Wood/epoxy composite moisture movement

[jmwoodring]

I've been reading some books on cold molded construction and came across some possibly conflicting information that I'd like to discuss here.

Reuel Parker advocates for the use of larger, single timbers vs laminations in structures such as the stem, horn timber, etc as the construction is more efficient and cost effective. He suggests polyester or Xnole sheathing to cover any exterior wood as this material is much more flexible than glass.

In Sam Devlin's book on the same topic, he argues that, even if sealed in epoxy, any dimensional lumber significantly larger than 1/2" can swell and overcome the elasticity of the epoxy, resulting in cracks and failures. He advocates soley for laminations of plywood for any members larger than 1/2".

Perhaps this has been discussed here before, but do you agree or disagree with either of these arguments? While I tend to agree with Devlin's statements (water is insidious), Parker's advice seems practical. What are your thoughts/experience?

 

[Fellies]

I suppose if you use a bigger piece of timber without laminates inbetween the expansion would affect the layup holding it down and would lead to cracking on both polyester and epoxy. You would need a stronger and thicker layup to avoid the expansion so might aswell use epoxy to make it lighter.

Water ingress would be coming in regardless on a layup thats snapping and isn't thick enough or strong enough. I would use layers in between and Personally would never use polyester over timber. The polyester is most definitely snapping if the timber expands regardless of whether it looks like or not.

If its pre coatings you talking about as in a primer on the wood, there is almost no ways polyester would be less porous than epoxy. Pulling a vac bag and holding it for 20 or 30 minutes to get air voids out then pulling resin through would be the best solution on the part, if its a part and not on the boat.

Polyester is generally terrible on any timber.

Those are just my thoughts.

 

[fallguy]

Both men are correct, but encapsulating large timbers is certainly a challenge.

I built sacrificial wooden shoes for my beaching keels. They are 3 pieces of 1/2” ash about 4” wide by 12 feet long. They were bonded with epoxy.

Then, despite them being sacrificial, I decided to extend their life by laminating them with a single layer of 1708 on exposed edges. The obvious failure point was on the sharp edge because I did not want to radius the sides.

Big mistake. Water managed to find a way into the wood via the edges and the wood swelled and basically exploded dimensionally and now the entire laminate is looking somewhat compromised. I suspect I will remove anything loose on haulout and leave it to continue to its demise. The shoes are a horrible idea as they add a ton of drag I suspect removing them could yield an extra knot top speed, although my boat rises up on the top end, perhaps skating on them to some degree.

And.

very little ply on my boat, I decided to use plywood on two aft locker tops for downrigger backing

One of the lockers filled to the top with rainwater before I installed the drains and the plywood; despite epoxy, but no glass swelled and exploded in situ; requiring repairs after it dried out.

I also have a stitch n glue Gilpatrick designed canoe. This boat has performed well, a few times, the wood got wet after skin damage and swelled, but the wood is so thin at 3/16” or less, that it simply has little expansion capacity.

The short answer is putting an epoxy/glass skin on wood is risky.

It is the reason we have products like coosa bw26.

The best approach is to avoid wood.

If you decide to use it, encapsulation must be perfect and include skins; otherwise don’t.

Just to add, what difference is 3 half inch pieces of ash bonded to make 1.5” from a 1.5” piece if water hits either? The answer is the 1.5” glue up has a greater potential to fail.

 

[Fellies]

I suppose if you use a bigger piece of timber without laminates inbetween the expansion would affect the layup holding it down and would lead to cracking on both polyester and epoxy. You would need a stronger and thicker layup to avoid the expansion so might aswell use epoxy to make it lighter.

Water ingress would be coming in regardless on a layup thats snapping and isn't thick enough or strong enough. I would use layers in between and Personally would never use polyester over timber. The polyester is most definitely snapping if the timber expands regardless of whether it looks like or not.

