The ulitmate plywood discussion?

[wkisting]

"the amount of penetration epoxy has on the wood substrate has little to do with its ability to keep out moisture. The protection comes from no breaches in the coating, not the amount of penetration."

Par, I didn't realize this. If I'm understanding you right, you're saying rot is determined by whether water penetrates or not... not by how deeply it works its way into the wood. Would you mind explaining why it wouldn't be advantageous to limit the depth of penetration? I would still think that it would be best to seal the wood deeply with a thinned epoxy (something other epoxy makers have also been known to recommend). But again, I'm not sure if I understand how rot works exactly (oxygen and water, I know that... but...). Anyway, that's why I ask. The whole issue still baffles me because I don't quite understand quite how fungicides work either.

Thanks!

[Howard]

Seems like the common concerns on the durability side relate to voids and quality of layers....primarily with the fir and other domestic softwoods, but also the okume....at least in the lower grades.

When I think about plywood's use in boats, it's under at least three forms of stress: flexing, tension (pulling) and compression (being pushed),with risk of moisture and the ensuing rot thrown in for good measure. We'll call those F/C/T. Possibly a fourth property...relates to density of the wood and ability to absorb hits or blows without denting or similar such damage.

The strength of plywood apparently coming from the matrix of the wood fibers, in conjunction with the glue that holds them together. The overall quality of the matrix then coming from the properties of the wood it's made from. Fir, having some natural rot resistent, strong in both compression and tension and having the ability to flex...being traditionally, a good choice and is/was a product made from domestic sources. The mahoganies...with their consistent tight grain and resistance to rot...also good but having different properties.

As I think about the fir then, the tendency of the surface to check as it dries out is a problem. Maybe the plies in the old stuff was dried before it was glued? I know the one and only sheet of 3/4" marine I have purchased was green and wet when I got it. It's been inside now about three years, and it has checked pretty bad as it has dried. And now that I look....I see it has several voids in the plies.....7 of them by the way. The same age ACX exterior ply has 5 plies....and is much worse in the way of voids...but adequate for the shop shelves it's being used for.

Under stress from flexing (this also includes bending)....if the voids collapse or worse give out and snap....I can see where you would not want that. Not much different from having a knot in long and otherwise perfectly clear stringer. When flexed...it will blow out at the knot. The more plys...the less likely a void or other weak spot is going to cause problem. One could increase scantling size to account for this....but then weight is an issue. So you go to something of better quality (no voids) to reduce the scantling size and save on weight. Better still if the wood fibers in the other product have equal or even better properties, as relates to F/C/T...rot and impact resistence (denting).

Even under compression and tension....once a void is present...a "weakest link" is present...weaker than one without voids. Again...if design scantlings assume no weak link.....and you have one....trouble is bound to follow.

So it's looking to me like from my original post...with references to Fir, Meranti and Okume...here is what I think I've learned:

Fir: potentially good stuff from the standpoint of strength in F/C/T, is dense enough to take a hit without denting and has some natural rot resistence....but the quality of trees available has resulted in manufacturers stretching it out...sneaking in the bad stuff (voids) in the inner cores, and generally lowering quality to the point where it's not the best option in relation to other choices. Presumably...the good stuff that used to be availlable back when is not available today at any price. IF you could find good, void free stuff, and glassed it to prevent checking....it would work. Plan B is to jack up the scantling size to account for the reduced quality, but you then suffer from the extra weight and reduced performance of the boat.

Meranti and Okume: The tight, consistent grains make it possible to create void free plys...and the qualities of the wood fibers also allow the matrix to handle the stresses of F/T/C (flexure, compression and tension). Meranti being a more dense and brittle wood than Okume...the latter being less rot resistant. I'm thinking this must be the difference in strength and rot resistance properties like you might assume if making comparisons to dimension lumber.....ash to white oak.....spruce to doug fir....or maybe a stick of SPF to Doug Fir or maybe even Western Red Cedar. Similar...but different.

So Okume....once glassed or epoxied...and even more so if treated...takes the rot issue out, and at the same scantling size can handle the F/C/T loads....and will not blow apart under stress. Having similar strength to Meranti, and being lighter...then becomes the ply of choice. Still fuzzy on Okume as relates to it's ability to take a hit or blow...without denting and the ensueing water instrusion that might result from that. Maybe a good epoxy/cloth coating minimizes this?

At least this seems to be what I can deduce from what I've read. Did we achieve the ultimate discussion?

