Rogerio, 

   I am wondering, why not just build the Hitia?  The shape of the Melanesia hull is not designed to trail a rudder or mount catamaran beams; the Hitia is a similar size and designed to do exactly what you want.  I bet you could sell your unused Melanesia plans if the money is a concern.

   Unless you've already built the Melanesia.  In that case, I'd say "go for it" and let us know how it works out :^)

--Rich

several pieces of 40 cm (the width of the hull of the Melanesia) left from my Tiki 30. I bought the plan to make a paddle. But as I will have a catamaran soon, I thought I would learn on a small. Because I just sailed the monohulls...

Hitia is the next...

The Melanésia cat start build