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Thread: Sheathing stitch and glue with glass -- outside or inside?

  1. #1
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    Default Sheathing stitch and glue with glass -- outside or inside?

    I'm building a 16ft stitch and glue dinghy (build thread: http://forum.woodenboat.com/showthre...Apple-16-Build). I am trying to keep it light, to an extent, but also am in the northeast, and we have plenty of rocky "beaches", so I don't want it to be too fragile. So I've (tentatively) decided to sheath _one_ side of the plywood with 6oz glass -- the seams on the other will be taped, and the plywood will be coated with epoxy (and of course will be painted). It seems like most people sheath the _outside_, with the idea being that you increase resistance to scratches (by putting more stuff to scratch through), but in the building instructions I have the designer describes that for strength, if you are only going to glass one side, the inside is better, as glass is much stronger under compression than tension -- so if you smash into something, the plywood will get bashed, but you will be less likely to have an actual hole. I've searched around on the forum, and saw a few comments that seem to back that up (and that the scratch resistance, at least of thin layers of glass, is very overstated), but I didn't find this direct questions asked.

    Also, I'm wondering if putting the glass on the inside increases overall rigidity better than the outside -- or if it's a wash, and either side would work.

    So, question for people who know: if you were going to glass only one side, which would you do, and why?

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Stitch and glue plywood panels are generally flat except where the joints are, so your basic strength is coming from the plywood unlike like a strip build where the curvature of the fiberglass on the wood strips becomes a composite. This can be argued until the cows come home but imo fiberglass scrapes and scratches just as bad as wood glass on the inside would save you an actual hole that could sink the boat. Thing is why would you build a wooden boat and then bang it up? When you can actually be a little bit more observant of your launchings and landings and protect your investment.
    Denise, Bristol PA, Oday30, Anchor Yacht Club, On tidal Delaware River. my current project; http://forum.woodenboat.com/showthre...0-Ducker-Resto

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    "glass is much stronger under compression than tension"

    Got it backwards, but a moot point since after all of that work you won't be bashing it on the rocks anyway.

    You will, however, be beaching it and you want to protect the bottom from abrasion so the thing to do is give the outside of the bottom a double layer of dynel. Except for taping the seams inside and out there will be no need to glass the rest of it.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    If you avoid the big rocks and can afford to just worry about abrasion resistance a layer of Dynel on the bottom panels might be the best solution for you. But I tend to agree with Denise...build to suit and be careful Nice build thread, by the way ! I'm on the other coast and we have everything from mud beaches to sharp gravel, depending on where you go. I just try to avoid beaching on the really gravely stuff and am thinking about picking up a couple of float bag/beach rollers for when I can't avoid the gravel. Which rig are you planning, by the way?

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    The one thing glass does do outside is provide a nice surface for paint that stays smooth. Ply can check.

    Also, in my experience running ply boats around on granite sand beaches is that sheathed boats fare a bit better than naked ply, as far as normal wear nosing up on beaches and such, but that scraping over a sharp rock is pretty much a draw, damage wise, except harder to fix if there is glass or other cloth present.

    Sort of a six of one, half dozen of the other type deal.

    You can also glue some poly rope to the stem area as a skid plate, of sorts, if you anticipate beaching it a lot.

    Peace,
    Robert
    Last edited by amish rob; 01-05-2020 at 09:06 AM. Reason: Typo that nagged...

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    The 14' Devlin stitch and glue recreational fishing boat I built 21 years ago is only sheathed on the outside. There are no docks at most launching ramps here in Vermont, so I have to beach it every time to board. I don't beat this boat up, but I don't handle it like a fragile egg either. Rocks, gravel, whatever, it has held up. The boat is still going strong. Sheath the outside of your boat. I only taped the insides and epoxy sealed the interior.
    I was born on a wooden boat that I built myself.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    "glass is much stronger under compression than tension"
    This doesn't make any sense to me either.
    Sheath the outside of your boat. Period.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Laying glass around inside curves is a real pain. Personally I'd glass the outside and then avoid running the boat into things. Also, honestly, if your bouncing off of objects with enough force to puncture the hull your PDF is the insurance, not glass

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    Default

    Quote Originally Posted by spirit View Post
    "glass is much stronger under compression than tension"
    This doesn't make any sense to me either.
    Sheath the outside of your boat. Period.


