Bottom Planking Too Thin

[jwatson]

http://i1120.photobucket.com/albums/l485/jwatson04/CrescentSmall.jpg

First time boat builder, and new at posting, so bear with me. I am building a 15 1/2' Crescent day sailer from a reprint of Boat Builders Handbook circa 1960. The plans called for 3/8" bottom, side, and deck planking. I decided to fiberglass the outside, so I planked the bottom and sides with 1/4" underlayment. I used polyester resin instead of epoxy (recession driven decision).

After painting inside and out, I was working in the bottom and heard a crack. the sawhorse was positioned putting pressure in the center of a plywood span. I found a small crack in the face veneer inside the bottom, but no crack in the fiberglass. Yesterday while fitting the wooden mast, I let it slip to the bottom and heard another crack. Still no crack in the fiberglass.

I am now considering grinding the paint (top secret 1 part epoxy) off the bottom, and adding another layer of fiberglass, or another layer of plywood and fiberglass. Any suggestions?

[jwatson]

Thanks seayou7. What was your fix. I hope you don't say trash it.

[Cuyahoga Chuck]

There are numerous kinds of underlayment. For instructional puposes, what kind did you use?

[Rational Root]

Mostly, the floor of a dinghy is not always designed to support a persons weight when the boat is not in the water......

Also, polyester does not bond well to Plywood. Not over time.

I have not enough experience to offer much more than that.

[BBSebens]

Remove the bottom, and put on the thicker stuff originally spec'd. Cheaper than building a new boat, easier than grinding off every fleck of paint to get a good glue-up.

At least it cracked now, and not out in the water somewhere.

[jwatson]

Thanks guys. Depressing news, but better now than never.

[David G]

Now, for a slightly more optimistic view.

You've put a lot of effort into what looks like a nice little design. I don't think it is, or necessarily has to be, a write-off.

As mentioned, a small plywood boat hull is not really meant to support weight while not in the water. You've made this even more true with your choice of plywood and resin. So you've given yourself a handicap. You've already begun the see the ramifications.

And - polyester resin on bare wood has failed miserably in the past. Lots of times. However... there are a number of proven instances where is has NOT failed. So there's hope.

Before I gave up, I'd try one of the following - separately or together:

A. Use some of your underlayment (or better grade plywood, if you have some scraps somewhere) to double up the weak/damaged spots as best you can. I'm thinking something like a 3" X 3" (or larger, if possible) doubling of those areas. Chamfer the exposed edges and glue onto your bare-of-paint hull. Use epoxy glue. It'll stick just fine to the polyester resin. Repaint. <btw - this regimen applies to all of the suggestions>

B. You could strip the paint off the exterior and add another layer of fiberglass - set in EPOXY resin. Again... it'll stick just fine to the underlying polyester resin.

C. I don't know if the design calls for any sort of keel. If not, I'd add one. Doesn't have to be big. Could be as little as 5/8" wide X 3/4" tall.

D. In the same realm - longitudinal stringers. It's remarkable how much strength and support a tiny little stringer will add when it spans an otherwise open and unsupported expanse of thin plywood. I'd add stringers (at least 2, maybe 4) running parallel to the keel/centerline. They don't have to go the whole length of the boat. But certainly have them span the cockpit area. Size can be as little as 5/8" X 5/8".

I can certainly understand the gloomy advice you've received so far. Any of the above steps could easily be called putting lipstick on a pig. However... to the extent that you do some or all of those steps... I think your boat can have a certain period of happy service. If you treat it gently. If you touch up every scratch in the paint within a day or two of when it occurs. If no water is allowed to set inside the boat for more than a few hours. If it's stored in a sheltered spot.

The issue I foresee is the polyester resin to plywood adhesion. You might not make it a year or two. If you make it that far... it could go on for years. But is it worth the risk of using additional time and materials and possibly only getting a year or twos use out of it? Only you can judge.

