"I think the two piece cone is little more efficient but the one piece is easier to produce."

That's about right, Mark. A two-piece cone has the potential to give most thrust, but the two parts have to be matched carefully for the best effect and that takes time and knowledge. The single, straight-sided cone gives the least thrust but is easy to make. The machined version we currently use on the 44s and 54s (not straight-sided but with a curve to it), is almost as good as a two-piece one - but the very best of the two-part ones might exceed it.

Sara Parish
Wren Turbines