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Thread: Dramatically different densities in a strip canoe

  1. #1
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    Default Dramatically different densities in a strip canoe

    I appreciate the comments from the forum on my previous questions, as I work my way through the education necessary to get started with boat building.

    A fellow is moving to the L48, and selling his stock of lumber. He has black planks he has labeled “African Cordia”. Based on my internet research they are likely not from Africa, but otherwise fit the description of Cordia, (almost black, black grain and extremely dense)

    If I got this Cordia, it would just be used for one or two racing stripes layered in with the cedar and cypress strips. And or be used as trim on the gunwals. If used in the hull planking, it would be incased in epoxy & fiber glass cloth. If used in the trim, just epoxy

    the seller is is an experienced wood worker, but has not built boats. He knows I am an idiot, and he is concerned with the vastly different densities. The cedar & Cordia would “tear its self apart”, as a result of the different densities adjusting to temperature variations at different rates. He said: “back when I was making cutting boards, the seams would split”

    If my work will either be incased in fiberglass or epoxy, will I still have these issues using the material for boat building ?

    i am thinking the reason he still has this old wood laying around, is because he couldn’t do anything with it. I understand working with different densities will present issues around fairing and sanding, but hope to manage those issues. However cannot contemplate seams splitting after a frosty morning.

  2. #2
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    Default Re: Dramatically different densities in a strip canoe

    You always run a certain amount of risk that the harder stripes will sand very differently from the cedar, resisting the sanding much more. This can make for a surface where the feature stripes are actually raised a bit and it shows. It's not the end of the world, but you have to be pretty careful not to dish the soft cedar as you are trying to fair a harder strip next to it. Some other fairing methods (like steel scrapers) might perform better, but you would have to try it and see how it goes. Ideally the hard stuff is probably better off for gunwales, thwarts, decks, etc.There may also be a certain amount of adhesion difference, as hard wood doesn't absorb the resin as well, but I wouldn't anticipate a serious problem for just a few strips.

    Personally I would never epoxy coat trim like gunwales, seat frames, thwarts or decks. It looks cheap and is hard to repair with no real benefits.

  3. #3
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    Default Re: Dramatically different densities in a strip canoe

    What Todd said about working and finishing.

    FWIW, the problem with different species tearing them selves apart has a lot more to do with moisture than temperature causing shrinkage and swelling, but the end result is the same. The wood database (link below) has shrinkage values. If one wood shrinks a lot more than another one that is glued to it, there will be problems in a cutting board. If you are looking at relatively thin strips encapsulated in glass and epoxy, the moisture content is not going to change radically, and the glass will constrain the movement. No worries.

    There are about 300 species of cordia according to Wikipedia. Based on little more than the color, density, and availability, (and totally unreliable guesswork) I would say you might be looking at bocote, or less likely, ziricote. http://www.wood-database.com/bocote/. There are African Cordia species that are much softer and lighter in color https://www.woodworkerssource.com/wo...+Now+%3E%3E%3E
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  4. #4
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    Default Re: Dramatically different densities in a strip canoe

    It seems to me that the only way they would try to separate would be if they changed lengths at different rates. Lumber doesn't shorten or elongate much at all with changes in either temperature or moisture content, so if it's well sealed and kept that way I think you're good to go.

    Perhaps, just for the peace of mind, use a more flexible glue like GFlex for those strips.

  5. #5
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    Default Re: Dramatically different densities in a strip canoe

    I also question whether his wood is “African Cordia”. Seems true African cordia is not primarily used for the manufacturing of wood products. (More useful fruit and shade tree). It is rarely found in the exotic wood trades. It is most likely bocote, Descriptions of bocote are all over the board. Most descriptions include shades of gold/brown wood with distinct black grain Some descriptions mention it gets darker with age.


    But what wood this is, is kind of secondary.the facts are: It is physically heavy as a lead pipe, compared to the Cedar, and This guy had trouble gluing

    ill check out the flexibility specs on the wood database web site
    thanks to all that shares thier thought.
    Attached Images Attached Images

  6. #6
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    Default Re: Dramatically different densities in a strip canoe

    Cut a strip out of it and one out of cedar and put it to the test. Heat it, freeze it, soak it, oven dry it, see what happens.

    If you want a dark accent strip you can probably find some chocolate colored red cedar.

  7. #7
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    Default Re: Dramatically different densities in a strip canoe

    I just got my hands on some small pieces of African Blackwood for use as knife scales. It is a very dense, heavy, wood like ebony, and it is used in musical instruments etc. The grain in your photo reminded me of it. My pieces are closer to black, but I understand there is variation in color.

    http://www.wood-database.com/african-blackwood/
    "Wherever there is a channel for water, there is a road for the canoe. " - Thoreau

  8. #8
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    Default Re: Dramatically different densities in a strip canoe

    Glue joint failures on stripper cores, inside the fiberglass layers are pretty rare. When they do happen it is usually due to insufficient glass weight on the inside of the hull. Rookie builders think the inside doesn't matter much and they use really light fiberglass layers on the inside while trying to save weight. Big mistake! When you run over something solid, or pound through big waves these things push upward on the hull's bottom. The tensile strength of the inside fiberglass is what has to resist this force and keep the core from over-flexing and splitting lengthwise. As a result, when you run over a rock, the inside glass is what is actually keeping your boat together. How does this relate to glue potential joint failures between woods with different hardness? Well, unless the wood-to-wood glue bond between these strips is absolutely horrible, any lengthwise split would just as likely (or probably more likely) be along a weak grain line, not a strip joint. There really is no reason to torture test strip bonds. With decent quality construction it doesn't demonstrate anything that is likely to actually happen once the stripper is built. Sure, you can glue a couple test chunks together and check to see that the bond seems decent, but doing a lot of torture testing isn't going to tell you anything pertinent to the issue.

    I think the only time I ever mixed hardwood strips in with softwood strips was in the bottom of this one. The light colored keel strip down the middle is white ash and the dark strips are redwood. The reason was that there was a 3/4" x 3/4" hardwood runner screwed onto the outside using wood screws and finish washers from the inside. I wanted something more solid than redwood to screw through. It worked fine (the light feature strips on the topsides are Sitka spruce).



    I think the weak link in terms of expansion and contraction for typical wood strip/fiberglass, canoe-style construction is always going to be the fiberglass itself. Long before anything dramatic would happen between differing pieces of wood you would see problems in the glass layers. Fiberglass laminates don't tolerate stretching well at all. A small expansion and then re-contraction (if that's a thing) of the wooden core of the canoe will leave permanent mini fractures down inside the weave of the clear fiberglass/epoxy. These will appear a bit white-ish, they'll have some visible and feel-able weave texture where there was none before, and they are not going to go away or be easily repairable. I once had this happen to strippers hung in the rafters of the garage when it got really hot up there in the summer sun and later cooled off. They did not really present any sort of serious structural problem, but they were visible on large areas which had never had them before. I really doubt that expansion will be an issue on your build as a result of mixing the two types of wood. If the mixture's hardness difference presents a problem, I think it will be in the fairing stages.

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