Dense Boat Building Douglas-Fir Weight

Discussion in 'Materials' started by abosely, Jul 10, 2015.

  1. abosely
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    abosely Senior Member

    I've seen the weight of Douglas-Fir usually listed as 32lbs cu/ft at 12% moisture and specific gravity of .45 .51, but some places list it as 35-36 lbs sq/ft air dried .45-.51 SG, not sure on the moisture % tho.

    How does the common spec for D-F of 32lbs cu/ft weight average, compare to the close ring, denser wood that is usually used for boat building? We're usually picking thru the lumber to find the close ring, dense wood to build with.

    So wondering what the approximate weight per cu/ft the D-F usually chosen for boat building weighs per cu/ft? I see the Specific Gravity of .45 to .51, but know how that translates to a given amount.

    Reason for asking is to get a rough idea of how much extra weight I'll have when using Albizia Lebbeck wood over D-F for my Narai Mk IV.

    The Albizia Lebbeck is listed as 39-40lbs cu/ft at 12% moisture & .51 to .63 SG, but is stronger than D-F. So at least I'm gaining some strength along with the added weight.

    Which isn't too bad as she's intended as more of an expedition style boat, not a high performance one. The Narai Mk IV is 7700lbs empty bare and 15,400lbs normal gross, so she shouldn't be quite as sensitive as a light boat. Also I'll have plenty of it so I can pick the lighter pieces to use.

    Curious as to what the actual (well rough idea) weight is of nice dense D-F compared to Albizia Lebbeck is.
     
  2. alan white
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    alan white Senior Member

    Depending on the building process, the lighter fir may be stiffer IF it is thicker. By the same token, balsa could be ever stiffer yet, if the wood is skinned with cloth/resin. Do you need wood more for strength or stiffness?
    If the fir were meant for a beam or a structural frame that was not skinned with glass, you might improve its strength by substituting white oak, e.g. even if the oak was cut smaller in dimension. Imagine if a glass/wood/glass composite used white oak at 57 lbs per cu ft. It would be so thin, it would flex far more than a thicker, lighter core would allow and it would fail sooner.
    It seems that lighter woods of larger dimensions work better as a core material but heavier woods do better at reducing the dimension of parts (overhead cabin beams), standing up to wear (teak decks, e.g.) and at carrying high loads (winch mounting base).
    In other words, no matter what you read about the qualities of the wood, the application is often as important as the species. Your heavier wood will likely be ideal SOMEWHERE but not everywhere. Don't choose a hull core wood for strength, but for the attributes you want in your hull, a combination of strength, stiffness, and resilience.
     
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  3. SukiSolo
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    SukiSolo Senior Member

    In my experience Doug Fir is definitely heavier than Sitka Spruce. Depending upon type so I probably have only used the 'Coast type' which is listed as having an SG (or SD if you prefer) of 0.53. This would seem to be about right, from hands on working the stuff. The fast grown stuff can really pick up even with a razor sharp plane, equally it can work beautifully. Personally I rather like the heartwood colour too, lovely warm pink hue.

    Alan's wise advice should be followed.Select the right piece for the job by using any natural features an individual piece may have. In certain areas it may pay to laminate small flanges in a lighter timber, to get glue area and keep the weight down. Sort of like a half I beam.
     
  4. gonzo
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    gonzo Senior Member

    To be really accurate, weigh a board and divide by the volume.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Weights provided by most data bases are for a specific amount of moisture content. The range on moisture content can easily double the weight of the sample. In fact most will list the weight as "average dry weight". Sitka averages about 27 lbs. per cubic foot. Douglas fir about 32 pounds per cu. ft. Most of the time 12% moisture content is the figure they use for the dry material weight.

    As far as Abosely's conversion needs, this is a simple engineering problem and some basic math. You'll match up the new species with the previous, matching rupture, elongation, tension, etc. as best as practical, with dimensional differences. It would be nice to just say make 1", 1.25" if using Douglas fir, but it doesn't really work this way, though some "averages" can be used, it's best is "sized" appropriately.
     
  6. Deering
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    Deering Senior Member

    You likely won't see 12% MC for air-dried in most environments. 20% is more typical. You can get to 12% or below by kiln-drying, which will obviously decrease the density.
     
  7. Jammer Six

    Jammer Six Previous Member

    A kiln is essential for other reasons.
     
  8. gonzo
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    gonzo Senior Member

    Kilns kill the spores that start rot, so they are a good thing.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Maybe for curing the ceramic pipes you're smoking through?

    Naturally seasoned stock can be gotten down to 12% or lower, though the ambient environment has a lot to do with this. Certain areas of the country can only dream of 12%, so they must used a forced environment, such as a solar kiln or simply a environmentally controlled shed.

    A solar kiln will provide accelerated natural seasoning, for little effort. I have one and use it in the wet months. 6 months out of the year, we can get 12% with natural seasoning and environmental protection (shed with a fan). This is just because of our climate, where the winter has little to no rain and very low humidity levels. Again, you can force these levels and avoid the pitfalls of production kiln dried stock.

    As far as killing spores, well yeah this works, but there's also other ways to get this effect, without forcing all the internal defects to expose themselves on the table saw.
     
  10. SukiSolo
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    SukiSolo Senior Member

    Having air dried some Douglas Fir, I have had zero problems with the heartwood. There has been a very small amount of mildew on the sapwood, simple really, I just cut it off!. It gets down to around 14% and I can get down to 12% by simply bringing it and leaving in the workshop for two or three weeks. W/shop is insulated, double glazed abd dry.

    I don't mind kilned timber at all - unless it has the collapsed cell problem from poor drying.....:mad:
     
  11. PAR
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    PAR Yacht Designer/Builder

    I don't have much trouble with kiln dried either, though I'm sure we're pretty selective about pieces we buy, which helps.
     
  12. Jammer Six

    Jammer Six Previous Member

    I took my solar kiln apart and burnt it. It was a waste of effort and space, as well as incredibly inefficient. It looked like a kiln, people thought it was a kiln, it pretended to be a kiln, but I didn't know what a kiln could do until I built a real kiln. The only thing it was good for was as a landmark on my learning curve.

    A real kiln is about control over wood.
     
  13. PAR
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    PAR Yacht Designer/Builder

    An intelligent if terribly subjective reply, from Jammer. Must be a full moon or something.

    Sorry your solar kiln didn't work for you, though a solar anything in Seattle seems self defeating at best. Not dinging the place, but sunshine isn't it's big selling point, which is what makes a solar kiln effective. I can season 2 by stock from green to 15% in several weeks (6 to 8 depending on species). 1 by stock is less than a month.
     
  14. Jammer Six

    Jammer Six Previous Member

    I'm busier than that, even in retirement.
     

  15. alan white
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    alan white Senior Member

    Thus began the kiln wars.
     
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