Short post with main points :



Beamloading is about 1000 kg to 1500 kg in a static sense already, this means that these numbers are a minimum, real (static + dynamic) stresses are most probably higher !

I seriously doubt whether foam is enough to bend the inner beam into shape. I fully expect the foam to be compressed and the inner beam to work itself some slack in which to stay straight while the outer beam bends.

If you have to do this then do it alu on alu like an 80x2 outer beam and a 75x2 inner beam. That is how unstayed masts are made on landyachts.

I don't think your idea of tube with significantly different diameters and foam to fill the space between them has merit. I'm sorry.

Wouter