Compound plywood bend in stitch&glue construction?

The hull presented in post #70 has one major defect,there is only one buoyancy compartment.It needs more,ideally two more so that the boat will float reasonably well if damage should occur while sitting on those rocks we have been warned about.I would suggest adding another transverse frame in the wing area too.The mast step and retaining collar aren't shown and yet they will be the most highly stressed parts of the boat.The comment about ugliness needs to be tempered with the realisation that for racing purposes,speed is more useful than beauty and then.as if by magic,the boat that succeeds on the race course will be considered beautiful.Speed of building is quite useful too and that is helped by keeping things simple unless there is some good reason for complication.

 

Ok, here are my thoughts about tortured ply, and how I think Gary was able to obtain that shape. First of all is experience and a good eye, he is able to visualize the shape the ply will take. Second is a long slender hull with almost straight topsides and not a lot of rocker.
There is plenty of separation between bends in different directions, there is no true compound bending, there are flats in between, even if they aren't all that obvious.
A key to success is probably the dry run, this lets one better plan the individual scarfing locations between sheets.

As I understand it one way of trying out the shape is to build a scale model using a material of similar stiffness. The thin 2mm aircraft ply seems to be a good match of how the real ply might behave, and if changes can be made to the hull to accommodate the bends.

 

It doesn't have to be flats,a single straight line that is common to one conic element of the panel and it's adjacent conic section would be sufficient and there may be no practical upper limit to the number of conic sections in each panel.The challenge is to form the hull in such a way that the panel adheres to the shape that you would like it to be.It is possible to introduce a small measure of deviation from these ideal conic shapes and this is the torturing process.How much distortion may vary from one species of wood to another and from one batch of plywood to another.Model building is a very good guide,but not an absolute guarantee of success.Thinner plywood may be more amenable to some distortion and will mean that closer supports will have to be provided in stressed areas.To get a good idea of what may be achieved it really is worth seeking out build sequences of Moths from the lowrider era or Int canoes.The Magnum series mentioned throughout this thread had 3mm Okoume/gaboon hulls and no glass sheathing.They did need careful handling but were amazingly fast.They also needed to be built over a well designed male former to ensure fidelity to the design shape.The challenge for this build will be to create the best shape that can be created without requiring excessive construction of a former.The simple solution is to use pure conic or multi-conic shapes and accept the aesthetic compromise while retaining the speed producing shape.The best reference to creating the multi-conic shape that I ever read was in this book ; https://www.amazon.co.uk/Aluminium-Boatbuilding-Ernest-H-Sims/dp/1574091131

 

Thank you, Rumars, these are some really good tips. But like I mentioned before, balsa is not available where I live, only very small pieces (typically 10x20 cm sheets, sometimes a bit larger) get imported in Lithuania. As for cedar, I called the place that I found that imports it - all would be great, except for the price, which is astronomical. 4600€ / m^3, or ~$150 per cubic foot. And they only sell 0.5 m^3 minimum, no scraps or off-cuts. And they are the only dealer. So yeah, I guess this option is out. I also took a drive to 3 lumberyards that sell aspen, and the situation is pretty bad too. The quality is total crap (warped, cracked), and the price is 2000 € / m^3, which is also VERY expensive compared to construction lumber which is ~300 € / m^3. Plus, they only sell aspen in lengths of 1.5 or less, which means at least 4 scarfs per boat length. The weight savings with aspen are very small in comparison. I called a few more lumberyards, described what I saw and asked if they have any better stuff, and the answer is no. It is what it is, I guess.

So, cedar is out, aspen is expensive, extremely bad quality and would require tons of scarfing to get anything usable. Couple that with the complexity of building the mixed-method boat, and this option is starting to lose it's appeal...

I am sorry to be shooting down your ideas, guys, I really appreciate the time you already invested into guiding and helping me with this project, and I am looking for ways to make your suggestions work, but the reality of the material unavailability keeps cutting me off at every turn. I will continue searching for any suitable strip planking materials from what you suggested, but it's looking pretty bleak at this point.

Thank you for making that sketch, I see your point now. Truth be told, I never hike like that, I always keep my butt on the edge of the boat, which is why I'm making the boat with these wide flares. The reason for that is because I almost exclusively sail inland (lakes), where wind conditions are extremely shifty and unpredictable, often with little indication of changes on the water surface due to surrounding terrain (hills, tall trees, etc.). This means that if you hike in the position you drew, and the wind suddenly dies, you fall over to windward, as you have no leverage to get back inside the boat (trying to do so induces a turning moment which turns the boat over even faster). With your butt on the chine, you can slide longitudinally and very fast, avoiding creating that turning moment. Your point is still valid though, and I'll see if I can make that batten wider.

