Planking alternative for small daysailer

Discussion in 'Boat Design' started by longfellow, Mar 2, 2010.

  1. longfellow
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    longfellow Junior Member

    I am lofting a thirteen foot E. Schock design from Mystic's collection which is drawn up for solid, half inch planks, laid carvel. It is a vee bottom daysailer with pieced together frames, solid keel, half-decked, about 100sf of sail (knockabout rig). I'd like to swap in plywood and am hoping that the design is cpmpatible. In Gerr's book, the Sn is actually so small for this boat that the tables are difficult to use but he does give the general statement that plywood may be substituted and the thickness may even be reduced. I happen to be a fan of Schock and do have some other plans of his for similar boats in which plywood is specified and it appears that the boat would be just fine structurally (I intend to use his most often specified thickness for his small vee-bottoms - 3/8 inch). The other concern is whether the hull design describes strictly developed surfaces which would make plywood a viable option from an assembly standpoint. From reviewing Schock's own analysis of 'plywood compatible' designs (excellent article in an old Sport's Afield Boatbuilding Annual), this boat should take the ply just fine. The upper and lower hull section lines appear to be parallel with their neighbors. The boat may even have been designed intentionally with developable surfaces to allow the builder the option. Although the frame spacing is clearly arranged for solid carvel planking (eleven inches compared to 14-18 for other daysailers between ten and eighteen feet OAL which specify plywood), I would leave frame spacing alone, let her be a bit overbuilt which should add less than twenty pounds (including slightly heavier plywood), erring on the safe side. Is there anything else I should be looking at or check out on the plans or construction drawing before comitting to the swap?
    Comments?
    Thanks all,
    Ed
     
  2. alan white
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    alan white Senior Member

    Frankly, the change to plywood introduces nmore than a thickness reduction. In fact, no frames (except bulkheads) would be needed if you built her taped seam, and she'd stay lighter by far if that method were used, as epoxy would seal the wood completely.
    The boat would be stiffer as well.
     
  3. longfellow
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    longfellow Junior Member

    Thanks Alan,
    Edson Schock was a big fan of teh vee bottom and also of using plywood, at least for his knockabouts and skiffs from ten to eighteen feet. I am out of my league in pulling out frames though I am guessing this is not cause for concern for folks with more experience. This is only my second boat and I hesitate to even do what I am considering. Is there anything other than checking that the hull lines are 'plywood friendly' that is 'developable' that I need to look at? While I am at it I will ask one other question. No mention in any of Edson's excellent texts have I found instructions NOT to fasten plywood (This applies only to pywood) to the frames but in Gerr's text he specifically states to only fasten plywood to keel, chine and at the sheer of the frames with no fasteners anywhere along the upper or lower sections of the frame members. This really struck me as unusual.
    Ed
     
  4. alan white
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    alan white Senior Member

    I think what's at issue is the avoidance of hard spots where panels cross transverse frames. Certainly plywood thickness and maximum width of hull panels figure in. Each design is different in those respects so if you feel that you can't work the details out, get some good advice.
    My preference would be to place longitudinal stringers halfway between chines to support panels and any transverse ribs or frames would support the stringers and chines as opposed to lying directly against the hull skin.
    Can you post drwings?
     
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    Plywood is dimensionally stable, unlike solid wood planking. In carvel construction considerable force develops when the boat is in the water as the planks expand and the seams close and seal. The heavy framing is needed to withstand those forces; also the strength of the boat is largely in the framing.

    Typical plywood construction is different. Early ply boats were built as ply on frame using fasteners, until modern glues came along. Ply expansion and construction are effectively nil so the seams can be fastened firmly along their entire length, typically using glass tape and epoxy, sometimes using a chine log which is just a continuous batten glued along the inside of the chine. This creates a monococque body that is very stiff and needs only minimal framing if any.

    A monococque body will still twist unless closed, so a deck is often used. Try twisting a shoe box with and without the lid to see this effect. You didn't mention your background so I apologise if this is over-simplified.

    Generally, when a design is converted from a traditional construction to plywood for the purpose of building a new boat (rather than modifying an existing boat) the designer starts fresh using only lines of the traditional boat. That is what I would do. The plywood skin thickness can be based on normal practise for the boat size and application if the designer does not have the knowledge to calculate it. It would usually achieve sufficient strength from the skin alone, and frames would be added where needed to create attachment points for items such as decks or to distribute forces from the mast, centerboard, rudder etc.

    There are so many great boat designs out there usng every possible construction method, and it is so much cheaper to buy a design than to build it, that I wonder why you are taking this route. Is there something unique about this boat?
     
