weight comparison, Carvel v cedar strip

Discussion in 'Fiberglass and Composite Boat Building' started by Roland Stockham, Nov 10, 2022.

  1. Roland Stockham
    Joined: Nov 2022
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    Roland Stockham Junior Member

    Hi all. I am a COVID casualty. My classic 32ft traditional wooden boat got stuck in central America for the duration and suffered so much water damage the hull is a write off. I am currently stripping her and can salvage most of the gear and ship it home in a container.
    I am looking at building a new hull to the same plans but using cedar strip instead of traditional Carvel planking (teak)
    My question is what is the hull weight difference likely to be? I am assuming it will be lighter but it would be great if anyone could help with an approximate value, is cedar strip generally X% lighter than carvel. I can do all the propper scantlings and calculation when I get home. Dry weight of original was about 15,500lbs
     
  2. gonzo
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    gonzo Senior Member

    The answer is "it depends". You can carvel plank with cedar, so the comparison is skewed. Are you re-using the interior and building a hull around it?
     
  3. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Roland.

    That is sad news re your classic Covid casualty :(
    Re how the hull is a write off, is she currently out of the water?
    And it sounds like her hull planking is teak?
    If so, I think it would be be well worthwhile taking all the planks off the old hull, and putting them in the container as well to send home with the other parts - or are they literally too far gone to salvage?
    What type of wood are the frames, and are any of them salvageable?

    What design is your classic boat? She is rather heavy, at almost 7 tons on 32' length.
    There should definitely be a significant weight saving if you build a new hull in cedar strip.
     
  4. Rumars
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    Rumars Senior Member

    Strip planking is a general term for a collection of methods, so it depends on how much you redesign the scantlings for a particular type. To make this more clear, you could choose to simply keep all scantlings the same, and just use cedar to strip it. Cedar is half the weight of teak, but planking is only a small part of the boats weight. To put it in numbers, teak is 610-750kg/m3 while WRC is 340-380kg/m3. It's unlikely that your boat has more then one cubic meter of wood in the planking, so we are talking about 350kg (~772lbs), wich is 5% of the original dry weight, and even less if we consider cruising weight.
    On the other hand, you could choose to redesign the entire structure to cedar cored fiberglass sandwich, wich will affect weight more significantly.
     
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  5. Roland Stockham
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    Roland Stockham Junior Member

    Thanks for the prompt reply. I have seen stuff you put on other forums!
    On salvage from the hull unfortunately the combination of a plywood deck and inadequate deck drains that always blocked in harbour plus floors and chainplates as well as other reinforcements being galvanized steel has led to rot and nail sickness all over. I doubt I could salvage 25% of the planking and none of the frames. It may be possible to salvage the stem and stern post plus some deadwood. That is definitely worth doing as the are at least 12 x 12 teak!!
    If I am going to be a couple of tons lighter on the new build I will need to add more ballast to bring her down to her marks.
    Plan is to use epoxy coated cedar. Simplifies construction Andrew produces a light strong shell without the need for multiple ribs. I am guessing that the wight of resin and glass will be similar to the weight of ribs and fastenings. Will also bond in some of the internal fill out which will further support the hull. Thanks for those numbers though and it does put it onto perspective. At this point I was just considering if the idea was practical and if I would be able to reuse the keel or need to adjust its weight. I am also planning a bit more freeboard so I think the difference will be minor and within a range that can be trimmed with internal ballast.
     
  6. Rumars
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    Rumars Senior Member

    No, the resin and glass will not compensate for the entire framing weight. In fact the entire structure will need to be recalculated from scratch, the planking thickness will also not stay the same. It's much simpler to keep the ribs and only switch the planking from carvel to strip. You can laminate the ribs and all other framing members.

    If you are asking if it's worth salvaging the keel, probably yes if it's cast iron, and most definitely if it's lead.
     
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  7. DCockey
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    DCockey Senior Member

    Strip planking using a light wood such as cedar covered with cold molded wood can result in a lightweight hull. It's a method used by some builders of boats such as Brooklin Boat Yard.
     
  8. rangebowdrie
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    rangebowdrie Senior Member

    It might help if we knew the design/designer, and knew what he specified for planking,
    I doubt that the original design called for teak planking, if it did that would be a rarity.
     
