hull strength

[Howard]

I thought the same thing regarding epoxy. And it seems like all the rot problem from absorption would be below the waterline. So would it be possible to just use something like CCA or treated plywood below the waterline and if it gets wet so what?

Otherwise, maybe you do a hybred boat. Maybe use 1/4" or less plywood skin as a form for below the waterline and put a 1/2" or so of glass over that, and use coated plywood above the waterline?

I know that even with fiberglass boats, they routinely core decks and topsides, but seldom core the hull. If they do, they are using something like foam that doesn't absorb water.

And again, I'm thinking of all this for a boat that is going to stay in the water. Not be dry sailed. For a trailerd boat, I don't see the water infiltration as being a problem.

[Frank Hagan]

Actually e[poxy is far more resistant to water absorption than polyester resin or even gel coat is by a long shot.

The way I have heard it' date= the epoxy resin itself is not as hydroscopic as a polyester resin, but it will allow water to pass through it easier ("hydroscopic" meaning that the water infiltrates and stays in the resin). So a certain film thickness has to be built up with the epoxy to counteract its greater tendency to allow water to pass through.

[edited to add the below info 7:55am]

I found one of the studies I based my belief on, and its clear that the epoxy resin absorbed more water. However, Ray is right in that the water does damage to the polyester resins, whereas it doesn't cause as much damage to the epoxy. The attached chart shows the absorption, but the water is at 50

[Frank Hagan]

The full URL to the study I snagged the chart from is at http://www.coe.montana.edu/composites/documents/18%202000AIAA%20Selection%20of%20Resins.pdf. Its a study of composites to be used for turbine fan blades, but the water absorption charts are the same for other studies (the link I gave above has simplified language in it, and they include a similar chart).

Polyester body filler includes talc, which is hydroscopic, so it will fail in contact with water. Straight polyester resin, which absorbs just slightly less water than epoxy, is also degraded by the water. The epoxy is not degraded as much. There's a reaction within the cells of the polyester resin that leads to blistering when water permeates the resin, and with epoxy, that reaction doesn't happen.

So why does a barrier coat work? Its a thin coating over the more susceptible polyester resin, so you would expect the water to go through the epoxy and still affect the polyester underneath. That's why I wonder if the elevated temperature for the testing is softening the epoxy (122

[PAR]

Frank and Ray, I saw the SP study when it was first released and it reaffirms what is already known about epoxy, polyester and vinylester.

This is the second sentence under the heading "Mechanical Properties"

" After a cure period of seven days at room temperature it can be seen that a typical epoxy will have higher properties than a typical polyester and vinyl ester for both strength and stiffness. "

That same section goes on to describe why epoxy is less likely to shrink during cure, compared to the 'esters'

The next section is about when a laminate reaches the load limit in stress. Just before failure, cracking can be seen in the laminate. The section title is "Micro-Cracking" and the diagram shows epoxy a 2:1 advantage over it's best 'ester' counterpart.

The next section is "Fatigue Resistance" this is the third sentence;

"As a result, epoxy based laminates tend to show very good fatigue resistance when compared with both polyester and vinyl ester, this being one of the main reasons for their use in aircraft structures."

"Degradation from Water Ingress" is the next section and the one of interest here.

"An important property of any resin, particularly in a marine environment, is its ability to withstand degradation from water ingress. All resins will absorb some moisture, adding to a laminate

[Howard]

So as relates to the original question, is enough water going to get through an epoxy coat to do damage to the underlying plywood, if the boat is left in the water year round?

I also notice that West Marine is now selling what I think is a micro-plate epoxy additive....which maybe the same stuff Interlux puts in their 2000E product.

Perhaps a thicker coat, with xynole cloth and these type of additives in the barrier coats does the trick?

Seems to be the ultimate goal is the ability of the boat to make contact with things in the water (glancing blows....not direct hits on unmoveable objects), including occasional groundings, and still have a skin surface that will survive that and protect from moisture intrusion at the same time. I'd actually be less concerned about hitting something than moisture working it's way through the small scratches and nicks that happen over time.

[Frank Hagan]

Well, the rule of thumb I have always heard used for amature boat builders is that if you put on a thick enough coat of epoxy to fill the weave of the glass cloth, then you won't have problems with water penetrating through the epoxy.