If its pre coatings you talking about as in a primer on the wood, there is almost no ways polyester would be less porous than epoxy. Pulling a vac bag and holding it for 20 or 30 minutes to get air voids out then pulling resin through would be the best solution on the part, if its a part and not on the boat.

Polyester is generally terrible on any timber.

Those are just my thoughts.

I think the second guy you mentioned Sam sounds like a better option. I actually find it nuts that someone would use polyester on a large piece of wood to specifically cater for the expansion of it.

 

[jmwoodring]

I think the second guy you mentioned Sam sounds like a better option. I actually find it nuts that someone would use polyester on a large piece of wood to specifically cater for the expansion of it.

Sorry for the confusion. The thread is meant to be about epoxy sheathing only. Reuel Parker advocates using a polyester fabric, also known as Xynole, instead of fiberglass.
Xynole 4oz x 55" https://fiberglasssupplydepot.com/product/Xynole-4oz-x-55.html He likes it's flexibility and workability, and it won't make you itch!

So we can dispense with the discussion of polyester resins.

Regardless, these are two excellent books, both:

Devlin's Boat Building Manual: How to Build Your Boat the Stitch-and-Glue Way, Second Edition: DEVLIN: 9781260467673: Amazon.com: Books

Devlin's Boat Building Manual: How to Build Your Boat the Stitch-and-Glue Way, Second Edition [DEVLIN] on Amazon.com. *FREE* shipping on qualifying offers. Devlin's Boat Building Manual: How to Build Your Boat the Stitch-and-Glue Way, Second Edition

www.amazon.com

https://www.amazon.com/New-Cold-Molded-Boatbuilding-Lofting-Launching/dp/0937822892

Mostly, I am curious to know of everyone's experiences here. Of course, much depends on the situation, materials, and professional application of products.

With smaller boats and scantlings, it's probably not much of an issue, but I'm curious to know more about larger boats and dimensional lumber pieces. Should large pieces solely be laminated from smaller, more dimensionally stable stock like marine plywood? Or can dry timber and enough epoxy properly applied keep moisture at bay? Is this just an inherent flaw in cold molded boatbuilding that must be accounted for somehow?

 

[jmwoodring]

Both men are correct, but encapsulating large timbers is certainly a challenge.

I built sacrificial wooden shoes for my beaching keels. They are 3 pieces of 1/2” ash about 4” wide by 12 feet long. They were bonded with epoxy.

Then, despite them being sacrificial, I decided to extend their life by laminating them with a single layer of 1708 on exposed edges. The obvious failure point was on the sharp edge because I did not want to radius the sides.

Big mistake. Water managed to find a way into the wood via the edges and the wood swelled and basically exploded dimensionally and now the entire laminate is looking somewhat compromised. I suspect I will remove anything loose on haulout and leave it to continue to its demise. The shoes are a horrible idea as they add a ton of drag I suspect removing them could yield an extra knot top speed, although my boat rises up on the top end, perhaps skating on them to some degree.

And.

very little ply on my boat, I decided to use plywood on two aft locker tops for downrigger backing

One of the lockers filled to the top with rainwater before I installed the drains and the plywood; despite epoxy, but no glass swelled and exploded in situ; requiring repairs after it dried out.

I also have a stitch n glue Gilpatrick designed canoe. This boat has performed well, a few times, the wood got wet after skin damage and swelled, but the wood is so thin at 3/16” or less, that it simply has little expansion capacity.

The short answer is putting an epoxy/glass skin on wood is risky.

It is the reason we have products like coosa bw26.

The best approach is to avoid wood.

If you decide to use it, encapsulation must be perfect and include skins; otherwise don’t.

Just to add, what difference is 3 half inch pieces of ash bonded to make 1.5” from a 1.5” piece if water hits either? The answer is the 1.5” glue up has a greater potential to fail.

Good example. Constant submersion vs occasional vs deck/uv exposure are all different use cases and failure modes.