Well damn, I guess I've been lucky to have never damaged any of my boats to THAT extent I guess the compartments could be separated, but it would make getting water/moisture out much more difficult during normal operation. I typically have a single continuous compartment throughout the boat with hatches at either end, so the moisture can be driven through A->B point circulation.

Thank you, I have been waiting for this comment for a long while now. Very good observation about separation of different bends. I am just not sure how to achieve that with CAD software when the goal is to have a smooth hull with C2 continuities all over, rather than build it in discrete sections. You said "straight topsides" though, but Gary's hull has something very similar to my flares/wings built into them, bent from the same sheet of plywood. No matter how I look at these pics from these two big threads you and I linked before, I can't quite grasp how could bend directions be separated like you described...

And yeah, I'm all for building a scale model, but without knowing what shape I'm trying to achieve, I can't build the frame, and I can't build the frame without knowing how the plywood will behave. It's a bit of a chicken and egg problem. I still feel that there is a lot to learn here in terms of pure theory so I can at least get in the ballpark for scale model building.

Thank you, the multi-conic method was something I've kept seeing all over, but so far my brain can't figure out how to actually design a hull with just cones (and then slightly deviate as you suggested to smooth things out). That books looks extremely promising. I'm not sure if I'll be able to get it before I have to build this boat (summer is just around the corner, and my feet are burning), but even if I have to save this for the next boat, this resource is very very useful.

EDIT: I found that book in PDF, so I can start reading it now after all. I still bought it to support the author, but I won't have to wait for shipping


Okay, so with the material unavailability, I am unfortunately forced to shelf the mixed-method design for now, and reconsider pure plywood build, as it appears to be the only realistic option right now. That likely means raising the weight limit to 40kg. 4mm Okoume is the lightest and thinnest that I can get. If I can fully uncover Gary's method of achieving these crazy complicated shapes, then I can in theory build the outside/bottom of the boat from a single scarfed sheet of plywood, and use very few bulkheads, like he did. This would yield round-chine hull like I wanted, and considerable weight savings.

If I can't figure it out, then I have to sacrifice round chines and go with stitch&glue, using trapeze bottom with slit V-shaped middle panel, which is heavier due to more filleting and glassing the seams, and probably needs more bulkheads. I'm reluctant to return to this after you guys gave me so much great advise with alternative build methods, but if materials are not available, then I have little choice in the matter.

Anyway, to avoid sitting idle with analysis paralysis, I threw together a design with probably a few too many bulkheads, but structurally it seems to make sense overall. I included the mast supports and daggerboard case for a more complete picture and weight estimations.



Now, for the weight (I'll calculate a bit differently this time), the total dry (no fiberglass, no epoxy) weight of what you see above is 29.5 kg. This includes all the plywood, edge stringer, supports on the keel, etc.

Now, you said not to glass the inside of the boat, so the following numbers assume glass on one side. I'll add epoxy on the inside later.
The bottom of the boat and underside of the flares is 6 m^2. So with 200 gsm cloth that would be 0.200 * 2 * 6 = 2.4 kg.
The top of the flares, the floor, the deck (all the topside) is 5 m^2, so 0.200 * 2 * 5 = 2 kg (this could be either fiberglass or carbon fiber like you suggested)
The total inside area (everything that needs epoxy but no glass) is 13 m^2, so with epoxy at 0.200 g/m^2, that is 0.200 * 13 = 2.6 kg.
Fillets and seam tape probably 1kg more.
Fairing, paint and hardware 3kg.

So in total we're looking at 40.5kg. But this assumes no foam, and I think it is needed for floor support between the bulkheads. There won't be a lot of it of course, not like I've drawn previously. Rumars, I will test the plywood with your method to try and estimate exactly how sparse I can place it. But I assume there will be at least 1kg more.

So, I'm over the raised weight limit by some 1.5kg. I can probably shave that down a little, maybe 0.5kg, but probably not more. I just don't see where else I can save weight, the structure is pretty bare-bones as it is. Maybe one less bulkhead, but bulkhead is just 0.2 - 0.25kg on average.