[Howard]

I submitted the same question to System 3 and they replied this morning. They say it's a non-issue and to not even worry about it. That boats several decades old are still going and they know of no problems with plywood rot in epoxy coated plywood boats left in the water, so long as they were done right to begin with. And that didn't mean 1/2" of epoxy...just the appropriate schedule for the boat.

Sounds like they agree with Frank. Also sounds like I won't be able to use it as an excuse to not build a boat.

[PAR]

There are a number of things that can be used as reinforcement in epoxy, which will increase the water tightness of the material. Aluminum oxide is a common additive as is graphite powder, both increase the hardness of the matrix, thus helping the moisture intrusion, plus additional benefits of a fire retardant or slippery finish.

In most applications over plywood, the real benefit is gained in maintaining the coating, not making it harder. A breached hard coating isn't going to help anything. A quickly fixed breach in a thin sheathing of an epoxy/cloth matrix will serve the underlayment better then a thick skin that has dings and nicks that go unrepaired. Using enough to level the weave in the cloth is more then enough protection, for the average boat. Work boats will want much more protection from it's sheathing as will craft needing service in a rough environment. These are questions that can be only answered by the owners.

Moored/docked vessels have a more difficult life to live. It will be quite some time before breaches in the coating will be noticed, let alone repaired. This is the crux of berthed craft and typically why they have heavier planking schedules.

West System has a barrier coat additive (420 I think), though I don't know what the material is, the gray color suggests it's little more then aluminum powder. It's intended for repair work over blistered poly hulls. The current method of repair on a badly blistered hull, requires the removal of the gel coat and the outer laminations of the cloth/mat in poly hulls. This surface is then built back up with epoxy mat/cloth, then faired. Insurance companies will not accept any other type of repair, when the blisters cover a certain percentage of hull. This should give a good indication of the expected results shown towards the epoxy use.

As usual, it carries down to how good the prep was performed. Did the cloth float, was it under-saturated in areas, did the blush get washed/abated off in over coats, resin content too high/low, etc. Most sheathing failures I've seen can be found to be bad prep or neglect in maintenance.

[Guest]

Par, I think a lot of hysteria is being created with the issue of epoxy and water intrusion on this board. I can speak about both big and small boats when working with three types of resin. Yes there are some cases that laminating with vinylester may have its advantages, but most people on this board work by themselves, with laminated wooden cores, and just one issue needs to have access to a product that is being sold at the least in local and marine supply outlets catering to the retail customer. Most, if not all are trailer boats, with limited use as casual toys, where any problem is pretty limited for most here. Most all resin is also topcoated unlike a lot of polyester and vinylester boats with parts of the hulls bare for its life, sitting out of the water in most cases. I will leave it at that for now. If you have some thoughts that you would like to address with me, or feel a bit offended with my post, feel free to do so by open posting or by the PM feature. This is just my personal read from afar.

[Frank Hagan]

Mike, I didn't see any hysteria over the issue of water and epoxy ... just took another look to see if anyone was strident or out of sorts over the issue in this thread and didn't see it. But if anyone was offended by the discussion, I apologize for not catching it.

I think what we have all agreed on (mostly) is that epoxy isn't absolutely waterproof, and may in fact absorb more water than polyester at first. But its a moot point really, because the composition of the epoxy makes any water absorption much less an issue than with polyester resins.

In the end, its "builder's choice", and everyone should feel OK with taking or rejecting advice given here.

[PAR]

My point in posting was in hope to draw down the excitement over epoxy and other resins moisture absorbsion rates. Statistics and limited testing or studies can be misleading. I think the idea of moisture absorbsion in epoxy is misleading, for the most part. At least when it refers to the structures being built in all but the most controlled environments. Some, rather high end manufacturing facilities, will require closely monitoring absorbsion rates, but 99% of the epoxy users will not know if they have moisture gain.

A well bonded coated of good quality paint will go a long way to relieving any worries, but there are other options if so desired. Personally I rarely use them (for absorbsion reasons), though do use aluminum powder on enclosed engine spaces that see epoxy or graphite on the bottom of racers where the rules permit. I truly hope I've not added to any hysteria.