  6. longfellow
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    longfellow Junior Member

    Terry,
    Thank you so much for the information, No, it is neither too general nor too detailed. My background is mechanical engineering and regarding boat building, one ten foot traditional lapstrake skiff complete. This is my second boat.
    Based on your comments however I'd like to ask a follow up. As I mentioned earlier, those Edson Schock designs that ARE drawn up from the start as plywood boats, are not absent of frames but simply have a few less frames. This is far from the frameless concept that you mentioned is possible. Of course Schock never mentions any glass sheathing so is that where the true strength lay and is this added feature truly what allows the reduction or elimination of frames? Finally if I just want to use plywood but not sheathe in glass, should I then simply stretch out the frame spacing to approximate other similar designs of his? This would mean about sixteen inch centers as opposed to eleven with solid planks. Actually if I could make the eleven into twenty two then I'd be in great shape since this would allow me to leave all of the locational dimensions that are currently taken off of stations, alone. Basically the plans show stations every 22 in but there is instructions to add additional frames in between so just leaving out these in-between frames would be easiest but this certainly exceeds the designers tendency to put frames 14-18 in apart when designing for plywood. The more thought I put into this the more it hurts and seems risky without a genuine modification and authorization signature from a NA.
     
  7. alan white
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    alan white Senior Member

    I think you're overthinking this. For one thing, plywood/epoxy often obviates the need for any frames except major bulkheads. Take advantage of this. Wire the panels together and eliminate chines altogether. Use partial and full bulkheads (seat boxes, collision bulkheads, etc.).
    Plywood thickness depends on spans and fore/aft curvature. The wider the span the more the need to go thicker or to support with some kind of a frame member. You don't need to go thinner than solid wood------ the plywood can be just as thick, since you are saving weight on the frames and chines you eliminate. I like this approach because it's simple and bullet-proof. Not as light as a more robustly framed thinner-skinned boat, but lighter than the original plank-on-frame and longer-lasting too.
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    Alan has a lot of good information! Here’s a bit of history to go with it:-

    Edson Burr Schock lived from 1871 to 1950. Plywood become commercially available in the 1920's and given the reputation it had when I was a child in the 40's, quality took a little longer to arrive. Truly water-proof glues did not become commercially available until Schock was an old man. Naturally he designed boats with thickish ply planks attached to heavy frames to retain the plank fastenings. For solid wood planks the thickness may have allowed for counterboring, with plugs to hide the fasteners. He did not live to see the use of glass fiber or epoxy.

    The advent of marine plywood, glues like epoxy and glass fiber triggered a revolution in boat construction. Before that lightweight boats were mostly built with thin cedar planks held by copper nails through steam-bent oak ribs and roved, a method requiring considerable skill.

    I am not a NA myself, I speak only as a practical boat builder. Getting a signed-off modification from a NA would be the safest thing to do, but perhaps a bit over-the-top for a small boat that’s a modification of an existing design.

    Now to your questions:

    Since you have completed a traditional lapstrake skiff complete you are obviously familiar with the process of building a boat over a solid form that does not become part of the finished boat, unlike the frame which is integral with the boat. Boats can also be built without a form by the stitch-and-glue method that Alan describes, but you need the plank developments so the planks can be cut to shape while they are flat. Without those you need a frame or a form to bend the ply around while you mark off the shape.

    Assuming you prefer to stay with the Schock design rather than take the safe and easy route and purchase a more recent design complete with construction details, here’s what I would do. You have the station data and obviously should use it: frames at the station spacing of 22 in are sufficient to define virtually any hull shape. Seats, frames at the mast and other stressed locations can certainly be constructed per the Schock design, the others can be slimmed down but should be thick enough to retain the fasteners holding the chine logs.

    My 10 foot sailboat was built with 4 mm sheers and 6 mm bottom. That was lighter than the 6 mm/10 mm recommended by the designer but approved by him for light service with supporting structure. Working with a living, helpful designer is one of life's pleasures.

    6 mm/10 mm ply should suit your boat unless you expect to use it roughly or in heavy conditions when 10 mm (3/8") would be more typical. 12 mm or ½" ply would, IMHO be excessive. The chine log thickness can be about 4x to 6x the ply thickness (I use 4x); this is to provide sufficient glue area to ensure the ply-to-ply joint along the chine is as strong as the ply and it is determined by the cross-grain strength of the outer veneer that the glue sticks to; no need for more if the joint is snug. Clamping the first piece of ply to the chine log is easy; the second can be attached using small screws, and I replace these with brass screws when the glue is set but they are not necessary if you use chine log thickness 6x ply thickness when the holes can be simply filled.

