Strip plank construction in alternative materials

Discussion in 'Sailboats' started by Ivanthefool, May 11, 2017.

  1. Ivanthefool
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    Ivanthefool Junior Member

    Looking at a potential build of a sailboat in cedar strip planking, I see that there is always a potential of water intrusion. Strip planking has the benefit of enabling quick and simpler construction, especially of nondevelopable surfaces. But why not utilize other materials? For example, aluminum cut into strips? or, fiberglass sheets? I remember from my homebuilt aircraft days when we used epoxy instead of rivets to bond wing skins. Aluminum is fastened all the time in aircraft construction. Why not on a sailboat hull?
     
  2. upchurchmr
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    upchurchmr Senior Member

    If you are going to use aluminum what will you use for an adhesive?
    If aluminum and rivets, how will you get the shape of the strips? You know that there are a lot of partial strips that stop on the side of other longer strips - right?
    How much are you willing to have this thing weight? Aluminum will be really heavy if you have to overlap strips to get them fastened.
    And aluminum is really heavy compared to cedar strip/ fiberglass/ epoxy. The only thing you can do to make the weight equivilent is to make the aluminum so thin that it will bend when you touch it.

    Have you ever looked into what is actually done to build cedar strip boats?
     
  3. Ivanthefool
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    Ivanthefool Junior Member

    I would use aircraft grade epoxy and of course the strips would have to be trimmed.
     
  4. upchurchmr
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    upchurchmr Senior Member

    So you are going to edge glue the strips just like cedar strip?

    Aluminum is 629#/ cu ft.
    Cedar is 25 #/ cu ft.

    So to get equal weight you need .25" x 25/ 629 = .0099" thick aluminum strips or .01"
    One hundredth of an inch. Typical aluminum foil for baking is less than that (.0007 to .0004)

    Try edge gluing that together. Oh, and how much strength is that?

    OK, I ignored the glass/ epoxy coatings. Still, you can't make it work if you make the aluminum .05 (5x the weight of the cedar).
     
  5. Ivanthefool
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    Ivanthefool Junior Member

    Plans call for .35 cubic meters of cedar in 12mm by 26 mm strips. On your numbers of 25 pounds per cubic foot, that comes to 309 pounds of wood. Aluminum is 169 pounds per cubic foot-6.8 times heavier. I think this is where you are off. Steel is only 490 pounds per square foot.
    5083 plate has a yield strength of 34,000 psi. Cedar is 609.
    1/16" aluminum is the same weight as the cedar, but has many times more strength. Doubling thickness to 1/8th would be a 300 pound weight penalty, but I would not need the glass and epoxy coating. 1/8th aluminum plate is pretty strong. A lot of 50+ aluminum boats have 1/4" hulls.
     
  6. upchurchmr
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    upchurchmr Senior Member

    Your right, I misread the table for density.
    Show us your bonding of 1/16" aluminum.
     
  7. Ivanthefool
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    Ivanthefool Junior Member

    [​IMG]
    I will just ask one of my aircraft friends how they glued the skin on this Grumman AA-5. The newer stuff does not require an autoclave.
     
  8. upchurchmr
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    upchurchmr Senior Member

    Show us a section thru the area the skin was glued.
     
  9. DCockey
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    DCockey Senior Member

    My guess is the Grumman AA-5 skin is sheet aluminum, with most if not all of the aluminum surfaces being developable which can be formed with rolls or a brake. The sheets are fastened/glued to a framework, not to each other.

    Strip planking gets most of it's strength from the strips being fastened to the adjoining strips.
     
  10. messabout
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    messabout Senior Member

    Ivan, I doubt that you are a fool. With that let us explore the realities of small boat design and construction. ... A kayak or lightly loaded canoe will have a hydrostatic pressure of ten to eleven pounds per square foot. A power boat will have dynamic pressures of many time that of the paddle boat. We gotta' address the implications of that unit pressure number.

    A typical kayak or light canoe will use quarter inch thick strips. To compare the stiffness of a quarter inch strip with an equivalent weight of aluminum is not just a matter of weight, there is he matter of stiffness. If the ratio of weight is...let us be generous here, the ratio is about six. so if the thickness of the aluminum is one sixth of one quarter, then one sixth equals 0.250/6 = 0.0416. ...but the stiffness is a function of an exponent of thickness. to be sure the stiffness of aluminum is more than the stiffness of cedar then to be the most generous for plate analysis....0.250^3 = 0.0156 and the 0.0416^3 = 7.199 x 10 to the minus 5 which ain't even within the ball park of the cedar strip stiffness. The elastic modulus of cedar compared to the elastic modulus of aluminum tells us that the best, stiffest, lightest weight structure is cedar or similar wood. .... Wood is still the lightest strongest material for most of our structures that do not depend on CF or other space age prohibitively expensive fibers.
     
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  11. CT249
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    CT249 Senior Member

    It was done with the Peter Norlin designed 3/4 tonner Profilen around 1981. If I recall correctly, the "strips" were special extrusions with edges designed so that they could clip together. It was, I think, an experiment by the large alloy manufacturer SAPA, who may have built one later boat.
     
  12. upchurchmr
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    upchurchmr Senior Member

    Lots of thing "have been done" but how costly/ successful was it?
    Any information that actually tells us what happened? How heavy was the boat compared to the current standard construction.

    One-off demonstrations by a group who has lots of money doesn't really suggest it should be done again. If it had of been successful we probably would be seeing boats built that way still
     
  13. jorgepease
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    jorgepease Senior Member

    You can strip plank with foam core
     
  14. CT249
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    CT249 Senior Member

    Light enough to be competitive at the worlds, although Norlins were never super-light and the 3/4 ton size tended to be dominated by boats that were heavy by the standards of the time, like Dehler dBs and X 102s and 3/4 tons. Also light enough to retire from one race with hull damage IIRC.

    Was it successful? The fact that they did (just) one more boat seems to say something. Around the same time there were also a couple of alloy/foam sandwich boats; one of the Yeomans and (I think) one of the Intuitions. They both went well but I haven't heard whether they are still around. Most of the alloy IOR boats of the '70s seem to be still thundering around but the foam core ones????? Dunno.
     

  15. rwatson
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    rwatson Senior Member

    The other obvious answer is that wood strips are cut out of already narrow planks, while cutting up 4' wide plywood is a waste of effort. Likewise, cutting aluminium sheets or fibreglass sheets into narrow strips is also a waste of time.
    If you need compound shapes, its eminently more practical to lay the fibreglass resin on the mould, than spend hours cutting and shaping strip material. 9 times out of ten, the mould is made up of shaped and curved and possibly developable flat panel sections with filler at the intersections.
     
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