G'luck!

[Bill Mercer]

I'd just add to what David said that I've seen FG and polyester over plywood that could be pealed off about like peeling off a bandaid, maybe even easier. That was on an older boat, so the FG and plywood had been through numerous wet-dry cycles. Also, polyester resin is sort of famous for transporting water through itself to the underlying wood. . .

Do you know what wood the underlayment is made of, and whether marine/exterior glue was used? Luan ply I think has a poor reputation for durability.

[Bob Smalser]

Y'know, many of those plywood Pacific Surf Dories used both commercially and for sport down on the coast use polyester resin instead of epoxy to keep the costs down. They get the snot kicked out of them with every launch and recovery, and it seems to work there.

http://lh3.ggpht.com/_jn8AXRuZo1U/SQyae7jT8FI/AAAAAAAAAJk/q_sfoeL6Stk/s400/kiwandadory2.jpg

I agree with David that a quarter-inch bottom isn't a show-stopper. I'd simply reinforce it from the inside with another payer of thin plywood bedded in epoxy. The epoxy layer alone will stiffen it noticeably.

But "underlayment" doesn't mean diddly to me. Is it Doug Fir CDX? Gold Bond OSB? What specifically, as both are constructed using phenol-resorcinol (exterior) resins? I go through DF CDX by the pile for subfloors, sheer walls and roof decks....it runs around $13 a sheet wholesale and I wouldn't hesitate to build a utility boat from it. Remember that the first two letters in over-rated, over-priced, imported "marine" plywood are "BS".

[jwatson]

Thanks Bill and David. The underlayment has exterior glue with birch faces. I'm not sure what the interor plys are, but seem to be clear of voids. I may try the patches for this summer, and then remove the bottom and replank with 3/8" , fiberglass, and epoxy next winter.

[TerryLL]

I suspect the reason for the cracking is that wide areas of the bottom are unsupported by interior framing, and the 1/4 ply isn't stiff enough to bear the load without excessive flexing. This is a common problem in aluminum boatbuilding, where thin plate is used to reduce weight and cost. The result is flexing and cracking.

Aluminum boats with thin bottom plate rely on extensive light-weight interior framing to support the thin plate and eliminate flexing. Before you rip the bottom off you might consider beefing up the interior of the bottom. Here's a shot of a 23' skiff going together. The spaces between the framing will be filled with foam and then an aluminum deck will be welded over everything.

http://i325.photobucket.com/albums/k365/TerryLava/Alhull015.jpg?t=1295831795

[Todd Bradshaw]

The interior cracking is a classic example of the compression/tension scenario with fiberglass - which many builders (especially those dealing with fiberglass sheathing) don't seem to understand. In a nutshell, you built your hull from a thin, rather weak material (luan) and the fiberglass reinforcement didn't help - because it's on the WRONG side of the plywood to do so in a situation like this (or when out on the water when using the boat).

The polyester resin had nothing to do with this. It does have some serious limitations and vulnerabilities for use on wooden boats, but they really don't apply here at this point. The strength of fiberglass is tensile strength. This means that when you pull on its fibers, they are quite strong and will resist that pull until you have applied enough tension to finally break them. A proper, fiber-rich ratio of glass and resin creates a thin skin with good tensile strength and a decent amount of flex and recovery if needed (higher than that of the wood itself). But by putting your boat up on horses, the boat's weight, and the weight of anything in it aren't pulling on that outside skin in the sawhorse areas (putting the glass in tension) instead, they're pushing on it. The same would be true if the hull was climbing over waves or being dragged up on a beach. That pushing force is putting the outside skin in "compression", not in tension. Fiberglass isn't very effective or strong in compression and being on the outside of the hull, it can't add any significant strength to your plywood in this type of "pushing inward" situation. That force is instead, stressing the inside surface of the hull in tension - and oops...there is no fiberglass there to reinforce the flimsy luan. Heavier, stronger plywood might have survived fine without glass, but you can put a sheet of luan on shag carpet, walk on it and hear it breaking under your feet. If you intend to use it, it had better be properly reinforced.

