How waterlogged is a wooden boat?

Discussion in 'Stability' started by John Goldman, Jun 16, 2021.

  1. John Goldman
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    John Goldman Junior Member

    I have been searching naval architect books and the internet for information about the effect of waterlogged planking on wooden boats. The only effect mentioned is the swelling of the planking.

    As far as I know, waterlogged oak is 50% more dense than dry oak, and l presume it is most likely to occur below the waterline.
    The changes in displacement and VCG may have a noticeable effect on stability.

    Does anyone know how much waterlogging would occur in an old wooden carrack?
    Is it just in the outer planking, or also the inner planking and frames? Would it just be below the waterline or extend up to the heel line?

    The example I was looking at is the MaryRose, which I have put on the updated boatcad website
    BoatCAD https://boatcad.web.app
    The waterlogging effect on GM can be seen in the boat trim loading table.

    Do modern wooden boats have waterlogged hulls, or are they fully waterproofed?
     
  2. Will Gilmore
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    Will Gilmore Senior Member

    If the boat is well caulked, only the outer planking should have water and not all the way through. As wood grain takes on water, the cells swell and close the space between them. Water logged planks are pretty water proof.
     
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  3. John Goldman
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    John Goldman Junior Member

    Thanks for your information.
    What puzzles me is why this is not more widely discussed.
    I did read that canal barges used to be recalibrated as the lightship waterline changed with waterlogging.
    Surely this must have been an issue in the days of wooden ships.
    Also the bilge water is running around the inner surfaces of the hull, and the ballast in the hold may have been wet.
     
  4. DCockey
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    DCockey Senior Member

    Source for this information that "As wood grain takes on water, the cells swell and close the space between them"? Are you claiming that when wood is exposed to water the moisture content only increases near the surface exposed to water?
     
  5. Will Gilmore
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    Will Gilmore Senior Member

    They are, admittedly, not perfectly waterproof, but wooden ships spend a great deal of time in the water and if you keep them out of the water for too long, they can start to leak upon their return, simply because the planks have dried too much and gaps open up between the planking. They take time to swell again once back in the water.

    This is common knowledge and what I base my statements on. Wood swells and shrinks with the moisture content of the air around them. They do so based upon a number of factors that include their initial dry point. They also do so unevenly, from the outside in. Kiln dried lumber forces wood cells to collapse beyond an air dried point. The moisture inside the cell shrinks or swells the cells away from or against their neighboring cells. The species of wood also makes a difference. Some species are very porous and moisture passes right through while other species are much less permeable. I was speaking particularly of woods typical to wooden boat planking. Because a well caulked boat should stay dry except through condensation sweat, I know they are not permeated through with water.
     
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  6. Rumars
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    Rumars Senior Member

    Wood has a fibre saturation point, after it's reached the wood does not change physical and mechanical properties anymore. This point is around 30% humidity for most woods, all water it absorbs afterwards is so called "free water", it only increases weight, but has no other influence. How much water a wood can absorb above the saturation point, depends on the species and individual tree. You can find specific numbers listed under "basic density", that's green/soaking wet weight.

    You don't find many mentions about this because it was not really important at the time. Defining displacement to the last pound was not really possible, most yacht designers would specify correcting weights to achive the desired trim. Boats with inside ballast simply compensate the waterlogging by removing some ballast. Workboats have variable displacement ayway, they move fish or cargo. The waterlogging is all underwater, so all weight increase adds to stability.

    Modern wooden boats have several construction techniques. Some rely on beeing completely dry (epoxy encapsulation), while others are just as wet as any ancient one, and some are in between.
     
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  7. John Goldman
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    John Goldman Junior Member

    Thanks for your advice.
    I found your reference to wet/green timber etc.
    It confirms what I thought, that oak can hold up to 50% water.
    I can find no reference to the effects of this on boat stability.

    In the case of the Mary Rose the absorption of this much water corresponds to an increased weight of up to100t. It is not just a few lbs.
    Including this extra weight the ship still needed 200t of ballast for stability.
    There is not space in the hold to store 300t to throw 100t out!
    I am wondering if the old time ship builders knew about waterlogging and allowed for it in their designs.
     
  8. DCockey
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    DCockey Senior Member

    The Mary Rose may have been built with relatively wet wood. It would not have been kiln dried.
    Why would they not have known?
     
