glass over ply

Discussion in 'Boat Design' started by Mupwi, Apr 23, 2007.

  1. Mupwi
    Joined: Apr 2007
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    Mupwi New Member

    hi guys

    I am planing on building a small trimarang and was wondering about the weight and strength of ply construction mainly the advantages of skining it with fiberglass how much stronger is a skined boat would doing this strengthen it enough to reduce from a 1/4 inch ply to 1/8 could this be done just on the outer of the hull or dose it need to be skined on both sides to create a stronger composite .

    thanks in anticipation
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    FAST FRED Senior Member

    On most boats the plyhandles 100% of the load , the glass is only an outer skin that adds abrasion / puncture resistance.

    Epoxy is the best glue , and fine woven glass cloth uses less resin than mat or roving.

  3. jimslade
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    jimslade Senior Member

    built ply before ( I like what he He said)
  4. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    You can use 1/8 inch (3mm) ply instead of 1/4 (6mm), but then you need maybe 300 grams of biaxial glass (and same weight of epoxy!) on both sides. Since glass and epoxy is heavier and cost more than even high quality plywood that doesn't make sense in most cases. If you engineer (calculate) very carefully and doesnt use more epoxy than necessary, you might save a few grams, but it will cost much more work and money.
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    In most cases, sheathing plywood is to provide abrasion resistance to the plywood. You can decrease the plywood thickness, but now you're building a cored structure and plywood is a poor choice for use as a core material. A core material should have excellent compressive strength and good bond properties with the sheathing, which plywood is good at, but at a pretty severe weight penalty. This is why foam, balsa and other materials are typically used as a core material. A cored structure will require an inner and outer sheathing of fabrics to be effective as a load bearing ellement.

    So, in the end what you are asking is if the plywood can be stiffened/made stronger with a sheathing, yes, but not necessary. Can you reduce the ply thickness and replace it with fabric. Yes, but it will be heavier and require more effort to build, plus changes the build type from plywood, likely a stressed skin structure, to a cored structure, which doesn't use the material choices to best advantage (poor core material selection).

    If you're building from plans, you'd be best advised to stick to the plans in the hull shapes and construction, engineered into the design.
  6. frosh
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    frosh Senior Member

    The initial post asks about reducing plywood thickness from 6mm to 3mm and sheathing with glass. It would be necessary to know more about the dimensions of the boat including sail area and expected all up weight with crew. I built a 5.6 m long tri using 2mm marine ply for the main hull but it has been very carefully engineered to make the hull strong and the panels stiff enough. The floats are composite using 5 mm Klegecell, woven carbon fibre inside and double bias glass outside. Give more information and you will obtain more reliable advice from the forum.
  7. Chris Ostlind

    Chris Ostlind Previous Member

    Going Lighter

    Apologies for being late to the game on this. The comments by Frosh are right on the mark. Give some mor info and we can give some specific advice as to what works and what doesn't.

    It all has to do with what rig, what weights and what sailing speeds you intend for this boat.

    The comments by Par, that there are better core materials is also correct with but one caveat. That being that most people grow-up with some rudimentary knowledge of working with wood, while foam and Balsa cores are fairly obscure materials left to committed enthusiasts of their sport.

    You guys who have worked in the world of composite cored finished products are not of the everyday boatbuilding environment and that includes me. I have to remind myself that most small craft are built from wood by homebuilders so that I don't get ahead of the curve on the design end of things.

    Mupwi... you can do what you ask... it's just that we need to know a few more things before we can predictably recommend the suggested process for your end use.

    Help us out here.

    As an example, the attached images are of a sailing canoe/trimaran with 115 sq. ft. of sail area. Not earth shattering in sail area, but quite nicely rigged to become a fairly quick and fun boat at 175 lbs. all-up. This boat is built in 4mm Okuome marine plywood with 9oz. glass/epoxy laminates inside and out, carbon/epoxy aka beams, carbon foils and lots of other carbon goodies to sweeten the overall effect.

    So, yes, it can be done, but you have to think through the process so that it does not become a structural disappointment in the making.

    Chris Ostlind

    Attached Files:

  8. Mupwi
    Joined: Apr 2007
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    Mupwi New Member

    thanks for the informative replys I was looking at building somthing along the lines of the Trilars from duckworks but hopfuly to build it lighter and hence faster easyer to car top my experiance with composites so far has been in repairing surf boards and rowing racing shells annother idea I had was to build the boat like the plans but out of foam boards instead of ply then glasing inside and out is this a feasable option if so what thickness of core and skin bould be the equivlent of what thickness ply or am I over simplifying this

    thanks again
  9. Mupwi
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    Mupwi New Member

    btw Chris have you got any more details on that tri maby a web page
  10. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Yeah, Chris it looks interesting!

  11. Syed
    Joined: Sep 2005
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    Syed Member

    I made a glass over ply some eight years back. Glass did add quite a weight. Some reinforcements (ply) had to add in the original design to avoid flaxing. I have heard that options other than glass are available.
    Wishing you best of luck with your project,
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