Ply- Glass- Ply

[Fritz Koschmann]

I have been looking at a couple designs for ply on frame boats where the designer call for two layers of plywood on the sides and bottom. What is unusual, for me anyway, is that the design calls for a layer of glass and epoxy between the layers of ply. I haven't seen this before and am wondering what the advantages are. I believe the second layer of ply is installed before the glass/epoxy has set.

[David G]

Doesn't make any sense to me yet. Maybe if you told us a little more.

What design? Link?

Fotos... or scans of the drawing detail, or instruction snippet?

[Fritz Koschmann]

Here is a link to a pdf study plan.

http://www.spirainternational.com/study/NewfieStudy.pdf

[David G]

OK, I see the detail you're referring to. It's unusual to have not only fiberlass/resin inside and out of a plywood hull, but between the two layers of plywood. I'm gonna guess that it's a way to ensure that home builders don't manage to build in voids between the sheets.

But there's really no reason to solicit speculation here. Jeff Spira is not only a very experienced designer... he's quite accessible. Simply ask him.

[James McMullen]

That is a ridiculous way to use fiberglass. You will be adding all of the weight and the cost of a layer of glass while avoiding having it be where it's strengths can be put to best use.

I also think that this particular design looks like it will incorporate all of the disadvantages of the St. Pierre dory while avoiding most of the pluses. This is not a promising study plan at all, at all!

[Breakaway]

My guess is that the designer intends to help ensure both enough glue (resin) in between the layers of ply and ensure no dry spots. Wetting out the glass properly and seeing it go clear provides a good visual prior to applying the second layer of plywood, particularly for an amateur builder more likely to be unsure about how much adhesive to use.

Kevin

[James McMullen]

David, Jeff Spira is only an experienced designer in the sense that he successfully markets lots of ultra-simplified boats to first time builders. I ain't seen a single one of Spira's designs that I would call better than maybe adequate. That particular dory is way too tall for its waterline beam. That is going to be a slow and rolly boat. Look at the successful Nexus Dory for comparison. . .

[David G]

David, Jeff Spira is only an experienced designer in the sense that he successfully markets lots of ultra-simplified boats to first time builders. I ain't seen a single one of Spira's designs that I would call better than maybe adequate. That particular dory is way too tall for its waterline beam. That is going to be a slow and rolly boat. Look at the successful Nexus Dory for comparison. . .

Yes... well... we weren't being asked to critique the design - only the construction detail. I agree that putting glass dead center in the layup is not normal, and not optimum from an engineering standpoint. However, I can easily imagine him specifying it for the reasons that Breakaway mentions, and also because he WANTS to add hull weight to keep that dory shaped hull tamed down... But, again, I'm just speculating. Best to quizz the designer.

[James McMullen]

There are ways to add weight to a bottom that are not only cheaper than an internal layer of glass but also stronger. Think of that unfortunate construction detail as a clue to how well figured out the rest of the design may be . . . .

[David G]

There are ways to add weight to a bottom that are not only cheaper than an internal layer of glass but also stronger. Think of that unfortunate construction detail as a clue to how well figured out the rest of the design may be . . . .

While I'm inclined to agree about that particular detail, I certainly haven't examined the design in enough detail (actually at all) to make a judgement. I'm guessing you haven't either, but are responding to your first impressions. The detail is certainly 'idiosyncratic' - but there may be a perfectly reasonable (if unusual) explanation.

[James McMullen]

Check out the link in post 3, David. Then guesstimate out the beam at the waterline based on likely displacement. I dare you not to get seasick yourself from this exercise. That boat will have diddly for initial stability and a wicked roll. It would not be kind or moral to Fritz to let this go by without mention. Even relatively good St. Pierres are prone to rolling, and this one has not nearly enough rocker to get the metacenter down low enough to help with squat. For the same stack of dollar bills he can do much safer.

[Fritz Koschmann]

I was more interested in the layup detail rather than the design. I am not considering building the boat for the reasons already mentioned.

[David G]

I was more interested in the layup detail rather than the design. I am not considering building the boat for the reasons already mentioned.

There ya go.

Did you get enough information, or will you be calling Spira to enquire further?

