Compound plywood bend in stitch&glue construction?

That's what I always thought about strip planking, that it's one of the most laborious ways of building a boat. But I always thought that only applied to wood, because of having to rip and profile so many strips and joints, requirement to have very close fits for the glue to work, etc. With the foam though, what would take so much time? The mould seems to be just a bunch of temporary frames on a strongback, looks like something that could be cut out and set up in a day. Foam strips seem not to need any special forming on the edges due to expanding PU glue. And it should be easier to sand than wood anyway. Glassing is probably about the same as on a regular stitch&glue boat. Am I missing something? I've never done strip planking, so my assumptions could be entirely wrong, I'm just wondering what takes that much time with this method?

Wait, you mean a single layer of 400gsm fiberglass on the outside, and single 300 gsm on the inside? That's all that's needed? I would have expected far more weight and layers. And that is really enough? How much did the boat from your photo weigh in the end? Also, is that foam you mentioned close-celled? If the boat receives damage and water gets to the foam, won't it cause all sorts of issues (osmosis, massive additional weight, etc.), since it's not really possible to dry out?

I didn't quite understand the procedure you described for making a raised floor. Did you mean that one side of the foam would not receive fiberglass at all? Or that the entire volume between the bottom of the boat and the floor would be completely filled with foam? And with your suggestion to fill XPS between the station moulds, would it still be possible to pull the station moulds out afterwards, or did you intend for them to remain permanently installed in the boat as bulkheads?

 

Yes that's all, you could bump it up a bit if you wanted to be tougher. Bear in mind that an additional layer would be required at the bow and along the keel. A canoe would only need the added reinforcement at the ends but your rocket ship is a bit different. All of the foams mentioned are close celled; beaded polystyrene should be avoided like the plague. I swore I'd never use the stuff again after building a few hotwired foam core craft decades ago, broke that vow once to make the wingsails for Nomad. Another foam to avoid is polyurethane, it's friable and will slowly decompose over time. The 12' EasyB canoe in the picture weighed 28# finished with a minicell foam seat, probably weighs a touch more now, boats like people tend to gain a little weight over time...

What I envisioned was making a solid XPS core from the floorboard to the inner face of the 8-12mm foam strips. This core would be fastened to the mold stations that had been cut off at the floorboard level. Stripping the thing might be interesting at the transition from mold stations to foam, not sure about how to best hold the two foams together between the mold staions where the XPS is really thin; the two can easily be held together while glue sets with long thin gage nails pushed in at an angle where the XPS is thicker.

Time to build depends on a lot of variables. Decades ago I could build a 14' wood stripper in 40 hours easy. Did not include making a strongback, already had that, but ripping strips, cutting mold stations, stripping, fairing and glassing took less than 40 hours for a boat that was clean, good looking but not quite ready for concours competition. A foam core craft would have been quicker to build but such didn't exist back then. Now days I am almost infinitely slower ;-)

 

Not sure. Nobody could guarantee that. Need to do some kind of calculation.

 

I looked back on my phone and the original 12' EasyB was built on six weeks; strongback, molds, the works. I already had the mahogany and cypress bead and cove strips which had not been carried away in the tornado. Boat was built fairly quickly May and June of 2019 because I didn't have a boat to paddle. The later foam core version off the same mold was built over the winter a year later, no rush.



Boat weighed 32# as I remember. Recently gave it to my son since they live on a lake without rocky shores. The strap holding the boat down on the racks of his truck with a camper top broke while on the highway home. Fortunately no one was close behind him and no vehicular collision. He tells me there was no structural damage, just road rash.

Here's a quick drawing of what I had in mind for an XPS floored boat. A couple of 1/4" x 3/4" are shown let into the mold stations to srabilize the XPS and keep the pegboard mold station in plce vertically.

 

Laborious? Not, I made my 35' strip planking hull in 200h and it include sawing the strips from 36x120mm planks. Needs a good amount of clamps thou. The amount of GF on size of your design, for structural loads even 200 gsm would be plenty but not for point loads and for wear and tear and that's on you to decide..

