow water boating you would have to go in and add extra layers of glass on the inside of the hull under the seat, because the area had become so soft from repeated flex and the resultant fiber damage.

    No matter what weight/durability range you're shooting for with your stripper, you need enough hull stiffnes to limit bottom bounce and the damage that it will eventually almost certainly cause. Unfortunately, I don't think anybody has published any formulas for strip size and glass weight vs. boat beam, bottom shape, etc. Thicker cores are stiffer than thin cores, heavier glass weights are stiffer than lighter weights and more importantly, stronger. If the bottom does get flexed, they're more able to take it without breaking, as well as more able to limit or prevent flex in the first place. Wider, flatter bottoms are less rigid than rounder, narrower ones and may need more reinforcement, just to hold their shape. As you can see, there are a lot of variables and practice and past experience seem to be about the only way for a homebuilder to really get a feel for just how much fiberglass weight a given design will need.

    Personally, I never ever use layups that are lighter on the inside of the hull. I generally match the inside and outside layers of the sandwich or in some cases, as mentioned, beef-up the inside even more. Think of your stripper hull as if it was an I-beam. Two fairly light load-bearing plates (the inner and outer glass skins) held apart by a non-compressable core (the strips). If we apply a lot of force to the underside of an I-beam, it will eventually start to break. What will likely happen is the bottom plate will bend (in compression) and the core and top plate will rupture (the core will crush and the top plate is in tension and will stretch until it breaks). Our bottom plate is just being bent, and not even that much, but the core and inner plate are being destroyed. If we then decide to save weight and we make the top plate only 2/3 as thick as the bottom plate the beam is going to break even easier. The same thing is basically true on a strip canoe with a lighter interior lay-up. Switching from 6 oz. cloth to 4 oz. cloth on the inside (top plate) reduces the ability of the boat to survive force applied from below (water pressure and/or rocks). The six oz. outside is going to get flexed or creased some, but like our I-beam's bottom plate, it gets off pretty easily. The other layers get the real stress. Other than a slight gain in fabric and filler coat thickness to possibly protect a bit better from rock abrasion, the outer layer isn't in a position to add much strength in terms of something trying to push in on the bottom. Essentially, the boat isn't much stronger than it would be with four-ounce cloth both inside and outside. It would actually be stronger inside-out with the 6 oz. forming the top plate of the I-beam. The ideal layup is probably something with more glass weight on the inside and the inside/outside cloth weight ratio would need to be coordinated with the thickness of that particular core and how far apart it spaces our top and bottom plates. Until somebody does a whol lot of work and gives us those formulas, it's generally pretty safe to match the inside and outside skins on most canoes and less safe (sometimes a lot less safe) to skimp on the inner layers.
     
  6. OP
    OP
    pklonowski

    pklonowski Unrepentant Canoeist

    Ouch

    Engineer speak -- I love it, especially when I'm not at work!!! :)

    Many thanks again for the great responses. I understand how I-beams work (and fail), and the analogy to strip-built boats is excellent.

    I've downloaded this to a text file, and put it in the file for my next (2nd) boat, which won't happen soon, but will get there.

    I noticed there's a WCHA booth at Canoecopia next month -- I put it on my docket of "Must Dos", and will join there. This discussion alone is worth a lifetime membership! Thanks again!

    Paul
     
  7. jackbat

    jackbat Jackbat

    one last opinion

    Hi Paul,
    The kind of crack that you have is not all that uncommon. The laminates on the outside and the inside of the canoe, or any boat for that matter does not have to match. A wood strip canoe, once it is sandwiched in epoxy and glass is a fiberglass canoe which just happens to have a wood core to it. The idea behind the glass is to keep it as light as the duty will allow and still make sure to protect the core. Epoxy is a very brittle substance by its chemical make up. If you use too fine a cloth with epoxy and there is any flexing at all you will see areas of white "heringbone" or cloth start to show through over time. This is actually fractures int he epoxy which then start to pull at the cloth. I am not sure where you read that 4 oz cloth was a good thing but I would not use that for anything but the most delicate of boats. Using 4 oz cloth on a 16 foot canoe instead of 6 oz probably only saved you 5 lbs or so and it is inevitable that it will crack.

    Cloth weight is generally determined by the use of the canoe. You mentioned that you are banging around on rocks. So the above recomendation to double layer your bottom is a good one.

    It is generally compression that causes cracks like the one in your picture. Because of the yoke there is very little if any compression from the sheer to the bottom of the boat. However there is a good deal of compression from the stem to the stern of the hull. That is why you were probably told at one time or another never to step into a canoe or kayak on dry land. Because there is no water pushing back up at you the compression is tremendous.

    You also mentioned that there was a trough there. It was probably there when you built the boat (also not uncommon in the inner curves of a boat) and those are the most difficult place to get a good adhesion of the epoxy to the wood becasue it wants to pull away which leaves a void underneath.

    I would just feather in a patch and it will probably be fine. That having been said, I would still be concerned about a boat with only a 4 oz layer on the inside. If you used Epoxy (I assume you did), you can lay another layer of 4 oz cloth right over the one that you have. Epoxy is simply an adhesive and as long as you wash down the inside well, you will get another layer of glass to adhear to it just fine.

    Just my two cents.

    Jack Battersby
    Sandy Point Boat Works
    WWW.sandypointboatworks.com
     
  8. OP
    OP
    pklonowski

    pklonowski Unrepentant Canoeist

    Big crack... done!

    Many Thanks again! I did feather the interiior glass right down to the wood, put a big patch over it (3in. either side of the crack, 4in. off each end), sanded to feather that to the existing cloth, and it looks pretty good, considering. While varnishing the outside, I did find a pretty good-sized bruise where I must have hit a rock fairly hard, or maybe someone stepped into my boat on dry land when I wasn't looking. The bruise is large and distinct, but not terribly discolored, which is probably why I didn't see it brfore varnishing. So the repair is complete, I'm just waiting for spare time to put it back in the water.

    Overall, I guess I should be nicer to my boat, and should probably get a Royalex one for those rocky streams...

    Thanks again!

    Paul
     

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