Laminating two sheets of plywood with layer of Kevlar between

Just wondering if there would be issues with sheathing a hull like this.

On a Wharram type vee hull with vmoderate curve along length of hull that plywood easily follows the curve.

Usually sheathed with 9mm or sometimes 12mm plywood.
Sheath hull with 4mm ply then epoxy 5oz Kevlar, after epoxy gets tacky apply 9mm ply.

Would apply enough epoxy over Kevlar to fill weave and pre coat the 9mm plywood, then use screws with 2” blocks as big washers starting from middle and using working outwards while using roller to help press air bubbles out as putting screws in.

Reason for thinking about this is to have Kevlar away from impact on exterior of hull.

In Gougeons book said a layer of plywood over Kevlar added significantly to puncture resistance.
Don’t remember the specifics of increase, but was noticeable increase, something about preventing Kevlar from shattering & delaminating.

Mostly just curious if a layer of 9mm could be laminated over Kevlar sheathed plywood.

The Kevlar and 9mm plywood would be applied in basically same operation.
Soon as Kevlar epoxy went off and could apply the next coat of epoxy, epoxy coated plywood.

Would put thin coat of epoxy on 4mm plywood, let get tacky, then lay Kevlar down,
squeegee down Kevlar since it tends to float on epoxy.

Or possibly put Kevlar down dry, apply epoxy squeegee it down to wet out cloth, roll on another coat of epoxy, then lay 9mm plywood with wet coat of epoxy down and screw & roll it?

Not building at moment, currently building Gary Dierking Ulula Nui 28’ double hull sailing canoe.
Just thinking about this.

Weight of the two layers of plywood isn’t an issue, that will be within perimeters.

Cheers, Allen

 

The Kevlar layer being the puncture resistant layer, I’d want it on the outside of the hull, as a first defense of the plywood core.

 

Kevlar is strong in tension but not so much in compression.
That is reason puncture resistant material is put on side away from impact.

The outside will get two layers of glass and one layer of Xynole up to above water line a ways and lapped seams so two layers along keel.

Cheers, Allen

 

You can't apply a thicker skin over a thinner one because the shape won't be the same. The thinner sheet will have "hard spots" - areas of high curvature where it lies over frames. The thicker ply will have a more uniform curvature over the span of frames and won't meet the thinner inner sheathing between frames. You would have to built the entire layup flat on a laminating table if you wanted that layup. It simply won't work as you described.

 

Well thats why I thought I would ask. I didn't think of that, but it certainly makes sense.

If pre-fitted both thicknesses, predrilled screw holes & make alignment holes, then put kevlar on thinner sheet and then the thicker sheet, rolled them together with roller, then put them on hull and screwed them from center out while epoxy is green. That would work correct?
Actually one set at a time on a flat table and using roller to remove bubbles then attaching them to hull in one piece would have advantages.

 

Why do you want the kevlar between two sheets of ply in the first place? Is there some desire for a varnished plywood interior?
Normally you build the plywood hull then laminate the kevlar on the inside. If the kevlar layer has to be continuous you glue in the bulkheads afterwards. If you wish a wooden look you fair the kevlar, then veneer over using vacuum.

 

My first and immediate thought is first of all; the 4mm will lose shape when you try to bond 9mm to it. @philSweet explained it well..

But then reversing and laying the 9mm first, then the kevlar is not without challenges and Phil sort of say so.

The problem is you simply will not get sure bonding and gets lots of voids. The pressure exerted by the wooden washer will create a pucker in tbe 4mm panel. The amount of bonding paste required is also very high.

There is kevlar on my Skoota.

I put one 6" wide piece buried in the beaching keel laminate and the beaching keel also has about a 1" sacrificial timber.

The idea is just impractical on a large scale.

Glass inside a couple pieces of wood can be done and is done on smaller scale to ensure bond thickness or to add a layer of safety to prevent a reef hole, etc.

You would possibly be able to lay a piece of kevlar along the bottom of the hull on a keel and bury it under a couple layers of 1708 and hope it is never needed. If it frays; it'll be a pain to fix.

 

If Kevlar is sandwiched between two sheets of plywood (inside plywood doesn’t need to be same thickness as outer) the inside plywood helps the Kevlar from shattering and delaminating from outer layer.

Don’t have the testing results handy, got them saved somewhere.
Tests were done to see what makes a plywood panel more puncture resistant.

Plywood then Kevlar and second layer (can be thinner than outer layer) had substantially higher puncture resistance vs plywood with layer of Kevlar only on inside.