Also, Look at your layup. When you weight the boat and bend the plywood over a sawhorse (or a wave or a rock) the outside of the hull moves and bends a little bit and the fibers (wood and/or glass) on the outside surface are being pushed upward and together. But..... the inside surface moves a lot more. The tension there is trying to pull the surface fibers apart. The thicker the core and farther the inner and outer surfaces of the hull are from each other, the more they're being pulled apart because they have to move, stretch or deform a lot more than the outside skin does as the ply bends. You don't have any fiberglass on the inside, so the luan is left to resist this force all by itself. Luan isn't very strong, so it breaks.
http://webpages.charter.net/tbradshaw/TENSION.jpg

A - ply and fiberglass at rest and not bent. All is hunky-dory.
B - As force is applied to the bottom, the hull bends. The glass on the outside doesn't really get stretched much at all, but the inside surface is getting a lot of tension, trying to stretch or break it.
C- The thicker the core, the more those inner tension forces are trying to tear the inside surface apart. Notice though, that even though the inner surface is really getting stretched, the fiberglass (blue) layer on the outside still hasn't been bent or stretched much at all. This is why your fiberglass was still just fine after the inner wood had been stressed so much that it broke. The glass never even came near to getting that much stress - it just bent a bit and pretty much returned to normal after the stress was removed. Loading up more fiberglass on the outside won't fix this flex/internal-tension problem until you have almost enough glass to make a stand-alone hull, so that's not the solution.

On a cored composite hull, we add the fiberglass layers to both the outside and the inside, so that both sides have good strength to offset forces applied in tension. Depending on a boat's construction, interior layout and internal structure, this may or may not be do-able on a plywood boat (or needed, if the scantlings call for sufficiently strong ply). In that case, the exterior layer of fiberglass that is often called for is just there for abrasion resistance as it's harder to wear through than paint alone or wood.

In this case, you either need stronger bottom plywood or additional reinforcenemt inside the hull - which is probably hard to add at this point. I hope this post makes sense. It's kind of hard to explain in a few paragraphs, but if you're going to be sheathing hulls in fiberglass, you really need to understand what that sheathing can or can not do. Otherwise, you end up with boats that break.

[davebrown]

stringers. this is by no means hopeless, although the glass and resin issue (which i have never done) is reputed to be guaranteed failure--AT SOME POINT. but we live in an entropic universe, do we not?

[Tom Lathrop]

Lots of good advice from lots of people that want to help. One thing not questioned is that you went with the 1/4" cheap underlayment and fiberglass/polyester resin bottom for economic reasons. Without a doubt, 3/8" plywood and paint costs far less than what you used and would not have had the problem of cracking, unless it was really bad plywood. If you want to keep this boat for years, the best thing is to remove the bottom and replace with thicker good ply. I don't know what is available in your area but Meranti would be a good investment for both strength and longevity.

[Cuyahoga Chuck]

In modern S&G construction vee-bottoms are often specd at only ¼" and covered with glass when more stiffness is required. It's necessary because going to thicker ply would make bending and twisting planks into shape a gargatuan struggle for a home builder. The exterior glass stiffens the ply because it acts like additional layers of veneer. What really strengthens the bottom are the internal bulkheads which break the bottom up into small spans and lay off bottom loads onto the sides and decks. If the spans between bulkheads are too long the bottom panels has to be stiffer to avoid oil-canning. To have the bottom actually break just sitting on horses indicates there are quality problems with the materials. A vee-bottom made from quality ¼" ply with a layer of 6-9 oz. glass on the outside should easily be the equivalent of unglassed 3/8" ply.

