Now the planking is getting closer. For my own part, I think it is good to fill the space between the frames with balsa.
On the long sides, where there is no bending to speak of, I have used approx. 8-10 mm balsa and where it bends more, I have filled in completely.
I think the hull will be smoother and more stable. The first hulls I built I didn't fill in and then some moldings sank in and even when the hull was planked I could sometimes feel that it was failing. Probably due to the fact that I did not get all the moldings to be completely glued to each other.