The design I've showed here has no compound curvature on any panel except the bow deck and the top surface of the flares. These panels will be installed last, when the boat is already rigid, so it should be possible to torture them a little for extra stiffness.

I ordered some 1.2mm model aircraft grade plywood (3 layers) to make a scale model, but it will take a few weeks to arrive. In the meantime, I 3D printed a simple structure with 3 bulkheads and some flat panels separately, to try and get a feel if this shape can be done with stitch and glue.







I didn't do a great job of gluing it all together, but in essence it seems to work, the panels conform to the expected shape with just 3 bulkheads, whereas in my design I have 6. Of course the model is way too small to make any definitive conclusions, it's just 31 cm, I need to make one at least twice as big and use real materials instead of plastic, but this is what I had at hand.

Again, not too happy about having to ditch round chines, but I did try very hard to make this multi-panel hull as smooth as possible, the angle between adjacent panels never exceeding 20°. Hydrostatics need improvement, but it's difficult to get enough displacement without increasing Cp, wetted surface area, and losing fine bow entry angle. Quite close though.

Disp. 140 kg.
LWL 4.42 m.
BWL 0.78 m.
Wet surf. 2.45 m^2
L/D 5.66
LCB 0.43
Cb 0.33
Cp 0.52
Cw 0.64
Entry ang. 16.6°

 

Addition to previous post: foam ain't looking too hot either. With this arrangement (200-280mm spacing), the foam comes out to 2.6 kg of additional weight, way more than I previously anticipated. And I'm not even sure if that spacing isn't actually too sparse, need to make that foot test that you guys suggested. This assumes 20mm thick foam at 50 kg/m^3, which is what I measured XPS to be from the samples I got lying around in my home. Not great at all.

 

After careful deliberation I have decided it's impossible to not find foam cores in Lithuania, so I exercised my searching skills.

Putplastis Atlas F60 10mm | Poliesterinės,epoksidinės, PUR, dervos, gelkautai,silikonai,glaistai https://www.topplastas.lt/putplastis-atlas-f60-10mm-2169-1 this company sells a foam/honeycomb hybrid that's designed for wind turbine nacelle covers.

Apie mus | UAB "Compositus" https://www.compositus.lt/page/apie-mus If my translator isn't broken this company offers you to deliver whatever r-g has in their catalog, and they most definitely have cores.

 

4mm ply no glass inside?

depends on chine widths, not gonna work over say 10”, if you ask me .. Rumars correction, I will accept..

 

Re your "fine entry angle" . In my experience that is at most a very low secondary consideration. In the world of marathon racing canoes I've had more than one design that was rule limited for length, for example a 3 man Ruta Maya canoe can be no longer than 20'. A competitive 3 man team is fastest in about a 27' or so boat. An optimised design for the 20' boat kinda snubs off the finely pointed end as the best compromise for minimum drag at design speed. The guys that built the first one said it's too blunt in the bow and reshaped the ends. I argued the point to no avail. A second boat was built as designed. I always wanted to see the boats match raced, swapping teams to see which was better. Two realities I believe one the difference would have been slight if measurable and secondly a 27' would be better than either but still barely measurable.

 

Other than aesthetics,I don't understand the reluctance to build with chines.For a comparison,look at the comparative speed of a Nethercott design International canoe and their more modern descendants.The more recent boats have chines and go faster.In fact they are probably the best source of inspiration for this project,but with the deletion of the sliding seat and the adoption of flares there isn't a huge conceptual difference.Light narrow and intended for performance,they have been evolving for a long time.

The arrival of some plywood for a larger model ought to be significant and should iron out any shape concerns.While it may give some data about shapes,I don't think too much reliance on various coefficients will be useful.Since Frank Bethwaite wrote about multi mode sailing of high performance boats we have to consider that the alterations in trim he mentions will mean sailing with underwater shapes that can be changed in different conditions and it might be an interesting exercise to compute the way the coefficients can vary accordingly.

 

Sometimes I'm amazed that I can't find anything in my local google search, I searched for F60 multiple times and nothing showed up, even now after you showed it to me. Either my search is broken or I'm dumb. In any case, thank you very much, I'll call them and if it's all good, I'll see if I can afford this option both in terms of money and the time I got left to build this boat.

The widest chine panel I got is 8" between unsupported areas, if we don't count foam as support. I had similar spacing in my previous boats, seemed to be fine, but I'll test some more to make sure, thank you for that reminder. Maybe I will add a few strips of fiberglass on the inside of the boat in a few key areas anyway.