[Tom Lathrop]

I'm sure you know that the air is full of tiny spores looking for a place to land and multiply. We breathe them and don't know the difference. Let a twig die and immediately some spores take action. They need some water, oxygen and a reasonable temperature range to do their thing and don't do well in deserts, deep freeze places or other places where one or the other of the necesaries are not available.

Water vapor penetrating wood is not a rot problem unless the spores are there and alive and oxygen also has a way to be there or get there. Fungicides are poisons that kill and keep on killing rot spores. We used to slap on Cuprinol or Woodlife before painting our boats before epoxy became prevalent and might ought to do that again, provided that epoxy adhesion is not affected.

I find no argument between epoxy covering and paint. Epoxy is paint, just a little different and better except for solar degradation. For me, sealing exposed plywood with epoxy is a no brainer. Adding fabric just makes the protection better. In the 60's epoxy paints were available from many marine paint manufacturers. The chalking issue probably did it in although it is still available and I use it for many parts of a boat. I know of no other paint that can touch epoxy paint for toughness, sealing and long life. One boat that I built in 1994 was reported by its owner a couple years ago as beginning to need some paint work. You can read about this much traveled boat "Loon" on Graham's web site.

Rot spores are not a bad thing. Think about how the world would look if nothing rotted. We would be living in our own landfill. Entropy lives

Added for Howard,

I think you have a better handle on the plywood thing than many who have been around the various forums for a long time. One thing is that this particular forum is almost all related to plywood boats and does not attract the nay sayers that pop on other woodenboat forums.

I have 24 foot planing power cruiser with 1/2" Okoume on the bottom. I have hit at least one significant deadhead while planing. This occured about 30% aft of the bow where the deadrise is about 20 degrees. Only damage I can find is a chip in the boot top paint. This boat is sheathed with Xynole which I consider the best of fabrics for this purpose.

[Howard]

The only other thing I can think of on the durability issue is a comparison of wood to fiberglass.

I've heard it said that fiberglass doesn't decay or go bad....it wears out. It is naturally weak from the standpoint of wanting to flex...so it has to be built in compound curves and reinforced with stringers and bulkheads to keep it from flexing. And when/if it does flex...the small fiberglass strands that make up that matrix of glass cloth and polyester resin snap. Larger scantlings and heavier layups are thrown into help reduce the flexing and tendency to snap.

So wood also flexes...but doesn't snap. It can handle the cyclic loads and flexing. And some woods may flex, without damage, more than others.

Fir being a "springy" wood can handle it. I presume Okume...being so willing to bend without breaking can also apparently handle it. Don't know about Meranti and the other more dense and brittle exotics.

Meanwhile....MDO lurks in the shadows.

[PAR]

Epoxy's penetration into the fibers of the wood will take place if you work under reasonable conditions (warm enough to insure the goo flows) I know of no epoxy manufacture who recommends their products be thinned, though they usually go on to say, "if you do thin, use this . . . and only below these percentages . . .", etc. I have epoxy made up as a proprietary mixture for my needs here in wet, wild central Florida, by a local company. Their formulations provide me with 45 minutes of pot life in 90+ degree heat of the day in the summers (which runs from March to October) I'm on the phone with their chemist reasonably often and he doesn't want this stuff thinned, rather insisting on temperature and humidity controls to increase viscosity.

Wood gets it's strength from the way it's constructed. It's tubes of sugar, in a mostly aligned cellular structure. These cellulose fiber tubules (the "ose" in cellulose means sugar) make up about half of the wood's weight. These are long chain, linear polymers, having a high molecular weight and are typical of regular sugars. The cell structure is bound together with "lignin", a tough glue like material, which is very similar (structurally) to the resins we use in 'glass laminates. Lignin adds about 25% to the weight of wood. A lot of the cellulose in the lumber is crystallized (the high molecular weight thingie) and helps make wood water resistant.

Basically it's a uni-directional composite (thank you mother nature) because of it's tube and fiber structure, which also accounts for it's light weight and stiffness.

The woods we generally use in boat building, range between 24 and 44 pounds per cubic foot in density, or about half that of water on average. Steel is about 500 lbs/cu. ft. and light weight aluminum is 168 lbs/cu. ft. Wood fibers are stronger in tension then aluminum and almost as strong as steel. It's also self preserving under load. The cellular structures buckle rather then crack (like metals and made laminates do) under stress. The lignin prevents the collapsed cell walls from further crushing by diverting additional stress loads to other cells, which helps wood to be self stabilizing. The net result is, wood (pound for pound) can absorb far more energy before fracture then metals or man made laminates.

