The classical two cone origami kayak

[magwas]

I guess anyone who have played with origami building technique have already built this model.

But has it more merit than being a teaching tool? I.e. anyone have ever built such a boat, and was it worth the effort?

Here is a DXF (use the "plates" layer only), and a postscript of the beast.
And a very bad quality photo of the paper model.
Just for a reference.

[magwas]

The postscript didn't got through. Here iit is in PDF.

[Jeremy Harris]

There are a few ply canoes of this type around, I made a few models using this technique before opting for a more traditional looking hull. This thread on the Song of the Paddle forum has some good photos of the build process: http://www.songofthepaddle.co.uk/for...uild-with-pics

Jeremy

[magwas]

Well that one looks very nice. It is a logical adaptation of the "canonical" plan for plywood and canoe.
That means framing elements and four sheets (not two as in its title) instead of one. Cutting to fore and aft means a conceptual difference: there is only one direction of bend, not two. And cutting to two sides means that the bow and stern can actually be shaped from plywood.
And maybe this is the difference between a teaching tool and an actual vessel...

[Jeremy Harris]

I've built one, and it is actually two sheets, not four. The only reason for splitting the sheets again is to get the extra length from just two 8 x 4 sheets; the slightly diagonal cut across each sheet is joined together down the keel, with the only real cutout at the bow/stern.

The major problem I found was getting the ply to curve at the centre join, to overcome the very nasty looking bump where the two conical sections join. I found that the ply can be persuaded to stretch/shrink enough, with some internal support, so that the horrible bump is a little less obvious.

Even with the extra work involved this is a cheap and quick way to build a simple canoe.

Here are some photos of a model I built to check this build method out:

First, here's a scale 8 x 4 sheet of ply, cut diagonally to increase the length around the gunwales:





Next, the two sheets are put in place under the keel frame. You can see from this photo that joining the two diagonal cut lines gives more length from a single sheet of ply:







Next, one of the sheets is pulled down to meet the inwale rails. The wires are just there to hold the straight edges of the ply together whilst its forced around the curve. Note that there is no three dimensional bending here, the ply is always following a part of a cone, but it doesn't look like it! The force needed to bend the ply at this stage is modest, it doesn't need to be forced into place:







This is an internal photo after the keel has been cut away. In my view, the big keel section is only needed as a building jig, once the ply is glued in place the only stiffening needed at the bottom of the hull is another layer of thin ply, or, perhaps, some glass cloth:







For this model I chose to add some ply sections to reinforce the bottom of the hull:






I found this to be a fairly easy way to make a reasonably nice looking canoe hull. I went on and modified the method to produce a narrow slipper launch type hull, which I was intending to use for my "efficient electric boat" project, documented on another thread here:









In the end I never built the full size hull to this design, but opted to buy a proven hull with a similar shape. Nevertheless, it was an interesting experiment in what can be done with flat sheets of ply bent into sections of cones!

Jeremy

[kerosene]

Great job Jeremy!

[magwas]

Very nice!
I like the shape of your modified hull.

[Jeremy Harris]

Thanks for the kind words. I was hoping that the modified hull shape would retain the low resistance of the canoe hull form but stiffen up in roll as the flatter section aft started to immerse. I'm no boat designer, though, and decided in the end to trust someone who knew more about hull design to build me a somewhat similar hull.

Had I gone on to build the full size, two sheets of ply, slipper launch type hull, then I was planning to change the design to raise the sheer somewhat aft, and deck the fore and aft ends. I've just taken a few pictures of the final model I made of this proposed "two sheet hull" design. You can see the ugly bump where the fore and aft conical sections meet in one of them.







Jeremy

[magwas]

I am wondering about this "ugly bump'.
You have separated the fore and aft sheet. If it is not done, there is a little section in the middle which bends with a transverse axis, not a longitudinal one as the rest of the boat.
This makes the bottom of the hull a bit flatter, but I guess the ugly bump would be more pronounced in the sides.
If there would be two cuts near the middle, the boat would be a cone-cylinder-cone one.
Or keeping the middle cut and doing the teo near the middle, it would be a cone-smaller angle cone-smaller angle cone-cone one, thus the bumps would be smaller.
I attach a picture of the four cone development to make it more understandable.

[Jeremy Harris]

If you look at the second photo I posted you can see that the hull bottom is flat and that the whole boat could be cut from a single sheet as you suggest. The only reason for making it from four panels was to squeeze the maximum length, beam and hull depth from two sheets of standard size plywood.

To reduce the ugly bump means finding a way to take material out longitudinally I think, around the centre region where the turn of the bilge is most pronounced. I had thought of cutting two lengthways vees on each sheet, from the central join towards the bow and stern, either side of the keel line, right where the turn of the bilge gets tight. This might reduce the bump.

At least one similar origami style boat like this does this, although in that case the designer included a shallow chine at that point.

Jeremy