Plywood skiff construction methods

Discussion in 'Boatbuilding' started by JRD, Dec 13, 2010.

  1. JRD
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    JRD Senior Member

    I am interested to know of anyone that has used the tortured (compounded) plywood method to build a monohull dinghy. What I am working on is a singlehanded trapeze skiff, 4.2m LOA and 1.15m Beam at the chine and less than 55kg hull weight). I want to cut down on the number of frames and stringers to save weight and possibly give a construction method that reduces the construction time.

    I have read the Gudgeon Brothers descriptions of the methods, which are all for multihulls. Their Tornado catamaran construction gives a significant degree of compound curvature, and provides a nice fair shape for a cat.

    Whilst ply would clearly struggle to provide for the full shape required for a beamy full round bilge monohull, I can't see why this method would not work for formation of the hull bottom which would terminate at a chine with topsides formed over frames along the chine and gunwhale stringer as per standard ply construction. What I have in mind is to scarf 4mm gaboon ply to give a full length panel in two halfs and glue fillet along the keel to provide the initial floor anlgles, then pull the edges up and in to form the chine curvature, and compound section curvature in the bottom. The finished shape I have in mind is a safer version of the I14, flat floored to the transon carrying the curvature close to the chine aft and slightly vee'd in the front third of the boat.

    I'd appreciate any thoughts on this.
    Cheers JD
     
  2. PAR
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    PAR Yacht Designer/Builder

    There are a number of tortured plywood designs available. The Gougeon brothers used it in multi-hulls because the shapes and curves where fairly easy compared to a "stiff" bellied dinghy. In the long term, what has been learned from these types of designs is you can easily exceed the panel strength limits with the pre-loaded stress from torturing. The usual result in these cases is a catastrophic panel failure, along an unseen defect, which can really piss off a builder.

    Since, many have developed what they consider safe radius they can force specific thickness panels into. If interested I can email you a list of the radius figures I consider safe, though I don't know how these would compare to some New Zealand designer's ideas of light and capable. I've tested each and you can't go much tighter in radius without the panel "complaining" a bunch, which is the pre-load of strain "talking" to 'ya.

    If you're attempting to do what I think you are, you should consider molding, which would permit you to use the shapes you want without most of the tortured plywood limitations. Two layers of 3 mm ply over a form, makes a tough, light little boat.
     
  3. JRD
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    JRD Senior Member

    Thanks for your suggestions. Your comments and a look at some of the other posts around the site may have dampened my optimism about the compound ply method. I guess with a molded construction of the compound curved parts I will end up with exactly the shape of my design instead of what the ply decides it wants to do.

    Would you just use a frame / stringer mold to form the 3mm ply strips over? As this will be a hard chine design, I would need a method of joining the topsides to the bottom of the hull, could this be a stitch and glue, or would you recomend some other method?

    Will the molded hull shell hold its shape with out frames and stringers when I take it off the mold, or is it still going to rely on some internal framing to keep the intended curvature?

    With molding of this kind of hull, would you just butt-join narrow spiled sheets from keel to chine, then laminate on the second layer with the buttjoins offset by half a sheet width or would you run them diagonally?
     
  4. PAR
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    PAR Yacht Designer/Builder

    Without knowing what you are doing, some rudimentary design parameters and expected loading, I'd just be guessing at scantlings, particularly for a light, preformance oriented craft.

    Without offending you, how can I suggest you're out of your "comfort zone" in regard to the engineering aspect of the design? I think what I'm trying to say is your few questions are about a "loaded" as they come and would need considerable refinement to answer properly.

    Yes, I would do a diagonal plywood veneer molding, over a jig of furring strips (closely spaced longitudinal stringers) and station molds. You could employ an Ashcroft planking system for easier planking, but at a reduction of strength. A fore and aft planking regiment wouldn't be my first choice with just two layers of plywood. You could do an outer layer fore and aft with the inner layer on the diagonal. This would be weaker then typical double diagonal, but not by much.

    Taped seam would be the logical choice for joining the bottom to the topsides. A healthy fillet and tabbing over lap will offer some insurance.

    Yes, the molded plywood will hold it's shape without frames, though some reasonably equally spaced bulkheads and seating partitions are usually required to help hold things where you want them, also to resist slamming and rigging loads.