 

Okay, so I see my option as this:

1) If I want a smooth-chine hull, the foam strip planking seems like the best option, given the unavailability of quality lumber in my country, but it would be a lengthy and risky build, since the majority of strength comes from the composite on top of the foam, and it is very difficult to perform reliable calculations or FEA simulations for such sandwich constructions, meaning I wouldn't really know how stiff and strong the hull will be until it is already finished;
2) Go with traditional stitch&glue, tried and true, and try my best to maximize hydrodynamic efficiency through clever panel design despite hard chines, and make my peace with the fact that I won't be able to hit my 35 kg hull weight target.

Since option #2 is pretty clear, I wanted to further investigate option #1. I did some research, and unfortunately it seems that GPET foam is not available where I live, unless it is branded differently (but I'm not sure how to search for it then). SAN foam is available, but extremely expensive. PVC foam is also available, but also pricey, and the density doesn't match what Skip said. Here is a website that sells them in my country (sorry, they don't have English version of their site, and google translate messes it up), and the price for say 10mm (0.4") sheet is 28€ / m^2, which is ~$3 / sq. ft. Not sure how that compares to your prices, but yeah, I consider that very expensive. Problem is the density - it is around 500 kg / m^3, or 31 lbs / cu. ft. That is insanely heavy, about the same as lumber or plywood. Obviously I'd be using MUCH thinner foam then, probably no more than 4mm / 0.15", but from Skip's photos it seems that you are using much thicker foam. And the density you quoted is much lower than this PVC foam I found. I am not yet sure how to explain this, perhaps I am looking at the from kind of foam after all? Here is the material property table they provide:



Also the operating temperature is awful - seems like there might be issues on a hot sunny day.

Now, if I understand you right, Skip, you meant using GPET for the planking, and XPS for the fill of internal volumes between the bottom and the floor of the boat. I did some research into XPS, and it's use on boats seems to have been debated to death on this forum, and the general consensus is that this kind of foam is not structural at all due to lack of compressive and shear strength. Fallguy, I see that you posted quite a lot about this, discouraging people from using this foam for boats due to multitude of good reasons.

I believe that, but decided to put it to the test just to see what happens, since I had some XPS lying around and it did feel kind of stiff for it's weight. I cut out a 30 x 10 x 2 cm (~12" x 4" x 0.8") rectangular piece, sanded it with 100 grit paper, cleaned thoroughly, sandwitched with 3 layers of 200gsm (6oz) 3K twill carbon fiber cloth on each side with some regular laminating epoxy, vacuumed, and cured overnight at 50°C. Next day I knocked off the edges and stray bits with 100 grit sandpaper, and set up a very un-scientific cantilever bend test. One end clamped between a few pieces of wood/plywood, the other end cushioned with some cork and a bag hung on it with some weights in it. The piece was supported by 5cm (2") on each end, and the "free" length was 20cm (~8"). Yeah, I know that is too short for this material thickness, but I wasn't interested in specific numbers here, but more into the failure mode.

The piece endured about 8kg (17.5 lbs) in the bag before I heard the first signature crackling sounds of carbon fiber, and noticed the foam starting to compress at the clamped end, indicating that the sandwich was thinning in that area in order to reduce the distance between the carbon skins and therefore reduce the stiffness of the panel. Carbon seems to have failed in a very local "break" across the width of the test piece, which to me indicates that the foam was not rigid enough to provide decent enough support to distribute the load across a larger area of the skin, which is probably why it failed with such a small load. The other explanation is that my test was crap due to improper clamping and too short a piece. I was not able to break the piece completely though, since my clamping setup ain't rigid enough.

Here are some pics from the prep, the test, and the results.

The initial foam piece


The layup setup



Wetted out (50:50 fiber to resin ratio), bagged and vacuumed:



Cured and finished piece (the forbidden Oreo)



Shady test setup



Failure





Additional, after this test I also tried punching the piece, stepping on it with all my weight on one heel, kneeling on it, placing a stone on the piece and stepping on that stone, and in all these scenarios I could hear the foam crushing and sometimes even a bit of crackling from the carbon fiber.