    Simple example: sheath two plywood planks. Try to bend one with the glass on the outside of the bend ( tension). Then, bend the other with the glass on the inside of the bend ( compression) Let us know which one bends easier.

    Kevin


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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by Breakaway View Post
    Simple example: sheath two plywood planks. Try to bend one with the glass on the outside of the bend ( tension). Then, bend the other with the glass on the inside of the bend ( compression) Let us know which one bends easier.

    Kevin


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    Glass has no compressive strength of its own, it is yarn after all. The resin and wood provide any compressive strength that the lay-up possesses. So you are dealing with the modulus of three different materials here.
    It really is quite difficult to build an ugly wooden boat.

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    Default Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by Peerie Maa View Post
    Glass has no compressive strength of its own, it is yarn after all. The resin and wood provide any compressive strength that the lay-up possesses. So you are dealing with the modulus of three different materials here.


    Understood.

    So, to the question: does glassing the inside or outside of the boat make it, "stronger." ( Leaving aside issues of abrasion-resistance and a potentially better surface upon which to lay paint)

    Kevin


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    There are two kinds of boaters: those who have run aground, and those who lie about it.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Assuming one decides to put cloth on the outside only, would you do all of the hull? I am considering just doing the bottum and over the lower chine on a pram I'm building right now, done this make sense, or should I do cloth from gunnel to keel? I've never done cloth over a large area and was hoping to simplify the process while still building in sufficient protection.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by Breakaway View Post
    Understood.

    So, to the question: does glassing the inside or outside of the boat make it, "stronger." ( Leaving aside issues of abrasion-resistance and a potentially better surface upon which to lay paint)

    Kevin


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    Since glass on the inside has to stretch for an object to pierce the hull it makes it more puncture proof. Glass on the outside won't help much with that.

    Glass on both faces makes it more rigid overall though. That can be helpful at times, especially with 1/4" cedar planked kayaks. I've used glass on both sides of lapstrake planks and strip planking in order to use fewer and even no frames.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by Ober27 View Post
    Assuming one decides to put cloth on the outside only, would you do all of the hull? I am considering just doing the bottum and over the lower chine on a pram I'm building right now, done this make sense, or should I do cloth from gunnel to keel? I've never done cloth over a large area and was hoping to simplify the process while still building in sufficient protection.
    What do the plans call for?

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Gib,
    In this case the plans do not call for any cloth, inside or out. I'm building this from a kit that came from Granta Boats sometime back in the 80's I believe. I bought the kit complete still in the box. I'm inclined to think I should add some glass to the bottum at the very least.
    Thanks
    Darin

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    Default

    Quote Originally Posted by Breakaway View Post
    Simple example: sheath two plywood planks. Try to bend one with the glass on the outside of the bend ( tension). Then, bend the other with the glass on the inside of the bend ( compression) Let us know which one bends easier.

    Kevin


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    You could actually sheath one plank and try to bend it both ways.

    Sent from my CPH1851 using Tapatalk

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    What Rob said, post #5
    And what Denise said about curved panels and cows coming home #2.

    For a variety of cloth types and general information about abrasion;
    http://forum.woodenboat.com/showthre...olyester-cloth
    http://forum.woodenboat.com/showthre...ion-Resistance

