First cut is the deepest...

You can make an internal sleeve from the same size tube with a sliver cut out from it - I had one inside Tystie's mast - but it's hard to do, and I'm not convinced it adds much strength. I think it's better practice to use a sleeve of uncut tube, and if it's too big, fill the gap with casting polyurethane.

Yes, the maximum bending stress is at deck level.

I can think of two 8 metre boats with 6" x 3/16" masts - Fantail and Fanshi. However, these masts are hybrids, and the tube portion is of a stronger spec than the usual 6063 of a spun-tapered flagpole. I think the spun-tapered tubes will be stiff enough; and will be strong enough for cruising in a fairly relaxed manner. For cruising to wild and remote areas, perhaps the lower end could be reinforced by an external alloy sleeve extending from the heel up to the tack area of the sail, if one can be found that is a reasonably close sliding fit. 

Nice project, Thomas! Good to read that you start tackling it! :)

Of course I understand your point about that mast is what you habe... but still: On Ilvy, the mast is 177x5mm for roughly the lower half, at 35m2 sail ares. It is quite bendy, I wouldn't feel comfortable with an even lower diameter... 

Do you know any further specs about the aluminium? Which one is it?

PS: backstays unfortunately rule out some major junk rig advantages...

It looks good.  I had a very similar, 35 sq m sailplan on 5-tonne Arion, and found it more than enough, very powerful once the wind was free, from a close reach to a run.  I could reef a panel down in 15 knots on a broad reach and not slow down, and often had two panels reefed down in 20 knots of wind.

I did have a heavier mast, 200mm dia at the partners and 110mm at the truck, with a 5mm wall thickness, spun tapered.  150mm dia seems a bit light to me, but then I am a conservative sailor. I am hoping to find something similar to Arion's mast for my latest project, Arion II, in a year or two.

Anyway, lovely project, and I look forward to hearing of your progress.