Epoxy's only real benefit other then it's use as an adhesive (a job it's quite good at) is it's ability to stick really well to wood. The result of this is the sealing of the surface cellular structure and it makes a wonderful (if costly) primmer for this reason. Once a coating of epoxy has completely (without exception) sealed the wood, moisture can't get in with sufficient quantity to cause problems. The coating doesn't need to be more then a surface treatment (no penetration) to work, though epoxy's ability to penetrate increases the likelihood that a ding or scratch will not breach the coating. I suppose if the penetration is deeper it would offer slightly better protection, in the event of a ding, but that's a function of a mechanical breach and not the fault of the epoxy. In reality, a rough landing at the dock will likely breach any coating, if just a few cells deep or several cells deep into the wood.

The bottom line is, penetration is measured in thousands of an inch. What's the difference if you have 3 thousands or 10 thousands of an inch penetration, if the hard recovery on the trailer, in a good chop, bounces the keel off one of the roller supports, removing a 1/4" deep chunk of well coated wood. The coating is breached and the repair needs be made in either case. Sure, a sheathing will offer considerable, additional abrasion protection, but we still manage to bash up our boats in normal use to breach these too. Which raises new issues of the sheathing getting a localized delaminating causing a small ding to become a much larger repair area, if gone unchecked, which happens more then we're usually willing to admit.

[Joe Nelson CS#35]

"the amount of penetration epoxy has on the wood substrate has little to do with its ability to keep out moisture. The protection comes from no breaches in the coating, not the amount of penetration."

This is why covering with a layer of fiberglass is so important...as long as you get the glass transparent, you have a full encapsulation (as long as you have no bubbles between the glass and the boat).

[Barry Pyeatt]

OK Charlie,

I'll try to remember to space my paragraphs and/or open things up a little to make reading them easier. No problem. 8)

I don't have that issue too much, my monitors are set so that I type in information at 200% of normal sized and I have huge monitors because I do a lot of computer work.

I've used CPES for a number of rot repairs on boats over the last few years. I've had excellent results with it. And no call backs for further damage to those areas. That is the most important criteria in my mind.

I also do, as I think Charlie does, with epoxy. Rather than thinning it I heat the material being coated first and then the epoxy as well. Two reasons for doing this.

First is heating the wood which opens up the cellular structure of the wood and allows moisture to off gas. Then as the wood cools it draws the resin back into the cellular structure and helps to make a better, longer lasting and considerably stronger bond with the material.

Second is that heating it thins it out some and allows for better coating and some degree of penetration and better bonding when coating or using for adhesieve between flat surfaces. It does shorten working time so you need to use a slower cure time hardner.

No problem working with different species of plywood or even slightly different grades of plywood within the same project. Economically it makes sense for the types of boat building that I was primarily involved with. Structurally, I insist on the greatest number of layers (plys) that I can get with them balanced. No 2-ply (yes it does exist), or 4 ply, or even numbers of plys. Also want to see reasonably thick outer plys as they carry the initial impact and bonding with the glass or finish.

I don't have any issues with Okoume as long as it is rated for marine use with a BS rating. Never had any rot issues with it other than where it was breached and not repaired correctly or quickly. No strength issues with it either. We used both Cuprinol or Woodlife on most plywood before finishing it in our boats as matter of course. It was just good insurance. But then we didn't coat with epoxy over it either. So I can't speak from experience on that matter.

I've had boats last for much longer than we ever expected. The first boat I ever built was a Sailin' Surfboard from Boy's life plans back in the 1950's when I was 9 years old. We used Sitka Spruce for stringers and 1/4" Douglas Fir Marine Ply in 12' sheets. Recorsinal glue and oil based primers, paints and caulks. Finished inside and out. Some nice Phillipine Mahogany for centerboard, grab rails and rudder/tiller and either bronze or stainless fasteners. That same sailboard is still teaching my cousins grand children to sail this year. It has had the hull breached a couple times and repaired, new masts, new centerboard, rudder, etc as well as a dozen sets of sails and rigging. Somewhere along the way one of my cousins took it down to bare wood and glassed it with polyester resin and about 6 oz. glass. Then painted it with porch and deck enamel.

Several paint jobs later it still looks like new and kids are still learning to sail with it. Never expected it to last this long but it has with care and minimal maintenance.