  9. gonzo
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    gonzo Senior Member

    Teak planking used not to be a rarity. The same for true mahoganies (Honduras or Brazil)
     
  10. Roland Stockham
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    Roland Stockham Junior Member

    Hi all. So done the initial design ideas and worked out the changes I want, mainly increasing the hight of the sole by 12" which means raising the shear line and increases the LOD by 4 ft. I plan a load water line 6" deeper. Voya was initially built for racing but in cruising trim with all the extra gear always sat at least 4" below her design waterline (waterline raises at 6.5" per ton calculate and confirmed in practice).
    Construction is going to be cedar strip plus cold molding in cedar epoxy glass coated on the outside only. I am not a fan of fully encapsulating wood as it is to difficult to ensure correct moisture content during build unless you can build indoors with climate control. It also only takes an inverter breach in the encapsulation to saturate the core and cause rot. I would rather have the hull breath to the inside.
    In order to develop the final plans I now need the scantlings for the hull. The original drawings specified 1 1/4" Iroko planking on 1 1/2" elm frames with 4 bulkheads at a max spacing of 9' 6" I am planning a similar arrangement of bulkheads and floors but essentially thinking of this as a monocock construction.
    The problem I have run into is getting hold of scantlings tables to establish hull thickness. I could simply go with laminating ribs as per the original drawings but think this is unnecessary and would prefer a cleaner line to the interior so does anyone know of suitable tables or have experience of a successful construction I could base it on? My primary concern at this point is the hull thickness as I need the to do the weight and cost caculations.
     
  11. gonzo
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    gonzo Senior Member

    That idea that wood "breathes" is not true. If it was, water would seep through the hull of every wooden boat. Proper encapsulation is not hard. Measuring moisture in wood is straightforward with a moisture meter, which is a minimal investment considering the job.
     
  12. bajansailor
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    bajansailor Marine Surveyor

    But if we take this literally, then every layer of cedar in your hull will be fully encapsulated apart from the very first layer.
    So what is the point of allowing this first layer to 'breathe' when everything else is encapsulated?
     
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  13. Roland Stockham
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    Roland Stockham Junior Member

    Water does not pass through wood or epoxy but moisture does otherwise you could not season it! The rate of transportation varies with the material and an epoxy glass layer has very low but not zero moisture transportation rates. Epoxy mixed as a glue has a higher transportation than the glass coat (it is thinner), paint is significantly higher than epoxy and wood is relatively permiable. As a result a fully encapsulated hull even with no defects will slowly absorb moisture if left in the water. If the permiability of the inside skin is the same as the external skin then the vapour pressure needed to move moisture out of the hull into the interior will need to be high so the equilibrium moisture level in the core will be high. If the vapour permiability of the inside is lower than the outside moisture can move out of the hull to the interior faster than it can move through the outer skin leading to low hull moisture levels (assuming the inside is kept reasonably dry, for example using a dehumidifier during winter storage or lighting the hot stove regularly) This is why traditional wooden boat hulls do not become saturated. Establishing an effective moisture permiability gradient is a fundamental part of designing wooden building in the construction industry and there are many vapour barrier materials available with different translation rates to achieve this. I am simply applying the same principals (as do Spirit yachts I believe)
    The second issue is the initial moisture content of the wood. Kiln dried wood should have a moisture content around 7 or 8% but as soon as you move it out of the kiln that will start to increase unless you can build in an environmentally controlled building. Given that I expect to take about a year to build the boat I don't think I would be able to garentee to keep the moisture content below the critical 12% threshold for rot to develop and hence would be sealing moisture in not out by encapsulating. The only way I can see to avaiod this is to buy kiln dried timber and immediately shape and encapsulate each individual piece. I don't t think that is practical and getting kiln dried cedar in my area is difficult and expensive. Air dried timber is available at the local mill and they will select and reserve 'boat Grade' pieces for me.
     
  14. Rumars
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    Rumars Senior Member

    For a simple approach use ELEMENTS OF BOAT STRENGTH https://www.gerrmarine.com/ELEMENTS_OF_BOAT_STRENGTH.html and make sure to download the errata. Presuming you have a boat of conventional proportions it will give you a safe structure, but not the lightest. There will still be some laminated frames present.
    If you want a pure monocoque with bulkheads and floors only its more complicated with your proposed strip covered by cold molding approach, and you would need proper engineering.

    May we know what the original design was?
     

  15. Roland Stockham
    Joined: Nov 2022
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    Location: West coast Canada, New Hazelton BC

    Roland Stockham Junior Member

    May we know what the original design was?
    Checkstone 32, by JJ Bickerstaff. Built in 1972 in Exeter. The designer is long dead and the boatyard no longer builds boats although still doing repairs last I heard. She is a medium heavy displacement with long keel and basically a scaled up folkboat. Anyone who has sailed a traditional folkboat will understand why I want to increase the freeboard, water belongs in the sea not on deck!!
    Thanks for the reference, I will look it up.
    I am planning laminated floors extending up to the cabin sole (about 2/3 of the keel to gunwale measurement) to distribute keel loading at each keel bolt and a couple up to deck level in the saloon and bow where there are high stresses or long intervals between bulkheads. There will be bulkheads either side of the mast base forming a twin ring frame to support main mast an shroud loads and transfer them to the keel plus a single bulkhead at the forward end of the cockpit supporting the mizzen mast.
     
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