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Re: [AtkinBoats] Elon Jessup plywood

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  • John B. Trussell
    Bear in mind that free advise may not be worth more than it costs, but... A major advantage of cross planked skiffs is that the bottom is stiff and does not
    Message 1 of 5 , Dec 16, 2005
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      Bear in mind that free advise may not be worth more than it costs, but...

      A major advantage of cross planked skiffs is that the bottom is stiff and
      does not require any framing. (A disadvantage is that unless you get very
      crafty, cross planked boats dry out on a trailer and leak horribly. They
      need to be kept in the water.) A fairly obvious solution is to plank with
      plywood. However, plywood is fairly heavy (at least compared to cedar) and
      it is a fairly limp material. If you substitute plywood for cedar by
      weight, the bottom will flex. Two options are to increase thickness of the
      plywood (building a heavier boat) or to beef up the framing.

      I am currently contemplating plans from a New Zealand designer for a flat
      bottomed plywood skiff about 6 inches shorter than Elton Jessup. It is all
      in metric, of course, but the approximate scantlings are:

      Planking 6mm--1/4" plywood
      Keelson 60mmx20mm, roughly 1"x3"
      Sister keelson 40mmx20mm, roughly 1"x2"
      Frames under each thwart--roughly 3 ft apart--60mmx20mm
      Keel 40mmx20mm

      The sides are lapstrake with two strakes (this looks good and effectively
      doubles the thickness of the sides where the strakes overlap. In addition,
      there is a 10mmx30mm (1/2x1 1/2) half round molding strip at the outside
      bottom of the top strake and a 20mmx40mm gunwale.

      I think this will produce a fairly light, stiff boat.

      I have used Bruynzeel and like it very much. Published information on
      Sapele is that it is heavier and stiffer than Bruynzeel, but I have no first
      hand experience with it. I recently built a boat using MDO plywood. This
      is the same stuff used to make highway signs and obviously Highway
      Departments have more clout with plywood manufacturers than boatbuilders.
      It is awfully nice stuff, but is normally available only in 3/8" and 1/2".
      You might want to consider this.

      Good luck.

      John T
      ----- Original Message -----
      From: "rljssn" <rljssn@...>
      To: <AtkinBoats@yahoogroups.com>
      Sent: Friday, December 16, 2005 1:36 PM
      Subject: [AtkinBoats] Elon Jessup plywood


      > I've been trying to work out which skiff and what material for too
      > long. I like these old skiffs but the traditional woods in decent grain
      > are turning out just as expensive as the marine plywood in my area. In
      > the description story on Jessup it mentions 1/2" rotary cut Bruynzeel
      > plywood as a substitute. Is there any more information or a precedent
      > from boats built as to how to make this substitution? Is there anything
      > in the scantilings table about the planking? 1/2" ply in the sides and
      > a double thickness on bottom maybe?
      >
      > I think I can live with the non-traditional materials but I don't want
      > to lighten this design and change the load waterline characteristics.
      >
      > thanks for all the great advice,
      > at least I've got a favorite skiff picked out.-Russell
      >
      > ps-I'm thinking of using sapele instead of the $$$Bruynzeel.
      >
      >
      >
      >
      >
      >
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      > If you set out to build an Atkin boat, please do not modify the plans. If
      > you stray from the plans you do so at your own risk and Atkin & Co. will
      > take no responsibility for the performance of the resulting boat.
      >
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      > <http://www.atkinboatplans.com/>
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    • JJ Johnson
      Before I say anything, just remember we are NOT supposed to change the Atkin designs structurely.. Somewhere in all my books and articles I have the formulas
      Message 2 of 5 , Dec 17, 2005
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        Before I say anything, just remember we are NOT supposed to change
        the Atkin designs structurely..

        Somewhere in all my books and articles I have the formulas on how to
        do this. The problem is I can not remember who wrote the
        article/book. It might have been David Gerr, or Jim Michalak, or Ted
        Brewer or who knows, I just do not remember.

        Also many of the older designs are designed and built to survive oh-
        my-god situations. Which is not saying that this wrong, it is
        absolutely right! Just that, in a small pond or stream, it is not
        necessary.

        However if you do make structural changes you should redo the frame
        spaceing (read stringers and frames) to support the skin you decide
        to use, AND, the speed at which you want to operate your boat, AND
        its designed displacement. The faster or heavyer the boat, the
        stronger the framing/skin combination must be to stop oil-canning and
        structural failure. This may be done by increaseing the skin
        thickness without changing the frame spaceing, or by decreasing the
        skin thickness and frame spaceing. The latter will normally result in
        a lighter, less expensive boat that needs ballast. So why not take
        the savings and build with the better materials, add ballast in the
        way of personal gear and go cruising... hehehehe