I did find this linked from another thread, that discusses the concept of wood epoxy bonds and their limitations:

Are epoxy-wood bonds durable enough? | US Forest Service Research and Development

research.fs.usda.gov

I would welcome any recent or current publications on the best practices of wood/epoxy composite boatbuilding as well as any builders currently working with the medium.

 

[Fellies]

Ok im with you, the fabric is polyester. I just dont understand why someone would sheath wood to account for its expansion when the epoxy itself would snap long before the flexibility of that fabric. It structurally makes no sense. If the resin crosslinks, its not an elastic adhesive to cater for expansion.

To veer off the topic slightly, Ive never had such a problem with outgassing woods like Okume if you talking about bonding 2 ply wood on the hull.. On larger jigs, epoxy with peeply ply or a tight screet of micro fibre/silica epoxy once its bent on the jig to seal off pores or pockets for a secondrey bond works fine if. On the flats you could just pre infuse the wood with resin for with a 20 minute pre hold on the bag to get as much air out and then pull resin. second layers glued with similar epoxy mix. Ive put glass in between the wood on bigger flat pieces to try to completely avoid the expansion with a bag which is fully my aim. Its counter intuitive to use a fabric to allow more expansion and micro cracking on the resin to avoid water ingress surely?

 

[Rumars]

What's confusing you is the loose terminology that people use. You are actually referring to different problems that are somewhat related.
All boatbuilding glues are stronger then the wood, the failure is usually at the glue/wood interface or directly next to it on the wood side. All woods move somewhat with moisture, some more then others. Depending on species, grain orientation, etc. there is an upper thickness limit to what the glue can hold together. You can either use thinner wood layers or prevent water absorption by coating the wood. The coating has to have a minimum thickness to work, that's why you see the usual recommendation of three applications of unreinforced epoxy. Inhibiting water absorption also has the side benefit of allowing non rot resistant woods to be used in applications they would otherwise not be suited for.

Everything else has to do with other things, not specifically connected to this two topics. Thin laminations are used often because the piece is curved and the wood would not bend otherwise. Fiberglass and other reinforcements are often used for structure or abrasion purposes. What strategy you use has more to do with this other things then the actual glueing strenght or waterproofing ability of a certain epoxy formulation.

I have yet to see a test demonstrating that epoxy reinforced with a polyester cloth like Xynole is more flexible then the equivalent amount of epoxy reinforced with fiberglass or whatever else. Thing is, any polyester cloth and Xynole especially needs way more epoxy then fiberglass of the same dry cloth weight.

 

[fallguy]

The idea anyone would build a stem and think coating it with 3 coats of epoxy would prevent ingress borders on humor. No disrespect to Rumars; his contributions here are stellar.

Consider all the failure modes or a few:

1. Damaged in handling/install.
2. Fastener pathways for ingress.
3. Post perfect install and handling, rubbing at sea through a 1 mm (at best) coating is relatively likely.
4. During the course of other work while vessel is in use; bumping the part is easy.
5. UV. No uv coating is another way to destoy the 3 layers.

And this is a 2 minute off the cuff rundown.

The best way to use wood and epoxy is thin okume ply in fully encapsulated one-piece hulls.

In plank on frame construction; one must be careful using epoxy because sometimes it can be your enemy entrapping water.

 

[Rumars]

The idea anyone would build a stem and think coating it with 3 coats of epoxy would prevent ingress borders on humor. No disrespect to Rumars; his contributions here are stellar.

That's exactly why I said the strategy to use depends more on other factors then what the epoxy can actually do. For the outside of a stem there are roughly two main scenarios: one is to use a stable rot resistant wood in thin laminations with minimal epoxy over it. If the epoxy coating gets damaged the piece won't instantly rot, split or loose strength. The other way is to make sure the coating survives impact and abrasion, so you use a thick reinforced overlay (can be fiberglass but it can just as well be a metal band), in wich case it doesn't really matter how the stem is buildt.
Boatbuilding should ideally be an integrated system, but this is often not the case because designers prioritize different aspects and some simply don't bother with advanced optimization. In an ideal world a thick reinforced epoxy coating for abrasion or impact should also be calculated to contribute to strength instead of beeing considered dead weight.
I praxis there are a plethora of systems combining elements from several different boatbuilding technologies all running more or less under the same common names, all with different degrees of success. Often it's very difficult to determine why a certain combination worked in one boat and failed in the next.

