Another Plywood Thread!

Discussion in 'Boat Design' started by RagnarTheDane, Nov 12, 2016.

  1. RagnarTheDane
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    RagnarTheDane Swabbie's Boot Shiner

    Howdy folks!

    I've read a bazillion threads (or maybe 30) on the Exterior Grade Plywood vs Marine Grade Plywood.

    I've searched all over, and can't find the 'exact' boat design that I'm wanting, so I've gotten several plans, done a few months of study, and have decided if I want what i want, I'll have to build it:)

    But to keep this post simple, It's a Flat Bottom design with lots of little changes.

    I'm eyeballing two 'shopping carts' and the one with Marine Plywood is 550.00 and the one with Exterior grade is like 190.00. ....
    I'm going to fiberglass the whole boat, inside and out, each seam etc.
    I'm REALLY struggling with the price difference.

    (Boat will be 18' x 70" with 24" sides and a Hunt Deck (spauldings?) on the back (so total of 20')

    if I'm coating this thing with fiberglass, does the Marine Grade REALLY matter? ... really? .... if it does, can you explain it to my wife?
     
  2. Tungsten
    Joined: Nov 2011
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    Tungsten Senior Member

    The amount of money you're going to spend in epoxy and glass will exceed the plywood cost x 3.
    Go to one of Pars posts, at the bottom he has a link for building tips.Everything you need to know is there.
     
  3. Barry
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    Barry Senior Member

  4. mydauphin
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    mydauphin Senior Member

    My understanding is that modern exterior grade plywood is almost as good as marine plywood. It is all about the glue. But when I was building boats 20 years ago, we only used marine plywood. Not sure what is happening today. But don't skimp of the resin/epoxy. Being cheap there makes the whole thing a waste of money. I can't tell you how many times I have seen a great boat good bad. They got cheap on the glass in a place and water got into the core, and whole boat became a mess.
     
  5. Barry
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    Barry Senior Member

    APA The Engineered Wood Association sets various specifications for products but do not really cover off marine plywood in much detail though if you google "The Difference Between Marine and Exterior Plywood" there are many unofficial opinions, here is mine

    Marine Plywood and Exterior grades share the same glue but there are grades with this glue and the marine glue is supposed to be at the head of the list

    If you order Exterior grade Fir as an example, the inner plys can be chosen from a variety of filler plys that do not have the same strength as the fir outer ply. Most important to a marine environment, the inner plys can have voids and the inner plys can be butt joined down the longer length

    If you order Marine Grade Fir ply, all the plys are fir, the inner plys cannot have voids and the plys cannot be butt joined across the 4 foot dimension.

    If you order cherry, or other ply, the inner plys would normally be Okume, no voids, no butt joints

    There was a time, maybe 30 years ago that you could order 16 foot lengths of Marine ply to limit butt joints, albeit, at a premium
     
  6. PAR
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    PAR Yacht Designer/Builder

    It's more than just the glue that holds the veneers together that makes the difference. Panel construction is the real butt kicker and once you bend a construction grade sheet, compaired to a marine, it becomes obvious, often with a loud bang. This is particularly true of the thinner 3 layer sheets, which is common up to 1/2" plywood.

    Simply put, the marine grades have more veneers, tighter controls over the amount of defects permitted internally and externally and the quality/species of the veneers themselves.

    You can still buy custom lengths of plywood and it's always been costly.

    When it comes to building a cheap, small boat, particularly a powerboat, you can use BS-6566 grade, which is nearly as good as the BS-1088 grade, but often half the price. On the other hand, if you elect to use construction grades, the strength, ability to bend around curves and remain intact in service, as well as finishing (Douglas fir is a bittch to smooth) can cause you to wish you'd spent the money on the good stuff, even if all it might take is a few gallons of filler to fix it. When it comes to powerboats, the engine, controls and steering will easily cost many times the hull planking, so is the stuff that keeps your socks dry, the place you'd like to skim on costs?

    I use construction grades frequently, but typically on not highly loaded or heavily bent internal structures. It's great for furniture and hull partitions, some bulkheads, etc. and this is where you can save some bucks.
     
  7. philSweet
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    philSweet Senior Member

    Well, if you are going to cocoon the thing in fiberglass, I would definitely go with marine grade because once it gets wet, it will stay wet forever. If this boat will live on a trailer, or can be covered when not in use, just glass the outside, keep the inside clean, covered and painted, but not glassed, and you can get a good number of years out of the exterior ply. In order to handle the inconsistent bending of exterior ply, use generous chine logs and sheer clamps. Let them control the curvature. And use generous taping on the seams. I'd use at least three staggered layers of 9 or 10 oz seam tapes inside and outside, then fair everything, then drape the skin over everything. If you have a bit of a vee bottom for drainage, I do recommend a 12" strip of glass inside down the vee.

