Hi Kevin - you'll have gaps like that in cases where the chain of edges comes to a sharp crease between 2 edge segments. In this case the sharp crease is where the 2 halves of your rudder come together, there is about a 10 degree deviation in the end tangents of the edges in that area.

A multi selection blend needs to have all of the edges be smooth to one another to generate just one surface as the output.

- Michael

Ok, thanks for the explanation Michael

Fortunately you've included extract isocurve and combined with loft-loose-exact works very well to give a nice blend between fuselage and fin.