Had several plywood boats that I built along the way some from fir plywood and some from Okoume and Doug fir or Spruce for solid stock. All of them glassed on the exterior only. Most painted with porch and deck enamel or polyurethane. Only a couple with marine paint where they sit in the water year round. Most of these are over 30 years old and still holding up well. But all of them were built with the best materials I could get at the time at a reasonable price.

The plywood future is not going to get better, it will get worse, particularly here in North America where there is greater and greater demand for construction grades of materials and less demand for marine rated or specialized use grades. Off shore sources will become more prevalent and expensive. We have foreign exchange to thank for that a bunch as dollars become worth less on the world market.

Rot prevention is a common issue, more so in areas with widely varying temperatures and humidity. Nothing beats air circulation thru the entire hull area. Keep it dry if it is a trailerable boat and it will last a lot longer. Keep it cleaned out and don't let things sit on the floor boards or against wood surfaces that will trap moisture. Great conditions for mold, mildew and rot to begin. Store it upside down over the off season if possible with the bottom side open to keep it dry. Open up hatches, and allow air to move thru all areas.

Good Plywood will last a long time if you take care of it and prepare for long life with some simple building techniques. They take more time than just throwing it together, but save you a lot of time, times the number of years, and then to some factor beyond that in savings in repairs and maintenance. Pound for pound and thickness the material is perfect for what we are doing. Those that have been there before for the long run will understand. Those that are new to boating or wooden boat building will simply need to learn. Just part of the ownership thing.

Good discussion guys.

[alkorn]

Tom Lathrop says " The idea that you should epoxy only one side of planking to let the wood breathe makes no engineering sense."

I think he's wrong there, though I admit that "let the wood breathe" is not a good description of the reasoning behind the outside-only school of epoxy application.

The idea is this: The outside of the boat is usually WET. The inside of the boat is usually DRY. If you epoxy both sides and then get a small hole in the outside epoxy, water will get in and be trapped there. If you epoxy only the outside and then get a small hole in the outside epoxy, the influx of moisture through the small hole is compensated for by efflux of moisture through the fully open inside surface. Moisture doesn't build up.

The same sort of reasoning applies in installing house insulation: you put the vapor barrier on the side where the air will be moist, and put NO vapor barrier on the dry side.

It seems to me the main counter-argument to this is that the bilge of a boat isn't really dry, so you're better off with epoxy on both sides. My preference, though, is for epoxy on the outside of plywood only. In hidden spaces leave the inside wood bare or apply preservatives. In exposed spaces paint, since paint is much more permeable than epoxy.

[Charlie Jones]

well- I seriously disagree with leaving the interior uncoated. I think coating INSIDE might even be more important than coating OUTSIDE, since moisture tends to accumulate in the little nooks and crannies down deep in the bilges and under floorboards, etc. The inside of most boats usually get wet, and stay wet, and THOSE places are really the places that seriously need coating to protect from that moisture. Otherwise you get rot. Paint doesn't do it, I've cut too much damaged wood out of older boarts where rot had gotten a foothold in there to believe that paint will protect.

On the other hand, I built a 35 foot trimaran in the late 70s, early 80s that was well coated on the inside of everything. That boat is still passing surveys today and as far as I know has NEVER had an inccidence of rot inside. It DID have one place on the stern where it was damaged, the plywood breached (and the coating) and not repaired ( after I sold it) Rot got in there, but was surgically removed and now is fine.

I did a plywood fuel (diesel) and water tank, coated heavily on the inside, and both tanks are still in use, 23 some years later.

You can argue it which ever way you want, but the Gougeon's have done a bunch of testing of the epoxies. They recently did a survey of the first boats they built, going on 30 years old now. Those boats are all still in good shape, PROVIDING the repairs are done in a timely manner.

Like any other method of building, NOT doing the repairs as needed will kill the boat, no matter WHAT it's built from.

The boats I build in my shop, for my customers, and for myself will be epoxy coated on the interior AND exterior. I'm firmly in that camp. Proper maintenance, and having any breaches repaired in a timely manner will lead to a very long lived boat.

[Frank Hagan]

Charlie, what's your opinion on epoxy coating timbers or laminated boards (such as in the Weekender keel) that are subject to wood movement? I've always felt that timber on a trailer sailer will always move due to the wet/dry cycling that is the nature of a boat that lives on its trailer, and the epoxy isn't strong enough by itself to hold up against the forces of nature.

