Compound plywood bend in stitch&glue construction?

CF will make a lighter, stiffer craft than GF but it will be somewhat brittle whereas the slightly heavier and still plenty stiff GF craft will bounce off the impact that cracks the CF craft.

 

CF is generally accepted as a mismatch for a marine foam and epoxy boat. It is used when the failure mode is not a concern. The Americas Cup boats often trade weight for durability and use CF as they are used one time, for example. My boat uses zero carbon fiber and is all epoxy and foam. I used CF as a decorative feature on a wood backer for my helm. CF does align well with plywood; your original choice of hull material; however the curvature demands you make do not match plywood. The alternative is wood strip. Wood, as long as it is thick enough will work with CF, but if you push the thickness down; the CF will also fail with a wooden skin that flexes the 1-2% I referred to in your test. So wood strip is also not a panacea. It will need to be probably 10mm or so on my simple gutcheck. What you will want to do is make some weight estimates for fair comparables.

The way to deal with rocks underfoot is to either accept delam or to put enough skin down in footwells and always get bits of rock out. My decks are built with san corecell M80. They have two layers of db1200. I also have a deck built with plascore and decks of 1708. When I see small rocks, I immediately get them off my boat. The point loading of a small rock is just very high as the surface can be so small. I weigh 100kg or 220 pounds. A small rock has a point of perhaps 0.01 square inches. My whole load on that rock on the skin is 22,000 psi! Most epoxy and glass on wood will also fail. What prevents a total catastrophe is the sheer rating. You see; a small point load delam has occurred on my boat. The reason it does not expand is the required force to shear is not present. Now, that said, the hull bottom is different. Delamination on the bottom of the boat is very bad. The reason is hydraulic pressures can provide the needed forces to shear the delaminated rock point and make it larger. The way to prevent this is to make the skin a bit heavier and more resistant to both hydraulics and rock damage. This can be done simply by using an offcut ans extra glass over the areas where rocks are likely.

 

Very useful info, thank you. I am surprised that you manage to build a boat strong enough JUST foam and epoxy, without CF / GF (apart from the decorative use you mentioned). That foam must be insanely strong then. Unfortunately it seems that there is no way to get it in my country. I called 8 different companies that sell various foam products, described what I'm looking for and they said they know it, but they don't ever ship that to Lithuania, there is no market for this stuff here... I'll keep searching though. But it seems that I might have to go with plywood again this time. Probably a similar design to that Paperjet that I posted as reference when asked about the mast partner, except for narrower hull with trapeze bottom and larger wings. And I'll make sure to use GF rather than CF, because I'll have to stick with thin plywood (4mm) to keep weight down.

 

see my edits above

yes, going very thin plywood will mean you’ll want to accept more movement than CF will allow ~ I foresee a build failure, though for you. 4mm plywood cannot be form sanded. In those areas where you know you’ll be sanding move to 6mm or even 9mm. On the inside; you’ll need to use putties to adapt to the changes in ply thickness. It won’t be pretty.

 

Thanks for that additional info, I'll keep that in mind. And I'll see if there are any EU sources with reasonable shipping for that specific foam you mentioned. As for form sanding, what do you mean? Fairing the hull after it has been stitched and glassed? I've built 3 plywood boats before (2 of them stitch and glue) with 4mm ply, never had the issue of sanding through even a single layer of the ply. Or did you mean something else?

 

I mean any pre-fairing prior to glass. If you decide you want to build with plywood, but want the flares rounded; you sand too much away with 4mm. It’ll get so thin the skin recommendation will be insufficient.

all strip boats usually get a bit of preliminary sanding before skins are applied and more with complex shapes

 

You boat can really be made with M60. M80 is a little tougher.

But you can also use Divinycel or other ovc foams. Even Gurit makes a pvc with very good mechanical properties.

avoid xps for all but temp uses .. I used it to insulate my livewell.. I made the top of the well with xps and will accept delam and skinned it well enough..maybe

 

For foam you need to find a specialist suplier. Brands available in europe are ArmaPET, 3D Core, Airex, Gurit just to name a few.