[James McMullen]

I'm sorry, but I'm not nearly as sanguine about your chance for even medium-term success as some of the others here. You reduced your scantlings to 67% of what was specified, and are using inferior resins and materials to boot. Those were all uninformed choices. You can probably get it to work in the short run, but don't compound your error by throwing good money after bad with successive band-aids that don't actually solve the underlying problem. In all honesty, I wouldn't spend anything more than necessary on this boat now other than for things that can be stripped and re-used on your replacement boat. Consider this a learning process, start saving up for the next one which will turn out much better, and always wear your lifejacket.

[Tom Lathrop]

A vee-bottom made from quality ¼" ply with a layer of 6-9 oz. glass on the outside should easily be the equivalent of unglassed 3/8" ply.

Sorry Chuck but that is just not so. Having built, and tested, boats with both materials in those sequences, I can say that 3/8" ply is stiffer than 1/4" ply plus 6 or 8 oz glass cloth. While glass fibers are high in tensile strength, woven glass cloth is not for small movements, especially with the glass cloth on the outside. Biaxial and unidirectional glass is in another class and does make for a very stiff panel. It is an easy experiment if anyone wants to test this. If you want to stiffen a bottom ply panel and greatly increase rupture resistance, do as Todd recommends and put the sheath on the inside.

[erster]

http://i1120.photobucket.com/albums/l485/jwatson04/CrescentSmall.jpg

First time boat builder, and new at posting, so bear with me. I am building a 15 1/2' Crescent day sailer from a reprint of Boat Builders Handbook circa 1960. The plans called for 3/8" bottom, side, and deck planking. I decided to fiberglass the outside, so I planked the bottom and sides with 1/4" underlayment. I used polyester resin instead of epoxy (recession driven decision).

After painting inside and out, I was working in the bottom and heard a crack. the sawhorse was positioned putting pressure in the center of a plywood span. I found a small crack in the face veneer inside the bottom, but no crack in the fiberglass. Yesterday while fitting the wooden mast, I let it slip to the bottom and heard another crack. Still no crack in the fiberglass.

I am now considering grinding the paint (top secret 1 part epoxy) off the bottom, and adding another layer of fiberglass, or another layer of plywood and fiberglass. Any suggestions?
First off a one part paint is not an epoxy paint these days. Read the ingredients on the label. Read up on the types of thinners that is required for thinning the paint.

Next its important for all projects to incorporate proper engineering into a project. A carefull selection of both the plywood and glass is needed. If you are not experienced in that area, you do yourself and all of your passengers a disservice and quite possibly will generate a safety hazard when reducing either of the parts thats required to create a sound hull.

When choosing the plywood, the amount of cores plus the thickness of the cores and the direction can vary across a lot of different types of plywood even if the overall thickness is correct.

Increasing interior framing or adding additional grid components may give you some added margin of error. but this still does not address the issues of impact resistance and quite possibly a failure for sure.

This is a good example of using structual glass in conjunction with sealing and finishing glass which does increase your overall freestanding structure without adding incredible amounts of additional framing, which rarely does a lot for you by comparison.
This is a structual glass, biaxall which gets skinned over with finishing cloth. Finishing type cloth rarely does much other than seal and stabilize a hull except in some cases that shape comes into play for liteweight hulls too and works together such as the strip canoes. You must always consider all your components and not depend solely on one part if any design depends on both or several additional parts to make a boat sea worthy for sure.
http://img.photobucket.com/albums/v356/Bateau1/DSC09262.jpg

YOu need to go back and do some research indeed on all of your parts, not withstanding the resin type, if your present hull broke sitting in the shed. Polyesters for sure work with glass to seal the deal. But also take a look at many of the pure fiberglass production boats. in almost all cases if the hulls are sound, the thicknesses by comparison is thicker to compensate for the actual weakness of some of the glass thats used in their constructions too even when using the polyester resin.