    Glass tape along the chines adds shock resistance. Glass sheathing significantly increases the strength of thinner ply, especially its resistance to puncturing. For thicker ply its value is more for its ability to resist abrasion and dents. I do not use it and consider it unnecessary for a boat intended for light recreational use, but not everyone agrees with me. Whatever: it is not the glass sheathing but the integrity of the plywood skin that allows the reduction or elimination of frames.
     
  9. longfellow
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    longfellow Junior Member

    Terry,
    This is great. Thank you for taking the time to include the level of detail that I was looking for. The chine log already meets your guidelines for thickness and I am going to seriously consider the suggestion to cut the frame spacing (and quantity) specified in half and go with 22 inches, use 10mm/10mm and loft the frames, keel and chine log rabbets with this thickness deducted. As long as I have the option, in your opinion, to either glass the chine only or the entire hull, I would lean towards the chine seam only. Iwill have to practice a bit since I would want to make sure that I can fair out the abrupt edge where the cloth ends for a good looking job.
    One final question: What about the statement in Witt's book on plywood boatbuilding that fasteners go only along chine, keel, stem, transom and the upper ends of the frames, and NOT all long the frames from sheer to rabbet?
    Thanks again. Your suggestions are going in my journal for this boat for adoption.
     
  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    I don't know why that is specified and can only guess. Again, bearing in mind the era that the design dates from, there may have been some strage ideas about newfangled plywood going around.

    The strength on your boat will comes from the epoxy (I'm assuming) rather than fasteners, with fasteners used primarily to hold the glue joints in place while the glue sets. I would certainly use glue between the planking and the frames, as otherwise the frame is held only by screws through the chines etc. However, fasteners into the frames may show through the finish and give the appearance of a riveted hull, I would omit them for that reason.

    I always "dry-fit" planking to ensure the joints are a good fit before gluing. If a gap forms as the planking is bent, I would add a temporary external batten to pull the planking into the frame with a couple of extra long screws into the chine log and keel ar other longitudinal member. It never happens to me, I think that is because I glue the chine logs and inwales to the ply sheers while they are flat, so when they are bent the ply between the chine log and inwale has a very light saddle shape curving in toward the frame, which applies enough pressure for gluing.

    Good luck! One last point: we expect a reward for all this! So post pictures of your progress and final product!
     
  11. alan white
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    alan white Senior Member

    As said, presumedly, the screwing of the plywood to the transverse frames might create an unfair bottom. Not so if screws are located only at the chines, sheer, etc..
    In any case, while one can build a hull out of plywood the old way, using no fiberglass tape or cloth, no epoxy, etc., once using the most recent methods, old advice doesn't apply in every case. Epoxy can be coved into bulkhead/hull joints where it creates a perfect cast-in connection between the two parts, never pulling anything out of whack like screws might do.
    Once coving is stiff enough, glass tape is built up into the cove, and the end result is supreme strength and a fair hull panel.
    I'd say a solid-planked boat as small as 10 ft could be redesigned for plywood easily right here on the forum and I'd wager the finished product would be relatively devoid of (especially) transverse (rib-like) frames, or in fact any frames except a minimum number (maybe four) of major plywood web frames that also serve as end seating, center thwart, and so forth.
    When you change materials to plywood from solid wood, you cvhange a lot more than just the hull skin. Solid wood has less tensile strength and more tendency to cup due to all of the grain going in one direction. Plywood won't cup for lack of a flat stiff piece of wood behind it. All of this affects framing issues and generally plywood construction allows for the omission of most framing that an old carvel hull couldn't live without.
    What's useful to building a plywood version instead of the original old technology method is the lines drawing, position of seats, and little more. Therefore, don't look at the original plank on frame details unless you're going to build the boat with solid wood as it was designed.
     
  12. Gilbert
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    Gilbert Senior Member

    Are your plans for the Sea Scout boat HALF HITCH listed in the Mystic book?
    The description in the book for that design states that the design was intended for plywood construction.
    If that is the boat you are talking about, it looks to be a really nice looking boat.
    I would certainly think that one quarter inch marine plywood would be all you would need for the panels, especially if it has floor boards to walk on.
     

  13. longfellow
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    longfellow Junior Member

    Gilbert,
    I should take a look at the boat you mentioned, just because I'd be interested in any of his designs. But no, I am guessing not. I am working on his design #30, 13' x 5'6" with no name. I don't believe that it has ever been published in any periodical or book either.
    Ed
     
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