Regarding wood, since the softwood is of such poor quality the solution is to switch to hardwoods. Suitable hardwoods are:
1. Paulownia: limited availability and relatively high price. You need to look in hardware stores for shelves and other panels and cut them up. For example (currently unavailable of course): https://www.senukai.lt/p/sienine-le...-x-25-cm-x-1-5-cm/kblh?exp_variant=v3_variant Weight around 300kg/m3.
2. Balsa: same as above, some hardware stores have it, usually lenght limited. Weight around 150kg/m3.
3. Aspen (populus tremula), (drebulė). Weight 350-450 kg/m3.
4. Black alder (alnus glutinosa), (juodalksnio). Weight 500-550 kg/m3.
5. Linden (tilia cordata) (liepa). Weight 500-550kg/m3
6. Willow (salix alba, etc.), (gluosnis). Weight 330-560 kg/m3. Commercial availability is very spotty, but I included it here in case you happen to stumble upon it.

Where do you find the wood? You need someone supplying cabinet makers not the construction trades. Alder for example is a classic furniture wood because it can be easily stained to look like the much more expensive cherry. Even the pine and spruce that is sold to them is a lot nicer.
I for example found this people who say they do retail Mediena stalių gaminiams https://www.zilinskoimone.lt/mediena-staliu-gaminiams/# using Medienos pramonės įmonių portalas | Medis.lt https://www.medis.lt/ as the agregator.

For kayaks and canoes the usual layup would be 6mm thick wood with 200g/m2 plain or twill glass each side, so that's what I would use. For more stiffness increase the wood thickness. For more abrasion protection put on more glass.
Weightwise it's a wash, birch ply can be as heavy as 800kg/m2, that's 3.2kg/m2 in 4mm thickness. With 500kg/m3 wood, 6mm is 3kg/m2. On both you still have to add glass, epoxy and paint.
I would start looking for aspen/poplar, it's probably the lightest and cheapest option, but alder and linden are equally good. Before anybody mentions it, none of the woods mentioned in my post have any natural durability, and it doesn't matter.

 

Second Aspen which should be easily available in Baltic (at least Baltic aspen is sold to Finland). Easy to find relatively knot free and it's light, close to 420kg m^3 half of the weight of birch ply. Besides you can mix wood and foam strips, I've done so on the decks, bulkheads & dinghy etc..

 

I would avoid balsa as I know of too many instances of it absorbing a lot of water,becoming heavy and ultimately rotting.Is there really any intention of using ply anywhere now or is it a case of an online debate to arrive at a laminate schedule?I know of a good number of 4mm ply hulls that have survived several years and have owned a few myself.ls there tie to allow for the miserable and laborious task of filling and fairing over a laminated surface?I know of few more miserable tasks and it eats up time.At least if you paint the hull,there needn't be the numerous coats of clear resin that the canoe strippers have to apply and then get anxious about on stony ground.

If foam is chosen,something like 8mm 65kg/cu.m PVC of some sort would be a good start and structurally doesn't need much more than 450 gsm of glass on either side for sailing loads and maybe an extra ply of glass on the bottom in areas where stone damage is a possibility.I'm more concerned about containing the forces of an unstayed mast and the leverage of a helmsman sitting a good distance from the centreline.some carefully considered bulkheads will be essential and some thought should be given to a drain bung for the double bottom section.

 

Right, I think you misunderstood me there. I was weighing strip planking VS regular stitch&glue, NOT strip planking with plywood. If I were to go that way then yeah, 4mm would definitely not be enough, I see why plywood is not the material for strip planking... Sorry for the confusion.

I searched some more, but it seems there is nothing out there in Lithuania of these foams, only the PVC that weighs as much as construction lumber, so it's clearly not the kind of foam that you guys had in mind... At this point it seems that the only way to do strip planking is with hardwoods like Rumars suggested:

Thank you very much, the summary of the suitable woods you listed here is priceless, I had never come across anyone using anything but cedar for strip planking; this gives me new hope. By the way, I might have asked you this before, but are you by any chance from Lithuania too, or one of the neighbor countries..? You seem to know all the terms and suppliers.

I managed to call several lumberyards before the end of the work day, asking about these hardwoods you mentioned. Paulownia seems to have been available from that link you sent, but I can't seem to find anything else. I will look some more, since the density is very promising. Balsa is only available in very small pieces, usually from model shops, no lumberyards here have it in larger sizes (since it doesn't grow in Lithuania), I suspect these online shops import it as finished product from abroad. Linden and Willow aren't available as far as I've asked around, Black Alder is, but it's very heavy, about the same as pine/spruce... It seems that the only available lightweight option is Aspen - it is available, several lumberyards have it. The only issue is that it's sold at short lengths (2-3m at most), so I'd be scarfing a lot, and the price is about 8x that of regular construction lumber. I will call around some more, maybe there is some cheaper seller. But in any case, this seems to be the only realistic option for strip planking if I were to go that way. I will continue searching of course, 1 day of calling around is nothing.