    Glass is stronger in compression than tension, but we aren't talking about solid glass. Fiberglass is strong in tension, and once you glue enough fiberglass together with epoxy, it is decent in compression. You have seen fiberglass arrows? It's all about the size, thickness, shape, buckling, etc. All that engineering doesn't apply to the original question because it was about a single layer of glass on one side of a (how?) thick panel. In the face grain direction, you won't see any difference in stiffness unless it's only 3mm thick. In the cross grain direction, it comes down to whether the face veneer is that paper thin stuff or something thicker.
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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by Rich Jones View Post
    The 14' Devlin stitch and glue recreational fishing boat I built 21 years ago is only sheathed on the outside. There are no docks at most launching ramps here in Vermont, so I have to beach it every time to board. I don't beat this boat up, but I don't handle it like a fragile egg either. Rocks, gravel, whatever, it has held up. The boat is still going strong. Sheath the outside of your boat. I only taped the insides and epoxy sealed the interior.
    +1
    Did the same thing on my stitch & glue gunning dory.
    If you need proof - make up a sample of each. One painted ply and one glassed and painted ply. Scratch with a screwdriver or whatever...and see what happens...
    Doing the whole hull is not as hard as you might think.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Thanks all -- so sheathing the outside probably makes more sense: obviously I'll do the best I can to take care of it, but rocky shores are pretty much all we have around here, and part of the point of having a small boat is being able to poke into whatever random place that looks interesing! Dynel sounds exciting but I'm not sure if I'm confident I could do it well -- probably I'll stick to glass!

    @MN Dave: It's 6mm, decent quality (no paper thin veneers) marine okoume. Does that mean there won't be much difference in stiffness?

    @Hugh: The lug yawl, though not will the massive mizzen.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    E2FA3C84-939E-4111-A030-A03E9D7BCC9F.jpg
    Stitch and glue. Glassed only on the bottom and up over the chines. Not even glass tape.

    176C03CE-2353-47F2-B21A-3BB30523AB19.jpg
    Repainted, but with the same ugly “sail”.

    Peace,
    Robert

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by dbp1 View Post
    Thanks all -- so sheathing the outside probably makes more sense: obviously I'll do the best I can to take care of it, but rocky shores are pretty much all we have around here, and part of the point of having a small boat is being able to poke into whatever random place that looks interesing! Dynel sounds exciting but I'm not sure if I'm confident I could do it well -- probably I'll stick to glass!

    @MN Dave: It's 6mm, decent quality (no paper thin veneers) marine okoume. Does that mean there won't be much difference in stiffness?

    @Hugh: The lug yawl, though not will the massive mizzen.
    Stiffness varies as the cube of thickness. A layer of 6oz cloth set in epoxy is about as stiff as the wood under it and about .005 in. (.127mm) thick. The plywood should gain about 6% in stiffness in the face grain direction, which is in the mud between one bare panel and another. Across the grain, if the face plies are thick, the thin, stiff layer of glass cloth will have a more significant effect. I don't know how much. One of these days I really need to set up some samples and measure this.

    Wood is much stiffer in the grain direction than the cross grain direction, so plywood with a thick face veneer will be noticeably more flexible in the cross grain direction because the thick layer on the surface is not very stiff in that direction. You will notice alot more sag under load if you use plywood to make shelving and cut the wrong way. When the face veneer is very thin, the next layer down is closer to the surface with the grain running crosswise, so it should be a little stiffer perpendicular to the face ply than the plywood with the thick face layer. The paper thin decorative veneer hiding the defects in the crummy inner plies in a cheap plywood make for an overall inferior product, even if the stiffness is a little more balanced. I will point out that this is an educated guess that is not backed up with hard data. The extreme case is wacky wood, AKA bending plywood. It has very thick face sheets and a very thin central cross ply. It is very easy to bend in the cross grain direction.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    " I will point out that this is an educated guess that is not backed up with hard data."

    You short change yourself Dave. It is a conclusion based on hard data and experience.

    You don't fool us you know. We know that you know your sh t (and a bit of engineering too), and we know that you know that you know it too. What you don't know
    is that we know things about you that you don't know we know!


    Know what I mean?

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by Gib Etheridge View Post
    " I will point out that this is an educated guess that is not backed up with hard data."

    You short change yourself Dave. It is a conclusion based on hard data and experience.

    You don't fool us you know. We know that you know your sh t (and a bit of engineering too), and we know that you know that you know it too. What you don't know
    is that we know things about you that you don't know we know!