        JJ








        --- In AtkinBoats@yahoogroups.com, "adharvey2" <adharvey@m...> wrote:
        >
        > Russell, I've been confronting the same issue with Elon Jessup. Soon
        > as I read that bit in the plans I went and bought all of my 1/2"
        > okoume ply and 3/4 for the bottom. That gives you about the same
        > weight as the Cypress. Then we had a discussion on this site about
        > lightening the design with 3/8 ply for the sides and 1/2 for the
        > bottom. But I'm not convinced that a 4 1/2' wide frameless 1/2 sheet
        > bottom would be a very satisfactory replacement for a cross planked
        > bottom. And I have my doubts about the 3/4 ply as well. Also I'm
        with
        > you on not wanting to change the boats weight without a good reason.
        > On the other hand, there are a lot of articles and books out there
        > that preach lightness - especially for flat bottomed boats! And
        > there's no doubt you COULD build her lighter.
        > I also have been thinking that a double bottom might be way to
        go,
        > but what thickness? I hope you can sort it out for both of us. You
        > might look at the numerous earlier posts on this subject.
        > Andrew Harvey
        >
        > --- In AtkinBoats@yahoogroups.com, "rljssn" <rljssn@y...> wrote:
        > >
        > > I've been trying to work out which skiff and what material for
        too
        > > long. I like these old skiffs but the traditional woods in decent
        grain
        > > are turning out just as expensive as the marine plywood in my
        area. In
        > > the description story on Jessup it mentions 1/2" rotary cut
        Bruynzeel
        > > plywood as a substitute. Is there any more information or a
        precedent
        > > from boats built as to how to make this substitution? Is there
        anything
        > > in the scantilings table about the planking? 1/2" ply in the
        sides and
        > > a double thickness on bottom maybe?
        > >
        > > I think I can live with the non-traditional materials but I don't
        want
        > > to lighten this design and change the load waterline
        characteristics.
        > >
        > > thanks for all the great advice,
        > > at least I've got a favorite skiff picked out.-Russell
        > >
        > > ps-I'm thinking of using sapele instead of the $$$Bruynzeel.
        > >
        >
      • John Kohnen
        ... If your Elon Jessup is going to live on a trailer using a plywood bottom, at least, is the best way to go. For some time I ve been meaning to put something
        Message 3 of 5 , Dec 23, 2005
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          On Fri, 16 Dec 2005 10:36:10 -0800, rljssn wrote:

          > I've been trying to work out which skiff and what material for too
          > long. I like these old skiffs but the traditional woods in decent grain
          > are turning out just as expensive as the marine plywood in my area. In
          > the description story on Jessup it mentions 1/2" rotary cut Bruynzeel
          > plywood as a substitute. Is there any more information or a precedent
          > from boats built as to how to make this substitution? Is there anything
          > in the scantilings table about the planking? 1/2" ply in the sides and
          > a double thickness on bottom maybe?
          > ...

          If your Elon Jessup is going to live on a trailer using a plywood bottom,
          at least, is the best way to go.

          For some time I've been meaning to put something together showing what
          scantlings the Atkins used when designing for plywood, to help those who
          want to convert to that kind of planking. I finally got a start in that
          direction, and here are some examples of plywood planked skiffs:

          Katelyn
          14' 3" x 4' 10" x 3' 8 1/4" bottom width
          Sides and bottom 3/8" plywood
          Keel batten 3/4" x 8" plywood
          3/4" x 2 1/2" oak bottom frames on 30" centers

          Sunshine
          17' 1 1/2" x 6' x 4' 9" bottom width
          Sides and bottom 3/8" plywood
          Keel batten 3/4" x 8" oak
          3/4" x 2 1/4" oak bottom frames on 32" centers

          Willy Winship as built by The Boatman
          13' 9" x 4' 10" x 3' 6 1/8" bottom width
          Sides 3/8" plywood
          Bottom 1/2" plywood
          Two 3/4" x 2" white cedar keelsons plus two 1/2" x 2" oak bottom skids
          Frames on about 29" centers

          Jasper
          14' 2" x 4' x 3' 1 1/2" bottom width
          Sides 1/2" plywood
          Bottom 5/8" plywood
          Keel batten 3/4" x 6" pine
          No bottom frames

          The Boatman Willy Winship was obviously overbuilt! I don't know if they
          got those scantlings from John, or if they thought them up themselves.

          Bottom frames are horrible to live with in a flat-bottom skiff! :o( If you
          must use them, make the limbers big, and put limbers both at the chines
          and next to the keelson, otherwise you'll have a heck of a time keeping
          the boat clean and dry. Don't ask how I know... <g> If I was building an
          Atkin skiff I'd go with the cross-planked style bottom, but using plywood
          sheets laid crosswise. I think the scantlings for Jasper are a bit heavy
          -- I'd use 3/8" ply on the sides and 1/2" ply on the bottom for a skiff in
          the 14-16' size range. Elon Jessup would probably be fine with no bottom
          frames and a 1/2" bottom and 3/8" sides (I'd do the lapstrake planks in
          plywood), but a 5/8" plywood bottom might be better for heavy use.

          --
          John <jkohnen@...>
          No man will be a sailor who has contrivance enough to get
          himself into jail; for being in a ship is being in a jail, with
          the chance of being drowned... A man in jail has more room,
          better food, and commonly better company. <Samuel Johnson>
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