Sometimes all it takes is a different perspective or experience. I should have checked my email earlier too! My posting about the diagonal bracing from yesterday produced a comment that turned on the light bulb in my head. Here I was forgetting the basics of geometry and trying to come up with a clever reason, besides I was biased because the diagonals "don't look nice". The comment Jim left was exactly what I needed; along with a visit to wikipedia to refresh my knowledge of tetrahedra, now I don't think I need to lay awake all night long ideas floating in and out of my head. I even have an idea on how to beef up the stem as Jim suggests, the answer is in the stern of the boat, interior to the rudder skeg. Thanks Jim!
posted by tsunamichaser at 11:32 PM
4 Comments:
You're very welcome. It's a little payback for the enjoyment I'm getting from following your progress!
You'll find precious little in the way of triangulation in multihull design. Wharram is better than most particularly since moving to triangular beams for the tikis.
There's a neat way to turn those beams into a simplified octet truss if you would like to know it.
Aahhg, That "griffon" is me, i.e. Jim of the tetrahedrons.
(Preview is my friend!)
Jim - here I am up in the 0500 hour anyway! I'd like to know more about octet trusses. I don't know of any beam failures in Tiki's but it is clearly a weak point and failures have occurred here on other multihulls. I'm planning on using roller reefing for the jib. As I understand, hull flex which is a plus in some aspects plays havoc with rig tuning.
Sorry I didn't check back. Wadya doin' up at 5:00 anyway?
First up, I'm not suggesting the beams as designed are not up to the job. This is merely how to make them stronger by quite a margin.
The old Tiki (21) beams were "I" beams which have low resistance to torsion. So if you want the boat to flex then they are the way to go, I suppose.
The later beams such as you have are triangular as you know. This shape is very resistent to torsion although not as good as a round tube. The tube will distribute the torsional stresses equally around the whole surface.
But I dont think the triangular beam will anywhere near as well. And herein lies the rub!
If you have a beam shape that doesn't like flexing as in torsion, it's best to make it VERY resistent to flexing. By placing triangular bulkheads inside the beam in such a way to create tetrahedrons and half octahredons (4 sided pyramids plus base), you create a structure that will spread any load equally along the entire structure. i.e. the skin will be loaded equally and will not ripple or buckle leading to failure.
To picture it, imagine you have joined the bottom plate with the vertical plate and with the angled face open.
Now put two ply triangles in the beam a little apart and lean them towards each other so that the edges that are across the open side touch along their length.
Underneath these two triangles you now have a tetrahedron (for other readers, that is a three sided pyramid with base making four triangular sides).
Now move along the beam and repeat this placing the bottom corner next to the bottom corner of the last triangle bulkhead.
Now you have two tetrahedrons under the triangular bulkheads and between them you also now have an open four sided pyramid whose square base will be formed when you place the angled beam face on over the top of these little bulkheads.
The four sided (plus square base) pyramid is half an octahedron. If you made another triangular beam which was a mirror of the first and placed the angled faces together, you would have an octet truss which would be a square beam in which all the internal space was taken up with octahedrons interspaced with tetrahedrons; the strongest self supporting structure there is.
The beam I described is only half an octet truss but it would be a lot stronger than the original. Again I'm not suggesting that the original design is deficient. I have no way of knowing that.
I hope I have explained this well enough. It's very difficult using words. An octet truss is difficult to picture properly even after you are familiar with it.
Make a scale model out of some cardboard and you will see it come together. If that doesn't work for you, I'll try another way. Perhaps with my tongue sticking out as I type; maybe that will help!
I believe it is hard to keep the jib luff taught with the flexing structure (actually, it's a mechanism. If it was a structure it would not flex. Mechanism bad. Structure good!).
The shrouds tend to whip as well!
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