[Ian McColgin]

The plan states quite clearly:

SIDES - 2 LAYERS 3/8 PLY
FIBERGLASS BETWEEN
AND COVERING

Looks like a rational structure with good strength to weight and possibly better stiffness than the same amount of material but with the wood layers just glued together and the outside having two layers of glass. I’ve made some glass/wood/glass/wood/glass structures. No scientific testing on my part but they stood up to plenty of abuse and were nice and stiff. Especially glued over a curve, seems smart to me.

[Keith Wilson]

The glass/resin layer is stiffer and stronger than the plywood, and to get any advantage in stiffness or bending strength, it needs to be outside, not in the center. The center of the sandwich is just about unstressed in bending. It might give you some improved impact strength, but no more than putting the glass on the outside. There's no real advantage over glue alone in fastening the sheets of plywood together. I think it's a bad idea.

[pipefitter]

Laminated planking to avoid torture, glass as a filler to cancel the cost of the amount of filler it would take to keep the epoxy from squeezing out at the peak. That would work nicely. Just enough epoxy to float the glass, and end up with a uniform glue line. Might even be easier to repair in the event only one layer is compromised. Could work, and probably does work.

[James McMullen]

No scientific testing on my part. . .

Ian, if you are interested, you might look up what structural engineers who actually have done extensive scientific testing on composite structures have found out. Your alternately layered structure is both weaker and more flexible than if you'd put all of the wood on the inside and all of the FG on the outsides. This has been proven both by theory and in practice.
I encourage you not to just take my word for it, but to research the topic yourself.

[Ian McColgin]

I'm aware of the sandwich structure theory and generally go with it. I'm not really sold on the notion that a layer of glass adds much strength to plywood but it does add a bit of scuff resistance and so far as I've seen plywood with but a single layer of glass on each side is not a sandwich structure - certainly not as compared to glass structures that are balsa cored where the glass is much much thicker.

So, does the club sandwich do anything? If two lighter sandwiches are stronger than one heavier, then perhaps if the two lighter sandwiches share a skin makes some sense. But that still begs the question as to how much structural strength the glass is doing anyway. Maybe we'll here from engineers who use this method as to their rational.

[JimConlin]

I can't figure how a ply of glass in the middle of a plywood laminate adds anything but work, weight and cost. It certainly doesn't help panel stiffness or local impact resistance.

[David G]

I can't figure how a ply of glass in the middle of a plywood laminate adds anything but work, weight and cost. It certainly doesn't help panel stiffness or local impact resistance.

From an engineering standpoint - you're correct. If you are going to add a third layer of glass, the stiffness would be most improved by putting it on the inside of the hull (assuming that, in use, a boat hull gets banged on the outside more than the inside), and least improved by putting it on the outside - though an outside layer would add stiffness against flexure/impact from the inside (eg. dropping an anchor, or a mast butt, or a human jumping aboard). A middle layer provides almost nothing - as Keith notes - to the theoretical stiffness. Though, if you assume (as I do) that your typical hull gets more load from the exterior... the middle layer provides some (but still not much).

From the standpoint of designing for the backyard builder - maybe it makes some sense. As I mentioned earlier, and pipefitter reiterates, a layer of glass in the middle might help a builder regulate the amount of glue and clamping pressure. By adding the glass to the glueline, one thickens it. Thereby making it less likely that a builder will inadvertently leave voids from insufficient glue/clamping pressure. Alternately, a layer of glass makes it harder to create glue-starved areas thru too much clamping pressure.

And... if you combine that with a hull-form that tends to be a bit corky anyway, the added weight (with no great increase in stiffness) might become less a penalty than an asset.

Again... I have no idea whether this is actually what the designer was thinking, but it's one conceivable scenario. That's why I recommended discussing it directly with the designer.

[Michael Wick]

In The Gougeon Brothers on Boat Construction Appendix E, pages 394-5 gives thorough discussion of lab tests. How the location of synthetic fibers in a laminate affected resistance to impact.
"It may sometimes be desirable to laminate a layer of cloth close to the interior of the structure. Panels that had synthetic fibers located near the panel backside were substantially better in resisting impact loads than panels that had only exterior sheathing."

[Ian McColgin]

Thank you Michael.