The plywood joints get 6” wide butt blocks on inside, this will tie the two panels of Kevlar together.

one option is to put Kevlar on each sheet of plywood, wait till tacky, then glue plywood/Kevlar to stringers, putting screws ever 6-8” with 2” plywood ’washers’, when cured remove screws, fill holes and sheath like usual.

Or if can laminate the two sheets of plywood with matched screw holes drilled with Kevlar between them, on flat table, that makes it simple to use weighted roller to expel lair bubbles. Few small screws to hold panels together as setting panel laminate on hull, then screw starting in center & working outward.

The inside of 4mm plywood will be coated with multiple coats of epoxy and sanded, ready for varnish or paint.

Sure beats trying to put multiple coats of epoxy & to seal plywood when vertical and down low in hull and sanding.

 

Ok it looks like my idea isn’t a practical way of doing this.

That’s why I asked to see if my idea was flawed, which it is.

It isn’t critical to have layer over the Kevlar. If it was practical to do then it would be nice to have a bit of additional puncture resistance added.

Just laminating layer of Kevlar on plywood before attaching plywood to hull looks to as good as practicable.

I appreciate the input and knowledge shared.

 

Laminating two ply panels 4'x8' with a piece of kevlar or aramid is certainly possible, but the bending character of the finished panel will change greatly. The ideal bond thickness is about 1mm which is a LOT of bonding paste and best results would be under vaccum of about 9-10 " Hg. I've found thru some testing that a 1/16" vee trowel is about right and applied to each surface; or about 4x8/(12x16) or 1/6 cuft of bonding paste. This paste is applied after a neat coat of epoxy is applied to each surface to avoid the plywood starving the joint. In addition, the kevlar would also be in the joint and using resins.

You could probably try to use a resin rich laminate and get away with just the kevlar, but then the bending character is still uncertain.

A 4mm and a 9mm ply panel sandwiched with kevlar is not going to bend easier than 13mm plywood. It would require testing, but it will be something more like 15-18mm plywood.

And the process would require vac and take basically about a day per panel for most people.

For most builders, it becomes an issue of effort vs reward and reward simply not there as a 15mm panel has lots of puncture resistance...

 

If, testing reveals a layer of fabric has a puncture resistance equal to say 3mm of plywood, then a layer of fabric added between two sheets of 3mm ply makes it into a theoretical 9mm panel or an improvement of 9/6% or 50%. But a 9mm and 4mm panel end up equal to a 16mm panel which is only 16/13% improvement or more like 23% only. And, if this value is lower like 2mm of plywood, the improvements are far less...

The risk/cost and return or reward are suspect even for exceptionally skilled laminators.

 

If the option has now been considered impractical,which it is, why not simply glass inside and outside the single thickness plywood skin? It will be bad enough to achieve a good finish on the outside because it always is a miserable job but you will have no doubts about possible voids in the basic plywood-beyond those that came from the factory. The inner glass skin doesn't have to be brought to a high finish and adds the strength,just as the inner skin of a stripper canoe does. Avoiding Kevlar is a good thing in most instances and I had thought it was hardly used these days due to the problems of it going fuzzy and needing special shears to cut.

 

Yes this was an idea that I thought I would explore by asking about the realistic practicalities of it.

Turns out it’s not a good idea.

One other question…
Would it be practical to epoxy Kevlar on a sheet of 9 or 12mm plywood and put it on hull while green, let epoxy go off enough to hold Kevlar in place while setting and screwing plywood to stringers?

Didn’t know if the Kevlar would make panel stiff even when epoxy has just started to firm up or not.

Probably better to use (wiser?) to just use 12mm plywood and call it good.

Maximizing puncture resistance isn’t a big concern.
If there was a relatively straightforward was to add layer of Kevlar on inside, would be nice. But not necessary whatever benefit gained wouldn’t be worth the effort.

Will just use 12mm instead of 9mm and call it good.

 

If minimising puncture resistance isn't a big concern, leave Kevlar out of your thoughts because that is it's only advantage. It is harder to cut either dry or laminated than glass.As for laminating any material onto a boat length panel and then adding it to the hull,have you ever manouvered a panel of that size? Not only that but you will need to do it with glue curing as you try to get it located and fixed.

 

For sure..puncturing 9mm plywood is actually going to require a massive impact and kevlar is not likely going to help.

The hour is late, but the elongation of kevlar at break is roughly 3%. Anything penetrating the 9mm ply is elongating the kevlar past its breaking point. So it breaks into the hairy mess..