Compound plywood bend in stitch&glue construction?

IMHO you'd be better, easier and faster doing strip planked hull.

 

Strip plank.

-Will

 

Faster than stitch&glue? I kind of doubt that! Well in fact I'd love to try it, but unfortunately there is a huge shortage of good quality lumber in my country for such a build. Even pine/spruce is total crap these days (heavy, knotty, fast-grown, poor dried, checked, full of sap). And for strip planking you want something even more lightweight, like cedar - and that has never been available in my country. Plywood is cheap and available, though. I suppose one could do strip planking with plywood... Very narrow strips... But that feels kind of silly. Feel free to prove me wrong

 

Cedar Wood Wholesale Suppliers, Manufacturers & Distributors in Lithuania | Bulk Prices & Deals https://www.go4worldbusiness.com/find?searchText=cedar-wood&BuyersOrSuppliers=suppliers&countryFilter%5B%5D=lithuania&cityFilter%5B%5D=&FindSuppliers=

-Will

 

Yeah, thanks, but if you look at the results, none of them actually sell cedar Trust me, I've searched, called around (for my previous projects). No chance of getting that for anything even remotely affordable.

 

The classic way to deal with compound bends in s&g is thinner strips over a jig, sanding and fairing.

Otherwise, next is strip plank.

There is no defying gravity.

 

Okay, well, how "dense" the jig should be to make these compound bends viable? Suppose we take that design in my first post. It has the bulkheads with curved edges that induce the compound bend, kind of performing like a jig, but there are very few of them. How many additional temporary bulkheads would be required to produce that shape? How densely spaced? Would I also need stringers between the jig bulkheads? And if I manage to get the panels onto that shape with these bulkheads + jig, and then glass the outside, how do estimate the amount of springback (if any) when the boat is removed from the jig?

 

If I was going to build such a shape (not likely at 81) I'd go with foam strip construction. Your cad system should easily crank out cross sections at whatever spacing you specify. Make the formers out of pegboard and strip the thing out of 8-12 mm foam.

 

Hmm, that seems doable. What kind of foam is that, what density? The glue you used is polyurethane, correct? How much fiberglass/carbon did you use on the outside and the inside?

 

The foam is Gpet, the current inexpensive (relatively) replacement for San or Pvc foams. Gpet is recyclable and as much as 40% less expensive at a cost of of about 80% of shear strength of San. I've used them all over the years; San is sold as Corecell here in the US, Pvc branded as Dyvinlcell or Klegecell. Densities are around 5-6 #/ cu ft. Yes it is polyurethane, the only place I recommend using the stuff.
That particular boat is covered with a 7 oz/ sq ft hybrid basalt/innegra on the outside, 5 oz kevlar inside. In the past I usually used 5 oz kevlar covered with 4 oz s-glass on the outside.
An advantage of using the foam is you can have a 4' x 8' sheet cut into 48" x 24" or 32" sheets and drastically cut down on shipping cost. I used to be able to buy corecell off cuts at a very reasonable but alas no more.

 

For strip planking you don't need 1st class wood as you can always cut of knots too big and go on. Besides spruce and asp (both good for strip planking) are much lighter than birch ply so you can do 6mm strips instead of 4mm ply (cut first to 7-8mm and sand down thus not needing 100% alignment laying the strips). On a boat that size you can use temporary frames and remove them after outside get's 300- 600 gsm biax over then glass the interior with 300 gsm, no frames needed but what you need for partners.

 

Interesting... Neither of these brands are available here, but I'll try to find something equivalent by density and material properties, surely there will be something. Very useful info, thanks! I wonder though, how well does that foam do in localized compression? Say for example that you fall on your knees onto the floor of the boat , would the core under the composite survive, or get locally crushed?

Yeah, you're right, but right now here it's difficult to get any more than 1-2 foot length of any dimension pine/spruce without knots or other heavy defects. To get enough planking material I'd be doing hundreds of scarf joints to avoid all the nasty stuff. I say this because I've done it a few years back when I built a birdsmouth mast. I had to throw away more than 80% of the lumber I got after all the scarfs were complete, and it took an insane amount of time..

As for no framing, that is an appealing option ,but since I do want a double bottom (for open transom), I suppose I would need to have at least some framing to support that floor, right?

 

Typically, you start off with something rational on spacing and then add BH/jig as needed if the strips/ply won’t cooperate.

18” or 1/2 meter would be my starting point knowing problem areas

 

If you do decide to strip plank,I'd be surprised if you could get a highly finished boat completed before the autumn.Which isn't to say that building one can't be done,rather it can't be done rapidly and the two factors that make it so are building a jig to plank over and then filling and fairing over the cloth.

 

400 gsm biax on the outside and 300 on the outside should be plenty. Probably no frames will be required but perimeter of hull neets stiffening and at least some deck to provide torsional rigidity. If you fall on your knees repeatedly in the exact same location while the hull is sitting on some rocks it is possible to cause some damage. I did the equivalent driving my canoe repeatedly into rock shoals ~ 120 times over a 4 day period on a remote river trip fully loaded with gear. The foam was compressed on the outside in line with the front of the minicell seat I sat on.

I missed the earlier comment on your raised floor. Consider making molds for the hull from floor up and use XPS foam for the bottom, Hotwire the foam in section to match your station spacing. Glue the the XPS blanks together and fasten to mold, fair lightly. Strip it all, glue the strips directly to XPS. Should make a slightly lighter truly unsinkable hull a bit quicker. The inside might be easier to dent though ;-)