    If this is a power skiff, one other thing is worth mentioning. Building out out of okoume (7mm or so, depending on engine power) marine ply can easily save you 100 pounds vs exterior ply. It might save you 2 gallons of epoxy at $45/gal (due to much better surface finish). Plus it might save you 3-5 hp on the engine, which is another $300 - $400 if buying new.
     
  8. rwatson
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    rwatson Senior Member

    Hint - add on to Pars comments.

    Quite often the cheaper ply will snap when you are trying to bend it.

    I found this out when building a skate ramp.

    I also found out, that if you apply 6 oz glass and epoxy to the outside of the bend before you fasten it, it will hold together.


    The decision on ply quality depends on how long you want it to last, and how proud you are going/not going to be. Phils advice is very sound if you want to go really el cheapo.

    For a 'posh' long lasting job, the quality paint and epoxy will be nearly as much cost again as quality plywood. You don't want to hang expensive plastic and paint on a void filled, damage prone standard of plywood.
     
  9. PAR
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    PAR Yacht Designer/Builder

    I'm working on a "big box" powerboat design currently, to see if it's possible to get the strength, stiffness and cost effectiveness with construction grade materials. I've selected 15' 6" as it's length to permit two lengths of plywood for the sides, without a little filler and kept the bottom width just under 4', again to best utilize plywood size.

    I've discovered you can use multiple layers of underlayment to make the panels, eliminate butt blocks or Payson butt joints and still remain 30% - 40% cheaper than the cheapest APA marine grade of plywood and considerably cheaper than BS grade pricing.

    2 layers of 1/4" underlayment, glued together presents 6 veneers and an extra glue line. A typical lauan or poplar underlayment sheet is about $15 nationally, though I can get it for less than $13 locally, still retail. This places the price of a composite 1/2" sheet at less than $30, which is cheaper than the usual pricing on APA PS1-95 grade marine (AA marine). Naturally there's more work associated with this approuch, as you'll need to make double (or more) layers of planking, but this is fairly easy and there is a significant savings.

    Ultimately, the question you need to ask yourself is; "what do I want from this boat" . . . If it's a throw away kind of thing, maybe getting a few to several years out of it, before it's done, worn out or tossed, well just about any grade of plywood will do, with minimum care, assuming a waterproof glue (exterior plywood). If you want a little more durability, possibly a lighter end result, you'll need to think about the marine grades and Okoume.
     
  10. rwatson
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    rwatson Senior Member

    Attached Files:

  11. bidonv
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    bidonv New Member

    I have built a sailing dinghy with cheap plywood I mean 5 mm, and the result is still pretty resistant, it's all about epoxy and fiberglass..............here is my dinghy...............:D
     
  12. motorbike
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    motorbike Senior Member

    The difference is $340, that tells me you don't have any money otherwise you would not even be asking. In short you get what you pay for, what you save up front you will make up for with extra sanding, painting, etc etc.

    You will save $340 in other places so easily by DIY that its really not an issue. My advice is to build a proven design to the best of your ability with the best basic materials. You can easily replace $340 but not your time. Once you are into your build you will realise that shortcuts on materials are very shortsighted.
     
  13. Easy Rider
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    Easy Rider Senior Member

    I built a plywood boat (28') an it performed well .. mostly because it was light. I didn't use fiberglass or multi part goo. I did use recorcinal glue and more "ringnails" than screws. All the plywood was Canadian marine ply.

    So IMO an economical boat would not include fiberglass.

    I wonder if double planking would minimize the downside to using non-marine ply. The downside to that IMO would be finding a way to fasten the two pices of ply together.
     
  14. RagnarTheDane
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    RagnarTheDane Swabbie's Boot Shiner

    I might name my next kid Phil!
    This is great material to think about. Thank you
     

  15. RagnarTheDane
    Joined: Jul 2016
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    RagnarTheDane Swabbie's Boot Shiner

    actually, i'm spending 7k on the motor, and my desire is to make the strongest boat I can as it will face some abuse here in North Texas.
    What I don't want to do, is waste money on materials that I don't need.
    If you'd like my personal finances, feel free to PM me, I'll send you a copy.

    I'm doing my research now, to prevent 'costly shortcuts' which is why I posted here. If i could find exactly what I was looking for, I would gladly build strait from blueprints, but alas, i cannot, so I found the closest thing to it, and am modifying it to fit my needs.

    Thanks for the advice, I'll keep it in mind.
     
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