Stitch & Glue Questions

Discussion in 'Materials' started by ken.H, Jul 19, 2016.

  1. ken.H
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    ken.H Junior Member

    I have been doing research on stitch and glue building and design,
    Devlin's and The new instant boat, and the www

    The stitch and glue plans I am looking at use 0.25" others use 0.5" for the hull on the same size and design for a small planing power boat.

    Some of the plans I have looked at are for aluminium construction.
    From reading Aluminium has great strength but needs lots of bracing to prevent flexing.

    I have read that 0.25 aluminium is equal in weight to 0.75" plywood

    weight vs strength
    0.25" with an extra layer of fibreglass, what ounce of cloth?
    or
    0.5" with one layer of fibreglass

    What would it take to have a epoxy plywood equal 0.25" aluminium hull.

    Thanks
     
  2. upchurchmr
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    upchurchmr Senior Member

    A 4x8x3/4" panel of Okume marine ply is 52#.
    4x8x1/4" panel of aluminum is 115#.

    Hope my calculation is correct.

    Its not really easy to pick a conversion on an actual design, because the stiffness of each is also vastly different. The material stiffness of aluminum is much higher than ply, but the thickness for equal weight is much much thinner, so the aluminum can be more flexible in actual practice (depends upon the geometry of the design and extra stiffeners).
    You would be better off asking the designer about what he would specify for a conversion - really - just get a design made for what you want to use.
    There are too many details of the design that need to be changed.

    I recently had a co-worker who made a pirogue from 1/8" aluminum (his brother welds for the marine industry). It came out great looking but 100#.
    We talked about making something similar in plywood. It came out to 40# (1/4" home depot plywood and glass). I personally thought that was over built, but he is happy.
     
  3. ken.H
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    ken.H Junior Member

    Let's throw out the aluminium plans

    What would be the disadvantages and advantages between the two choices

    I am probably over thinking the problem.
    The issue is in the lakes around me both plans would work out OK in open protected water, the issue would be in certain areas there is a lot more fallen trees and flooded trees. If the 0.25" is good, then I would just do the 0.25.

    Choices
    #1- 0.25, plywood and one layer of fiberglass cloth,
    what the designer calls for and the lightest of the three choices

    #2- 0.25, plywood and two layers of fiberglass cloth
    Extra fiberglass and extra weight, but stronger than choice #1

    #3-0.5", plywood and one layer of fiberglass cloth
    Strongest and the heaviest, but is it overkill

    Thanks
     
  4. upchurchmr
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    upchurchmr Senior Member

    #2 with glass on inside and out (1 ply 6oz each).

    If you hit something or get hit, its the inside of the ply that breaks first. a ply of glass on the inside strengthens the hull where it is weakest.

    There was a recent thread which showed a Gougeon (West system) report on strength / weight for variations in ply and glass thickness. I'll try to find it.
     
  5. gonzo
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    gonzo Senior Member

    The aluminum is stiffer per thickness, but the wood is stiffer per weight.
     
  6. TANSL
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    TANSL Senior Member

    That difference is difficult for me to understand. Any clarification or demonstration, please?
     
  7. tom28571
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    tom28571 Senior Member

    A great error, often repeated here and other places, is that a layer of woven glass cloth increases stiffness in a panel. It ain't so but most believe it anyway. Simple experiments will prove that but many will still not be convinced. Its true that glass fiber has higher tensile resistance than wood fiber but the fibers in woven cloth are not arranged to take advantage of this factor.

    FG fibers run over and under warp and woof and are not straight. That means that they must be somewhat straightened out before resisting tension. Until that point, tension is resisted mainly by the resin matrix. Non woven cloth like biaxial have fibers that are straight from the get go and do greatly add to stiffness of a panel.

    Stiffness of a panel or beam varies by the cube of the thickness. A 5/16" panel is therefore almost twice as stiff as a 1/4" one. A 3/8" panel is about 3.75 times stiffer and your 1/2" panel is 8 times stiffer than your 1/4" one. This should tell you where to put your money to increase stiffness and it is not on woven glass cloth.
     
  8. PAR
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    PAR Yacht Designer/Builder

    I'm not sure what the OP's goals actually are, but playing around with conversions and scantlings requires some engineering understanding. I perform scantling conversions with some regularity, but arbitrarily making adjustments, just don't make much sense.