[Guest]

Not a problem with me . I just went back over the thread and read some real concerns, again, beginning with the provided post, in particular, on the second page. There are some issues with the rule of thumb of encapsulation of plywood hulls with some people, not pointing towards PAR at all] that have not experienced this technigue over more than just a plywood decks, in particular on boats of any age on them across the net.

Building cold moulded hulls, and even skin coating hulls, in backyard settings, quality control is the number one issue, combining a bit of knowledge with costly materials with limited knowledge of glasswork before tackling these hulls. Speaking from my personal experience, horror shows have been created when building with "ester" projects by limited ability folks,. This is not a put down of anyone, but more of a realist when projects are steered towards the "ester" materials.

Keep in mind, this board in particular caters to two different building tedchnigues with the Weekender and most of Grahamn's hulls incorporating epoxy and glass as its main ingredient, with the wood aas a form, for lack of a better description, unlike coating the more traditional framed hulls[used lightly] as the Weekenders which has long been built with skinning being a sidenote and folks can get by with most "ester" products. Hope this clears up my angle of the read in print. Regards to one and all.

Howard

Joined: 06 Dec 2004

Posts: 49

Location: Missouri

Posted: Tue Apr 19, 2005 7:44 pm Download Post Post subject:

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Having read all the posts on this thread a couple times, I have a question about plywood. They say old fiberglass boats get heavier over time due to water absorption. How well would a plywood boat hold up with a skin coat of glass cloth/epoxy, if the boat is left in the water year round? Will it absorb enough moisture through the skin to wet the plywood and start trouble? I don't see plywood being a problem on a trailer sailed boat that is only in the water for a few days or weeks at a time. But year round?

Follow the posts on the second page, combining the issues of water absortion through the epoxy and this is just my read. Without cherry picking and placing them in order, lets read just one comment:

In other words, a hundred pounds of cured epoxy may pick up a few more ounces of water they the 'esters' but it will not cause the laminate to break down like it will in 'esters'. So epoxy's absorbsion ability is really a moot point at the absorbsion levels and expected reactions we're talking about.

It boils down to epoxy having reliable results (which includes some absorbsion) whereas the 'ester's will stave off the moisture, for a short while better then epoxy, then start to break down, letting in more and more moisture, until failure or repair. This is how epoxy fixes the blisters. It provides a stable Band-Aid over the area, that doesn't change it's properties in contact with moisture.

[Howard]

I'm not sure I understand the controversy, but from the standpoint of the guy who kicked this sleeping dog, I think I got my question answered. And that is basically so long as I do a good job of it, I can build a plywood boat using an epoxy coating, and it will hold up long term without damage to the plywood core. My concern was water coming from absorption through the epoxy coating into the plywood, eventually causing rot, and not as a result of collision damage. That fear was apparently unfounded. I personally would not build a boat using "Ester" or any of her cousins. I would use epoxy only. Don't know if the concern was I was being led astray? If so, I wasn't.

But at the same time, I enjoyed reading the discussions too. It's obvious you guys have a wealth of experience. Lucky that I'm able to tap into it.

Howard

[mustangermatt]

Over seven years with Victoria Clipper we hit logs on a VERY regular basis. The hull plating is about 3/8" aluminum and we travelled at 30-42 knots. Small logs about 1'-3' x 10'-15' would be forced down by the stem and bounce back up again to slap midship, then bounce their way aft (passengers hate that one .

The longer ones would break. I only saw hull damage on one occasion where the plating was cracked ahead of the collision bulkhead. The motion dampening fins that hang 4' under each bow got torn off a number of times though.

[Garry]

Howard, I don't think moisture absorption through epoxy would lead to rot. You also need oxygen to support the bacteria causing rot. Moisture will be present in any wood at any time at different levels. Its having the right combination of moisture AND exposure to air that leads to rot.

The concern is cracks or joints that are not properly sealed. Fiberglass strengthens the epoxy and wood surface to avoid cracking. Over-drilling bolt holes and filling with epoxy will avoid reliance on sealers. Older wooden boats almost always rotted from the inside out in the bilge area where they were wet and exposed to air.