[David G]

In The Gougeon Brothers on Boat Construction Appendix E, pages 394-5 gives thorough discussion of lab tests. How the location of synthetic fibers in a laminate affected resistance to impact.
"It may sometimes be desirable to laminate a layer of cloth close to the interior of the structure. Panels that had synthetic fibers located near the panel backside were substantially better in resisting impact loads than panels that had only exterior sheathing."

I don't have it in front of me, but this fits with my understanding as well. Both that there are sometimes reasons to include a central laminate of cloth... and that the back side of a glassed-two-sides panel resists impact better than the face (impact side). At least with fiberglass cloth - this is because a fg/epoxy matrix is stronger in tension than in compression. So it resists bending less on the impact (compression) face than on the opposite (tension) face. My understanding is that is less true with the polyester cloth (Dynel, Xynole, etc.).

What's not clear to me is to what degree a theoretical central ply of glass/epoxy would add to stiffness. Using the example from this thread of the three plys of glass surrounding and seperating two plys of 3/8" plywood... does the stiffness of the central glass ply come into play before the backside glass ply begins to take the tension load? I imagine the calculation for that issue come down to the details: how thick are the ply layers; what weight glass; how abrupt is the impact (maybe).

[dorydude]

In The Gougeon Brothers on Boat Construction Appendix E, pages 394-5 gives thorough discussion of lab tests. How the location of synthetic fibers in a laminate affected resistance to impact.
"It may sometimes be desirable to laminate a layer of cloth close to the interior of the structure. Panels that had synthetic fibers located near the panel backside were substantially better in resisting impact loads than panels that had only exterior sheathing."


True, but I think it would be just as effective to have 2 layers of 1/2" ply with no glass between.

A heck of a lot easier to build, probably cheaper and similar strength. It may be a bit heavier , but boats like this benefit from heavy bottom panels as many of the above comments suggest.

[J. Spira]

Interesting thread to read. As David G points out it does nothing, or lets say, very little to the stiffness and bending strength. That's not why it's done. Some background on my philosophy about plywood for my boats might help: http://spirainternational.com/tipsandtricks/081122_marine_plywood_boatbuilding.html So, yes, it is there to make sure the two pieces of low quality ply, perhaps even with knotholes, are well bonded together. Yes, you could do it with a single sheet, but bending 3/4 or 1" ply isn't always practical. If you were using prime marine ply, a good, well spread out thin coat of adhesive would be ample as well, without the sandwich.

Back in the early 1970s, I used to help build Radon Craft boats in Morro Bay. They were tack and tape built of knotty old lumberyard 1/2" ACX ply with lots of polyester filled glass inside and out for the commercial abalone and urchin divers. If boats were bigger than 27x9, we used two layers of ply with a layer of glass mat in between. Those boats were incredibly tough. One broke loose during a southerly storm in Port San Luis and bounced up and down on the breakwater rocks for 4 hours. The outdrive was completely trashed, but the hull was essentially undamaged. Nothing that a bit of sanding and some new paint couldn't fix. They were unbelievable. I'm sure most are still in daily use, and that was about 40 years ago.

I realize I give the oakum and caulking mallet set the vapors with many of my designs and material selections, but they're intended for first time builders who aren't sure they can build a boat. Maybe someday they'll graduate to the hand hewn, steam bent air dried English oak frame crowd. In in the meanwhile, they get a good, safe, usable boat in the water that they can be proud of with a minimum of time and expense. Many who do know what they're doing elect not to follow my material schedules, and do their own thing - that's the beauty of building a boat yourself instead of buying what someone else builds. Still others write or call and ask if they can do things in a different way. I'm always frank with them and let them know what I was thinking - like I did here. So if you want to bond two good pieces of marine ply together - sure - it's plenty strong enough. If you want to carvel plank it and caulk the seams with oakum - sure why not. There's nothing sacred about the materials I select. I do it so the average Joe who hasn't built anything since high school wood shop can get in the water at minimal expense.

[James McMullen]

I do it so the average Joe who hasn't built anything since high school wood shop can get in the water at minimal expense.

Yes. That seems to sum it entirely. Ease of building over any other consideration. Gives me the vapors, though.