    As Tom rightly points out, a layer or two of cloth just doesn't do much to strength and stiffness. Substituting unidi, biax or triax can improve this to a degree, but as pointed out it's fabric weight that's the real factor to consider, so by the time you have enough fabric to make a significant difference, you've added a notable amount of weight, which may be best applied elsewhere in the scantlings.

    A better understanding of what the goals of this exercise is necessary, before any real advise can be offered.
     
  9. ken.H
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    ken.H Junior Member

    I am looking to build a lightweight stitch and glue planning hull aprox 12’-13’

    An example would be Glen-l bingo, if anyone has a better plan or if there are problems with that plan, let me know, I am not set on a set plan.

    The bingo material's would be
    8- 4x8x1/4” plywood
    FIBERGLASS CLOTH: For exterior sheathing, 7-8 oz. treated boat type.
    4-1/3 yds. - 50" width (covers bottom in full width and laps over transom)
    4-2/3 yds - 38" width (split to 19" to cover both sides lapping transom)
    6 yds. - 38" width (decking)

    So we are not talking huge amounts of plywood or cloth.

    It would be used in protected areas of local lakes and some of the creeks that run into the lake.

    I am sure the ¼” plywood would not have any problems in open water but, there will be areas where there are flooded and downed trees, abrasion wise sand would be the soil the hull would come in contact with.

    Don’t want a barge or ultra-light racer, but would like to build it with a balance between strength and lightness.
     
  10. upchurchmr
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    upchurchmr Senior Member

    The plans show fiberglass. Epoxy will work best

    Is there any reason to change the design?
     
  11. PAR
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    PAR Yacht Designer/Builder

    The Bingo is a typical puddle scooter, built with light plywood, taped seams and a few athwart stiffeners.

    Additional cloth on the exterior will not help if you hit a hefty branch or something at speed, you're going to be holed regardless. A layer of kevlar on the inside of the hull, maybe in the contact patch and forward sections, where the likely locations of an impact might occur, will offer some protection, but attempting to protect for every possible contingency is impossible.

    Will you run over stuff, yep, you can bet on it. Will you hole the boat, maybe, it depends on how hard you like to bash into stuff, but most things will deflect away, assuming you don't see it underway and steer to clear it. A light 'glass sheathing will offer good abrasion protection, which all most folks need. Some boats (typically not a puddle scooter) need more protection, like those that are dragged up on beaches, over rocks, oyster beds, etc. and some rub strips can help. These additional layers of abrasion protection will add weight and costs, detracting from performance too. If you are moving at displacement speeds in the backwater areas, to avoid smashing into logs and the like, very little real damage will occur. Yes, some dings and divots could be expected, but these are repairs that you'll perform seasonally.
     
  12. ken.H
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    ken.H Junior Member

    So after reading up on Kevlar and properties,
    Tensile strength
    Compression
    Young's modulus elasticity

    In Toms post he brought up the stiffness differences with different thickness
    Of plywood, Base 1/4" .
    5/16 twice as stiff
    3/8 3.75 as stiff
    1/2. 8 times as stiff

    How would changing the thickness of plywood in the plaining surface areas
    Compare with adding a layer of Kevlar to the inside of the hull.

    If I changed the planing areas to a thicker plywood instead of the 1/4", It would add more weight but how would it change the puncture resistance vs the Kevlar.
     
  13. tom28571
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    tom28571 Senior Member

    Using commonly available materials, it would be hard to get more puncture resistance than with Kevlar on the inside, no matter how thick the plywood within reason. Puncture resistance is greatly increased with any common sheathing material on the inside.

    If you hit floating stuff, the common result is a glancing blow that caused no real damage. Most of us have had that experience if we have been on the water very long. If you hit a rock, shame on you. Worse floating object is a deadhead that is pointed at your boat.
     
  14. SukiSolo
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    SukiSolo Senior Member

     

  15. PAR
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    PAR Yacht Designer/Builder

    Ken, what are you trying to do? Significantly altering the scantlings on one of these puddle scooters, will really hurt it's capacity and performance potential. These boats are lightly built, so they can get up and scoot. You might be able to build a bulletproof hull, but burdened up, she'll just plow along like a scow, without tripling the HP. Hundreds of these have been built and sure a few probably smashed into something hard enough to sink it, they're easy enough to raise in fairly shallow water. Most of us cut our teeth on these as kids and I can assure you, we weren't gentle on them, but they survived, so did we.
     
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