    This sounds like an arc bottom design more then a round bilge. The transition from bottom to topside planking will be a butt joint, but the joint will be ground back, rounded over a tad, then filled and taped. This, in combination with the internal fillet and tabbing will hold the bottom to the topside planking. How much structure experience do you have?
     
  5. gonzo
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    gonzo Senior Member

    I prefer to use logs at the chines, because it makes it easy to join and allows for a large radius. Stich and glue seems like a time consuming method. I have never seen the advantage, in time or weight, compared to using a wooden log or framing.
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    I am with Gonzo on the value of chine logs.

    There are several methods. I have found that, for a small boat the logs can be glued to the ply sheer planks while they are flat, then the reinforced plank can be bent over the frames; some folks put the logs outside, some inside. I put them on the inside, and also add the inwales to the sheer planks so they are stiffened along both edges; a great help when using light ply on long floppy planks.

    For a canoe with 3 mm ply I use about 3/8 x 3/4" logs. i scaled that up to about 3/4 x 1-1/4" for 4 & 6 mm planks on a sailboat but found the sheer planks were too stiff and had to slit them: next time I may try the same logs as for 3 mm and add 3/8" thick battens after completing the main hull construction.

    This approach avoids all the drilling and stitching, taping (although you can still add tape if you want), filleting, sanding etc of S&G which saves a great deal of time, and reduces the cost by using less epoxy: I think it looks nicer too.

    Dora, the canoe (my log is at http://theancientkayaker.weebly.com/index.html) had bilge planks and a flat bottom which is an alternative to rounded bilges; I don't see why a skiff cannot be done the same way - which is shown on my blog.

    The downside of using logs is, you cannot glass the interior; I don't glass my boats anyway, but they are treated with care.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Stitch and glue is a technique using wire, cable ties, etc and lots of holes drilled along the seams to help join the panels. It permits the assembly of cut panels with out the need of a jig, mold or traditional ladder mounted station molds. There are different and in my opinion better assembly techniques then stitch and glue.

    I mentioned taped seam which is the actual build method (of which stitch and glue is but one seam assembly technique) as the best attachment. It's lighter and stronger then a solid wood log and it's not prone to hold debris, dirt or moisture such as can be on top of a log and it's a lot easier to clean out with it's smooth and fair transition from topside to bottom planking. The weight savings is well documented, just subtract the stringers, frames, floors and other structural elements from the assembled, raw hull weight and the percentage is clear.

    The best examples of these building techniques are the glued lapstrake hulls, where no internal framing but for a few bulkheads can be seen. Placing one glued lap skiff next to another of traditional lapstrake construction makes the differences quite obvious, especially when you have to carry one, which is 20% to 50% lighter as a result.

    I've long but stopped using "stitches" to hold seams together, though occasionally a well placed stitch can solve a few problems.
     
  8. JRD
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    JRD Senior Member

    "Without offending you, how can I suggest you're out of your "comfort zone" in regard to the engineering aspect of the design? I think what I'm trying to say is your few questions are about a "loaded" as they come and would need considerable refinement to answer properly"

    Not offended at all - I guess my question in reply was a bit loaded (unintentionally) as I have a few pre-conceived ideas based on boats I have owned over the years. Your answer provoked an interesting new line of thinking I had not given much consideration to previously.

    I have sailed boats such as Farr 3.7s, 12 foot skiffs and NS14s over the years, along with the attendant modifications, repairs, new decks, etc. But have not built one from scratch.... yet
    The 3.7s are 3mm ply over a permanent stringer frame, built to 50kg and the original one built by Bruce Farr is still sailing after 30 years, for me this is a starting point for what will stay together.

    My initial tendency was to use a composite foam sandwich construction which I am familiar with for small scale work and is normal practice for small race boats these days. But have been getting more interested in the way that some more traditional methods could be applied to a modern hull shape and also keep the cost down. Most likely I would ended up with a ply on stringer frame but suggestions so far do pose some interesting alternatives.

    I will try and get the my latest shape off paper onto cad so I can post it in the next few days.
     
