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Thread: Calculating board feet for stripper canoe from offsets?

  1. #1
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    Default Calculating board feet for stripper canoe from offsets?

    I've spent the last hour searching and have found nothing. I have to believe this is out there somewhere but my Google fu is failing. Feel free to smack me if there's a thread I missed somewhere.

    I'm prepping for my first strip canoe build this summer - totally stoked for it. I've found sources for everything and am getting ready to order, and I realized I have no clue how many board feet to order for the strips. I'm looking at the Huron Cruiser from the wonderful Canoecraft book. The study plans from Bear Mountain Boats and the book itself don't give any details on the amount of lumber. Perhaps that is something that the complete ordered plans would have, but it doesn't sound like it from descriptions I've read.

    The closest calculation I've come across is from CLC: http://www.clcboats.com/shoptips/str...ll-i-need.html

    That's a great approximation, I understand how it works mathematically. I also know I can just use a measuring tape once I have the station molds. However, I'm hoping to get a discount from the lumber yard by buying everything at once - meaning I'd like to order the plywood for the stations at the same time as I order the boards for the strips.

    From the data available in the offsets, knowing the length, beam, etc., how can I best approximate how much lineal board feet I need for strips?

    (I just realized I can draw the largest station on paper and measure it ... maybe that's the easiest way?)

  2. #2
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    Default Re: Calculating board feet for stripper canoe from offsets?

    That's the way I would do it.
    Are you doing bead and cove or rectangular strips.

    Bead and cove actually takes more material due to the overlap of the bead and cove.
    Additionally, I found more waste in B&C strips because they are more easy to damage - which I throw away.

    Personally I've always used particle board for station forms. On a kayak, but I think there is little difference.

    Realistically, there is very little wood in a canoe or kayak. Don't think you will have enough to get a volume discount.
    Your biggest costs will probably be epoxy, then glass. Unless you are going expensive on the wood (like mahogany). Or if you are paying for pre-made B&C strips.

    But have fun anyway.

    I hope you provide pictures of your build.

    If you haven't noticed, Todd Bradshaw is a forumite who has lots of good suggestions about canoe building. He had professional experience.
    Might be worth a search for his comments.

  3. #3
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    Default Re: Calculating board feet for stripper canoe from offsets?

    Haven't decided on bead and cove yet, but the CLC formula takes that into account. I hadn't considered particle board for the stations, perhaps I might. I'm going to be filming and blogging all my boat adventures this summer, absolutely will share here!

    I'm ordering lumber for a couple projects, not just the canoe, and the lumber yard has some pricing for me. I'm getting free home delivery at the very least!

  4. #4
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    Default Re: Calculating board feet for stripper canoe from offsets?

    Quote Originally Posted by nloding View Post
    Haven't decided on bead and cove yet, but the CLC formula takes that into account. I hadn't considered particle board for the stations, perhaps I might. I'm going to be filming and blogging all my boat adventures this summer, absolutely will share here!

    I'm ordering lumber for a couple projects, not just the canoe, and the lumber yard has some pricing for me. I'm getting free home delivery at the very least!
    You sound like you are really trying to minimize material costs, but you need to be careful. Nothing is more frustrating than running out of stock near the end of a build, especially if your boatbuilding lumber comes from a not-close supplier. CLC's 20% overage is for milled strips, not raw stock.

    I may be pointing out the obvious but don't forget to take saw cuts into account. A 1/8" cut done many times will dramatically decrease the yield of strips from a board. I'm not sure how you will be cutting and milling the wood but 1/16" blade in a circular saw or table saw saves quite a bit of wood.

  5. #5
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    Default Re: Calculating board feet for stripper canoe from offsets?

    I don't remember ever having a working formula, but for a typical 17'-18' recreational canoe we used to get 28-30 strips per 1 x 12 board and use around 70 strips (about 40 board feet worth) per canoe. These were straight edged strips. We never saw any need for bead and cove, so I don't know how much they would reduce your coverage. This would give us a few extra strips, which is good. You might crack a couple here and there while bending, or need a couple to use for battens when aligning forms, striking a sheer line, etc. The quality of your saw system and kerf of its blade will also make a difference in the yield, as will the width of the boards you can find. Narrower boards may generate a bit more waste. If the strips were shorter than the boat, we just used simple staggered butt joints, done on the forms. They work just fine and we thought they looked better (less obvious) than scarf joints anyway.

  6. #6
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    Default

    I would model the problem something like this.

    Given...

    L = max length on the skin, inches (maximum length, measured on the outside surface of the hull). Or just measure LOA and scale up by a factor of 20% or so.

    G = max girth, inches. Alternatively, model the cross-section of the hull as a demi-circle: the circumference of a circle is 2 * PI * R or PI * D, where D is the diameter and R is is the radius (D/2), using the maximum beam as the diameter. Note that the demi-circle approach will get more and more off-kilter as the hullform deviates from a semicircle (e.g., skimming dish or plank-on-edge).

    T = strip thickness, inches

    W = overall strip width, inches (including the bead if bead+cove)

    K = saw kerf, inches

    WF = waste factor, (K * T) / T ... this assumes you're ripping strips off the edge of the board.

    Compute net board feet as

    NBF = (L * G * T) / 144

    Compute gross board feet, accounting for saw kerf, as

    GBF = WF * NBF

    Compute the number of strips required as

    N = (L * G) / T


    Don't forget to scale these numbers up by a factor of 20%-25% to allow for Donald Rumsfeld's "unknown unknowns"
    You would not enjoy Nietzsche, sir. He is fundamentally unsound. P.G. Wodehouse (Carry On, Jeeves)

  7. #7
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    Default Re: Calculating board feet for stripper canoe from offsets?

