How many planks to save?

Howard Caplan

Wooden Canoe Maniac
If I end up doing what I thought may need to be done from the begining, how do I do it? As I inspect and remove cracked planks from this 1946 Penn Yan, I see there are maybe, 3 or 4 planks that probably don't need replaced.
As planks come off, I see a lot of flex in the ribs that is making me nervous. Should I start nailing in new planks, starting at the bottom and working my way up before totally risking the integrity of the canoe?
Or am I overly worried about structure?
If I understand the question, the way I do it is to replace one plank at a time. Two or three may let the ribs move too much? If you take off all but the 4 or 5 good planks the ribs could go everywhich way.
That's the answer. I must start replacing.
As I nail in the new planks, because I took so many off, I will need to eyeball the ribs before nailing to make sure they didn't drift....YES?