    Know what I mean?
    I spent a year running the lab in a sewage treatment plant. I KNOW my sh_t! In agonizing detail.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by MN Dave View Post
    I spent a year running the lab in a sewage treatment plant. I KNOW my sh_t! In agonizing detail.
    That's funny shhhtuff

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by timo4352 View Post
    That's funny shhhtuff
    shhhtufff anyway. Not funny really. The good news is that thanks to the pumps in the lift stations, no one ever saw a 'brown trout'. What was funny was the initiation for the new field guys involving about a million palmetto bugs pouring into a lift station. The newbie sort of rocketed out of the manhole covered in roaches.

    I found some scraps of 3mm okoume in the shop. The two pieces are about 2 fingers wide by a span long and the bolts weigh about the same, I checked with a balance, well a stick, a fulcrum and a tape measure, but they balance well enough. One is cur perpendicular to the face grain and the other is parallel. The parallel one deflected around 1/4". I swapped them just to show that they are close enough. No fiberglass for a few months, it was a balmy 28° above zero today. It will go the same below before it warms up.
    grain orientation.jpggrain orientation 3mm.jpggrain.jpg
    Last edited by MN Dave; 01-05-2020 at 08:59 PM.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Instead of 6 mm okume , perhaps use 9 mm meranti with no glass.
    okume is soft and weak on its own , easy to puncture and poor rot resistance.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by wizbang 13 View Post
    Instead of 6 mm okume , perhaps use 9 mm meranti with no glass.
    okume is soft and weak on its own , easy to puncture and poor rot resistance.
    Bit late for that decision...



    and the bends at the bow were _hard_ (required some glass and a bolt on one strake), so I'm not even sure the design could be done with 9mm -- it's certainly not an option presented by the designer.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    If you have a situation where you hit a rock which tries to penetrate the side, glass on the inside will dramatically increase the damage resistance. Since it does not splinter as easily as the plywood.

    I personally want glass also on the outside for abrasion resistance and to limit the damage to the wood.

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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    CLC Chesapeake 16 s&g sea kayak, 4mm meranti, glassed all over on the outside, glass tape along the inside of all the joins, and they recommended glass on the floor of the cockpit, which I did. If I built another, the glass on the outside would only be on the botton planks.
    JW lapstrake Pathfinder, glass right over the bottom, and up to the top edge of the lower strake on the outside. Glass on the floor of the cockpit. The bottom panel on these is 12mm ply, the cockpit is the only place where you are standing directly on the bottom.

    Glass both sides adds a lot of stiffness to a plywood panel if that's required, but also a lot of weight and $ if you overdo the resin. It won't really hide any mistakes, and IMO, is more work to fill/fair and paint than bare plywood.

    Pete
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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Quote Originally Posted by upchurchmr View Post
    If you have a situation where you hit a rock which tries to penetrate the side, glass on the inside will dramatically increase the damage resistance. Since it does not splinter as easily as the plywood.

    I personally want glass also on the outside for abrasion resistance and to limit the damage to the wood.
    I was surprised to find any test data at all. More surprised at how badly plywood performs. What was least surprising was that the only test with glass on one side had the glass on the strike face. I wonder if the falling dart test is the best test here. Rather than try to sum up the information here, I will leave it to anyone with the patience to plough through the paper in the link. I would hesitate to read too much into it without more information. I think that the speed of the dart might have been a major factor. You can shoot a nerf dart through a 3/4" sheet of plywood if you have a supersonic air gun, but that rarely happens.

    https://www.montana-riverboats.com/f...t-strength.pdf
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    Default Re: Sheathing stitch and glue with glass -- outside or inside?

    Way back in the fifties my dad built two prams in the basement from Hagerty furniture kits. One was for a dinghy to get to and from our moored sailboat, the other was for me. He glassed the outside of both. When we drove to the shore for the summer, the first was strapped to the roof rack of the station wagon. Somehow the roof rack detached and it and the pram flipped off onto the highway. A line was attached to the back bumper so (luckily?) there we were dragging the boat upright down the breakdown lane until we could stop. One corner of the chine on the bow transom was shaved off a bit through the glass sheathing but otherwise all was okay! I wish we still had that boat. When they sold their home I helped clean out the basement. There were the molds and patterns. I had no way or place to save them so they went for kindling.

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