Marine grade plywood ?

Discussion in 'Wooden Boat Building and Restoration' started by Travis Grauel, Feb 8, 2020.

  1. Travis Grauel
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    Travis Grauel Junior Member

    So I’m in the process of building my first wooden 18 foot boat. I have constructed it out of all red oak and soon will be sheeting it. I want to use marinade grade but the sheer cost is outrageous but I do not want to cheap out on this step just to save a few bucks. I have 3 options and I will be fiberglassing the hole outside and epoxying and painting the whole inside. My 3 options are regular 1/2 plywood, Home Depot “sande” plywood which is a cheap “marine” style plywood or true 1/2 marine grade sheets. What are your opinions? Thanks !
     

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  2. fallguy
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    fallguy Senior Member

    No matter what plywood you use; it must pass a couple of tests.

    30 minute boil without coming apart

    screw directly into it in 10 places and make sure it does not split in between the veneers; a cracking sound or lift off of a veneer is a sign of a poor glue or a poor process and I have encountered it a few times

    I am a little surprised at the 1/2" spec; that is a bit much for the whole hull..
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the difference in price ?
     
  4. fallguy
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    fallguy Senior Member

    A sheet of 6mm okume is $220 plus tax here.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds ridiculously high. This is from a ply supplier near where I live

    Premium Marine Grade Plywood “BS1088” THICKNESS 4MM 3PLY 6MM 5PLY 9MM 5PLY 12MM 7PLY 15MM 7PLY 18MM 9PLY 25MM 9PLY PACK M2 $10.00 (100) $14.00 (80) $20.00 (70) $25.00 (50) $30.00 (40) $37.50 (35) $50.00 (30) LOOSE M2 $15.00 $20.00 $25.00 $30.00 $35.00 $45.00 $60.00
     
  6. Travis Grauel
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    Travis Grauel Junior Member

    Thank for the help? How close should the 10 screws be? And I thought 1/2 for a wood boat was standard 3/8 could probably work by 1/4 is fairly easy to break this boat will be in the bay not a river or lake
     
  7. Travis Grauel
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    Travis Grauel Junior Member

    1/2 standard is about 25
    1/2 sande is 35
    1/2 marine is 69
     
  8. fallguy
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    fallguy Senior Member

    just keep them apart to not void the test and don't test on a panel edge.....I have had panels fail to stay together under screws; it is my personal test and not always advised, but I want the glues to avoid shear pressures

    thickness depends on design, glass plans, etc
     
  9. Barry
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    Barry Senior Member

    You used red oak on the frames instead of cheaper fir or (SP). I would say, don't cheap out now on the ply for the difference in price. Marine grade ply has all the voids filled and probably
    the glue for this application. It would be a shame to see delamination in a few years. I would that thought that the supplier of your plans would have provided a thickness for the ply. 1/2 inch
    seems overkill
     
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  10. Travis Grauel
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    Travis Grauel Junior Member

    My grandfather designed this boat and he’s just a carpenter not a boy builder he has years of experience but 1/2 was his suggestion
     
  11. messabout
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    messabout Senior Member

    The bottom is the most stressed of the panels. The pressure on the bottom is a function of velocity and there will be some impact loads when striking waves or wakes at speed. Whether half inch ply is adequate for the bottom structure is dependent on the unsupported spans. That is to say that the spacing of frames and stringers are a deciding factor.

    You could build the boat nearly bulletproof by using three quarter or one inch bottom ply with fewer frames and stingers. The sides would probably be adequate with three eights ply (if you do not mind a bit of noise). The dynamic pressure on a bottom is a function of the square of velocity. If you intend to use a lot of HP for impressive speed then the bottom had best be pretty stiff. Consider a velocity of 20MPH..... the impact velocity factor in feet per second will be something like 900. At a speed of 30 MPH the impact factor will be about 1900. You can see that the speed that you intend will have a lot to do with your choice of bottom thickness. There is a disproportionate bottom load that varies exponentially as a function of speed. ......This is an oversimplification of the way things work but it is the general idea.
     
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  12. Barry
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    Barry Senior Member

    A couple of other questions
    Has this set of framing had plywood attached on a previous build. Ie the plywood has to be able to conform to the frames and make contact at every member and be fair
    If you use 1/2 inch, will you be physically able to bend it to conform even if the frame geometry creates a developable surface?
    If you do not know if the frames provide backing as a developable surface, you can take a sheet of say 1/4 inch plywood and a bunch of clamps and see if it will conform.
     
  13. Travis Grauel
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    Travis Grauel Junior Member

    My grandfather built this exact boat at 16 foot (2 feet shorter) and used 1/2 ply
     
  14. fallguy
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    fallguy Senior Member

    Is there glassing or sheathing in this build?

    Power plan?
     

  15. Travis Grauel
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    Travis Grauel Junior Member

    Unfortunately no...do you think 3/8 would be a better route? The whole outside is getting glassed
     
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