This confirms to me what I read in all these posts all over this forum - this foam is definitely no good for anything structural. However, I understand that Skip meant using this foam only as a filler between much stronger GPET foam, meaning it would never experience any point loads, which is probably fine. And as I understand, GPET is actually a structural foam and would not fail like in my test, at least not with such laughably small loads.

Skip (or anyone else), could you please comment on my findings regarding the specifics of that GPET foam that you used? From what you said earlier I gathered that it should be very similar to SAN / PVC foam, but the density stated by my local seller is way off from what you said, and I can't find GPET specifically here. Like I said, maybe it's branded differently, but I'm not sure how to recognize it. Any comments would be really appreciated.

EDIT: one more quick point load test I made... Placed an M3 nut on the board, put a steel ruler on top, and stepped on it. Here is the result. It needs no comment Definitely not using XPS anywhere near the skin of the boat.

 

You mean you used wood planks? What kind? And yeah, point loads really worry me... That happens quite a lot on boats. Wear and tear can be protected against fairly easily, but predicting behavior for point loads - not so easy... Probably needs physical testing with various number of GF / CF layers. Then again, I suspect point loading is much more about the core rather than the skin, or is that wrong? So far in every test I've done it's always the core that fails in point loading, either directly or by failing to provide adequate backing to the skin, causing it to bend locally beyond limit (meaning the skin is essentially "alone" in trying to resist the load, rather than being a part of a sandwich).

 

Gutcheck here for you.

First of all, foam is easier, but there are downsides. The main downside is hull rigidity prior to glassing the inside. It’ll tend to be floppier. So best to build on athwart floors. And those should be glassed first before attaching the foam to them. The challenge here is build order. If you attach the hull core to the floors; you cannot glass the hull inside as a monocoque, but if you don’t; the hull will loss shape with only an outside skin. The way to win this battle is probably to use some sacrificial blocking. You’d set the preglassed floors(athwart frames) on a strongback and then build a sister for them. Hotglue the sister to the frame in a few spots allowing clearance for the hullshape. After all the sistering is done, you plank the hull. I’d use 12mm core minimum. 10mm might suffice, but the 12mm will allow more forming at a lower risk of needing more skin. So, plank it. Spot hotGlue the planking to the sisters (less is more), just enough that it isn’t floppy and can be formed. Glass the outside. Flip the hull. Then, you remove a section of floor and sisters or blockings and glass a few feet and then bond the frame back in to reduce losing the shape. Alternatively, you can bond the hull to the frames while planking and glass to them inside and tab the floors which is probably easier. The goal of all of it is to not lose shape after flipping. Those wide flares best friend is gravity,

I don’t quite ‘get’ the design or the reasons for the wide flare. You cannot load on the flare and when stepping aboard; you have to step over the flare or the boat is going over sideways. Maybe I have a misunderstanding about the design, but it looks like you will be raising vcg some with that weight up high and create some stability issues.

San or other marine foam would be the way to go. I’d tend to go with a db glass since you pickup so much strength in two layers of glass.

I prefer 12oz db glass for these curves. adding a sole? Go 12 oz all the way and use 12oz biax tapes for tabbing. It’ll be easy to overbuild her. If you find it at all floppy post interior work; reinforce locally using offcuts.

 

Never use xps. It works for coolers and outriggers, but not made for hulls.

 

Your test has an agriegous error. The elongation properties are a mismatch. In foam design, the properties of the glass and core and resin must align.

For your test, the elongation properties of the carbon are like 1%. The resin and core much higher like 8% or more; especially a day after bonding; the epoxy had not fully cured.

All your test did was show that bending the structure more than 1% or so did result in the expected failure of tbe carbon. The carbon failed; nothing else. It did show your lack of understanding of composite structures, but chalk it up to learning.

Other portions of the test were valid but misapplied. The sheer or compression rating of most xps is about 25psi. Your point load test was sort of also wrong in this regard. Most good marine foams would have failed the bb test. The reason is simple. Let’s say your bb was 1/4” in circumference. The very edge of the bb is infinitesimally small. Let’s call it 0.01” sq. At 100psi rated core, let’s say you are very small and weigh 100 pounds. When you stepped on the ruler, tbe effective loading was 100/0.01” or 10,000 psi. No resin or foam core system would have passed your bb test.

