Hi Bob,

It's now middle of the night, so I have to wait till the morning for taking the pics. (My wife was already growling).

Yes I tried to make some static calculations but things went very complicated because of the diamond section shape which I had in mind. That section shape was a must for me because I'm used to sail fullspeed from the water on the beach. A nasty habit of a solo sailor.

I first glued the inner HOLLOW wooden core from redcedar planks (special sawn with the grain longitudinal, don't know the english expression). Planks of about 1.7 cm thick. Then rounded the corners a bit off before start laminating the core with layers of carbon and expoxy.

Used square woven and one-directional carbon. The one-directional layers were always packed between the square woven ones. The numbers of layers was a bit of a gamble. After I stopped and let the whole thing cure, I tested the beam by placing it on 2 telefonebooks on each side and jumping with my full weight in the middle.
The goal was no swing-movement in the beam what so ever.

I had to repeat this procedure another 2 times. (Ofcourse sanding up the beamlaminate very carefully before the next layers!). Ended up with total beamweight of 10 kg. each.

So it was a sort of trial and error method which IMO is also usefull for your 20 feet beams.

The pics will follow soon.