You may be right, I based this decision on some papers I've read on catamaran hulls, where it said that hulls with very small entry angle and L/D ratio of 7 or more can exceed theoretical hull speed without planing much easier than more traditional hulls. I was not able to reach L/D of 7 with this design (I'm even struggling to reach 6) because the length of my workshop where I'll build and store the boat is limiting LOA), so I thought that at least I'd try to keep entry angle as small as possible. From my anecdotal evidence I've noticed a pattern in regattas that boats with finer bows create a smaller bow wave and less splashing, although that may not be an accurate representation of overall performance. But of course this is all theoretical.

I also noticed that modern designs often have round chines at the bow and then transition to hard chines towards the aft, like RS Aero:



I'm seeing similar pattern on IC as well:



I suspect the reason for this is the expectations that these boats will perform at or above planing speed most of the time, so they keep front end with smooth chines to prevent early turbulence, and then transition to these hard chines to promote clean flow separation by the part of the hull that does most of the lifting. This kind of shape could roughly be approximated by a V-shaped plywood hull made from 3 panels like that PaperJet I mentioned several times:



The reason I didn't go for this is the expected wind conditions and boat speeds at which I'm expecting to race. From my ~12 years of experience in local regattas, 95% of the time we get winds of 3 kts or less, and planing conditions occur maybe once a season. So there is little reason to design the hull shape for planing. I tried that with my previous boat, and it didn't pay off, I got enough wind to plane maybe 4 times in 3 summers. Therefore, I designed this new hull with low wetted surface area, low CP and smooth chines to prevent bow turbulence at slow and medium speeds as much as possible. That was my reasoning. But maybe you're right, maybe I am overestimating the importance of it.

As for changing boat characteristics with heel/trim, yes, I do design for that too. With this design I tried to keep hydrostatics unchanging as much as possible between 0-20° of heel (including wetted surface area, to prevent the need to heel the boat in light winds), so the boat behavior doesn't change much during gusts, but keep hydrostatics adjustable with trim, since that is easier to control and is less affected by wind changes.


Well in any case, perhaps I have already taken too much of your time. Rumars, Fallguy, Wetfeet, Skip and others, you have provided a wealth of info for this project, and I am very grateful to you all. I have more than enough to work with, and I don't want to bother you guys at every step of the way. I will make my final decision in 2-3 days time, spend the rest of the week designing and preparing production drawings/templates, and begin building the boat hopefully early next week. Otherwise the summer will come around and I'll be still on the computer screen

Again, much appreciated. If I pull this project off, I will of course share the results with you.

 

Laukejas, please be careful with the Atlas F60 foam, don't buy it before you get the technical data sheet. From what I could find on the internet this isn't a "normal" PVC or SAN foam, it's a honeycomb/foam hybrid designed specially for a particular application. This doesn't mean you can't use it, but you need to confirm the properties. There isn't much data available in the public domain, but it seems the strength and sheer values are lower then common boatbuilding foams.
The guys that import from R-G are a safe bet, R-G sells Divinycell.
Prices are what they are.

BTW, I also found you a source of clear dry aspen battens, at least the website says they have it in stock. Stefa drebulė (gultai) 28×72 mm (AA) https://ejbaltic.lt/produktas/morta-juodalksnis-plautai/ You need about 6 or seven of this battens to get 3m2 of 6mmx28mm stock. Not exactly cheap (I calculated 1000€/m3) but on the other side you only need about 40€ worth of wood (includes sawing waste). The ideal situation would be to go to their warehouse with a small scale and actually weigh a bunch of individual battens and select the lightest. 400kg/m3 means a batten should weigh ~1.9kg. Wood has enough density variation that in a reasonable size pile it's likely to find a few battens under 2kg.

 

Will do, thank you. I didn't visit that place you linked to yet (very long drive), I called them before though, they said their aspen isn't nearly as nice in these photos nowadays (the wood quality went down a lot during the past few years), but maybe I can pick something. Even if I can't get enough aspen for the entire boat, would it make sense to replace pine with aspen for stuff like floor supports, gunwale strips, stringers, daggerboard/rudder blade core..? I read some sources saying that aspen fares much worse when it comes to rot compared to pine, so not sure about parts that will inevitably see damage (like gunwales), but using it would yield a considerable weigh saving anyway. Not for spars of course.