 

Thank you, I am not quite sure I understand the procedure you proposed yet (probably need to read it 10 more times and try to draw out on the paper), but it does sound quite involved. I would have expected that building the frames on a stronback (upside down, like in Skip's pic), adding foam between the frames, planking and glassing the entire bottom of the boat in one go would make it rigid enough to flip afterwards for planking and glassing the inside... Ok, I need to read what you wrote some more times, I'm clearly missing something here.

Can you clarify further on "marine foam"? Like, what am I looking for, exactly? Specialized stuff is extremely difficult to source here in Lithuania, so I need to have a very good idea of what I'm looking for.

As for the flares, well, the idea is to build a boat with an extremely narrow waterline and very high length/beam ratio, at least 6 (which translates to 29.5" waterline width), yet have wide enough cockpit area for hiking. The flares don't have to be high (just high enough to clear the water surface at a reasonable amount of heel), they just have to be wide. It's the same idea as on the Magnum series Moths, before they switched to foiling. And yes, I know the boat will be very unstable with such a narrow waterline, but I'm in good shape and can live with that for some extra speed. And yes, it will make climbing onto the boat more difficult, but I'm ok with that too. the main concern is to make these flares/wings strong enough so they don't break off.

Yeah, the cure time was short, but my epoxy specs say that at 50°C it should have completely cured in 6-8 hours (it's a fast curing epoxy), whereas I cured it for 12 hours. So it's definitely as good as it's going to be. But I agree with the rest, the foam is definitely miss-matched with this carbon, it's just what I had at hand, I didn't expect this to be a usable combo, just wanted to get a sense of how these things fail. And yeah, I don't understand composite structures well enough, that much is certain...

Re-reading what you said, perhaps GF would have fared better than CF with that foam..? Yeah, I know not to use XPS on a boat, I'm just wondering in general for educational purposes. GF has more elongation, closer to that foam, so perhaps it would have delayed failure? Or am I still thinking about it the wrong way?

 

I guess it was an M3 nut. Without going into a complex calculation; let’s say the M3 nut is 0.200”sq.

The same loading is 100/0.2 or 200psi. Most marine foam cores would fail this test. Gurit Corecell (san) M80 is rated for 168 pounds. The way around it is to add some more glass to the footwells. Keep in mind the crush rating of xps is only 25 psi, so your test was 8 times too much for the expected failure mode. Again, not understanding composites well. Not intended to rip on you here, but simply point out these failures were predictable without effort.

 

In my longwinded post, I was only trying to help you laminate a single skin on the inside; just read the alternatively portion in bold.

 

Marine foams will have a density of about 4 pounds per cubic foot or 60kg per cubic meter and compression and sheer ratings of 100psi or greater.

xps foams have compression and shear ratings of about 25psi ~ this means the glass skins will pull away too easily from the foam .. honestly the shear rating of an orange peel to an orange seems higher

 

Understood, no worries, feel free to critique, having worked very little with composites I do indeed lack the knowledge and appreciate being told where I'm wrong and why But if you say that even marine foam cores would fail this test, then it's indeed a very serious concern for foam core build. I can't remember how many times I stepped into the boat with some rocks stuck in my shoes. And while in this test I did have steel ruler as backer for applying the force, whereas a shoe would likely be much softer than the floor of the boat and would cushion most of the force as well, still, a small hard object being pressed against the floor (or the hull) by another hard object is still a very common scenario. I'm not sure how plywood compares to foams for this kind of point loading though, maybe I'm expecting the unreasonable here.

Okay, I will search for some foams that match the specs you said. I'm just wondering, given the choice, should I use GF or CF with SAN / marine foam? I have some CF stocked up from the other failed boat project, but what you said previously makes me think that perhaps it would be a worse match with such a foam, and maybe GF would perform better (at least for the same weight)?