As I said I don't regard polyester cloth to be better then fiberglass when combined with epoxy, and I wouldn't use it. Until I see some hard data I consider the "flexibility" argument to be just marketing.
By now we all know the basic factors of sheating already buildt (aka. old and tired) traditional plank on frame. Dry the wood, stabilize the construction by glueing the planks together, make sure the framing is solid and soundly atrached, etc. It doesn't matter if the sheating is epoxy, polyester or ferrocement, the basics remain the same. But success is also a matter of perspective, let's say I do a job with a heavy fiberglass skin set with polyester resin while disregarding all of the above. It's probably going to cost the same as a proper epoxy job because I'll have to use three times more of the cheaper resin and more glass. The difference is that I can do it right now and it takes me a week or so, instead of waiting three years for the wood to dry then do all the repairs, etc. So the rushed job gives me 10-20 years before a complete rebuild is in order, and if that's all the lifespan I want it's actually the better option.

 

[Fellies]

What question is this aimed at?

- Pre coating the wood to avoid outgassing
- Secondary bonding for the wood
- Replacing a structural layup with this polyester fabric instead of Eglass
- using it for non structural parts in thin layers to account for expansion ( as in spraying clear alwcraft parts )

 

[jmwoodring]

That's exactly why I said the strategy to use depends more on other factors then what the epoxy can actually do. For the outside of a stem there are roughly two main scenarios: one is to use a stable rot resistant wood in thin laminations with minimal epoxy over it. If the epoxy coating gets damaged the piece won't instantly rot, split or loose strength. The other way is to make sure the coating survives impact and abrasion, so you use a thick reinforced overlay (can be fiberglass but it can just as well be a metal band), in wich case it doesn't really matter how the stem is buildt.
Boatbuilding should ideally be an integrated system, but this is often not the case because designers prioritize different aspects and some simply don't bother with advanced optimization. In an ideal world a thick reinforced epoxy coating for abrasion or impact should also be calculated to contribute to strength instead of beeing considered dead weight.
I praxis there are a plethora of systems combining elements from several different boatbuilding technologies all running more or less under the same common names, all with different degrees of success. Often it's very difficult to determine why a certain combination worked in one boat and failed in the next.

Well said. That's sort of the essence of this exploration. Parker, Devlin, Gougeon brothers have all written excellent books on the topic, but I am curious about some of the differences I have found in their methods. My hope was to learn from the experience of those here.

As I said I don't regard polyester cloth to be better then fiberglass when combined with epoxy, and I wouldn't use it. Until I see some hard data I consider the "flexibility" argument to be just marketing.
By now we all know the basic factors of sheating already buildt (aka. old and tired) traditional plank on frame. Dry the wood, stabilize the construction by glueing the planks together, make sure the framing is solid and soundly atrached, etc. It doesn't matter if the sheating is epoxy, polyester or ferrocement, the basics remain the same. But success is also a matter of perspective, let's say I do a job with a heavy fiberglass skin set with polyester resin while disregarding all of the above. It's probably going to cost the same as a proper epoxy job because I'll have to use three times more of the cheaper resin and more glass. The difference is that I can do it right now and it takes me a week or so, instead of waiting three years for the wood to dry then do all the repairs, etc. So the rushed job gives me 10-20 years before a complete rebuild is in order, and if that's all the lifespan I want it's actually the better option.

I don't really have a dog in this argument, just trying to learn and think here.

