I've had an email from a non-member who read this topic, and put the question on a multihull forum. He got a reply from a multihull designer (whom I deduce to be Rob Denney) saying that "without having the RM of the L17 he could not give a precise answer, and he also thought a length of aluminum tube would have needless weight at the top. BUT he did say that he thought a 4 inch dia tube would get the chap sailing and it was quite likely that a bit less would do".

G'day,

I am currently building a 17 foot W17 Trimaran that I wish to rig as an unstayed Junk rig.

My designer and other sundry sources are of the general opinion that only a fool would attempt this, and this is like waving a red flag at at a very nasty bull.

Would anyone like to advise on a tube that would be right for approx 160 sq foot of sail. By my reckoning I would need about a 18/20 foot mast above the partners.  I cannot supply the displacement, however it is a light weight Trimaran that I suspect would take very little force to "lift an AMA"
I have noted on other forum's that someone recommended 3mm tube by 125mm
diameter.
Any advice would be appreciated.