[Bill Mercer]

The interior cracking is a classic example of the compression/tension scenario with fiberglass - which many builders (especially those dealing with fiberglass sheathing) don't seem to understand. In a nutshell, you built your hull from a thin, rather weak material (luan) and the fiberglass reinforcement didn't help - because it's on the WRONG side of the plywood to do so in a situation like this (or when out on the water when using the boat).



Just to be clear, I don't think that we know that it's luan--I suggested that, as luan seems to be a common material for underlayment, but as Bob Smalser said, it could just as easily be douglas fir, which would be quite a bit stronger.

[jwatson]

The plywood is 5 ply with birch faces. The inner plys look like birch also, but I'm sure they may be gum or some other fairly tight grained wood. It is not Douglas fir or luan.

[erster]

The plywood is 5 ply with birch faces. The inner plys look like birch also, but I'm sure they may be gum or some other fairly tight grained wood. It is not Douglas fir or luan.
I don't recall seeing underlayment in 1/4" with five plies. I use the Sure Ply or Super Ply for patterns, which now has a birch face. But its really not a full 1/4" and its three plies too.

[jwatson]

It is definitely 5 ply. The 2 faces are very thin, with cross bands and core thicker. I don't have a way to measure it, but the faces are probably 1/32" to 1/40". It is sold in Lowes in North Carolina.

[Chris-on-the-Boat]

There's a really good plywood sold out here at Gray's Lumber in South Beach, Oregon, with 5 plys in a 1/4 inch plywood. I don't know how many are in the 3/4 inch ply i'm using on this project....hang on...

Just counted it. 15 ply for 3/4 inch. Been getting this stuff for 5 or 6 years here....russian plywood, called "Baltic Birch"...5'x5' sheets, holds up well to our rain forest conditions. I think it runs 18 bucks for a sheet of 1/4", 40 bucks or so for a sheet of 3/4". No voids. And beats the tar out of the so-called 'marine plywood' the chain lumber store sells here for 60 bucks...

And I'm an old plank on frame guy, so the idea of anything less than 1.5 inch planks and 2x3" frames with tons of concrete below me gives me the willies...but good luck with the 1/4 shell...

[Bill Huson]

You could skin the bottom with 3mm marine, usually called "bending plywood" since it is often used for multi-layer cold molded hulls. A 4X8 sheet of 3mm weighs 8 lbs, figure a few more pounds for epoxy and you're done without adding a ton of weight.

For a 15 1/2' boat, had you used 5 ply marine Okoume you'd be done now. I've built 15' outboard skiffs Stitch & Glue with minimal framing and had no problems.

[James McMullen]

For a 15 1/2' boat, had you used 5 ply marine Okoume you'd be done now.

I think that's the real lesson of this story. Using inferior materials is never a good economy.

[erster]

This is Lowes underlayment, five ply compared to 6mm Okumne plywood. The inner cores of the underlayment are large and cross the running grain versus the equal plies of the five ply 6mm.
Your boat is flawed. Unless you reskin over the existing plywood or add several layers of structual glass, your boat is subject to a severe failure, IMO. Also notice the thickness of the face plies alone.
http://img.photobucket.com/albums/v356/Bateau1/DSC09270.jpg

[Bill Huson]

Yes, I built a cheap canoe with Lowes "5 ply" - hey, it is a CHEAP canoe! Probably the same stuff Mr Watson has since I live in NC as well. Face plys are dismal thin, very hard to make a clean scarf joint.

[Cuyahoga Chuck]

The plywood is 5 ply with birch faces. The inner plys look like birch also, but I'm sure they may be gum or some other fairly tight grained wood. It is not Douglas fir or luan.

2nd question. Are the veneers close to bring equal in thickness?

[Bill Huson]

2nd question. Are the veneers close to bring equal in thickness?

Look at the pic, Cuyahoga Chuck - face plys are very thin. On the "5 ply" 1/4" I had from Lowes the face plys were near paper thin. Lean on with #80 grit and you're through to the next ply.