[PAR]

Tom Lathrop's comment about poor engineering sense in regard to coating one side of planking material is correct, Alkorn. From the stand point of understanding how and why epoxy and wood interact, it's a valid remark.

The studies, tests (short and long term) and returns on repairs (insurance company research, for example) have been in for many years and epoxy coatings on wood and wood products (plywood). Total encapsulation is the only sure way; every other method hasn't proven remotely close in durability.

A common misconception is that plywood is wood and has similar properties. It's not, but is an engineered product that has wood in it, a large percentage, but none the less an engineered material. Epoxy starts out with base compounds of organic material, some manufactures use wood, but it too is an engineered product.

Epoxy can be formulated to be extremely flexible (CPES or other penetrating epoxy, for example), much more so than paint, but in instances you are not encapsulating, little more then an expensive primer. If you do encapsulate with a penetrating epoxy, the wood can move, vibrate, flex and all the things wood can do, except absorb moisture. The vast majority of wood primers don't seal out moisture, relying on the over coat for this protection.

Plywood doesn't need to breath like solid lumber does. There is some movement with changes in moisture content, but very little. Epoxy can adhere abrasion resistant fabrics to things, with great success, but the use of epoxy this way can change a project's scope.

It's one thing to use epoxy as an adhesive, binding the pieces together in lieu of fasteners (or in concert). It's another to begin coating pieces in an effort to prevent moisture absorbsion. These are the two basic uses for epoxy and the big problem for a lot of builders and repair people. To use epoxy or not to use epoxy. As an adhesive, it's an easy call; you're exposure is limited and the volume is small. As a moisture barrier, the epoxy volume goes up dramatically (several hundred percent or more) because each piece, each hole, each facet of each pieces, needs to be coated (with or without sheathing) or you're just using costly primmer. A whole new can of worms is added to the project. Not only is the application, effort and cost going to rise sharply, but the long term maintenance will require close scrutiny to prevent coating breaches from becoming big issues, unnecessarily.

An example of this would be an owner, pulling out his hole saw and cordless drill to install a new fishing rod holder in his side deck. The hole is cut, the three #10 oval head screws run into place, all looks good. The bedding will slow the process for a short time, but the coating is breached and it will be a problem later, as the only place water can get in or out of the otherwise well sealed wood side deck.

Coating one side has very little benefit. It doesn't help abrasion; epoxy requires reinforcement for that. In fact, regular epoxy is pretty helpless without the many different reinforcements we add. It doesn't keep out moisture, if the other sides and edges of the piece are exposed, particularly the end grain. The short of it is, moisture will get in, unless it's in a well maintained plastic sack, especially in the marine environment. You can work with the moisture content changes, like traditional methods employ or try to stabilize the material by sealing it up tight with a cross linked polymer and hope the goo doesn't get breached.

[Charlie Jones]

Frank- I must confess I kinda ignored the plans a bit when I built the Weekender. I epoxied the keel members together, with screws along the edges that I left in. Then once it was installed, I glassed over the entire keel with multiple layers over the bottom edges. I did that boat in 2000 and we haven't seen any problems with the keel at all.

Worked on THAT boat. Whether it would do the same on others I can't say. But were I to build another boat with a similar construction, I'd do the same thing.

I sorta figured the Weekender plans were drawn up in the 60s, well before epoxy came into general use, so I just used the techniques I've learned with epoxy to build the boat, rather than the construction techniques from the plans.

[capt jake]

I did the keel on my Weekender theh same as Charlie desccribes and have seen no problems. 2 years out in the water so far. I added brass 1/2 oval to the keel just 'cause.

[Frank Hagan]

The problems we've seen with the keel is due either to wood movement or shrinkage (I think Konrad has his glass crack because of a void caused by wood shrinkage ... when he cinched down his gunnel strap on his trailer the glass along the keel cracked).

I left mine unglassed, due to the fact that I knew it would shrink. I used "kiln dried" construction grade 2x material that was resawn into 3/4" wide planks ... and it was still very wet inside. Lousy kiln (but I've always suspected that the "kiln dried" construction lumber isn't REALLY dry). There has been a lot of shrinkage, but I will admist the epoxy fillet along the keel to hull joint held very well. Large cracks developed where the horizontal planks met the vertical planks at the stem, which had to be filled and faired.

Anyway, I'm a bit nervous about encapsulating solid wood. Probably unnecessarily so.