Kiln Dried vs Air dried Douglas Fir

Discussion in 'Boat Design' started by Thomas Wick, Feb 8, 2008.

  1. Thomas Wick
    Joined: Feb 2007
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    Location: Southern Oregon

    Thomas Wick Junior Member

    I am building a 30' semi-displacement power boat.. the strip plank method and would like to get some thoughts on my best options for hull planking. I can purchase air dried lumber for $3.50 a board foot,,air dried on the Oregon Coast, moisture content will be around 15%. Or I can purchase the same wood, (CVG grade) that is kiln dried for $2.50 aboard foot. I was told the kiln process took 7 days to complete. Is this to quick...are the cell walls brittle from excessive heat and rapid drying? I know from reading that air dried lumber is suppose to be supperior in strength, especially when bending, however I do not have any hard bends to negotiate. I have also read that one of the benefits of using kiln dried lumber is that it kills any fungus that may be present. Thoughts would be appreciated!

    Thomas
     
  2. RatliffFranklin

    RatliffFranklin Previous Member

  3. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I am an engineer that designs wood structures for a living. The reason wood is dried is because the more moisture in the wood, the weaker it is. The moisture fills and softens the soft wood cells, weakening it. All structural lumber will not meet its rated strength if it has too much moisture, the process of kiln or air drying is just one of production expedience. I think where I live in the Puget sound area it is difficult to get to 15% moisture (to gets its rated strength) when air drying most of the year, so the mills use kilns to speed the process and get more consistency out of each production run. the lumber is required to meet industry standard moisture content if it to be used in structural applications for safety reasons. That means virtually all lumber used in building construction. Also as wood dries it shrinks and changes shape, not exactly good for making cabinet or furniture, so cabinet makers also have their own moisture requirements to allow consistent production.

    I have never heard of drying wood too fast will damage it (presuming it does not burn), damage occurs from mishandling it so the grain is crushed (can be hard to spot unless you know what to look for, or test samples).

    If dried properly either way it should be good wood, too high a mosture content it will not have its highest strength, lowering the moisture content will restore the stregth. This would be true no matter how it was dried if left out in wet weather and it regained more moisture content. There are wood moisture meters that do not cost too much that can be used used to determine moisture content.

    I think all of the anecdotal claims of wood workers on which is better is simply folklore. Before kilns, all wood was air dried and because it was the 'old' way of doing it, therefore it must be better. The drier the wood, the stronger but also the more brittle. It does not matter how it got that way, air or kiln.

    I have heard from other wood boat builders that "green" wood (still has the "as grown" moisture in it) is easier to steam bend, so they desire green wood for their steam bending. I have found I can bend any type and age wood if I boil it rather than steam it. Boiling is much less sensitive to moisture content since you put it all back to soften it for bending anyway. Of course then you have to allow it to dry out before you finish it. I have built many small boats with very old salvaged lumber I find where they are tearing down old buildings, it has to be boiled if I want to bend it. It will regain its strength once dried out again.

    Kilns may kill fungus, but if it gets wet again, the fungus will grow again. Wood eating organisms form where ever there is air, wood and moisture, remove one of these three things and the organism dies. Keeping it dry will keep it from rotting. Dough fir has fairly good rot resistance anyway, if you use chemical fungicides it will resist rot even better.

    If it were me, I would get the lower priced wood if there was no other difference. If you think the moisture content is inconsistent, stack it up with space between it for a few weeks and it will all even out anyway. Doug fir has excellent strength to weight ratio properties, good fastener holding and fairly good rot resistance. It is a good economical choice for most applications.
     
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  4. Kay9
    Joined: Oct 2006
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    Kay9 1600T Master

    Wow. Learn something new every day....Thanks Petros, points awarded.

    K9
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Juvenile wood and mature wood dry at different rates. Rapid drying will cause additional internal stresses in the lumber, which weakens the wood and will appear as soon as it's cut. Everyone that has run a 2x4 through a band or table saw and had the cut turn into a long wishbone or pinch the blade, has seen this internal stress (as it's being released) first hand. There are also several other reasons to naturally season or at least use a slower method, like a solar kiln (my method). Yes, you can find internal defects and stresses in naturally seasoned stock, but it is greatly reduced.

    It's very common to see wood dried too quickly, with honeycombing, checks, splits, collapsed cells, deformed cells and cavities within the wood.

    Most carving is best done on seasoned stock and many species look much better if seasoned rather then forced dry.

    The only reason for kiln drying lumber is to get it to market as a useable product faster. In home construction and most furniture, these internal flaws and stresses aren't a big issue, but in boat building that can be a real problem.

    There was a time when the kiln operator was a careful man, constantly managing his load and monitoring the gauges. I've found this isn't the case any more and they shove lumber through their machines as quickly as they can. It's still possible to find a kiln operator that knows their stuff and will dry the lumber at the rate it needs and not force it to conform to their latest delivery schedule.
     
  6. Thomas Wick
    Joined: Feb 2007
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    Location: Southern Oregon

    Thomas Wick Junior Member

    Kiln or Air dried lumber

    Thank you to everyone for the info...most helpful!
    Thomas
     

  7. Petros
    Joined: Oct 2007
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    Petros Senior Member

    PAR brings up a good point. When lumber is dried too rapidly the moisture in the outer layers is driven out first, making the outer layer shrink around the inner wood, causing the "check" type splits. And also introducing internal stress between the layers. Obviously when you buy boat building lumber you will not use wood with large checks or splits that run down to the core of the wood. If you can not see any visible slits or checks, the wood should still be perfectly good.

    If you select the kiln dried lumber that does not have checks and is not warped too much, you can stress relieve any internal stress by stacking and seasoning it for a few weeks (or longer) preferably in a heated shop or shed. The moisture in the inner layers will working its way out, evening up the internal stress. After this no new checks or warping should occur when you recut it or plan it down to size when you expose the inner layers.
     
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