  9. JRD
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    JRD Senior Member

    Terry - Nice Kayaks, I like the simple methods for putting them together, you have obviously put alot of thought into this.
    I am wondering if we are thinking about different meanings to the word skiff (often debated on this site I think) I am refering to the NZ/Australian version of the word being a light planing dinghy with way too much sail area :)

    This said, I think the key point is about building a light easily driven boat and the points about new vs old methods of building lapstrake "skiffs" are totally relevant in terms of making a simple strong boat.

    I'll have to get off my computer back to the sketch book to figure through a few of these ideas. Im thinking the chine log vs glue fillet will come down to what is most practical at the time.
     
  10. Jethrow
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    Jethrow Junior Member

    Hi JRD

    Check out my blogsite where I describe the building of my International Canoe. It was built using tortured ply methods out of 3mm Gaboon.

    www.jethrowsicpage.blogspot.com

    Jethrow
     
  11. Typhoon
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    Typhoon Senior Member

    If it's going to be single handed, trapeze and 55kgs, just build a single or double chined hull. It'll be up and planing very early anyway.
    The later plywood 12ft skiffs here went to a single, high chine (above waterline) and curved the bottom panel, giving in effect a reasonably round bilge hull with a nice flat bottom. The chines also started about halfway back along the gunwhale from the bow, most of one side of the boat was one joined piece of ply.

    Regards, Andrew.
     
  12. Landlubber
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    Landlubber Senior Member

    JRD,

    ...another way to do it is to use glass pre formed sheets, made up on a flat bed, you can make them very thin, so they conform easily, they readily accept more strengthening as required and can be very light and strong. You can also make constant camber sheets with foam in between for stiffness, they will then be used to construct the shape that has been predetermined...
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    Generally near the bottom of the first page I revisit the first post to refresh my memory of what the thread starter wanted!

    After Googling the I14 it seems to have significant chine curvature. As it is, it would be more suited to molding than tortured ply as has already been mentioned. With the changes you propose, you may be interested in a method of construction that I am investigating. In my case I am considering making a rounded-bilge canoe and I want it very light and simple to build; I do not propose torturing the ply but that can be added to my concept.

    The hull has a ply bottom and sheer planks, with strip-built constant chine radius chines. My plan is to laminate half ribs to define the stations, glue these to the ply bottom per sketch, strip-build the chines, then use more ply for the sheer planks.

    For this boat at least, using the ribs allows me to dispense with the building mold and avoid glassing. Because the ribs are identical I can laminate a bunch of them at once over a ladder mold. The ribs are glued to the ply bottom at right angles to the edge of the bottom plank to avoid beveling the ribs to meet the strips and sheers, not quite as shown in the sketch. With this design the strips are not twisted, which makes clamping the strips easier. See bodyplan. The shorter ribs near the stems are cut down from full-length ribs and inserted after the chines and sheers are completed.

    I am not sure how many of those features are of interest to you but if it sounds good we can go into more detail.

    * the sketch illustrates this, although more ribs are required in practice.
     

    Attached Files:


  14. JRD
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    JRD Senior Member

    Thanks so much for all your suggestions.

    Jethrow - I have taken a careful look at your construction photos (Very nice looking boat, I hope its getting plenty of use this summer). What you have built is very similar to what I had in mind when originally posting this thread. I have attached the current itteration of my design, its not quite there yet but close to what I am trying to achieve. I'm not fixed on the concave flair evident in the topsides. I hope to find time to draft a hull shape that may be a little more suitable for folding up as you have done. Based on your build experience I am interested to know if you think this could be done with the hull I have posted with the exta beam and curvature closer to the chines. What was the final weight ready to sail?

    Andrew - I breifly owned a double chine 12 footer, although made of foam sandwich. This was about 20 years ago, which was about 15 years after its used by date. We used its rigs on a newer design hull with strip balsa core/glass sandwich. This was lighter, faster and required 2 hours of repairs for every hour of sailing. The ply 12s were a wee bit before my time. Were they a ply over stringer frame, or tortured ply similar to Jethrow's International Canoe?

    Terry - I dont think the ply over ribs fits with what Im trying to do, although I think it will work well on your new canoe. I look forward to seeing one under construction.

    Landlubber - I think a glass / foam sandwich would deffinitely be required for stiffness. This is the alternative I'm looking at, but would be likely be formed over stations before laminating the glass to the foam.

    Cheers, Jeff
     

    Attached Files:

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