Spira w a twist.

Discussion in 'Boatbuilding' started by Easy Rider, Nov 29, 2015.

  1. Easy Rider
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    Easy Rider Senior Member

    If I was to build a 24' Key Largo skiff w a plumb stem how much trouble would I have dealing w attaching the side planking w all that twist? Other than w single sheet plywood what other planking methods would deal better w the side planking? I'm thinking Ashcroft double planking would be a good alternative.

    http://spirainternational.com/hp_keyl.php
     
  2. gonzo
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    gonzo Senior Member

    Flat bottom skiffs are ideal to build with plywood. They only need three panels. There won't be any problem, but you need to fair the lines on the forward 1/3 of the hull. What do you want to accomplish? The forward lines will be blunter and the boat will be really wet.
     
  3. DCockey
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    DCockey Senior Member

    If built in sheet plywood the unrolled shape of the sides will change by a large amount. Based on the study plans and photos the sides appear to have constant flare with straight frames. To build in sheet plywood curved frames forward will be needed, and either the stem will also be curved or the forward sides will have an "interesting" shape.

    One alternative for planking would two layers of narrow strips of plywood. The layers could crisscrossed, or could be in the same direction overlapping by one-half width of each.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Yeah, that's what I see David. The forward sections will have a fairly predictable amount of belly along the topside futtock faces. If I was building this puppy, I'd make a model, at a reasonable scale, making the planking template(s) with a grid, so I could use these as a very close guide to the rough cut on the actual plywood. This will be an easy boat to plank, with some "talking to it" on the forward planking likely being necessary. The templates will also give you an idea of how much belly each topside futtock will need.

    I don't see a need to diagonally plank this overly heavy thing. If there was, I'd consider Ashcroft instead, simply so I could plank both layers in the same pass. This would be true of any other building method too, except for lapstrake, batten seam and carvel, all of which will do just fine on these shapes too.
     
    Last edited: Dec 1, 2015
  5. Easy Rider
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    Easy Rider Senior Member

    Thanks all,
    I intended to use the Spira Key Largo stretched a bit both in the midsection and the stern (to 27') .. as a substitute for this boat;
    www.atkinboatplans.com/Cruisers/Marcia.html

    The Spira looked simpler to build. All those frames to build and I don't make it any easier w a plumb stem and a lot of topside twist.

    I liked the flat (on both planes) bottom thinking it would deliver a more level ride w minimal pounding the elimination of rocker helping. The plumb stem providing some increase in WLL further leveling the ride. I have a 60hp Suzuki OB for power. Putting batts and fuel where Marcia's engine would normally be. This will be a trailer boat.

    DCockey,
    Yes the Spira boat as designed has constant flare fore and aft. With my plumb stem that will change a lot fwd. This is the unknown to me ..... How much twist is easy and graceful to do w various planking methods/design. Perhaps I should get some boards and do the twist? Your comment "One alternative for planking would two layers of narrow strips of plywood. The layers could crisscrossed, or could be in the same direction overlapping by one-half width of each." I like the overlap "by one half of each" method. I'm not crazy about lots of plywood edges to soak up water. That would require more frames (at least the vertical part) for fastening. Fairly large battens and lots of rivets? Don't see a good fastening plan.

    PAR,
    Usually I follow you quite well and am rewarded but here I don't know the "belly" and "futtock faces". Yes of course I see it as an easy boat to plank .. and perhaps at 75 I may even get-r-done in time to use the boat.

    gonzo,
    Making the stem vertical I foresee the fwd lines as less "blunt" and anticipate a drier ride. The flat bottom can't pound much if it's up and down motion is limited. The straight bottom of the Spira should ride smoother w less up and down. But in the vids of Spira boats w this flat bottom they seem to be riding on their midship bottom throwing spray out there not unlike a water ski. Marcia's rockered bottom should pound more as the fwd part of the boat will be prone to ride a bit (or more) higher.

    But mostly I'm looking for input on the planking w the twist or warped topsides. And fastening of double planks. I know I should just build the Key Largo as planed w the stretching to 27' and go looking for a trailer. Most of the time the fwd 1/3 of the boat will probably be out of the water however I build the bow.
     
  6. DCockey
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    DCockey Senior Member

    Three choices for the modified design with twisted sides:

    1) Straight frames and a non-developable surface. Sheet plywood construction would not be possible. Simplifies frame construction by complicates planking.

    2) Curved frames with a developable surface. Sheet plywood construction would be possible but frame construction is somewhat more complicated due to curvature of the side frames. Design for the developable surface is required - either using software, model, or possibly paper and pencil which may be less accurate.

    3) Curved frames with a non-developable surface. Sheet plywood construction is either not possible or may require "torturing" the plywood into position.
     
  7. gonzo
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    gonzo Senior Member

    Or, you can do it the old fashioned way. Lay the panels to their natural curve and then fit the frames in.
     
  8. PAR
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    PAR Yacht Designer/Builder

    You can usually force plywood to lie flat on straight frame faces, though it depends on how much torture you need to apply and the hull shapes. For example Eric mentioned it looks easier than the Atkins Marcia, but if you look at the Marcia, it's not a truly developed design, actually intended to have been batten seam, but the sections align favorably enough to permit torturing in plywood.

