Horizontal plank crack; a concern or not?


Curious about Wooden Canoes
I am replacing all the planking on a Thompson and all is going well so far...I think.

However as I was reviewing my work so far I noticed a four inch horizontal crack in the middle of one of the new planks. At first (after cursing) I assumed I had to remove & replace the plank. However after pondering a bit more I wondered how this horizontal crack is any different than the horizontal plank edges? If a horizontal plank edge is not a structural problem why would a (tight) horizontal crack be a concern?

How is this different than if I had installed two narrower planks instead of one wider plank?

Vertical cracks a concern, but a tight short horizontal crack is not?

Replace, repair or ignore? Maybe install some additional rib tacks as if the crack is a plank edge?

Thoughts, guidance and recommendations welcomed...thanks.
How fussy are you? Cracks sometimes happen. The way your planking is sawn, how dry it is, where on the hull and how much "curve" you are making with it influences whether or not it cracks. In the grand scheme it does not matter very much. You can (as you suggest) put a few more tacks in to secure it. You can opt to pull the piece and try another piece in it's place or you may choose to ignore it. Unless it's quite prominent varnish will likely fill it when you varnish the hull. On old hulls it is not at all unusual to have splits. If I am working on an old hull and if there are splits that don't seem to compromise the boat I leave them. I only replace a plank when there are multiple splits in the same plank or if the plank has been removed to work on the boat. I almost never put a split plank back on a boat.
As you proceed, you might be able to coax some of your planking to take the lines of the hull without splitting by using a hot iron and a damp cloth to set the shape of the plank before you tack it in place.