    Quote Originally Posted by Dusty Yevsky View Post
    You sound like you are really trying to minimize material costs, but you need to be careful. Nothing is more frustrating than running out of stock near the end of a build, especially if your boatbuilding lumber comes from a not-close supplier. CLC's 20% overage is for milled strips, not raw stock.

    I may be pointing out the obvious but don't forget to take saw cuts into account. A 1/8" cut done many times will dramatically decrease the yield of strips from a board. I'm not sure how you will be cutting and milling the wood but 1/16" blade in a circular saw or table saw saves quite a bit of wood.
    I'm not really trying to minimize costs per se; mostly I'm trying to make sure I get enough lumber up front. If my guesstimation is too low, then I end up in exactly the scenario you describe. I don't plan on buying exactly what the formula gives. I'd probably add at least one more board to the pile if not more. I'm still researching kerf blades for my table saw but I'm going to keep that as thin as possible, but will take that into account when converting the lineal feet into board feet.

    Quote Originally Posted by Todd Bradshaw View Post
    I don't remember ever having a working formula, but for a typical 17'-18' recreational canoe we used to get 28-30 strips per 1 x 12 board and use around 70 strips (about 40 board feet worth) per canoe. These were straight edged strips. We never saw any need for bead and cove, so I don't know how much they would reduce your coverage. This would give us a few extra strips, which is good. You might crack a couple here and there while bending, or need a couple to use for battens when aligning forms, striking a sheer line, etc. The quality of your saw system and kerf of its blade will also make a difference in the yield, as will the width of the boards you can find. Narrower boards may generate a bit more waste. If the strips were shorter than the boat, we just used simple staggered butt joints, done on the forms. They work just fine and we thought they looked better (less obvious) than scarf joints anyway.
    Interesting that you quote 40 board feet when most things say 50 board feet for a 16' canoe. That 50' board feet quote might be bead and cove, which shaves 1/16"-1/8" off, depending on how you do it. Good info to have, thanks!

  8. #8
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    Default Re: Calculating board feet for stripper canoe from offsets?

    Quote Originally Posted by Nicholas Carey View Post
    I would model the problem something like this.

    Given...

    L = max length on the skin, inches (maximum length, measured on the outside surface of the hull). Or just measure LOA and scale up by a factor of 20% or so.

    G = max girth, inches. Alternatively, model the cross-section of the hull as a demi-circle: the circumference of a circle is 2 * PI * R or PI * D, where D is the diameter and R is is the radius (D/2), using the maximum beam as the diameter. Note that the demi-circle approach will get more and more off-kilter as the hullform deviates from a semicircle (e.g., skimming dish or plank-on-edge).

    T = strip thickness, inches

    W = overall strip width, inches (including the bead if bead+cove)

    K = saw kerf, inches

    WF = waste factor, (K * T) / T ... this assumes you're ripping strips off the edge of the board.

    Compute net board feet as

    NBF = (L * G * T) / 144

    Compute gross board feet, accounting for saw kerf, as

    GBF = WF * NBF

    Compute the number of strips required as

    N = (L * G) / T


    Don't forget to scale these numbers up by a factor of 20%-25% to allow for Donald Rumsfeld's "unknown unknowns"
    Great formula, thanks!

    G is really the unknown value from just the forms. I'd have to draw it out on paper, which seems to be the best option. Shouldn't WF be (K + T) / T?

  9. #9
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    Default Calculating board feet for stripper canoe from offsets?

    Quote Originally Posted by nloding View Post
    Great formula, thanks!

    G is really the unknown value from just the forms. I'd have to draw it out on paper, which seems to be the best option. Shouldn't WF be (K + T) / T?

    Good catch! It was late. If you have drawings, you get get the girth by setting a pair of dividers to scale and walking it off the section drawing.
    You would not enjoy Nietzsche, sir. He is fundamentally unsound. P.G. Wodehouse (Carry On, Jeeves)

  10. #10
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    Default Re: Calculating board feet for stripper canoe from offsets?

    Interesting that you quote 40 board feet when most things say 50 board feet for a 16' canoe. That 50' board feet quote might be bead and cove, which shaves 1/16"-1/8" off, depending on how you do it.
    Part of that is likely also due to the fact that we built most canoes from 3/16" thick strips, not 1/4". My fur trade 22' canoe has 1/4" on the sides and 5/16" on the bottom and my drift boat was 1/4" on the sides and 3/8" on the bottom, but those thick bottom strips were mainly used to make wide bottoms stiff enough to prevent bottom bounce. Regular canoes were fine with 3/16" strips.

  11. #11
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    Default Re: Calculating board feet for stripper canoe from offsets?

    I tend to wrap my tape measure around the largest form for starters. I then figure for a 3/4" x 1/4" strip I'm going to lose 1/8" of that if I'm doing cove and bead, so 5/8". I divide my measurement from the tape by 5/8" and round up to the next even number and add 10%.

    For figuring out how many strips out of the board? Depends on your cutting method, but I usually find out what board widths are available and divide by 1/4" (if that is your strip thickness) plus the blade thickness you'll lose to the kerf. I tend to cut on a table saw with Freud Diablo 7-1/4" circular saw blades which have a 1/16" kerf.
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