Hi Wayne & Derek,
As you appear to be using balsa to plank with, I will offer the following based upon my experiances, also with balsa.
Ther timbers have their specific characteristics and are not as 'easy' to use as balsa.
What Derek is endeavouring to say I think, is when planking, full size boats, and our wee models, planking is widest at the centre frame and gets progressively narrower towards the bow and stern. The amount is usually marked on the frames, then transferred to the parallel plank.
Starting with the centre frame, or the frame at the widest and deepest point on the hull, the plank widths are marked as equal spaces between the keel and the gunwale. Then using this number of spacings each frame is marked into equal spacings. On the sketch I have used 8 planks, and as you can see the distance between each marking is less than those at the centre frame. With balsa we can then mark these widths along the plank, and trum so we end up with tapered planks, which can then be offered up and glued into place.
Sometimes we can get away with tapering only one side of the plank, (this because balsa can be very forgiving and allow the plank to bend in the wide direction, whereas other timbers will not).
I generally taper my balsa planks both sides of the plank, or as the plank dictates. You will soon find what works for your hull.
Using this method, you will not have to infill with tapered part planks to "fill the gaps", as you will soon find if you continue as you are doing.
Hope this is of some help
cheers
vnkiwi