Accurate plywood panels in CAD

What approaches do people use for creating accurate plywood panels to be used in stitch and glue boat construction? I want to model a chine hull form, and create accurate frame patterns using the inside faces of the hull surface, but if I just offset the outer hull surfaces, i get problem areas where the inner surfaces overlap.

I am guessing its important to first get a good representation of the inside surface of the hull before creating the bulkheads. I know that Sam Devlin and others actually overlap one plank with another, a bit like clinker ply construction. What I have tried so far (using Rhino 2.0) is to offsewt the outer surfaces, then match the inner overlapping edges using the average surfaces modifier, which means in theory the angle between the inner and the outer surfaces is exactly bisected. This means that for plotting the hull panels, i would use the outer hull surfaces. But it also means that for accurate fits, both panel edges would need to be bevelled by varying amounts along their lengths to get a tight fit. However, any frames taken from this surface would fit accurately.

Is there an easier, or more usually accepted way of doing this? It would be nice to have an accurate inner surface so that the frames fit, but not have to bevel the panel edges....(epoxy filler and glass tape would cover the gap on the outside)?

 

Andy,

If I understand what you're saying, you propose to forego scarfing the plywood joints and just butt the joints and tape and glue? Not sure I'd recommend that - scarfs are in there for a structural reason. You'd need a whole lot of tape and epoxy across that joint before I'd feel secure, then fairing would be a challenge.

For measuring, why not just measure the whole panel - stem to stern, then adjust panel sizes to accommodate the 8:1 or 12:1 scarf?

Either I'm oversimplifying, or you're overcomplexifying (to quote my president).

 

I don't use Rhino, but it seems like you are doing it the correct way.

Beveling the edges of the plywood with a hand plane would not take to long. You could easily get the precision you needed for the pannel/chine joint.

Using the outside hull pannel dimensions as you control line. You could pick up the bevel angle needed every 100 cm to use as a reference.

A tighter joint will use less epoxy. So you will save some time in mixing up the resin and silica.

As a second thought: you could offset the outside pannel dimesion line to reduce the pannel size by 2 mm. After that bevel it. This would make the pannel easier to fit and allow for some epoxy inbetween the pannel edges.

 

If we're talking stitch and glue,you should not need to be too precise about the chine joints,as you have noted filler and tape will do the job.I have no knowledge of Rhino and its commands but I would expect that if you work from a plane perpendicular to centreline and create lines in the positions where you wish the frames to be you would be able to project these lines onto the outer surface of the hull.If you then change the view,UCS,graphics plane or whatever the system describes it as so that you are working from either ahead or astern,can you offset the lines you have just created.If you can and if you can then trim the excess,you should have the frame shape you appear to be seeking.

 

Andy,
I have found much easier in Rhino to design plywood boat from inside surface. Then offset outside surface and blend (= filled while building) all the joints. Use loose offset to keep the surfaces simple. Use different layers for inside and outside surfaces. This helps a lot when cutting bulkheads and frames from the inside surface.

 

thanks terohalme - i took another look at the matched surfaces with the 'average surfaces' option and realised that both surfaces had been changed and the shell was no longer of constant thickness. I tried your suggestion and it seems to be the only workable one (unless anyone else has other ideas ) and so will continue with that. Seems a bit odd designing from the inside out, but until McNeel sort out a shelling method, itll have to do...

Andy

 

In Rhino 3.0 you can trim the offset surfaces against each other.
You should use something close to the middle of the plywood, the neutral axis when bending.

 

Sorry, I reread your post now.
To find the shape of the frames simply offset the surfaces and trim the results, then section for each frame. If necessary, the 2D drawings of the frames can be corrected n AutoCAD (or BricsCAD if money matters).