Okay, so just to clarify, as I understand the construction would look something like this:
1) Raise plywood frames on a strongback (4mm ply);
2) Glue XPS foam in between the frames, completely filling the entire volume of the boat, and sanding it down to the frames;
3) Strip plank on top of the frames and the foam;
4) Add 450 gsm or heavier fiberglass cloth all over the boat;
5) Fair, prime and paint.

Is that correct? Fallguy, as I understand at some point you were suggesting adding fiberglass not just to the outside of the boat, but also on the inside of the planking (in this case, between the planking and the frames/XPS foam). I'm not entirely sure, is that required, or not? I can imagine it would make building much more difficult, as I would have to use epoxy for gluing strip planks together and to the fiberglass, as regular wood glue (typically used for strip planking) wouldn't work here.

I also forsee several issues:
1) The flare/wings - I am not entirely sure if they would be strong enough if I built them from that Aspen as core and fiberglass on the outside, or they need to be much thicker, maybe with some frames extending into the insides of the wings. I have no idea how to calculate the strength of such a construction and mixture of materials.
2) Ripping the Aspen planks and shaping their edges to U/inverted U profile for joinery would mean a ton of ripping, and I don't have a decent enough tablesaw (nor space for it), and also my neighbors won't allow any such noisy tools being used. I am not entirely sure if all that plank prep can be done in some easier and more quiet way. This is maybe not the kind of issue that needs be discussed here, but I'm just mentioning it because it might be a showstopper. When we were considering foam, I expected to be able to rip it with a straightedge and a box knife, but that clearly wouldn't work with wood.
3) It also seems to me that the boat might still end up quite heavy. Rumars, you said Aspen is 350-550 kg/m^3, but I'd probably need 6mm at minimum (probably more since this is not a canoe), and the total area of the boat is around 12 square meters, so that's 3kg / m^3 * 12 = 36kg just for the core, not to mention the XPS foam fill, frames, fiberglass, fairing, paint, hardware, etc., so I'd probably be around 50kg when finished... That is insanely heavy. My current boat is 50kg. The whole reason I'm building this new one is because I want it to weigh 35kg max. That weight seems to be almost doable with stitch&glue... It could be done very easily if compound curvature was possible (the reason I started this topic), but since we figured out it isn't, it would need more structure and would likely end up more like 40kg. Still much better than 50 though. Please correct me if I'm wrong here, I really don't know jack about strip planking...

 

Well, like I wrote before, I am not yet convinced to go for strip planking due to the weight and build complexity concerns, my backup plan is to forgo the smooth chines and do regular stitch&glue with 4mm ply, probably something very similar to PaperJet. At least it would be reasonable lightweight, even if not as lightweight as I originally planned for... Low weight is the entire reason for this whole build. I already have a heavy boat

 

4mm thickness? no matter how much rigidity you can obtain from shape/form, the ability to withstand point loading is quite limited.
Looking at your design on page one reminds me of a line by a well-known NA, "It's so easy to draw something that's so hard to build".
Can you not find a design that's optimized for stitch-and-glue construction that will result in a boat that has a much greater level of general usefulness over its lifespan?
Exotic shapes are best obtained by multiple layers of thin wood strips, essentially, you're producing a piece of plywood in the shape you want.

 

No way I can see coming in under 100 pounds. I don’t have a good sense for your sizes, but that seems like a lot of boat. I made the mistake years ago of making my canoe with 3/16” strips and it does okay, but I see ripples underway. Too thin

 

I'll try to answer some questions as best I can.
1. Weight. The published densities of aspen vary in the above-mentioned range but the accepted norm seems to be 410-450kg/m3. This means 2.46-2.7kg/m2 for 6mm. Comes out 29.5- 32.4kg for the core. To this we add fiberglass on both sides, so 24m2x200g/m2=4.8kg, plus the same amount of epoxy, plus some paint and the frames. If it comes out under 45kg without rudder, daggerboard and mast it's a good day.
Western red cedar isn't much lighter, if I take 350kg/m3 the core is 25.2kg, it would be hard to keep the naked hull under 36kg.
Realistically you can't keep the hull under 36kg with birch ply either, if I use 750kg/m3 it's 36kg just for the hull surface without any epoxy fillets and fiberglass tapes or frames. I would be possible with a lighter plywood that weighs a maximum of 550kg/m3. That would be okoume or poplar ply. The problem is, okoume is expensive and maybe not available to you and poplar is seldomly available in the required quality (WBP glue and "no" voids).

