Because the ama on one tack works as ballast but on the other tack works as buoyancy;

What’s the usually suggested ama percentage of over-all displacement at the waterline,,, 25% like a proa?
What’s the usually suggested ama percentage of over-all displacement when totally submerged-to-the-deck be,,, 200% like a trimaran?
Where’s the usually suggested location of the ama’s stem relative to the vaka stem,,, even with it like a trimaran, or aft of it like a proa?

http://outriggersailingcanoes.blogspot.co.nz/2010/08/o-tahiti-nui-freedom.html

thanks Gary,
their ama is all the way forward,,, i wonder why? pitch resistance?
the print on that drawing was too small to make out the square sail area indicated next to the center-of-effort mark.

8 tons unladen - now that’s heavy.
i would think you could get the same length in under 3 tons using cold molding or strip planking.

I think the ama is all the way forward for the same reason that tris do it; to prevent diagonal capsize.  Not so much a problem with a cruising rig but a real one with a tall rig.
My Searunner 37 tri was about 4 - 4.5 tons unladen.  Going up to the next size Searunner 41 almost doubled the interior volume, so I can see a fifty footer built with average ply on frame construction to get close to 8 ton.

Gary

I forgot to include ama displacement when i thought 50’ could be under 3 tons.
Also, their vaka has alot of hull beam, while i was thinking a vaka with a one meter (3’) beam on the water line and 4.5’ (1.4m) max hull beam.
Though spartan, that could be under 4 tons and should be easily driven with 700sq.ft of sail.