In his book, Parker suggests that, compared to fiberglass, Xynole fabric:
-has higher tensile strength per unit weight
-better abrasion resistance
-not as stiff or brittle when epoxy impregnated
-wets out more rapidly and thoroughly
-higher peel strength
-higher impact resistance
-conforms easily to tight curves/shapes
-more expensive
Overall, he suggests it is more compatible with wood than fiberglass, because of it's flexibility and higher tensile strength, allowing it to avoid cracking. He offers his anecdotal experience as evidence of this.

In another thread, PAR (of legend) also suggested these properties of Xynole fabric, though whether he got this data from rigorous testing, sale brochures, or his own experience, I don't know.

sheathing BRS 17 with Xynole - how much epoxy

I am getting ready to order epoxy to sheath my hull with xynole. I've never used Xynole. Does anyone have a formula for determining how much epoxy it will take?

messing-about.com

 

[jmwoodring]

What question is this aimed at?

- Pre coating the wood to avoid outgassing
- Secondary bonding for the wood
- Replacing a structural layup with this polyester fabric instead of Eglass
- using it for non structural parts in thin layers to account for expansion ( as in spraying clear alwcraft parts )

I have been reading some different works on "cold molded" boatbuilding. As Rumars pointed out, there are a variety of similar boatbuilding "systems" that vary in nuance.

My question was mostly about the tradeoffs between using solid dimensional lumber vs smaller, more dimensionally stable laminations in boats with larger scantlings. In his book, Devlin describes some failures in some of his earlier boats and his subsequent choice to design all parts as laminations of marine ply. Parker on the other hand, is a proponent of using larger sawn lumber in places where it makes sense. It would seem that his confidence in the properties of the Xynole fabric dovetails with that philosophy.

 

[Fellies]

Maybe im not with the program. The last cold moulded project

- I infused the pieces with a bag and a prior 20 minute hold before pulling resin. Is this the sheathing you talking about. Sealing off the wood before use?
- Screwed the first layers onto the jig ( 12 mm ) Glued the peices then from the inside of the jig so I could remove screws and filled them with an epoxy mix
- Vacuum bagged e glass on and pulled as much air out on the breather
- Drilled holes in the second 12mm okume peices, the grain facing opposing still to keel and pulled an epoxy mix with a vacuum bag for the second layer of okume
- Sealed it off with epoxy and then vacuum bagged the outer layer wiht an inital small primer in the first layer of resin
- Flipped hull and vacuumed inner skins, buklheads and stringers.

I wouldnt lay one thing without a vac bag or insfuse it, which i couldnt over a jig, maybe your point is from a more traditional cold moulding process. Seeing the notable difference from Okume to traditional marine plies is also a huge issue with regard to peel strength. The quality of the wood and the glue in it is the main issue if you ask me. This polyester fabric seems like some sort of thin aramid I just cant understand why someone would market a fabric with such flex for the sole reason of expansion. Even if i were to use thinner okume on the hull i would do the same process of a much thinner barrier to seal it off priort to even using a layer of glass on the jig. If I to did wooden stringers, even if though the wood is acting as a former i would still cut it and run uni carbons down the middle, bag it and then encapsulate it for that specific expansion problem.

Everything shift and expands slightly however on certain parts thats fine if the parts itself moved with an elastic adhesive, within structural reason. Surely the main reason is to be able to compensate structurally with a strong layup and a build process to accomodate for minimal expansion, considering micro fractures and snapping on the resin is the direct cause to water ingress

 

[Fellies]

I have been reading some different works on "cold molded" boatbuilding. As Rumars pointed out, there are a variety of similar boatbuilding "systems" that vary in nuance.

My question was mostly about the tradeoffs between using solid dimensional lumber vs smaller, more dimensionally stable laminations in boats with larger scantlings. In his book, Devlin describes some failures in some of his earlier boats and his subsequent choice to design all parts as laminations of marine ply. Parker on the other hand, is a proponent of using larger sawn lumber in places where it makes sense. It would seem that his confidence in the properties of the Xynole fabric dovetails with that philosophy.

It seems like a strange trade off still as the expansion of the wood ruins the part regardless of what fabric you use, Just my Opinion