With foam you need more glass, since the foam only contributes stiffness not strenght. Using a standard 80kg/m2 and 8mm thickness it's 7.68kg for the core. With 450g/m2 glass on each side, it's 10.8kg of glass. Plus another 10kg of epoxy, frames, some fairing and paint. Can you keep the naked hull under 36kg, probably yes. The complete fitted out boat, maybe. You can go to a thicker core and lighter glass and make it a definitive yes, but you have to accept the resulting fragility regarding puncture and abrasion. O course the lighter you make the boat the easier it is to just lift it instead of dragging.

2. Cost. Wood cost increases the more processed it is. So cheapest is the round tree, then green flitches (boards with bark on the edge), dried flitches, rectangular boards (called equalised lumber), planed boards.
If you can wait a little you buy green flitches, if not whatever is cheapest dried. Flitches and boards usually come in 25 and 50mm thickness, you slice the strips from the side using a handheld circular saw with a fence set at 6.5mm, resulting in 6.5x25 or 6.5x50mm. You use them as is, no further machining, the thickened epoxy will take care of the joints, and the sanding after installation of the rest.

Of course you can pay someone to do the job for you, usually at some cabinetmaker shop. My advice for this, find a local cabinetmaker and tell him you need three aspen boards, 4000x250x50mm (or similar in cubic meters) cut into 6x50mm strips. Ask for a price out the door, with lumber and cutting.
Another thing, many of the wood sellers will tell you over the phone they only sell to businesses in cubic meters amounts. Don't be discouraged, go there, tell them yo need 3 or 4 clear boards for a desk, right now cash in hand, no receipt needed, how much please. Be prepared to have to pay and load them onto your car on the spot if it works.

Foam costs what it costs, PET is the cheapest, then PVC. PET has lower shear strength then PVC so you need a heavier grade. At the moment ArmaPET Struct GR100, 10mm can be had in Germany for 18.03€/sqm retail. The problem is shipping, it comes in 2448mm x 1008mm plates, so you need something local.
It's actually funny, Diab (makers of Divinycell) actually has a factory in Lithuania but the sales office is in Poland and the distributor in Estonia. Contact us - Diab https://www.diabgroup.com/contact/
One thing with foam, epoxy isn't a must, polyester and vinylester can be used and are usually cheaper. But, the neighbors might not appreciate the smell, so it's not always an advantage the home builder can use.

3. Strip planking: planed and bead and cove strips are used by builders who want a natural wood look (clear, laquered). Otherwise square, raw strips work very well, some filler and paint will hide all the sins. Never heard of someone using profiled strips with foam, only square, but that doesn't mean someone didn't use it.
Strips are fixed to temporary frames, usually 12-19mm MDF, chipboard, OSB or solid wood, with staples, double headed nails, clamps, fishing line, etc. After the glue between the strips hardens, fasteners are removed, the surface faired and sanded smooth, then glassed. This is the same regardless if you use male or female forms, wood or foam. Then the hull is flipped and the process repeated on the other side. Frames and other stiffeners are installed after the interior has been glassed, via fillets and tape.

4. Foam building: stripping isn't the only way to get complex shapes with foam. That's why double cutting is offered, but that needs a more advanced mold system. Some foams can be thermoformed. Also stripping doesn't need to be longitudinal, you can go transverse with much wider strips.

In conclusion, I can't tell you what to do. One possibility would be to go more Moth, replace the hard wings with a mesh or fabric on aluminum or carbon tube outriggers (maybe with small xps floats under them so you don't capsize instantly). With the reduced surface area of the solid hull the heavier materials might allow for your weight target at a lower cost.
If you can find some good poplar or okoume ply obtaining the compound shape might be possible with some carefully placed darts, and fiberglass reinforcement. You will want to revisit this thread, it's a similar shape buildt in 3mm ply, but it's a longer boat. 6.5 to 7.5 metre performance/cruise multihulls https://www.boatdesign.net/threads/6-5-to-7-5-metre-performance-cruise-multihulls.52655/