    The frames (for Eric) are made up of pieces, called futtocks.

    [​IMG]

    When the planking is developed and there's a bit of twist, the face of the frame pieces (the futtocks) have a belly in them, instead of a straight line.

    [​IMG]

    As you can see on these two hard chine frames, instead of the sides and bottom being simply straight lines, there's enough twist in the planking that the plywood will cup a little, as the edges are forced into alignment. It's the tension and stress concentrations along the edges that cause this cupping.

    On most of these style of designs intended for plywood, I'd skip the majority of the frames entirely and tape or batten the seams. It makes for a lighter boat and cleans up the interior quite a bit too. I'm not sure why Spira (and others) insist on all those frames on such a simple set of shapes, but some feel more comfort with them, knowing the "boat bit" count rises dramatically.

    As to your pounding concerns, well you can mitigate this to a degree with the design, but all flat bottoms will find a set of sea state conditions, that will just toss lots of wetness and pound. This can often be eased with a speed reduction and or modest course change.
     
  9. Easy Rider
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    Easy Rider Senior Member

    DCockey,
    Never thought about curved frames and coumpound curved sides.
    " Straight frames and a non-developable surface. Sheet plywood construction would not be possible." By "developable" you mean twist or warping? Seems I need to learn what a developable surface is and a developed design? I see now the problem w sheet plywood and Marcia.
    I'm thinking double plank mostly but I'm stressing over fasteners.

    gonzo,
    Thought of that using a rough temporary frame to start the sides w the right flare to get started. No bottom frame at all like PAR's "Digger". Vertical frame pcs (2nd futtocks?) could lay flush to the inside of the planking and thus require no beveling. Kinda like a one plank per side Pirogue. Could evolve into a plan. Seems I need to learn what a developable surface is and a developed design?

    PAR,
    On a simple flat bottomed skiff are the bottom horizontal frame pcs called first futtocks and the short near verticals 2nd futtocks? It looks to me like Marcia is a developed design and the Spira boat is not as there is only one simple curve in the Spira. In Marcia the sheer line is longer fwd than the chine. I can see where that would complicate a single sheet side installation. Would double planking eliminate this as a problem? Or would it just make it easier?
    For a double plank effort would you contemplate fasteners to hold the outer and inner planks together or just rely on glue or bedding compound and the fasteners to the 2nd futtocks (upper side frame pcs) frame members? Wish there was suitable pop-rivet for boats sometimes.
    PAR yes speed and course changes are anticipated.
     
  10. PAR
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    PAR Yacht Designer/Builder

    There are a few ways to denote the futtocks. I prefer to call them topside and bottom in single chine designs. When you have a large frame with a few futtocks, per side, then you have to give them some sort of order.

    A developed surface is one where the "sheet" goods can lay without distortion, stretching, kinking, etc. These tend to be cylindrical and/or conically shaped. If you want to get an idea of what this does, take a note pad's backer cardboard off the back of a writing pad, then using just the corners on the longer edge, try to put a bend in it, attempting to fold it in half, except with a big radius, instead of a crease. As you're doing this you'll see the portions of this board between your two fingers is curved, perpendicular to the bend you're trying to make. This is what's happening on the frames.

    Marica isn't a developed surface, though is close enough you can force the sheet goods (torture) to the shapes. I don't know the exact shapes of the Spira dories, though do admit I certianly don't like their weight and dead flat bottoms, but I suspect they're not fully developed, though (again) close enough to torture the planking into place.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    If that 24' dory isn't 100% developable, I'd be surprised. Or wonder why not.
     
  12. PAR
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    PAR Yacht Designer/Builder

    To be a "true" developed surface, the planking should "drape" all over the frames, touching everything, without having to force anything. In the case of the Spria dory, the bow sections would show a slight amount of cupping in the planks, which the frames would also emulate. The little information offered on the site suggests, the forward frames are straight sided, meaning there will be some torture involved. Now, we all use some torture to get things to hit, but in a perfect world (or a CNC kit) you'd work out these areas, so everything lands true to the molds and/or frames.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, so the plans provides for straight frames, I wasn't thinking that.
     
  14. Easy Rider
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    Easy Rider Senior Member

    Look at the twist in Marcia's fwd section.
    Would seem to me it would be very difficult to attach the upper planks w all that twist. Can't see it done w a 3/4" oak plank but w two 1/4" or 3/8" planks (Ashcroft) I see it as not even difficult. What's reality?

    Some of the text from the Atkin site.
    "The sections of design number 436, Marcia, are all absolutely straight; there is ample flare, but it will be noticed, no flam. The purpose of these straight sections is to ease the process of planking and to have a design that can be planked with light steel, waterproof plywood, or in the usual boatbuilder's manner, with wooden strakes and seam battens."

    Is "flam" curved flare? I think I need to buy tees plans and have a study. I copied the lines too but they wouldn't post.
     

  15. gonzo
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    gonzo Senior Member

    Flam and flare are the same.
     
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