For your hull shape, balsa blocks would be most effective between frames 2 and 3 since there are very complex curves in this section of the hull. The planks make two turns as you go from stem to stern, bending outward from frame 2 and bending inward as they pass over frame 3. As you go from keel to gunwale, there is another "S" curve in that same planking. Making a hull shape from frames that are this far apart makes you rely on the consistency of how each plank curves as you bend it over the frames. Here is where having consistently grained and good lumber (nice hardwood, not bamboo or knotty softwood) comes into play. Without support from balsa blocks, wood strips that do not bend at consistent curves do not always line up edge to edge as you plank. The balsa blocks ensure that they do, because the strips are forced into place by being glued to the filler block. Looking at the overall hull shape, using filler blocks prevents areas of sharp curvature from forming vertical lines or sharp folds that indicate where frames are underneath, and flat spots between these folds, sort of like seeing your ribs sticking out of your chest after being stranded naked and afraid on a desert island with almost no food or water. You can see all the frame lines in the hull where curves are sharpest or where plank curvature changes direction. You need wood filler to hide all these imperfections, and that takes more time. If the folds over the frames are severe, no amount of filler will perfectly hide them.
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