  9. mc_rash
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    mc_rash Junior Member

    In my opinion this is an interesting subject and it depends much on circumstances as caulking, paint, etc.
    I worked on an Excel-sheet where I did some theoretical assumptions, I know it's not what you would see in the real world but it might give a rough picture of what we could expect.
    My assumptions were:
    - rectangular, box shaped hull (L = 10 m, B = 2 m, D = 1 m) and skin-thickness of 0.02 m, there's no deck, just the bare hull
    - made from oak
    - density of dry oak (15% moisture) = 760 kg/m³
    - density of waterlogged oak (100% moisture, as fresh cut wood) = 1100 kg/m³
    (both density's came from Wikipedia.org)
    - the wood is not coated or caulked or something
    - hull is made from homogeneous wood
    I choose the moisture of the wood only changes under water. All parts above the waterline are dry wood.
    I really don't know how wood's moisture behaves if the wood gets exposed to water, that's the reason I choose the moisture of fresh cut oak.
    It's just a model, based on assumptions, with nearly no regard to reality.
    My expectation that the hull is going to stop getting still more moisturized at one time was confirmed and everything comes to a stable equilibrium.

    The question if there's any change in stability is answered with a YES. (In this theoretical case)

    A., B., C., ... G. show the iterations, where A. is the all dry hull in the beginning. After contact with water there's a rapid change ( -> B.) and after that there doesn't change much.
    upload_2021-6-18_21-46-23.png
    upload_2021-6-18_21-46-50.png
    upload_2021-6-18_21-47-37.png
     

    Attached Files:

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  10. Rumars
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    Rumars Senior Member

    Solid timber boats are normally buildt with "wet" wood, meaning it is well above 25% humidity. The tree is felled, transported to site (sometimes by floating it), sawn and installed. This is about all the time the wood has to dry out, there was no dry wood construction as we understand it today. It would have been impractical anyway, just consider the 1"/year rule for air drying, and apply it to something used in big ships. Why wait all those years, if the wood is going to end submerged? The exact opposite was true, with big timber pieces care was taken to keep them moist during the entire construction time. This has two very practical reasons, preventing the wood from checking (splitting) and keeping it soft for hand tools.

    As to how much water is in the wood, this depends on the wood thickness and its position in the boat. The space between the planking and the ceiling creates a chimney effect, air drying the planking and framing, so you would not have 100% saturation, more like 50%. Keel and deadwood have higher humidity, depending on thickness and immersion time, up to 100% troughout.

    Here is a link to a study done by the US Navy in 1946 regarding laminated keels. The raw oak came with 40-50% humidity from the yard, and the 6x6" keels were submerged for three months in salt water. On the humidity distribution graph you can see that in the first inch humidity goes down from 70% (near surface) to 42% (1" depth), and stabilizing at ~45% at 3". Report https://books.google.de/books?id=jLVWAAAAMAAJ&pg=RA14-PA2&lpg=RA14-PA2&dq=keel+laminated+navy&source=bl&ots=5648iGsfqv&sig=ACfU3U2NJI_ktO7DBgADjxwQUbQwCy0-fA&hl=en&sa=X&ved=2ahUKEwiYndOunaLxAhUHjhQKHQM7AQwQ6AEwEnoECBUQAw#v=onepage&q=keel%20laminated%20navy&f=true

    Wet wood is really not an issue when building traditionally. If you assume basic density for the underwater part when calculating displacement the boat will float high at launching. Maybe it will settle with time, maybe not, depending on specific conditions (for example while the deadwood waterlogs more, the topsides dry out). You must remember that even the published wood density is an average, individual trees can vary considerably, and the better tables offer a range. European oak (Quercus robur) can vary in density from ~500-800 kg/m3 ( https://www.researchgate.net/public...ur_and_simulating_the_silvicultural_influence ) and it is the job of the shipwright to select "good wood" for each individual piece of the boat. The designer has no ideea what weight will be installed, and the builder has no ideea until he sees the wood. That's why compensating with internal ballast is normal for traditional construction, displacement calculations are a near estimate.
     
  11. John Goldman
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    John Goldman Junior Member

    Thanks, very useful references, especially the keel strengths paper.
    It confirms what Will Gilmore was saying, that water does not penetrate beyond the outer layer (well certainly not in the first 3 months).
     
    Last edited: Jun 19, 2021
  12. John Goldman
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    John Goldman Junior Member

    mc rash:-
    Interesting, but you forgot to include some ballast.
    Then when the hull waterloggs, throw out just enough ballast to keep the initial waterline.
    I think you will then find the change in GM will be minimal!
     
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  13. rangebowdrie
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    rangebowdrie Senior Member

    Back in the early '90s, Charles Wittholz , was kind enough to send me a copy of his "Weight and Balance" study for a 35 ft. boat.
    With carvel construction/typical wooden deadwood, he indicated 500lbs for "Soakage", that's a fair number of gallons.
     
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  14. John Goldman
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    John Goldman Junior Member

    Thanks for the info.
    Assuming 1inch hull planking I reckon that he is allowing for 63% moisture content in wetted timbers which gives 40% increased density as compared to the air dry wood.

    I also found this in Wikipedia:
    "While the Cromarty had performed well in its limited service, one problem (as with all wooden-hulled flying boats) was soakage of water into the hull, with as much as 600 pounds (270 kg) of water absorbed after a few weeks of service."
     
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